New Challenges in Tunnel Construction

### Chapter 6

Transit-Oriented Development Interactions on Existing Metro Systems: The Need for the Design of Adequate Structural Monitoring System and the Experience from International Projects

Evangelos Astreinidis

### Abstract

Contemporary metro transport systems present unrivaled efficiency for the commuting population. The development of the urban environment is interwoven with the metro transit systems. The transit-oriented development (TOD) is an upcoming topic in the design of the contemporary and of the future city and metro system alike. It entails the development of a microcell of the city centered around the metro station. Typically, bulky TOD buildings rise over and around the station and tunnel. The structural engineering aspect of these mega projects is highly complex. Major part of the complexity is due to complicated interactions between the oversite building and the underlying tunnel or station with its track-rail system. A significant number of issues arise, like methods to bridge over the tunnel or station, structural isolation, induced displacements to the track-rail system, tunnel movements and impact to tracks, vibration induction to the TOD building, and a plenitude of similar problems. It is highly important to design a structural monitoring system that will provide a validation tool of the structural-dynamic performance of the closed system TOD-tunnel/station. The distilled experience from international projects is presented.

Keywords: metro systems, tunnel stations, transit-oriented development, rail, track systems, displacement field, vibration field, structural monitoring, BIM

### 1. Introduction

This chapter is dedicated to a special engineering topic having to do with the structural interfaces of the metro to urban environment mega structures. The transit-oriented developments (TOD) are typically large real estate developments over or in the close vicinity of the metro stations. In the following, by the term metro is meant the whole metro installation including the tunnels, Stations and Switchboxes and all other structures like pop-ups and entrances. The social and financial importance of the TODs is very significant as they provide to the owner

(usually the metro owner) great marketing privileges. The same time they are regarded as a major step toward the sustainable urban environment minimizing the use of cars (see for example [1, 2], also www.tod.org). High rise buildings for office complexes, residential or hotel apartments, schools and hospitals, large malls and all other elements of the contemporary city life are built over or in walking distance to the metro. Thus, the technical concept of the TOD is threaded together with major structural issues. On one hand, the metro must bear TOD rising above and sustain the construction and service life loads and displacements envelopes safely. On the other hand the TOD shall be designed and constructed so that the inevitable effects on the existing metro shall be minimum and in all times within acceptable limits, without reducing the Design Service Life of it. The same time, the metro activities must not be blocked during the construction of the TOD or any case during the service life of it, while the noise and vibrations need to be filtered out on their way up to the residential areas of the TOD. In order to succeed in all the mentioned difficult tasks, special structural engineering considerations must be made and construction methods must be employed. The significant degree of uncertainty regarding the existing infrastructure is combined with the sensitivity of the metro and track rail system as well as the ambitious superstructure of the TOD giving a very cumbersome engineering undertaking. This makes unavoidable to employ methods to monitor the structural performance of both, the metro and the TOD. The monitoring system, as will be discussed in the following pages must be considered to have high specifications reflecting the important aspects of the analysis and design. It should never be considered to be a construction task left to the discretion of the contractor alone. It should be rather designed tailor made for the aforementioned engineering challenges and construction methods.

A typical TOD-metro combination is depicted in Figures 1 and 2. The figures come from a purpose made BIM exercise. In Figure 1, there is a bird's-eye view, while in Figure 2 there is a bottom view, to show the distinct parts of the combined foundation of the TOD and the metro.

In the next pages some important structural considerations shall be presented, as they come from the experience of the TOD design over the metro of Qatar (Figures 3 and 4).

### Figure 1.

Typical BIM overview of the structural compound of TOD (in gray) and the metro station (in white) and tunnel (red).

Transit-Oriented Development Interactions on Existing Metro Systems: The Need for the Design… DOI: http://dx.doi.org/10.5772/intechopen.86923

### Figure 2.

The same BIM model seen from a bottom angle to reveal the TOD piled foundation around the metro tunnels and structures.

### Figure 3.

Note the compound foundation system of the TOD. Part A is sitting on the metro, while part B is resting on a pile group.

More specifically, the following issues shall be discussed:


### Figure 4.

Typical example of railway protection zone and critical zone around the metro tunnel.


The challenges that are highlighted in this chapter lead to new unexplored areas which, as will be discussed below are:

i. Simulation and computing demands

This includes issues regarding potential Static condensation capabilities in Finite Element structural software packages and numerical treatments like rigid link considerations

ii. Multi-sensing monitoring—data fusion—situational awareness of Structural complexity

Special attention is given to the rail itself.


### 2. The framework of regulations for real estate development in the vicinity of the metro

The starting point for the design of any structural intervention in the vicinity of the metro installations is the set of rail authorities regulations as depicted in technical guidelines and standards.

Transit-Oriented Development Interactions on Existing Metro Systems: The Need for the Design… DOI: http://dx.doi.org/10.5772/intechopen.86923

It is very important to make a reference of such regulations in this starting paragraph, as these constitute a prerequisite to any construction or structural activity near the metro and provide a glimpse of the structural issues of the metro-TOD interaction.

In most countries with advanced rail assets there are regulations regarding the permitting for real estate developments and in general construction activities by any third party in the vicinity of the rail-track.

Typically, there is a definition of the rail corridor which follows in a notional path on the ground surface the route of the rail track either, tunnel, at-grade or elevated.

Into the rail corridor, in general, there are conditions and requirements to be fulfilled in order to permit the construction activities. There is however a narrow zone, the critical zone, around the tunnel (in the case of underground metro) where the construction is in general prohibited. It may be allowed only after meticulous calculation of the effects on the metro assets, the tunnel and the rail-track system. In this case it seems that the most suitable analysis of effects may come from a Finite Element soil structure interaction analysis which should be very accurate in the calculation of the displacements of the rail.

In Table 1, a typical grading of the restrictions onto the railway corridor is depicted, depending to the proximity to the rail assets.

In general, into the Protection Zone there is a requirement for application for permit. During the review of the development all aspects of safety and operation of the railway are considered. There may be restricted construction activity, but the development is in general possible. The restrictions reflect the three major clusters of risks for the railway. The first is the risks related to potential damage to the structural part of the railway infrastructure like the tunnel concrete lining, the station, emergency exits, pop-ups of all kinds, etc. The second is the potential effects or blocking of metro operations due to construction activities close to entrances or pop-ups. A typical example is the risk of flooding to the existing metro installations due to excavations, earth moving and other construction site operations. But the most sensitive family of risks lays in the effects of the construction on the rail displacements. The tolerance to the rail displacements is always very small. The reader is referred to [3–7] for detailed presentation of the capacity of the track system to absorb displacements induced by the various construction phases. Especially the Deutsche Bahn standards are very sensitive to the lateral displacements at all expansion joints. For example, the expansion joints between the station to the switchbox, or even worse the tunnel to the station are very critical areas to check the induced displacements due to the construction program of the TOD.

It is the experience of the rail and metro authorities, that a soil structure interaction analysis will reveal the effects on concrete structure of the metro and the induced displacements on the rail. Moreover, some special construction restrictions


Table 1.

Grading of the construction activities depending on the proximity to the rail asset.

or requirements should be imposed. For example the piling into the influence zone and especially into the critical zone may be required to include sleeves in order to reduce the development of pile skin friction. The pile sleeves are usually required down to approximately the depth of the tunnel invert, or to a depth under which there is no practical effect on the tunnel or the rail.

Monitoring of the tunnel or station and especially of the rail is a very important requirement in this case. The necessary accuracy and precision is important to be validated, but equally important is the real time character of the monitoring and reaction time for application of mitigation measures. A more detailed analysis of monitoring requirements shall be made in Section 3.9 of this chapter. For the purposes of the current paragraph it suffices to emphasize the real time character of the monitoring coupled with readiness of suitable mitigation measures.

Therefore, for any real estate development project within the rail corridor, the rail authority sets the following considerations regarding the existing metro structural assets:

Critical structures and issues:


These considerations should be a prerequisite to any transit-oriented development structural design. Moreover, the structural monitoring requirements focused on the rail behavior, usually becomes the central part of the broader monitoring program of the TOD. The structural assessment of the existing Rail infrastructure becomes the starting point of the design phase. A more detailed presentation of the structural assessment of the existing structures shall be made in Section 3.2.

### 3. Transit-oriented developments over existing metro installations

### 3.1 Building extension over the metro station: the modeling issue

In this section it is asserted that indispensable part of the structural analysis and design of the transit-oriented development (TOD) structures is the study of the effects on the existing metro structures (metro). Moreover, as the structural response of the TOD depends heavily on the underlying metro, the numerical simulation must include both of them. It should be highlighted that structural

### Transit-Oriented Development Interactions on Existing Metro Systems: The Need for the Design… DOI: http://dx.doi.org/10.5772/intechopen.86923

interaction between the two structures TOD and metro exists even if the direct mechanical connection is partial.

Experience from TOD projects shows that it is a widespread belief among the various stakeholders that whenever there has been provision of a future Oversite building loading during the initial design of the metro, design work for the TOD could be eased and limited only to the superstructure part alone, i.e., the TOD itself. A widespread belief is that probably, the only care for the metro should be not to exceed the "allowable TOD reactions" for which it was initially designed. However, this is very rarely enough, as the existing metro forms part of the total structure. In other words the TOD and the metro are a combined structure. In this case, the stakeholders need to understand and consider the cost and risks that stem from the combined metro-TOD structure. More specifically, significant effort is needed to incorporate into the design of the TOD the metro as part of the total structure.

A dominant parameter for the design of the existing metro stations and tunnels is undeniably the future prospect of a major oversite building. This is reasonable and expected as a high rise superstructure connected to the underground metro installations, induces significant reactions and displacements. This is primarily due to gravity loads, but lateral loads (due to wind and earthquake) exerted on the building may also produce dominant loading combinations. Critical construction stages which may include one side excavations, leading to unbalanced lateral earth pressures, also turn out to be very demanding tests for the metro. Depending on the construction program the loading on the metro may be highly non-uniform, leading to structural loads for which it is not designed. At the time of the design of the existing metro structures, the actual architectural configuration of a future oversite building or buildings may have not been entirely known. Far less would be known the construction stages necessary for the actual TOD. Therefore, gross assumptions may have been taken into the structural design regarding the prospect transitoriented developments. These assumptions comprise an initial necessary condition for the structural design as far the bearing capacity of the metro is concerned. The satisfaction of the necessary and sufficient condition for the metro to undertake the effects of the TOD is checked by the simulation of the precise configuration of the complete connected structure of the underground metro installations and the TOD superstructure, following the real construction phases. Both the existing metro and the TOD along with their structural connections and TOD independent foundation elements should be simulated as accurately as possible.

The ideal method to perform this simulation would be to incorporate in one model the whole metro and the TOD superstructure. The continuous modeling of the total structure, underground part and superstructure, in one model provides an uninterrupted displacement and stress flow. Arguably, this leads to safer results than any set of separate analysis of the metro, the TOD and their subassemblies. The benefits of the unified approach are pronounced in analysis for lateral loads from wind and earthquake. The computational cost, however, becomes an unsurpassed obstacle for this. The trivial method is to subdivide to two substructures, the underground metro box and the superstructure TOD and perform separate analyses. In this case the risk lurking is the intermediate and tedious phase of manual insertion of reactions from one substructure to the other. Apart from being a very tedious procedure which leaves large room for error, the risk of inadequate simulation of the total-unified response is always there. The soil-structure interaction in this case proves to be far more complicated, especially when the construction phases of the station box and the TOD building are distant to one another. A comprehensive study of this subject has been elaborated by O'Riorden in [8].

A substructuring method, if available, using static condensation to form superelements for the underground part and the superstructure would provide better

control of the modeling work and better approximation of the structural response of the total structure.

### 3.2 Structural assessment of the existing metro assets

It is highly recommended to perform an initial analysis of the metro as it is, prior to the introduction of TOD or its construction phases. This should be part of an initial structural assessment of the existing structure. This initial analysis may help to understand the structural response of the initial structure and reveal sensitive areas and critical failure modes.

Significant parameters to consider at this phase of the design have to do with the standards with which it was designed, the foreseen and the actual loading conditions but also with the current structural condition of the metro. Especially the crack formations and potential water ingress through cracks, voids, and construction joints, need to be taken seriously into account. The corrosion level of the reinforcement should be considered in case there are signs of corrosion or even better if there are measurements for the corrosion potential. It must be born in mind in this case that an allowance for the structural capacity should be made to simulate the cracking and the reinforcement section loss. The designer should carefully consider the actual stiffness of the metro. Potentially, a reduction of the modulus of elasticity should be considered.

In the structural monitoring section of this chapter, it will be mentioned that corrosion monitoring and mitigation should be introduced and should become part of the holistic structural health monitoring schedule of the total structure (metro and TOD).

In order to confront such structural assessment and modeling issues, appropriate sets of standards should be employed (see for example [9–12]). The standards for the assessment should deal with the necessary investigative works for the potential concrete deterioration, precondition surveys, etc. as well as the bridging of the design standards used for the existing structures and the current ones with which the TODs are under design. It is very important also that the design code employed will provide guidance on special modeling aspects pertinent to the special structural system of the combined TOD and Station box. American set of standards like ASCE-41 [9] and FEMA 356 [10] are more adept to provide guidance for existing structures of such importance. They also provide significant guidance for the numerical modeling. In the Euronorm family of standards, one can seek limited guidance in EN1998-05 (although this standard is dedicated to earthquake engineering) and the Greek Code of Structural Interventions [11]. Qatar Rail has compiled a necessary set of investigation practices as well as structural assessment best practices in Ref. [12].

Realistic reanalysis of the structure must not be over conservative. If it's conservative it'll prove the tunnel or the station inadequate to withstand the loads or the induced displacements. Careful selection of assumptions must be made in order to depict the real structural condition. Typical example is the simulation of the massive concrete joints of the existing structures. For example the "knee" and "tee" joints of the station box and also the joints of the columns to the massive bottom and top slabs require careful design of a rigid link.

### 3.3 The transfer systems

The challenge of having a transfer system spanning over typical metro stations has been detailed in many significant publications, with some key references can be found in [13, 14]. The typical transfer slab is characterized by massive concrete and usually by a small span in comparison to its longitudinal direction. In what follows,

Transit-Oriented Development Interactions on Existing Metro Systems: The Need for the Design… DOI: http://dx.doi.org/10.5772/intechopen.86923

structural members made of massive concrete (MC) shall mean members with thickness more than 1.5 m and which are prone to exhibit high temperature gradient across the thickness during concrete hardening.

Figures 5 and 6 depict characteristic configurations of transfer slabs spanning over stations.

In the case of direct connection of the TODs to the station box a secure path of stress flow from the superstructure to the station box is needed. The station box acts to a great extend as a foundation to the TODs, as a multi-cell concrete box circulating the stress flow between the lining walls and the main slabs (base slab, concourse and roof slab). However, there are sensitive structural members of the interior of the station box, like internal columns and stress concentration points due to openings that set limitations to excessive deformation due to the effects of the TOD superstructure. The most efficient way to transfer the TOD's actions on the station

### Figure 5.

Typical configuration of a transfer slab over a metro station. Note the oversite development (OSD) building transfer columns and their load redistribution role.

Figure 6. Transfer slab (a) at elevated position and (b) transfer girder.

box's hard points is the utilization of a transfer system that passes the reactions of the TOD's columns and cores at selected anchor points of the lining walls of the station (see for example Figures 7 and 8).

This transposition of vertical forces can be done either in the form of a massive concrete slab resting right on top of the lining walls and spanning over the roof slab of the station, or at any level above and well into the structure of the TOD (see for example [14]). Massive concrete considerations must be performed, including thermal stress, early age and long term cracking, etc. Heavy post tensioning sequence steps must be considered given the span of the slab over the station. The massive character of the slab helps to absorb vibrations from the station before they enter into the superstructure of the TODs, performing thus as the first line of defense for vibration isolation. In the case of the transfer system at elevated position, a hanging system is formed, usually employing deep steel trusses.

From the structural system point of view, the method of the massive concrete transfer slab resting in close proximity to the station box seems to have great benefits. The center of gravity is close to the station-foundation and provides small lever arm to the top of the "knee" joints of the station box. The connection to the lining walls can be made to behave as a pin with little effort. A potential draw back of the position close to the soil surface is the exposure class for the durability design, which in this case is harsher.

On the other hand the pop-ups of the station box and other surface utilities may obstruct the position of the transfer slab at that level. The architectural and mechanical engineering requirements are many times such that the position of the transfer system at an elevated position is one way to go. The lever arm of the mass

Figure 7. Idealized "module" of TOD "loading" all along the lining walls of the station box.

Transit-Oriented Development Interactions on Existing Metro Systems: The Need for the Design… DOI: http://dx.doi.org/10.5772/intechopen.86923

Figure 8.

Actual TOD building configuration on top and besides the station box—in contrast to Figure 1 of the initial concept. The reader should notice in the cloud the half-treading of the building only on some part of the station.

of the transfer slab is then very great, producing significant bending moments during lateral loadings. The construction of the slab at an elevated position is by itself a significant construction task, involving difficult logistics and heavy falsework the design of which is a cumbersome undertaking (Figure 9).

### 3.4 D and B regions in concrete beam analysis

Flexural behavior of beams and therefore slabs is well coded in international standards providing specific conditions apply which signify a more-or-less known stress flow pattern. This behavior is generally known as Bernoulli or Beam type (B type) flexural behavior as it complies with Bernoulli conditions of flexural behavior. In contrast, areas of beams or slabs that do not fulfill the necessary conditions for a Bernoulli type flexural behavior present a more complex, unpredicted and spurious stress flow, which necessitates delicate computational mechanics analysis methods. They are usually designated as D regions, named after the disturbed or discontinuity regions (see Figure 10). The only closed-form analytical method alternative to expensive Finite Element methods accredited by international codes is the strut and tie method. The success of the strut and tie method relies upon the experience and "art" of the modeler. Usually, it is employed for validation purposes rather than for the main design procedure. Certainly, the criticality of the transfer slab and the risks/impact involved requires a fool proof method, capable for representation of the full stress field in every region. These requirements are very high, well above the capacity of the ordinary Finite Shell element method or the strut and tie method.

The transfer beams and slabs differ from the ordinary, so as to say, everyday beams and slabs due to the ratio of their thickness to their length. There is no doubt that these slabs contain areas that belong to the D regions. Their thickness to span

### Figure 9.

Typical examples of the diverse methods of transferring the load through slabs-girders—trusses.

Figure 10. Typical examples of D and B regions in transfer systems.

ratio and the MC joints produce stress flows that do not comply with Bernoulli beam bending theory. The classic Bernoulli beam bending theory may only be valid under certain conditions for it to be applied. Thick shell Finite Element methods can in some cases be a remedy, but the problem may persist in cases where the geometry does not allow discretization with shell or line elements anyway. The consideration of appropriate numerical treatment of D-regions, or in other words a nonshell-element FE area may be decisive for the correct analysis (Figure 11).

Moreover, all the mentioned D regions and concrete joints require dedicated methods of analysis, apt to simulate the Saint-Venant principle due to the

Transit-Oriented Development Interactions on Existing Metro Systems: The Need for the Design… DOI: http://dx.doi.org/10.5772/intechopen.86923

Figure 11.

Characteristic case where FE shell elements without treatment of critical areas is inappropriate. The bending moment at the connection of the wall to base slab is 66% higher when rigid link is used.

complicated stress bursting there. Posttensioning of the slab or girder contributing to strong stress bursting exacerbates the analysis complexity and increases the finite element requirements.

Methods capable to perform accurate simulation of the stress-strain flow of the transfer slab are either numerical simulations based on solid-3D elements or laboratory tests. The most accurate Finite Element methods to treat the MC and D region problems incorporate 3D and non-linear analyses. Thus, the work load and numerical effort needed increases significantly as well as the assumptions log.

The transfer slab analysis must take into consideration the massive concrete nature of the particular structural element, especially at its connections. The classic linear analysis, as this is offered by linear shell element analysis, may be inappropriate. The linear beam analysis assumptions, and especially the Bernoulli assumption of plain section rotations, usually are not applicable. The nature of this structural element is a thick shell. The infusion of stresses into the transfer slab is following a Saint-Venant bursting stress pattern that is not possible to be simulated with a mere rod-like line element approximation. Either an efficient local Finite Element analysis should be employed, or an appropriate strut and tie local model should be used to simulate the stress distribution in the joint.

To this point it is important to note that the concrete joint analysis of both the underground structure and the transfer system or other similar TOD massive structures must take into account rigid link analysis, wherever appropriate. As far as linetype elements, ASCE [9] and FEMA 356 [10] provide guidelines for the estimation of the length of the rigid link, with the ASCE standard being somewhat more sensitive to the actual concrete configuration. Birely et al. [15] offer a more delicate approach to this critical FE analysis issue (see also [16, 17]). Similar approaches should be followed for the MC joints formed by the surface members of the metro and TOD. The MC joints formed by the slabs and walls of the metro and the TOD transfer slabs and their connections are similar to the line element joints for which the mentioned literature is dedicated, although the actual engineering mechanics problem may be somewhat more complicated due to the influence of the stress components in the cross direction to the joint. It is to be noted that there aren't enough experimental or numerical analysis results published for this type of concrete connectivity.

In any case, neglecting the rigid link at the massive concrete connections may result in significant underestimation of the developed stresses and consequently the reinforcement requirements. Apart from the pure engineering mechanics fact that the core of the MC joint behaves differently from the field of the converging wall and slab, there is also a simulation necessity to incorporate a type of a rigid link. In Figure 12, it is evident that a refined FE discretization is not appropriate to simulate

### Figure 12.

A typical FE discretization of part of a station box. The actual concrete joint cannot be represented properly by the selected FE modeling. In the current configuration, which does not have any modeling treatment like a rigid link, the FE nodes that are located into the actual concrete joint do not represent the geometry or the stiffness of the joint. Obviously the shell-like representation chosen is inappropriate there.

### Transit-Oriented Development Interactions on Existing Metro Systems: The Need for the Design… DOI: http://dx.doi.org/10.5772/intechopen.86923

the core of the joint with the same type of FE as with the fields of the slab or wall. A similar geometrical problem is evident at the connection of the columns to the MC slabs—with more important the thick base slabs. If no rigid link is introduced, then the size representation of the column may be largely different. Potentially, an envelope of a lower and upper bound stresses corresponding to analysis with and without rigid link may provide a conservative approach.

### 3.5 Connection of the transfer system to the metro structures

In the case of transfer system at an elevated position, the connection of it to the station box has a further complexity. Necessarily there will be vertical elements connecting it to the lining walls, but in this case significant bending moments will be induced into the "knee" joints of the roof slab (Figure 13).

This is unavoidable, because of the lever arm of the transfer slab and the slenderness of the vertical connection elements. The analysis of the existing "knee" joint must incorporate the joint stress flow appropriately. A pure and genuine pin and bracket connection is usually prohibitive, due to the magnitude of the gravity loads of the TOD. The requirements for water tightness and durability add greater difficulty in properly designing and constructing such configuration.

On the other hand an existing concrete station box may not have been designed for such additional bending moments, as, in the usual structural provision for future extensions, only vertical and horizontal components are considered. Therefore, the structural assessment of the existing structure must include the investigation of the "knee" joints of the existing metro structure.

A typical approach for reducing the bending moment development on top of such "knee" joints of the existing structures, involves a doweled connection configured primarily for shear (see Figure 14). It is noteworthy, that such a configuration may be simulated by a pure pinned connection for the TOD building with sufficient accuracy. This however, does not apply for the underground structure. Parasitic bending moments shall be developed, which may prove to be significant for the concrete shell of the existing structure.

In such a case, it is necessary to investigate the actual rotational spring acting at the interface of the new wall or column resting on the existing "knee" joint. Either the actual moment rotation diagram of the concrete cross section may be used, or even a potential works analysis of the structural element rising above new structure may be used, to provide the rotational spring at that support of the TODs. A

### Figure 13.

Typical configuration of the transfer slab concept on top of the station roof. Note the close position of the rigid transfer slab to the top of the lining wall. In this case the concept of pinned connection may work sufficiently.

### Figure 14.

Typical development of parasitic bending moments due to dowel articulated connection between TOD supporting wall and station "knee" joints over the lining wall.

### Figure 15.

Connection of the TOD to the station "knee joints" of the station roof. The connection is meant between retaining walls and lining walls.

subsequent building analysis shall provide the parasitic bending moment, which in most such cases proves to be not negligible at all (Figure 15).

The usage of the transfer system smooths out the concentration of vertical forces induced by cores and arbitrary distribution of vertical elements in general. If, however, there is partial occupation of the plan view of the station area, then there is necessarily a bending effect on the station box that the transfer slab cannot mitigate. This is further aggravated by the lateral loads (wind and earthquake, if applicable) that will induce a torsional deformation shape to the station box. Openings and entrance interfaces will develop significant stress concentrations that need to be checked. A critical point may be internal columns. These may already have a significant utilization factor, due to the complex behavior of the station stress flow. Given the bending and torsional deformational behavior of the station box, due to the TOD effects, buckling and punching at the pier heads shall be checked thoroughly (see also [17]).

### 3.6 Thermal dissemination due to massive concrete hardening

One major challenge of MC is the significant thermal loading developed during hardening and the corresponding thermal strain gradient throughout the cross

### Transit-Oriented Development Interactions on Existing Metro Systems: The Need for the Design… DOI: http://dx.doi.org/10.5772/intechopen.86923

section of the member. As the thickness of the MC is significant there are great temperature variations between the core and the outer skin of the member. It is noted that, this type of temperature loading is a highly non-linear and depends heavily on the thickness of the structure and the construction phases and joints provided. In the overall performance of the member, post tensioning analysis should take into consideration the residual stresses and strains due to hardening thermal loads. When highly aggressive environments must be considered, like for example in Qatar and in the Arab gulf states, in order to combine the necessary mechanical strength required and also to withstand the local weathering effects by corrosive agents and sulfate attack, a typical choice is to use high strength concrete mixes that provide solutions to many of the mentioned challenges. Concrete mixes with high percentage of binder like Grand Granulated Blast Furnace Slag (GGBFS) becomes often the choice. Nevertheless, there is still high risk for internal cracking of the MC, and this requires careful thermal dissipation analysis, shrinkage restraint analysis and construction joints design. Multiple layer construction and post tensioning increase the analysis and design demands.

The restrains to the slab or beam contraction may also be an additional analysis task. This is especially true in the case of MC transfer slabs acting as pile caps as shown in Figure 16. The pile head rows provide multiple restrain lines as they act as anchor points. The shrinkage and thermal loads may impose to the slab significant contraction strains and consequently the piles may experience very high shear loads. Crack development of the slab becomes the major design concern. FE analysis of the restraint of the slab due to shrinkage should rather be made on conservative assumptions. As far as the early age thermal cracking, CIRIA 660 Report [18] on early age thermal crack control offers some guidance on the type of restraint

### Figure 16.

Arrangement of the TOD over the tunnels and the pedestrian walkway. The transfer slab resting on the heavy piling in this case has a significant dimension. The massive concrete thermal considerations must be combined with the restraining effect of the piles.

formed. The analyst however, should be very cautious in his simulation of the boundary conditions whether there are conditions favoring end restraint to the notion of CIRIA 660. Stress concentrations due to abrupt stiffness changes, thickness, holes, may favor localized cracking rather than a somewhat evenly distributed crack pattern similar to the edge restraint pattern implied by CIRIA 660.

It is very important to perform mock ups of the transfer systems for calibrating the whole concreting procedure. Fruitful results also become available for the detailed design. Even so, structural monitoring of the performance of the actual transfer system is necessary. This is further discussed in the later section of this chapter regarding monitoring. Here it suffices to note that the transfer system is probably the most critical structure of the TOD-metro complex. The actual final condition of it will define its own structural performance, but also the structural performance of the total structure. The challenges due to massive concrete concreting, the actual stress flow into the massive concrete joints, the post tensioning, and the laminated concreting make the structural monitoring an indispensable validation tool.

### 3.7 Noise and vibration issues

The train noise and vibration emissions due to the metro operation are a major concern for the near-by Real Estate developments in general. This is a highly specialized technical topic that deserves a dedicated presentation which unfortunately falls outside the main scope of this chapter. The main effort in this work is toward the more traditional structural analysis and design issues. It must be said however, that the TOD configuration dictates to a great degree the transmissivity of noise and vibrations coming from the operation of the metro. It is indeed very possible that the structural configuration as well as a wide range of material choices may be defined by it. Therefore, careful cooperation between architectural and structural design should incorporate also the noise and vibration transmission analysis.

It must be noted that the international experience shows that the vibration transmission can take place through the piles as well. Therefore, soil-structure interaction for the vibration transmission must take place.

The transmissivity of the vibrations through the station to the TOD is much different to the one from the tunnel to the nearby buildings, at the time of the tunnel boring. Therefore, even if there may be analysis and recording of measurements from the tunnel construction phase new measurements are necessary for the new compound structure TOD-metro.

The most widely accepted set of acceptance criteria is probably FTA Report Transit Noise and Vibration Impact Assessment [19] (see also [20, 21]) .

There are no effective mitigation measures known to amend a vibrations inflicted development a posteriori. The vibrations prediction must be complete and influence the structural/architectural design.

In general, a Finite Element model may be used for this. In some cases, the FE model is possible to play a dual role for structural-strength considerations and also for structural vibrations analysis. However, the discretization for vibrations analysis follows different and more stringent rules than the one for structural analysis. It might be practical however, to utilize the FE structural modeling for the vibration modeling as well.

### 3.8 Special geotechnical issues

The soil structure interaction may be so great that the structural effects could be savaging to the metro structural infrastructure. In Figures 17 and 18 below the

Transit-Oriented Development Interactions on Existing Metro Systems: The Need for the Design… DOI: http://dx.doi.org/10.5772/intechopen.86923

### Figure 17.

Plan view of excavation next to the metro, station-tunnel. The unbalanced lateral earth pressures need to be considered for each construction phase.

### Figure 18.

Due to excavation only from one side of the station box, significant unbalanced earth pressures raise high stress distribution at the head walls. Other distressed areas include the column pier caps and the tunnel to station expansion joint displacements.

effects of the excavation next to the station concrete box have been analyzed to show the unbalanced lateral earth pressures significant effects. The otherwise resilient thick concrete shell walls of the station box are experiencing significant stress development which needs to be checked for the main failure modes (cracking water ingress). Nonetheless, slender structural members like columns, need to be checked for their predominant failure modes, like buckling or punching through their pier caps. Due to the unbalanced earth pressures, an overall box torque-like

Figure 19. Desired "ramping" of the displacements.

displacement field may is induced which, among other stresses, increases the bending moments at the pier heads, making the punching check more demanding.

Regarding the very sensitive rail track displacement issue, Figure 19 is very depictive about the tight tolerances. In practical terms, the expansion joint opening tolerances may be limited to a tight 10 to 15 mm depending on the joint capacity and the waterproofing or gasket allowance. The allowable displacement envelope induced to the rail however is far more restrictive. Refs. [3–5, 7] sited in Section 2 should be considered regarding the rail displacement. However, the Rail authorities concern is usually, to grade the induced displacement to enough length. In general displacements of the order of 5 mm are tolerable, but the most important characteristic of the desired displacement profile is the spread over a significant length of the rail in order to achieve a tapered deformation profile (Figure 20).

Typical allowable requirements for the rail into the tunnel and at the expansion joints are displayed in Table 2.

Figure 20. The expansion joint opening tolerances may be limited to a just a few millimeters.

Transit-Oriented Development Interactions on Existing Metro Systems: The Need for the Design… DOI: http://dx.doi.org/10.5772/intechopen.86923


### Table 2.

Suggested warning and alarm levels of displacements into the metro tunnel.

The tunnel concrete shell is highly susceptible to induced displacements because of any third party construction activities, let alone the TOD loadings. Moreover, the tolerances of the rail-track system are an even greater challenge. In contrast to the concrete shell which can sustain displacements of the order of 15 mm, the requirements regarding the rail are far more restrictive. Depending on the Rail organization, approximately, a mere 5 mm are tolerable and that should be spread over a span of 10 meters over the rail length (see for example [22, 23]).

At the interfaces of the Station/Switchbox—Tunnels, or any other type of expansion joint, there may be a sharp rise of differential displacements due to the TOD. Significant design effort must be put to make sure that the differential displacements of the rail are within the allowable limitations.

Furthermore, in the case of ring segmental tunnel, it must be preserved at all times in compression, in order to avoid tension cracks or unacceptable movements at the segments' interfaces. Therefore, significant effort should be provided in FE analysis to accurately detail the effects of the TOD on the tunnels and especially at the interfaces to the station. If piles are needed to pass close to the tunnels, significant effects are expected to the tunnels in terms of displacements and stresses. Single or double sleeved piles could be used in order to reduce the pile-soil friction and avoid stressing the tunnel to the degree possible. A comprehensive numerical study of the simulation of the sleeved piles in such cases can be found [24]. It must be always remembered however, that the friction reduction methods could not eliminate completely the friction, allowing therefore, a significant stress field to affect the tunnel or even the station box.

The arrangement of the foundation system of the TOD alongside the tunnels poses a significant challenge. In order to avoid stressing the tunnel lining or inducing unacceptable displacements, it often required to use pile skin friction reduction systems. The sleeved system offers a reduced pile skin friction but does not eliminate it. It must be remembered that it does not get reduced to more than 50% or maximum 30%. It is very difficult to verify what exactly has been achieved during the construction of the pile. Therefore, it is strongly recommended to follow a very conservative approach in the design, especially regarding the friction developed at the sleeved part of the pile. The vertical springs that simulate the friction should be appropriately treated in the model, while the lateral ones, which simulate the lateral resistance, should not be forgotten (Figure 21).

The pile cap will be a major source of bearing pressure to the soil, if in full contact, despite the axial resistance of the piles that they connect. Therefore, if a separation cannot be achieved, a cushion material should be designed and suitably inserted to detach the slab from the soil, as shown in Figure 22. This is not an easy task at all, either to design or to implement in construction. The degree to which this detachment is achieved is difficult to be verified.

Since there are so many sources of uncertainty, it may be wise to consider in the design envelopes of upper and lower bounds of induced stresses to the tunnel,

### Figure 21.

Excavation for the TOD next to the tunnels. Experience shows that even at hard soils, like the Simsima Limestone of Doha, there may be significant displacements to the tunnels. And while the effects on the lining may be tolerable, the displacements to the track system may be prohibitive if no stabilization methods for the excavation are not in place.

### Figure 22.

FE analysis of effects of progressive excavation and TOD construction over the tunnels with segmental lining. As the piles are sleeved all the way down to the level of the tunnel invert, their representation into the FE model includes removal of the pile skin friction to a specific degree. This reduction is not suggested to be more than 30%.

representing the maximum and minimum friction sleeve isolation and soil pressure by the pile cap (Figure 23).

In Figure 24, a typical arrangement of geotechnical instrumentation as is shown. With such an arrangement it is possible to monitor the soil mass, and probably this may lead to reliable conclusions regarding the stress-strain conditions of the tunnel lining or the station (see also Refs. [25–27]). This can be achieved by comparing the readings of the instruments to the values of the displacement—stresses from the soil structure interaction analysis. But it becomes entirely indirect in the case of the more sensitive entities like the rail itself or even the expansion joints. In order to obtain the picture regarding the actual stress-strain conditions of the tunnel or the station it is important to have a direct instrumentation attached on carefully

Transit-Oriented Development Interactions on Existing Metro Systems: The Need for the Design… DOI: http://dx.doi.org/10.5772/intechopen.86923

### Figure 23.

Typical arrangement for pile debonding. Notice that the lateral support is not eliminated and should be considered in the simulation accordingly.

### Figure 24.

Plan view of new construction next to the metro station and tunnels. A typical distribution of geotechnical instruments to monitor the surrounding soil of tunnel and station. Notation: EXT: extensometer, I: inclinometer, W: water pressure meter, P: piezometer.

selected positions into the structures. A more direct strain-stress distribution insight can be gathered from the instrumentation of Figure 25, where a fiber optic instrumentation is shown to "wrap around "the tunnel lining for a critical length. Such strain instrumentation is useful to provide the picture of the tunnel lining distortion at a critical are of interaction with the TOD or other third party construction activities. But the cost in this case becomes too high. On the other hand, if a mountable type of sensor is used, then the instrumentation can be retractable and is

### Figure 25.

The original FBG sensor, which is fabricated to operate in axial tension, has typically excellent characteristics in terms of accuracy and repeatability. When used into a compound sensor, to monitor other degrees of freedom (like rotation) may have dramatically reduced its fine characteristics. Testing and calibration are indispensable in such cases.

not "spent" in the specific section of the tunnel. It then can be used for a period of time to measure the effects caused and when requested can be transferred to another section. Only the bases to which the sensors are fixed are spent. The drawback in this case is the accuracy of the mountable sensor. It must always be remembered that intrinsic effects of the sensors are the gauge length, the attachment and the fixation, even the rotation of the attachment to the desired direction.

### 3.9 Structural monitoring

The structural monitoring of buildings although not entirely rare does not take place often. The experience built up until now is not comparable to the one of the geotechnical monitoring. A comprehensive presentation of recommendations and best practices can be found in Ref. [28]. It is the intention of this chapter though to support that structural monitoring is a necessary and indispensable part of the design of the TOD-existing metro compound. It is not an issue to be left to the

### Transit-Oriented Development Interactions on Existing Metro Systems: The Need for the Design… DOI: http://dx.doi.org/10.5772/intechopen.86923

contractor alone, as it seems to be a widespread belief. There are many reasons why the designer should specify the structural monitoring instrumentation and its monitoring targets. These include verification of critical design parameters (and how these evolve in due service life time) and construction loading parameters which affect the existing metro structures and for which there may be many unknowns. In general, in the consideration of the compound TOD-metro structure there is accumulated significant uncertainty mainly due to the long assumptions list for the behavior of both parts but more importantly of their interaction. Tolerable displacements of each part of the compound structure may mean significant change in the boundary conditions of the other part. Moreover, the interface between TOD and metro, as configured in the transfer systems may have itself significant uncertainty built up. To give a few examples, concrete deterioration of the transfer slabs or girders may mean not only reduced service life of the members themselves or of the TOD but also altered reaction envelope for the metro, which receives them. Reversely, concrete deterioration of the metro may ignite change in the boundary conditions for the TOD. Concrete deterioration may be of limited or controllable character for the structural member itself, but may produce disproportional alteration to the stress distribution over the TOD-metro compound.

Exactly because such reaction changes at the interface of TOD-metro due to any damage accumulation or structural performance are unpredictable it is necessary to monitor the condition of the critical structural members. The transfer systems in this endeavor are high in the priority list.

The nature of the transfer systems makes the employment of advanced monitoring instrumentation necessary. The target of the monitoring should be the validation of the stress-strain levels, the displacement field and also the structural condition of the members monitored. In order to make possible to synthesize the stress-strain gradient picture throughout the total section of the member the instrumentation should have robust characteristics (Figures 26–28).

• Robustness and redundancy of sensors: As a basic rule, it's highly preferable to have not only robustness in the sensing systems but also redundancy of the sensors, despite the additional costs. Potential damage of the cabling of the sensors during the construction shall reduce drastically the life span of the instrumentation. Therefore, significant budget for SHM is dedicated to cable protection in the form of conduits, extra protection for cable splices, sensor

### Figure 26.

A triple layer construction sequence envisaged for thick slab transfer system of a TOD over the metro tunnels. Note the post tensioning tendons and proposed arrays of strain-thermal sensors.

### Figure 27.

In this transfer steel truss system the members marked with red symbol are of greatest concern to the designer. He therefore designed a strain monitoring system to validate the stress-strain development during the erection of the structure and building up of the loads.

### Figure 28.

A situational awareness visualization tool is necessary to obtain the overall picture and the same time to be able to locate on the BIM the section of interest to get the sensor analysis in detail. In the figure each green dot on the top of the structure represents a set of two sensors. Their stress flow history is shown in the bottom of the screen. On the right hand side of the screen their stress analysis level is compared to the green-to-red chromatic scale of alarm.

covers, etc. Moreover it's highly recommended to pass alternative or additional cable routes with redundant sensors at least at the most critical members/areas.


In order to achieve such characteristics, the designer should specify wisely the instrumentation characteristics. Parameters to be considered should include:


Investigate:


At the sensor points a member cross section analysis can be performed to provide the distribution of stresses and strains to compare with the monitored strain distribution. It follows that careful positioning and attachment of the sensors is necessary. In order to obtain a clear picture of the stress condition of the structural member and the notional rotation of the neutral axis it's rather necessary to use more than one sensor at every cross section.

A significant part of the structural monitoring, which deserves separate analysis is the corrosion monitoring. Corrosion of reinforcement and especially of tendons is critical concrete deterioration which must be early detected and mitigated. Instrumentation for corrosion monitoring may in some cases be combined by cathodic protection infrastructure as shown for example in Figure 29. The inspection port is meant to be used for incipient current cathodic protection. Otherwise, separate apparatus may be installed. This may be comprised by embedded half-cell potential wiring, reference electrodes, or chloride ingress detection devices. The risk of corrosion of the post tension tendons is especially high in structures close to the ground and to ground water. In regions where the soil is rich in chlorides like in the Arabic Sea states, even underwater there is significant prospect of corrosion, through formation of macrocell development. This is noted to happen in underground heavily reinforced concrete boxes of stations and switchboxes (see also [29, 30]). Macro cell corrosion, where the cathodic reaction taking place in an aerated zone of the structure and the anodic process taking place the depassivated area may produce non-expansive products making the phenomenon undetected until it will be too late. This has been found to be occurring in cases of post tensioned tendons and especially when the duct grouting is problematic.

Active corrosion protection is by far a more prudent way to secure the Design Service Life of the structure. The high costs for distributed anodes or incipient current protection pay off as a major concrete deterioration risk is removed. The hybrid type of anodes has shown excellent characteristics while they offer the advantage of periodic chloride ion removal (see also [29–32]) (Figure 30).

The application of such a method for active cathodic protection all over the foundation system of the TOD may be very costly. The critical structural elements however, should be seriously considered to be protected, especially when post tensioning is present.

### Figure 29.

Cross section of base slab to lining wall concrete joint. A monitoring terminal is shown. It is used for connectivity check, but can in theory serve for corrosion monitoring and incipient current.

### Figure 30.

(a) Distribution of hybrid anodes, (b) reference electrode, and (c) transmitter for remote sensing.

Transit-Oriented Development Interactions on Existing Metro Systems: The Need for the Design… DOI: http://dx.doi.org/10.5772/intechopen.86923

### 4. Verification, validation, and uncertainty quantification

From all the major points discussed in the preceding sections of this chapter it must have been evident by now that the structural compound of the TOD and the metro is a highly complex system. It is a complex system in the sense that it presents emergent properties: i.e., properties that are not apparent from their structural members when these are studied in isolation but which result from the relationships and dependences when they are finally constructed [33–36]. The assumptions log of the total structural analysis is being increased too much because of the interaction of structures and structural elements so different to each other. The predominant Failure modes of metro are different when the TOD is built over it, as the ones of the structural elements of the TOD when considered on a fixed foundation rather than on the metro. The displacement limits of the rail-track measured at millimeter scale are in significant contrast to the large scale Finite Element model of the metro-TOD compound which produces so great reactions in magnitude, has enormous computing requirements, it has many assumptions pertinent to large scale structural systems rather than the one comprised by the delicate rail-track system. Let alone the inherent uncertainties regarding the existing metro structural condition. The same time there are critical structural elements like the transfer systems that require a thorough verification of their functionality.

In order to keep the Uncertainty contained in acceptable levels, a rigorous Verification and Validation system is necessary to be employed. A far as design is concerned, it should not focus only on producing reliable simulations, in terms of Finite Element analysis and BIM, but it should focus on accumulation of evidence of the credibility of the simulation results. This should be an important element of the risk management plan for both the TOD and if possible of the metro.

It is asserted in this chapter that the laboratory testing of critical structural members and also data collection from structural monitoring are vital parts of uncertainty quantification procedure and that validation is achieved through compound simulation, testing and structural monitoring.

As has already been said in Section 3.6, mock up tests for concrete are very important. Mock ups of massive concrete would provide information about the actual heat dissipation flow of the specific concrete mix to be used. Strength build up, thermal gradient and thermal stressing are vital pieces of information that are needed for the most critical structural members of the TOD, like for example the concrete transfer slabs or girders.

Unfortunately, the uncertainty regarding the performance of them is deemed to built up with the size (not only the thickness), the reinforcement quantity, concreting conditions, weather conditions, concreting layers, construction joints, post tensioning and stress flow into the joints. Therefore, a good solution to cast light into all these complicated and intrinsic procedures, series of laboratory tests could be organized. The same time, data collected from structural monitoring are necessary to provide the picture of the actual developments of the monitored parameters.

A great benefit would be to identify the failure modes of the transfer systems. The laboratory testing should focus on each failure mode and organize precautions for them. The precautions should have the form of simulation calibration parameters for further analysis during the final Detailed Design phase, construction, construction sequence, maintenance and monitoring during the service time.

The risks from concrete casting in layers should be investigated. Thermal issues, de-bonding and post tension passing through the layers should be thoroughly studied. Again, guidelines for the casting and tensioning should be drafted.

The key attributes of simulation credibility are evidence of completeness and correctness, which must be communicated in an understandable and straightforward manner. In conjunction with simulation governance, the technical processes to build and assess correctness in simulations are verification, validation and uncertainty quantification.

This issue should be closely monitored with the structural monitoring means, to cast light in areas that will be critical in the actual TOD behavior.

The scope in this case of monitoring is not only the ultimate capacity, but also potential deterioration/change with time, as this would inevitably lead to magnified effects on the rest of the building or even the QR assets (stations-tunnels). Therefore and because of the massive concrete character of the transfer system, a wellstudied instrumentation strategy needs to take place. The performance of the instrumentation depends on the position of the sensors, the type of the sensors, the attachment method and many other factors. Fruitful results will lead to decisions about complementarity of the sensing systems, redundancy, accuracy of local measurement and overall structural response.

The simulation results are thus compared with sub-scale systems or portions of the complete system, such as subsystems and components of the total system. Formulation and approximation errors are commonly intertwined with uncertainties in the input data for the mathematical model. If for example, initial conditions, boundary conditions, or system parameters cannot be measured independently, then uncertainties in these quantities are entwined with model formulation and approximation errors. Therefore, model calibration becomes necessary. As a result, model parameter calibration and model validation involve an iterative process for cases where experimental data and data from structural monitoring are available. A potential procedural flow chart for Experimental and numerical simulation cross validation is shown in Figure 31.

There is one more thing however to consider regarding the VVUQ. This has to do with the method of the structural analysis itself. In order to utilize the VVUQ character described in the previous paragraph, it may be found practical to perform types of analysis resembling to Failure Mode and Effects Analysis, or even Fault tree analyses. This has to do of course with the critical structural members, the load paths utilizing them, and potential alternative load paths. A good paradigm of such approach is offered in Refs. [37, 38].

Ordinary structural analysis as ruled by international standards, (like the Euro Norms—EN1992 for example), does not categorize the structural members depending on their importance. It does not categorize or classifies potential failures, as the aim is to avoid either.

However, some structural elements are apparently more critical, or their integrity may be vital for the whole structure. Obvious examples in the case of the metro-TOD compound are the rail-track system and the transfer slab/girder respectively. But the complexity of the compound structural system pronounces the criticality of other structural systems as well. For example, a long span post tensioned beam is far more critical and should obtain a higher degree of structural importance than an isolated beam at a part of the structural system with large structural redundancy. Or equally, and to put it closer to the FMEA, a potential failure of a structural member that leads to progressive collapse is more important and deserves higher severity ranking than a potential failure of a structural member that will cause moment redistribution achievable by a robust and highly redundant structural system. The predominant failure modes of the transfer system of the TOD should be examined thoroughly from the perspective of progressive collapse, activation of alternative load paths, limitation-containment of collapse/failure and especially, away from the Station box and tunnel and at least away from the rail-track system. Obviously, the

Transit-Oriented Development Interactions on Existing Metro Systems: The Need for the Design… DOI: http://dx.doi.org/10.5772/intechopen.86923

### Figure 31.

Flow chart for cross validation of experimental and numerical simulation.

hierarchy of structural assets leads to prioritization of failure mode analysis and effects analysis.

In a potential FMEA there should not be a limitation only to controls for the typical Ultimate and Serviceability Limit states. Durability deterioration should be included in such investigations as well. Potential introduction of importance factors attributed to structural members should take into account the risk factors for damage or failure: Occurrence, Severity and Detectability.

### 5. Conclusions

This chapter is an effort to provide a glimpse of the multi-faceted design efforts for analysis and design of transit-oriented developments over existing metro structures. It has been pointed out that the existing metro stations and tunnels are very sensitive structures with very tight limitations for the displacement and stress envelopes induced by the TOD. The Structural interventions needed to

accommodate the transit-oriented development must be considered following a comprehensive structural assessment of the existing structures. The effort needed to model properly the combined TOD-metro structure is very significant. Careful procedures should be adopted for the modeling of the boundary conditions of the two distinct parts and of all critical structural members. Special Finite Element numerical treatments should be employed to approximate massive concrete. The importance of the Transfer system is highlighted, along with the potential detrimental factors that may affect its performance and that need special attention. The combined TOD-existing metro structure has got a long assumptions log which makes inevitable the structural monitoring and active corrosion monitoring and protection. The structural monitoring should be also considered as a vital tool for the validation of the design and construction assumptions and procedures.

### Author details

Evangelos Astreinidis Structural Engineering, Qatar Rail, Qatar, Doha

\*Address all correspondence to: eastreinidis@gmail.com

© 2019 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Transit-Oriented Development Interactions on Existing Metro Systems: The Need for the Design… DOI: http://dx.doi.org/10.5772/intechopen.86923

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### Chapter 7

## Innovative Concepts in TBM Tunnels

Silvino Pompeu-Santos

### Abstract

Tunnel boring machine (TBM) tunnels are increasingly used in the construction of transport infrastructure, allowing for reduction of the environmental impact and cost and time of construction. Despite these advantages, TBM tunnels still face major challenges such as further cost reduction, the structural safety under earthquakes, and the improvement of safety during operation in the case of traffic tunnels (rail and road tunnels). To overcome these challenges, three innovative and very cost-effective concepts for the construction of TBM tunnels were recently developed by the author: the tunnel of improved seismic behavior (TISB) concept for improving structural safety of tunnels on soft ground in seismic areas and the tunnel multi-floor (TMF) and tunnel multi-gallery (TMG) concepts for road and rail tunnels, respectively, which allow an even greater cost reduction and improvement of safety in operation. In this paper these concepts are presented as well as their application in some specific cases, emphasizing the obtained added value.

Keywords: tunnels, TBM, innovations, TISB, TMF, TMG

### 1. Introduction

Tunnels are increasingly used in the construction of traffic infrastructure for both rail and road networks.

The "tunnel boring machine" (TBM) technique is nowadays the most common, allowing for reduction of the environmental impact (in underwater tunnels allows for non-disturbance of the natural bed) and significant savings in costs and time. It has become usual to build more than 0.4 km of a TBM tunnel per month on average, depending on the specific ground conditions.

Despite great progress observed in recent times, the construction of traffic (rail and road) tunnels with the TBM technique still faces significant challenges.

Big issues are the improvement of the structural safety of the tunnels when built in soft ground in seismic areas, the decision on the number of separate tubes to form the tunnel with cost analysis, and the measures to provide safe evacuation of users in the event of an accident or fire inside the tunnel.

### 2. TBM tunnels

In the construction of tunnels bored with TBMs, the circular cutter head of the front shield of the machine excavates the ground as the erector mounts precast

Figure 1. Schematic view of a TBM.

segments around the excavated surface, which are clamped together, forming the circular wall (lining) of the tunnel (Figure 1). TBMs are of different types, depending on the characteristics of the ground to be bored (EPB, mix-shield, hard rock, etc.) [1].

The precast segments are made of high-strength concrete (C40 or higher), reinforced with steel bars or fibers (in steel or synthetic). The number of precast segments in each ring will be appropriate to form complete circles with pieces of a given weight, according to the capacity of the handling equipment; commonly medium size tunnels have 6–8 segments per circle. The thickness of the precast segments will depend on the surrounding acting stresses and on the thrust forces applied by the TBM; in common situations it corresponds to about 1/25 of the internal diameter of the tunnel.

After the execution of the tunnel wall, a fill is installed at the bottom of the tunnel, creating a platform for the circulation of the vehicles: trains in the case of rail tunnels or cars and trucks in the case of road tunnels.

### 3. Challenges

As is known, the TBM technique is suitable for the construction of tunnels in stiff ground (rock, stiff clay, compacted sand, etc.), since the tunnels built in this way have their stability ensured by the resistance of the surrounding ground (the precast segments mainly work as a finish), so they do not need to have significant resistance in both the transverse direction and the direction of the tunnel axis.

In the case of soft soil (mud, soft clay, loose sand, etc.), the TBM tunnels can be unreliable because, as the connections between precast segments are very weak (it is a kind of LEGO), the strength and the ductility of the tunnels are low, so there is a risk of sinking, even collapsing, particularly during earthquakes. Soil treatments, sometimes used to aid seismic behavior, are very expensive and often do not guarantee the reliability required for the tunnels.

Regarding the number of tubes, in the case of rail tunnels in order to meet the international safety requirements [2, 3], the installation of both directions of traffic placed side by side in the same tube is only possible in short tunnels.

In long rail tunnels (tunnels over 1 km in length), for safety reasons [2, 3], two separate tunnels are usually be built, each for a direction of traffic, and a complex system of cross-passages connecting the two tubes. In the event of an accident or fire inside the tunnel, users will leave the affected train and move to the other rail gallery to be rescued later by another train. In some situations systems of safety galleries and shafts are also built for local access of the emergency personnel.

As an example, the Gotthard Base tunnel (recently built in Switzerland), with 57 km in length, the longest in the world, is formed of two tubes [4]. Cross-passages regularly spaced along the tunnel and access galleries to outside in some locations were adopted (Figure 2).

### Innovative Concepts in TBM Tunnels DOI: http://dx.doi.org/10.5772/intechopen.87965

Where it is not possible to build access galleries or shafts (e.g., in underwater tunnels), three tubes are usually adopted. This is the case of the Channel tunnel (between the UK and France) or the basic layout of the Gibraltar Strait tunnel, where two tubes are used for rail traffic and a third tube, placed between the two, is used for access to emergency services and rescue of users, using special vehicles with wheels (Figure 2) [4]. The three tubes are also interconnected by a number of cross-passages, regularly spaced.

Also in the case of road tunnels, the installation of the two traffic directions in the same tube is only possible in tunnels with a single lane in each direction. When there are two or more lanes in each direction, the required diameter of the tunnel would become so large that it would be impractical. In any case, in long road tunnels (longer than 0.5 km in length), in order to satisfy safety requirements [5], placing bidirectional traffic side by side in the same tube is quite problematic. Hence, two separate tubes, each one for a direction of traffic, are nowadays usually built, so that, for ventilation and smoke removal purposes, air will circulate in one direction, the direction of traffic.

Generally, the two tubes are interconnected by cross-passages, regularly spaced, so that in the event of an accident or fire, users will leave the incident tube to the other, from where they will be evacuated by conventional buses, such the Westerschelde tunnel, in the south of the Netherlands (Figure 3) [6].

Where possible, instead of cross-passages, access galleries and evacuation routes are built along the tunnel, to allow local access and evacuation of users from the tunnel. This is the case of the tunnel of the south bypass of the M30 motorway, in Madrid, Spain, in which two large diameter tubes were adopted (Figure 3) [6].

Building of two (or three) tubes and the systems of cross-passages or access galleries makes the construction of the tunnels very expensive. In addition, although such layouts represent the most advanced solutions at present for rail and road tunnels, the long time necessary for rescue services to reach the scene may be too long, as has been seen in the recent past.

In order to overcome the abovementioned limitations, the tunnel of improved seismic behavior (TISB) concept for TBM tunnels on soft ground in seismic areas

### Figure 2. Common layouts of rail tunnels.

Figure 3. Common layouts of road tunnels.

and the tunnel multi-gallery (TMG) and tunnel multi-floor (TMF) concepts for TBM rail and road tunnels, respectively, were recently developed.

### 4. The TISB, TMG, and TMF concepts

### 4.1 The TISB concept

The "tunnel of improved seismic behavior" concept is an innovative solution for TBM tunnels, when the referred tunnels are built in soft ground (e.g., mud) in seismic areas, allowing the tunnel to be provided with the adequate resistance and ductility. It also allows the strengthening of existing TBM tunnels, using them as external formwork for the execution of the internal strengthening [7]. The TISB concept is a Portuguese patent [8] and is illustrated in Figure 4.

In the TISB concept, the tunnel is formed by two concentric tubes; an external tube (3), which is a conventional TBM tunnel, and an internal tube (4), which is subsequently executed, inside the external one. The external tube (3) is thus formed by precast segments mounted by the TBM, while the internal tube (4) is later cast inside the external tube (3), using the latter as exterior formwork. Within the thickness of the internal tube (4), longitudinal reinforcement bars (7) and transverse reinforcement bars (8) are laid, both in two layers, which are confined by confinement bars, so as to provide the tunnel with adequate strength and ductility.

Where the vertical actions in the tunnel can have a significant variation (e.g., due to the increase or decrease in the height of the overburden in underwater tunnels), the tunnel will be provided with supports, regularly spaced along the tunnel axis. Those supports are composed of groups of piles with great horizontal deformability and ductility, arranged in the longitudinal and transverse directions, which are anchored at the top in large blocks of jet grouting (5) surrounding the outer tube (3) and at the base in the stiff ground below, so to resist vertical loads, while allowing horizontal movements of the tunnel during earthquakes, functioning as a kind of "movable bearings."

The TISB concept thus leads to the obtaining of monolithic structures (there are no joints) with appropriate resistance in both longitudinal and transverse directions and great ductility under earthquakes. It will also be very effective if liquefaction and cyclic mobility phenomena occur. In addition, the structures obtained will present great structural redundancy, which can be useful in the case of unforeseen scenarios during the design phase.

Figure 4. Illustration of the TISB concept.

### 4.2 The TMG concept

The "tunnel multi-gallery" concept allows, with a single TBM tunnel, the creation of rail tunnels with completely independent directions of traffic and the installation of appropriate means that provide a dedicated and very reliable system for local access of the emergency personnel and the evacuation of users, in the event of an accident or fire inside the tunnel. The TMG concept is a Portuguese patent [9] and is illustrated in Figure 5.

In the TMG concept, the tunnel is constituted by the external wall (1) made by the TBM, a slab (3), placed slightly above the bottom of the tunnel and the entire width, and a separating wall (2), placed in the middle of the tunnel and its entire height, so as to form two independent rail galleries, disposed side by side (4) (5), one for each track, and a service gallery (6) below.

In both sides of the tunnel, vertical access galleries (7), regularly spaced and provided with escape doors (8) in both rail galleries, are also created, allowing for the safe passage of people to the service gallery (6), in the event of an accident or fire inside the tunnel. Inside the service gallery (6), emergency vehicles (9) of monorail type are installed, to provide local access to the emergency personnel and the evacuation of users to outside.

A variant B of the basic solution may also be adopted, in which the vertical access galleries (7) are placed in the middle of the tunnel, in the separating wall (Figure 4). Although there is a local slight reduction of the cross-section of the rail galleries, it avoids the need to make openings in the external wall of the tunnel.

### 4.3 The TMF concept

The "tunnel multi-floor" concept allows with a single TBM tunnel the creation of road tunnels with two identical road galleries, isolated and independent, and the installation of appropriate means that provide a dedicated and very reliable system for local access of the emergency personnel and the evacuation of users, in the event of an accident or fire inside the tunnel. The TMF concept is a Portuguese patent [10] and a European patent [11] and is illustrated in Figure 6.

Figure 5. Illustration of the TMG concept.

### Figure 6. Illustration of the TMF concept.

In the TMF concept, the tunnel is constituted by the external wall (1) made by the TBM and two slabs (2) (3), built at its full width, one placed roughly at half the height of the tunnel and the other placed slightly over the bottom of the tunnel, so as to form two superimposed two road galleries (4) (5), one for each direction of traffic, and a service gallery (6) below.

In one of the sides of the tunnel, vertical access galleries (7), regularly spaced and provided with escape doors (8) in both road galleries, are also created, allowing for the safe passage of people to the service gallery (6), in the event of an accident or fire inside the tunnel. Inside the service gallery (6), emergency vehicles (9) of monorail type are installed, to provide local access to the emergency personnel and the evacuation of users to outside.

### 4.4 Specific matters

The application of these new concepts in the construction of traffic tunnels raises specific issues that require the adoption of appropriate measures.

Tunnel cross-section. Regarding the rail tunnels, the internal diameter will depend basically on the speed of the trains and the permissible pressure variation inside the trains. Figure 7 shows relationships between the cross-section of the rail galleries of bi-tube tunnels and the permissible pressure variation inside trains of 12.4 m<sup>2</sup> of the front area (a common value in high-speed trains), for different speeds of the trains [12].

Admitting as acceptable a pressure variation of 5.5 kPa (appropriate value since there is no clash of the piston effect of the trains), for example, for speeds of 300 and of 250 km/h, cross-sections of 52 and of 38 m<sup>2</sup> will be required, respectively, in each railway gallery.

However, as this effect is only sensitive in the portal zones of the tunnel (at the entrance and exit of the trains), it can be overcome by adopting special arrangements in those zones, namely, creating openings in the separating wall, whose area decreases from the outside to the inside, acting as pressure relief (Figure 8), which allows a significant reduction (10–15%) of the cross-sectional area of the railway galleries [13].

When variant B of the TMG concept is used, the placing of the vertical access galleries in the middle of the tunnel will cause slight localized decrease of the

Innovative Concepts in TBM Tunnels DOI: http://dx.doi.org/10.5772/intechopen.87965

### Figure 7.

Relationships between the cross-section of the rail galleries of the tunnels and the speed of the trains.

### Figure 8.

Openings in the separating wall at the portal zones of the tunnels.

cross-sectional area of the railway galleries on those areas and thus an increase in the pressure variation inside the trains. However, as the vertical access galleries are inside the tunnel, outside the portal areas, this has no influence on the comfort inside the trains.

Regarding the road tunnels, the internal diameter will depend essentially on the number of lanes in each traffic gallery and their width and the permitted height of the vehicles. In Europe, where the height of the vehicles is limited to 4.0 m, a minimum clearance of 4.8 m will be, in general, adopted.

Execution of vertical access galleries. When the vertical access galleries are placed on the external wall of the tunnel, they will be built by locally dismounting the precast segments mounted on those areas and casting new concrete walls in situ.

In situations where there is water pressure around the tunnel, injections of cement grout will allow for the development of the works in safe conditions. In those situations special steel segments will be mounted on those areas, provided with holes to allow for the execution of the injections.

Although there are some risks, they are similar to those of execution the crosspassages in twin-tube tunnels.

Firefighting. The traffic galleries of the tunnels will be equipped with active detection and attack devices, acting jointly, instead of relying on conventional systems of attack by fire trucks. Heat sensors and smoke detection systems automatically activate high-pressure water mist nozzle systems, regularly distributed along the tunnel and grouped in sections, lowering the temperature on site. After this action, firefighters (who come through the service gallery) can then extinguish the fire.

Rescue of users. In the event of an accident or fire inside one of the traffic galleries of the tunnel, users will leave that gallery through the emergency walkways to the nearest escape door, from where they reach the service gallery below, down the stairs inside the vertical access galleries. Inclined platform lifts running along the stairs provide access to handicapped people.

Emergency vehicles of the "emergency monorail electric vehicle" (EMEV) type that circulate inside the service gallery will rescue the users to outside of the tunnel.

Emergency vehicles. The EMEVs are autonomous vehicles that receive wireless signals from the tunnel control center. They are battery powered, so as not to be dependent on the reliability of the electricity network inside the tunnel. They circulate suspended from the ceiling slab, in general, in two parallel lines. They are grouped in "trains," in numbers according to the needs. They will be parked at one or both of the tunnel portals, where users will have outbound exits.

### 5. Application of the TISB and TMF concepts on a proposal for a road tunnel crossing the Tagus River in Lisbon

### 5.1 Introduction

The Algés-Trafaria road tunnel, crossing the Tagus River in Lisbon, Portugal, aims the decongestion of the road traffic of the suspension bridge, which is currently 50% higher than its capacity. It will allow the closing of the inner ring motorway of Lisbon, constituted by the CRIL in the north bank, the Vasco da Gama Bridge at east, and the CRIPS (A33) in the south bank of the river. It will be located west of the suspension bridge (Figure 9).

The location of the tunnel is characterized by the existence of thick alluvial deposits along the riverbed, composed of various complexes of mud and sand, extending from elevation 29 m (the deepest level of the river) to elevation 75 m. Underlying the alluvial deposits, there are bedrock formations constituted by basalt and limestone that extend by the north bank. On the south bank, there are Miocene

Figure 9. Location of the Algés-Trafaria tunnel crossing.

### Innovative Concepts in TBM Tunnels DOI: http://dx.doi.org/10.5772/intechopen.87965

formations composed mainly of sand and clay. The very prone-seismic conditions of the area must also be noted (one must remember the 1755 Lisbon earthquake, one of the most destructive in history).

Preparations for the construction of the new crossing have been under way for a long time, and an immersed tunnel solution has already been studied by Lusoponte, the concessionaire of the Tagus road crossings in Lisbon [14]. However, for various reasons, no significant progress has been made.

The study concluded that the construction of the immersed tunnel is viable but presents significant risks, associated with the high probability of liquefaction of the sands that constitutes the riverbed, in the event of a strong earthquake, and also with the difficulties of the realization of the tunnel connections at its ends, which will significantly increase the overall cost of the project. The cost of the tunnel (at current prices) was estimated at 600 million euros.

A TBM tunnel solution for the crossing, based on TISB and TMF concepts, was, in the meantime, proposed by the author [15–20].

### 5.2 The proposed TBM tunnel solution

In the proposed solution, the tunnel will be a single-tube tunnel, 5.1 km in length with two superimposed road galleries and a service gallery at the base. At the deepest part, the bottom of the tunnel is at elevation 59 m, allowing for a soil overburden identical to the diameter of the tunnel. The tunnel runs through the alluvia along most of its length under the river (Figure 10).

The tunnel has an internal diameter of 14.2 m, with precast segments 0.55 m thick (Di/25) and 0.15 m clearance between the lining and the ground to be injected with grouting; hence, the excavated diameter of the tunnel is 15.6 m.

Despite the large diameter of the tunnel, it is significantly smaller than that of the larger tunnels being built, such as the SR99 in Seattle (USA), with a 17.5 m diameter, and the Tuen Mun-Chek Lap in Hong Kong (CN), with a 17.6 m diameter.

Inside the tunnel two concrete slabs are built, in order to create two superimposed road galleries (each one for a direction of traffic) with two lanes each (3.5 m wide and 4.8 m high); outer emergency lane; inner edge and emergency walkways on both sides, with a total width of 12.6 m; and a service gallery at the bottom, 2.0 m high (Figure 11).

To provide the tunnel with adequate structural safety under earthquakes, the section under the riverbed (2.25 km long) will be strengthened with an internal reinforced concrete tube 0.3 m thick, dully confined, in order to improve its strength and ductility, which are essential in the case of liquefaction of the sand (Figure 11).

The upper slab is supported laterally on continuous corbels executed in the precast segments or cast jointly with the internal tube (where it exists). The lower

slab is supported on two small longitudinal concrete walls cast in situ and on the TBM tube.

The road galleries have escape doors located on one side of the tunnel, spaced 400 m (less than the 500 m allowed by the EU rules) [5], which give access to the service gallery below, through vertical access galleries with 2.6 3.5 m of inner section (Figure 12).

Inside the service gallery, emergency vehicles of the EMEV type circulate to allow for the access of emergency personnel to inside the tunnel and the evacuation of users out of the tunnel in the event of accident or fire.

Figure 11. Proposed TBM tunnel solution for the Algés-Trafaria crossing. Current cross-section.

### Innovative Concepts in TBM Tunnels DOI: http://dx.doi.org/10.5772/intechopen.87965

Environmental impact. Once the tunnel is bored by a TBM, its construction will not cause any disturbance of the riverbed along the tunnel axis. Traffic on the Tagus River and on the port of Lisbon will not be disturbed either. The connections of the ends of the tunnel to the existing road network, on both sides, present no particular difficulties.

Safety in operation. The tunnel is provided with an advanced safety concept, which represents a step forward in the safety of road tunnels.

In the event of an accident or fire in one of the road galleries, users will leave the incident gallery by walking through the respective emergency walkways to the nearest escape door, from where they reach the service gallery, down the vertical access galleries.

Inside the service gallery, dedicated EMEVs that circulate in two parallel lines give access to emergency personnel and evacuate the users out of the tunnel. They are grouped in pairs, being parked at the portals of the tunnel. In such situations there will be no disturbance of the traffic flow in the non-incident traffic gallery.

Cost. The cost of the tunnel was estimated by considering a tunnel unit cost per cubic meter of excavation with a value appropriate to the tunnel layout and site characteristics. In view of these conditions, it is appropriate to adopt for this tunnel a unit cost of 420 euros per cubic meter of excavation [19, 20].

With an excavation diameter of 15.6 m, which corresponds to an area of the excavated section of 191 m2 , the unit cost of the tunnel will be about 80 million euros per kilometer. As the tunnel length is 5.1 km, the cost of the tunnel will be about 410 million euros, significantly lower than the cost of an immersed tunnel, 600 million euros, as mentioned above [19, 20].

### 5.3 Conclusions

The application of the TISB and TMF concepts allows to obtain a very costeffective solution for the construction of the Algés-Trafaria tunnel crossing the Tagus River in Lisbon, with very low environmental impact, improved structural safety under earthquakes, and low construction costs (significantly lower than that of an immersed tunnel), and provides an advanced safety concept for emergency personnel access and rescue of users in the event of an accident or fire inside the tunnel.

### 6. Application of the TMF and TMG concepts on an optimized TBM tunnel alternative for the Fehmarnbelt Fixed Link

### 6.1 Introduction

The Fehmarnbelt Fixed Link is a Danish-German project, in the Baltic Sea, 18 km long, to provide a direct link by rail and road between the two countries and Scandinavia (Figure 13). It is part of the expansion of the Trans-European Transport Network (TEN-T) of the European Union, being co-financed by EU funds. The project will be owned and financed by Denmark and to be repaid by the users. It is being managed by Femern A/S, a Danish state-owned company.

The project aims to connect the Lolland island (in Denmark) and the Fehmarn island (in Germany), through the Fehmarn Belt. It will be for mixed traffic, with two road galleries provided with two lanes each and two rail galleries for trains at speeds up to 200 km/h, keeping the pressure variation inside the trains within acceptable limits [21].

The geological profile along the alignment of the tunnel is shown in Figure 14 [21]. Both sides present smooth slopes near the coast areas, the deepest water being 34 m.

Figure 13. Location of the Fehmarnbelt fixed link.

Figure 14. Geotechnical conditions of the site.

Under the seabed the soil comprises an upper quaternary layer of post and late glacial deposits (clay and silts) followed by a Paleogene layer of highly plastic clay. The German side is characterized by Paleogene clay and some clay-till, the central basin by sand silts and clays, while the Danish side is dominated by thick deposits of clay-till.

Studies for this project began more than 20 years ago, in the 1990s. It has been studied in several variants, starting with a suspension bridge, followed by a cablestayed bridge. As both bridge solutions received much opposition, especially from environmental organizations, an immersed tunnel solution was also further studied.

Although the costs of the cable-stayed bridge and the immersed tunnel were broadly similar, in 2011 the Danish authorities took a preliminary decision to adopt an immersed tunnel in the link. A TBM tunnel solution was also at the time studied by the promoter but, being composed of three tubes (two tubes for road traffic and another for rail traffic), despite having less environmental impact than the immersed tunnel, was rejected because the estimated cost was higher [21].

The immersed tunnel solution has then been subjected to public consultation of the Environmental Impact Assessment (EIA) by the Ministry of Transport in

Innovative Concepts in TBM Tunnels DOI: http://dx.doi.org/10.5772/intechopen.87965

Denmark in 2013. Under the framework of this consultation, the author developed an optimized TBM tunnel alternative based on the TMF and TMG concepts, which proved to be much more cost-effective than the "official" TBM tunnel solution and the immersed tunnel solution [22]. However, this alternative was not accepted, and the immersed tunnel solution obtained its approval.

The EIA of the project was also submitted to public consultation in the state of Schleswig-Holstein, in Germany, in 2014, where, after a rather harsh process, approval was also recently granted. However, the future of the project in its current form is still uncertain, as several environmental organizations threatened to challenge this decision in the German courts.

### 6.2 The immersed tunnel solution

The immersed tunnel solution is a conventional immersed tunnel, consisting of a single prismatic tube approximately 18 km long, 42.2 m wide, and 8.9 m high, consisting of 89 precast concrete segments in general with 217 m in length (Figure 15) [21]. There are also cut-and-cover sections at the ends.

The tunnel is provided with four traffic galleries: two road galleries, 11.0 m wide and 5.2 m high, and two (ballastless) rail galleries, 6.0 m wide and 6.0 m high. It also includes a service gallery, placed between the two road galleries, 2.0 m wide, for the installation of pipes and cables and to be used as temporary refugee although not allowing to be used by vehicles.

The railway galleries are provided with emergency walkways on both sides, 1.3 m wide, while the road galleries have an emergency lane on the outside but have neither internal edge nor emergency walkways.

The precast segments are placed in a trench dredged in the seabed, on a bedding layer of crushed rock. A combination of locking gravel fill and sand fill is then backfilled along the sides of the elements, while a protection layer of stones is placed across the top of the elements. Part of the dredged material is placed over the protection layer.

The execution of the works presents significant risks, since they are developed at the surface of the open sea, in a zone of intense ship traffic, and using precast segments which are significantly larger than those used in prior projects.

Environmental impact. As generally recognized, the environmental impact of the immersed tunnel solution is very significant. Among others, the large area of natural seabed of the German Natura 2000 site that will be disturbed by the construction works is worth noting, a width of over 100 m along the entire tunnel length. The huge volume of excess dredged material that will have to be placed in reclamation areas (14.8 million cubic meters) is also noted [22]. Also its significant "footprint" is impressive, with the following quantities of the most representative

Figure 15. Immersed tunnel solution. Current cross-section.

materials used: concrete, 3.0 million cubic meters; rock, 3.1 million cubic meters; and sand, 5.1 million cubic meters [22].

Safety in operation. In the case of fire or accident, the rescue of users relies on conventional vehicles that will use the road galleries, to which they access through escape doors, spaced 110 m [21]. However, despite this low distance between escape doors, much lower than the 500 m required by the EU rules [3, 5], as it can be shown, this does not represent a significant added value for the safety of the users of the tunnel [27].

On the contrary, the safety concept of the tunnel presents several significant shortcomings [27], namely, (a) the road galleries have no emergency walkways, so, during escape, those from behind will tend to push the others ahead into the traffic lanes; (b) with the arrival of dozens, perhaps hundreds, of people at a roadway gallery, fleeing an incident gallery, there is the risk of disruption of the traffic flow in this gallery, preventing the arrival of rescue vehicles; (c) rescue of the passengers of a train (full of 600 people) will need at least a dozen buses, which can take several hours to have them at the scene; and (d) the traffic flow in the non-incident galleries will be significantly disturbed by the occurrence of any safety problem in one of the galleries of the tunnel.

A very serious question is how to escape from the outer railway gallery. Passengers will have to cross the railway line of the internal rail gallery to reach the adjacent road gallery, which will be very dangerous and therefore should not be acceptable (see Figure 15) [27].

Cost. The cost of the tunnel was estimated by the owner at 5500 million euros [21], which is identical to the tenders in the meantime received for the construction. Given that the project was granted with 600 million euros of EU funds, the financial effort of the promoter will thus be around 5000 million euros, to be repaid within a 36-year period.

### 6.3 The optimized TBM tunnel alternative

Based on TMG and TMF concepts, an optimized TBM tunnel alternative was developed [22–27] by the author. It consists of two separate tunnels, one for road traffic and the other for rail traffic (Figure 16), placed beside one another at a distance of about 15–20 m, that go deep into the ground to about elevation 63 m, complemented with cut-and-cover sections at the ends.

The rail tunnel is about 20 km long and has an inner diameter of 11.5 m, with precast segments 0.45 m thick (Di/25) and 0.15 m clearance between the lining and the ground, to be injected; hence, the excavated diameter of the tunnel is 12.7 m, a common size for TBM tunnels. An intermediate slab and a central wall are then constructed, creating two parallel, independent, and isolated rail galleries with

Figure 16. Optimized TBM tunnel alternative. Current cross-section.

### Innovative Concepts in TBM Tunnels DOI: http://dx.doi.org/10.5772/intechopen.87965

38 m<sup>2</sup> cross-sectional area, each for a direction of traffic, and a service gallery below, 2.2 m high (Figure 17).

The variant B of the TMG concept is used; thus, the emergency walkways of the rail galleries, 1.4 m wide (wider than those of the immersed tunnel solution), are placed on the inner side.

On both emergency walkways, there are escape doors spaced 400 m (less than the required by the EU rules [3]) that have access to vertical access galleries placed in the middle of the tunnel, at the separating wall (Figure 17).

Although the placement of the vertical access galleries in the middle of the tunnel causes a slight local decrease in the cross-sectional area of the railway galleries, since the vertical access galleries are outside the portal zones, the comfort conditions within the trains will not be affected.

The road tunnel is about 19 km long and has an inner diameter of 14.2 m, with precast segments 0.55 m thick (Di/25) and 0.15 m clearance between the lining and the ground to be injected; hence, the excavated diameter is 15.6 m (Figure 18), the same as of the largest TBM tunnels in operation, although (as referred above) larger TBM tunnels are still being built, such as the SR99 in Seattle (USA), 17.5 m in diameter, and the Tuen Mun-Chek Lap in Hong Kong (CN), 17.6 m in diameter.

Inside the precast tunnel, two intermediate slabs are constructed, creating two superimposed road galleries, independent and isolated, each one for a direction of traffic, 5.0 m high, and a service gallery below, 2.0 m high.

Both roadway galleries have two lanes of 3.5 m wide each, external emergency lane 2.2 m wide, inner edge 1.0 m wide, and emergency walkways on both sides 1.2 m wide, over a total width of 12.6 m, greater than those of the immersed tunnel (11.0 m).

Laterally to the emergency walkways in one of the sides of the tunnel, there are escape doors spaced 400 m (less than the required by the EU rules [5]) that have access to vertical access galleries, allowing the safe passage of people between the road galleries and the service gallery (Figure 18).

Environmental impact. Being formed of bored tunnels, the optimized TBM tunnel alternative will not provoke any disturbance of the natural seabed along the tunnel alignment.

The volume of excavated material that will have to be placed in the reclamation areas is about 6.2 million cubic meters, much smaller than the volume of the

Figure 17. Optimized TBM tunnel alternative. Cross-section of the railway tunnel.

dredged material that will have to be placed in the case of the immersed tunnel solution (14.8 million cubic meters, as mentioned). The spending of natural resources in the main building materials (footprint) is as follows: concrete, 1.9 million cubic meters, and rock and sand, non-significant, which is also much smaller than in the case of the immersed tunnel solution [22–27].

Safety in operation. The TMG and TMF concepts provide to the optimized TBM tunnel alternative advanced emergency systems, which are a great step forward in the safety of traffic tunnels [22–27].

In the event of accident or fire in one of the traffic galleries of the tunnels, users will leave that gallery by walking through the respective emergency walkway to the nearest escape door, from which they achieve the service gallery of the tunnel, down the stairs of the vertical access galleries.

Inside the service gallery, dedicated emergency vehicles of the EMEV type that circulate in two parallel lines will allow for the access of emergency personnel and the evacuation of users out of the tunnel. They are grouped in "trains" (of five in the rail tunnel and two in the road tunnel), being parked at both portals of the tunnels (Figure 19). In such situations there will be no disturbance of the traffic flow in the non-incident gallery.

Cost. The cost of the optimized TBM tunnel alternative was estimated on the basis of the estimated cost of the "official" TBM tunnel solution, considering appropriate unit costs for each tube.

Figure 18.

Optimized TBM tunnel alternative. Cross-section of the road tunnel.

### Innovative Concepts in TBM Tunnels DOI: http://dx.doi.org/10.5772/intechopen.87965

The "official" TBM tunnel solution consists of three tubes [21]: two road tunnels with internal diameter of 14.2 m, corresponding to excavated diameters of about 15.6 m, and a rail tunnel with 15.2 m of internal diameter, corresponding to an excavated diameter of about 16.6 m, which leads to excavated volumes of about 3.82 million cubic meters for each road tunnel and 4.32 million cubic meters for the rail tunnel.

Whereas the estimated cost of this solution was 6800 million euros, and considering that the unit cost of the rail tunnels is about 70% the unit costs of the road tunnels [25–27], unit costs of 450 euros per cubic meter for the rail tunnel and 635 euros per cubic meter for the road tunnel are obtained.

Hence, for the optimized TBM tunnel alternative, unit costs of 450 euros per cubic meter for the rail tunnel (the same as the one obtained for the "official" TBM tunnel solution) and 650 euros per cubic meter for the road tunnel (slightly higher than the one obtained for the "official" TBM tunnel solution, since there are the vertical access galleries to build) are assumed.

Whereas in this case, the excavated volumes are about 2.54 million cubic meters for the rail tunnel and 3.63 million cubic meters for the road tunnel, the estimated costs will be 1150 and 2400 million euros for the rail and the road tunnel, respectively. Therefore, the estimated cost of the TBM tunnel alternative is 3550 million euros, which is less than 2/3 the cost of the immersed tunnel solution [25–27].

It should also be noted that the unit costs obtained for the "official" TBM tunnel solution are significantly higher than those normally obtained in tunnels under similar conditions, so it has to be admitted that the estimated cost is exaggerated by at least 15%.

Thus, the cost of the optimized TBM tunnel alternative will probably be around 3100 million euros [27], so the financial effort of the promoter would be about half of that of the immersed tunnel solution.

### 6.4 Conclusions

Given the above considerations, the following main conclusions are drawn.

With regard to the environmental impact, while in the immersed tunnel proposal it is very significant, in the Optimized TBM tunnel alternative, it is very low; in particular it avoids any disturbance of the natural seabed along the tunnel.

Regarding the safety in operation, while the immersed tunnel solution has several weaknesses in its safety concept, the optimized TBM tunnel alternative presents an advanced safety concept, in which the rescue of users relies on dedicated unmanned electric vehicles operating inside a service gallery, so completely independent of the access conditions inside the traffic galleries.

With regard to costs, the cost of the optimized TBM tunnel alternative is about 3100–3550 million euros, which is less than 2/3 the cost of the immersed tunnel solution, and the financial effort of the promoter would be halved, allowing for an equivalent reduction in the tolls to be paid by the users or in the repayment period.

In summary, the optimized TBM tunnel alternative is undoubtedly much more cost-effective than the immersed tunnel solution.

### 7. Final remarks

The TISB, TMG, and TMF concepts are innovative developments that represent a step forward in the construction of rail and road tunnels executed with the TBM technique.

In addition to the intrinsic environmental advantages of TBM tunnels, they provide improved seismic behavior, reduction in the construction costs, and improvement of safety during operation.

Regarding the seismic behavior of the TBM tunnels built on soft ground, the TISB concept provides the tunnels with the necessary strength and ductility, avoiding the need for additional soil treatments.

Furthermore, although being formed by single tubes, each tunnel accommodates two completely independent and isolated galleries of traffic (for rail or road) and a service gallery below, which allows a very reliable safety concept, much more reliable than any currently existing.

With regard to costs, simply comparing the excavated volumes of the referred single-tube tunnels with those of the equivalent conventional solutions using two parallel tubes connected by cross-passages shows that reductions of more than 20% will be easily achieved.

With regard to safety in operation, the service gallery at the base of the tunnel provides a very reliable pathway for access of the emergency personnel and the rescue of users in the event of accident or fire inside the tunnel, through dedicated emergency vehicles (EMEVs), therefore independent of the availability of conventional rescue vehicles or the access conditions inside the traffic galleries of the tunnel.

In summary, the TISB, TMG, and TMF concepts provide very cost-effective and safe solutions that can be of great value in the construction of the rail and road tunnels of tomorrow, especially long tunnels.

### Author details

Silvino Pompeu-Santos SPS Consulting, Lisboa, Portugal

\*Address all correspondence to: pompeusantos@sapo.pt

© 2019 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Innovative Concepts in TBM Tunnels DOI: http://dx.doi.org/10.5772/intechopen.87965

### References

[1] Herrenknecht. Pioneering Tunnelling Technology for Underground Transport Systems. Herrenknecht AG; 2011

[2] UIC. UIC Code 779–9: Safety in Railway Tunnels. UIC; 2003

[3] EU. Safety in railway tunnels. EU Decision 2008/163CE; 2008

[4] Henke A. Experiences from the ground probing in the gotthard-base tunnel and their applicability in the gibraltar strait crossing. In: UN-ITA Workshop Systematic Ground Probing and Treatment in Mechanized Tunneling; Madrid, Spain; January. 2005

[5] EU. Safety in roadway tunnels. EU Directive 2004/54CE; 2004

[6] Herrenknecht. Masterly performance in Madrid. In: Keeping Things Moving on the M30 Highway. Herrenknecht AG; 2009

[7] Pompeu-Santos S. Tunnels of improved seismic behaviour. In: Proceedings of the FIB Symposium Taylor Made Concrete Structures—New Solutions for Our Society; Amsterdam, Netherlands; May 2008

[8] INPI. Túneis de Comportamento Sísmico Melhorado. Portuguese Patent no. 103421 (in Portuguese); INPI; July 2008

[9] INPI. Túnel de Galeria Múltipla. Portuguese Patent no. 103769 (in Portuguese), INPI; January 2009

[10] INPI. Túnel Multi Piso. Portuguese Patent no. 103748 (in Portuguese); INPI; August 2009

[11] EPO. Tunnel Multi-Storey. European Patent EP 2317074A1. EPO; November 2011

[12] Tielkes T. Aerodynamic aspects of Maglev systems. In: Proceedings of the 19th Conference on Magnetically Levitated Systems and Linear Drives; Dresden, Germany; September 2006

[13] Bopp R, Hagenah B. Aerodynamics, ventilation and tunnel safety for high speed rail tunnels. In: Workshop Tunnels for High Speed Railways; Porto, Portugal; October 2009

[14] Cancio-Martins. Nova Travessia do Tejo. Corredor Algés-Trafaria—Solução em Túnel Imerso. Report (in Portuguese). Lisbon: J. L. Cancio Martins Lda; January 2001

[15] Pompeu-Santos S. TBM tunnels of improved seismic behaviour—The TISB concept. In: Proceedings of the 15th World Conference on Earthquake Engineering (15WCEE); Lisbon, Portugal; September 2012

[16] Pompeu-Santos S. New trends in the tunnels of the future. In: Proceedings of the IABSE Symposium Engineering for Progress, Nature and People; Madrid, Spain; September. 2014

[17] Pompeu-Santos S. Sustainable TBM tunnels for tomorrow. In: Proceedings of the Second International Conference on Concrete Sustainability (ICCS2016); Madrid, Spain; June 2016

[18] Pompeu-Santos S. The long traffic tunnels of tomorrow. In: Proceedings of the IABSE Symposium Tomorrow's Megastructures; Nantes, France; September 2018

[19] Pompeu-Santos S. Grandes Projetos de Obras Públicas—Desafios Portugal 2030. Lisbon: Scribe; March 2019; (in Portuguese, in press)

[20] Pompeu-Santos S. The long rail and road tunnels of tomorrow. In:

Proceedings of the FIB Symposium Concrete—Innovations in Materials, Design and Structures; Krakow, Poland; May 2019

[21] Femern. Fehmarnbelt Fixed Link— Consolidated Report. Femern A/S; December 2011

[22] Pompeu-Santos S. Fehmarnbelt fixed link—The alternative TBM tunnel solution based on TMG and TMF concepts versus the immersed tunnel solution. Public consultation of the EIA of the Fehmarnbelt Fixed Link held by the Danish Ministry of Transport. September 2013

[23] Buxton L. Flexible solutions for Fehmarnbelt? World Tunnelling Magazine; July–August 2014

[24] Pompeu-Santos S. Optimized TBM tunnel solution for the Fehmarnbelt fixed link. In: Proceedings of the FIB Symposium Concrete—Innovation and Design; Copenhagen; May 2015

[25] Pompeu-Santos S. Optimized TBM Tunnel Solution for the Fehmarnbelt Fixed Link Based on TMG and TMF Concepts. Lisbon: SPS Consulting; February 2016

[26] Pompeu-Santos S. Meet the challenges of the Fehmarnbelt fixed link. In: Proceedings of the IABSE Congress Challenges in Design and Construction of an Innovative and Sustainable Built Environment; Stockholm; September 2016

[27] Pompeu-Santos S. Make feasible the Fehmarnbelt fixed link. In: Proceedings of the FIB Symposium High Tech Concrete—Where Technology and Engineering Meet; Maastricht, Netherlands. May 2017

### **Chapter 8**

## Design of Immersed Tunnel and How We Research Submerged Floating Tunnel

*Wei Lin, Ming Lin, Haiqing Yin and Xiaodong Liu*

### **Abstract**

This chapter begins with the discussion of the immersed tunnel design, concerning its reason of existence, historical review, general design, transverse and longitudinal design, the interaction, and the critical issues. The discussion is founded on the author's 10 year experience in building the Hong Kong-Zhuhai-Macao Bridge (HZMB) immersed tunnel as a site design engineer. The experience of building immersed tunnel is transferable to build the submerged floating tunnel, which has never been built. In author's opinion, the submerged floating tunnel (SFT) technique will be the next generation of IMT technique. In the second part of this chapter, the author proceeds to discuss the strategy of SFT research and the latest development in CCCC SFT Technical Joint Research Team.

**Keywords:** immersed tunnel, submerged floating tunnel, design, research, civil engineering

### **1. Introduction**

Immersed tunnelling is an art of guiding the great natural force, the water, to do engineering works: "guiding" buoyancy for transportation, "guiding" water weights for immersion, and "guiding" hydrostatic pressure for connection. Submerged floating tunnel (SFT) is an even more extreme form of this art, as the full weight of tunnel or most of it is balanced by buoyancy. This chapter discusses the method of immersed tunnel design and SFT research.

### **2. Design of immersed tunnel**

### **2.1 Reason of existence**

"In order successfully to conceive and to plan a structure or building of any kind it is necessary to investigate and to know well its reasons for existence …" is the first line of the book the *Philosophy of Structures* written by Eduardo Torroja. A city that has water barriers but has no bridge is like a mansion with no elevator, answered Strauss, the chief engineer of the Golden Gate Bridge. However, a bridge could have its limitations: its span could disturb the ship traffic, and its tower could disturb the air flight if the bridge were built close to an airport.

When a bridge crosses a harbour, the water salinity in the harbour may change due to the slowed water exchange between offshore sea and the fresh inland water, as the bridge piers disturb the water exchange, giving impact to the living condition of sea resident in the harbour region. In Øresund tunnel compensate dredging was performed to eliminate the said effect. In Hong Kong-Zhuhai-Macao Bridge project, the two offshore artificial islands, which connect immersed tunnel and bridges, have a minimum length so that the total water blockage ratio of the entire link is controlled minimum. The water blockage ratio was defined as the projected area of the link that disturbs the water exchange along the axis of the link divided by that of the total area of the water.

The above is the reason of existence for a subaqueous tunnel. Whether to build a bored tunnel or immersed tunnel varies and depends on specific project condition. One commonly seen reason to choose immersed tunnel is more cost-effective because the immersed tunnel is buried shallower than bored tunnel; the latter requires typically a buried depth not less than 1–1.5 times of the bored diameter for construction. In the island and tunnel project of Hong Kong-Zhuhai-Macao Bridge (HZMB Island-Tunnel Project), there were two main reasons for choosing immersed tunnel. Firstly, as both ends of the immersed tunnel connect to bridges via two artificial islands (**Figure 1**), the length of the artificial islands would be twice smaller, leading to smaller water blockage ratio. Comparatively, the bored tunnel would be buried deeper than that of the immersed tunnel due to its longer transition length. Secondly, the geology risk such as encountering boulder for bored tunnel is high, and thus the risk of time delay for the entire 55 km long link is high.

Despite the advancing of technology, the understanding about the marine environment is still limited; the risk of construction of an immersed tunnel in the offshore condition is relatively high. "Stories" of sinking, flooding, and damage exist such as [1, 2]. Therefore, one aim of the design of an immersed tunnel is to find a way to mitigate the construction risk by proposing the appropriate scheme and technical requirement.

**Figure 1.** *Overview of HZMB link from Hong Kong side.*

*Design of Immersed Tunnel and How We Research Submerged Floating Tunnel DOI: http://dx.doi.org/10.5772/intechopen.88169*

### **2.2 History and state of the art**

The earliest attempt was in 1810. The British Engineer Charles Wyatt won the competition by proposing the immersed tunnelling concept, using brick-made cylinders, each around 15.2 m long, and sinking them to a dredged river bed, and then backfilling them. A test was carried out with much care by another British engineer with two specimens, each 7.6 m in length and 2.74 m in diameter. The test results are positive. However, the cost was overrun, the project terminated. It was not until 1893 when three sewer pipelines (diameter of which is only 1.8 m) were made by this construction method. The first traffic immersed tunnel was built in 1910 [1].

The immersed tunnel is, as per the defined term of ITA WG11, a tunnel consisting of several prefabricated tunnel elements, which are floated to the site, installed one by one, and connected under water. **Figure 2** shows the working image of the HZMB Island-Tunnel Project. This tunnel consists of 33 tunnel elements and a closure joint (**Figure 3**). The immersion had been completed on May 5, 2017. Since then it becomes the longest roadway immersed tunnel. This record will soon be broken by the Fehmarn tunnel, which will be around 18 km long and consist of 89 tunnel elements.

### **2.3 General design**

The environment acting on the immersed tunnel depends on the location of the tunnel. Thus, the tunnel alignment needs to be fixed in the first place.

The plane alignment of the tunnel mainly depends on its two ends, the access point of the tunnel portal. For the vertical alignment, that is, the elevation of the tunnel, several considerations need to be taken. The elevation of the tunnel ends shall neither be too high nor too low. If the ends of the tunnel were too high, the immersion depth is inadequate for the hydraulic connection of the first tunnel

**Figure 2.** *Construction works of the immersed tunnel in HZMB Island-tunnel project.*

### **Figure 3.**

*Vertical alignment of the immersed tunnel consists of 33 tunnel elements in the HZMB Island-tunnel project.*

element that connects to the land structure. If the ends of the tunnel were too low, the risk of flooding increases as more massive amount of water could rush into the tunnel due to rain or overtopping. Moreover, the elevation of the middle section of the tunnel depends on the navigational requirement of ships passing over the tunnel. After the elevation of tunnel ends, the section under the navigation channel was fixed. The remained work is to "draw a line" for the vertical alignment of the tunnel. The principle as an experience by the pioneer is to dive down or rise up as quickly as possible. In this way, the tunnel length will always be the shortest, as can be proven in **Figure 4**.

As long as the tunnel alignment is fixed and the environment that will be encountered by the tunnel is thus fixed, the actions such as wind, wave, current, and water depth can be defined as well. In short, structural design can be done.

The immersed tunnel consists of one or several tunnel elements. Therefore, the design of the immersed tunnel is, in fact, the design of tunnel elements. The design of each tunnel element usually distinguishes from each other. One reason is that each tunnel element exists in a more or less different environment and the actions on them are different. The other reasons can be seen in **Figure 5**, as an example from HZMB Island-Tunnel Project.

As for the design of each tunnel element, the problem can be further discretized into several subproblems, as will be elaborated in Sections 2.4 and 2.5.

### **2.4 Transverse structure**

The transverse design needs to satisfy three aspects: the structural issue, the weight balance, and the interior space.

The structural issue is a familiar subject to structural engineers. Not only the permanent scenarios but also the temporary scenarios of construction shall be

### **Figure 4.** *Illustrative image showing the principle of drawing vertical alignment.*

*Design of Immersed Tunnel and How We Research Submerged Floating Tunnel DOI: http://dx.doi.org/10.5772/intechopen.88169*

### **Figure 5.**

*Uniqueness of immersed tunnel element design in HZMB Island-Tunnel Project.*

considered regarding the boundary condition and load and actions, because the permanent condition of the immersed tunnel may not govern the design as it would do for many other types of structures.

The weight balance means that the tunnel element can float when being transported and can sink for immersion and underwater connection; those are construction need. Further, in the service period, attention shall be paid to ensuring adequate safety factor against uplift, in case of extreme weather conditions. To note, the construction rigs may affect the freeboard of tunnel element when it was afloat (**Figure 6**).

The interior space requirement depends on the traffic clearance (i.e. the minimum space requirement for traffic defined by the relevant regulations/code), the space for interior installations such as ventilator and fireproof panels, and the extra space for accommodating construction tolerances from the prefabrication and immersion of tunnel element.

With the increased awareness of comfort design and life safety, more attention is paid to ventilation and evacuations, in addition to the above said three basic needs. **Figure 7** shows three ventilation solutions, namely, the longitudinal ventilation, semi-transverse ventilation, and transverse ventilation. The longitudinal ventilation

### **Figure 6.**

*Immersion rigs of pontoon sit on tunnel element and reduce the freeboard of the tunnel element while catamaran increases it.*

**Figure 7.**

*Ventilation solutions of immersed tunnel: (a) longitudinal, (b) semi-transverse, and (c) transverse.*

requires fans that increase the height of the tunnel, leading to a deeper foundation and more dredging works. The transverse ventilation requires special bores and thus increases the width of cross-section, also leading to more dredging volume. The semi-transverse ventilation was somewhere in between. Concerning the setting of the inner walls in the cross-section, **Figure 8** shows its relation with the safety concept. Also, purely from a structural point of view, the more inner walls, the less governing the largest span of the cross-section structure. In the 1990s Japan tunnel favoured a cross-section of two tubes with two galleries, and the double walls gave benefit to both the robustness of watersealing and the safety of the structure.

### **2.5 Longitudinal structure**

The longitudinal design needs to consider three aspects as well, namely, the structural system, element length, and joint configurations.

The earlier immersed tunnel had monolithic tunnel element. The cross-section is circular shaped; the structure type is steel shell. To increase the space use from 1937 to 1942, the first reinforced concrete box structure tunnel element was made. Around 10 years later, the segmented-type tunnel element made of reinforced concrete was developed in the Netherlands. In Øresund tunnel, factory method was

### **Figure 8.**

*Safety and transverse design: (a) two-way traffic in one bore, collision risk is relatively high; (b) two traffic bores separated by a wall, evacuated people could be hit by vehicle on the other bore; (c) two traffic bores separated by two walls; (d) the centre part is further split into two bores, one for evacuation and the other for smoke extraction.*

### *Design of Immersed Tunnel and How We Research Submerged Floating Tunnel DOI: http://dx.doi.org/10.5772/intechopen.88169*

invented to produce tunnel element of 55,000 tons in a production line [3]. That method was used in the HZMB Island-Tunnel Project (**Figure 9**) for the second time; the production line was capable of incrementally launching the 76,000 t tunnel elements (in which five of them were plane-curved tunnel elements with curvature R5500) without cracking them. In around 1990 in Japan, no more place along the shore can be found to prefabricate tunnel element. Further, the experienced concrete vibration workers were not adequate. In this background, the steel-concrete-steel sandwich structure box-type immersed tunnel element was developed as its concrete requires no vibration; the pouring of concrete can be completed in the floating stage of the tunnel element.

The length of the tunnel element determines the number of tunnel element, given the fixed tunnel length. On the one hand, the longer tunnel element reduces the total number of element and thus reduces the total number of immersion joint, the main works of which are bulkhead and its embedded part, Gina gasket waterstop, and so forth. Further, fewer tunnel elements mean fewer times of the immersion works and thus less risks of construction. On the other hand, the shorter is the tunnel element, the less is the total prefabrication cost as less area of land near the water is needed for prefabrication of tunnel elements, and the less sensitive of the tunnel element structure to the differential settlement issue, hence the less cost of the prestressing system. The above shows that the element length design is a matter of keeping a balance and finding the optimum.

The immersion joints need to ensure watersealing in tunnel's service period taking into account all the unfavourable scenarios such as earthquake, differential settlement, and accident like sunken ships; it also needs to provide a way of connection of tunnel element for construction. **Figure 10** shows immersion joints of a typical tunnel element in prefabrication yard.

### **2.6 Interaction**

To optimise the scheme, the works described in Sections 2.4 and 2.5 may be named as "analysis", and the subsequent work, on the contrary, as described in this section, can be named as "synthesis", that is, understanding the links between the factors and then looking for the most satisfying design schemes by means of design iterations.

In the transverse structure of the immersed tunnel, the three aspects mentioned in Section 2.4 are interlinked. For example, strengthened structure causes thickened slab or more densely arranged reinforcing bars, either of them would give additional weight to the structure; the weight balance is broken and thus needs to be rebalanced by adjusting the inner space of the tunnel. Taking another example,

### **Figure 9.**

*Factory for prefabrication of immersed tunnel element. (a) Photo HZMB Island-tunnel project and (b) plane layout.*

**Figure 10.** *Photo of immersion joint of tunnel element in HZMB Island-tunnel project.*

enlarged inner space leads to the increased buoyancy, which requires an added weight by thickening the walls to balance the extra buoyancy.

It does the same to the longitudinal structure of the immersed tunnel element. Moreover, the longitudinal structure is interlinked with the transverse structure. Following the above example, the thickened wall allows for larger shear key, which could increase the capacity of shear key; it also means the ability of survival of tunnel element against differential settlement increases. In this way, the tunnel element can be made longer.

Another link is that the design of immersed tunnel is related to time and space, as shown in **Figure 11**. The prefabrication of tunnel element, the installation, and the inner works of tunnel often take place in three different locations. Moreover, to complete the work, there are sequences to follow. This figure shows that the immersed tunnel design, to some degree, cannot be reproduced; hence, the nature of the design work for an immersed tunnel is indeed to eliminate the gaps of space or discontinuity of time of the immersed tunnelling works.

### **2.7 Two fatal issues**

The success or failure of an immersed tunnel project largely relies on the prefabrication yard and the water sealing of tunnel. For the former, in HZMB Island-Tunnel Project, great efforts were made to find a suitable place, six locations were investigated, and the final selected location was on an island for three advantages. First, the geological condition is hard rock, suitable for the incremental launching system of the factory method. Second, the transportation distance of the tunnel element for immersion works is shortened to only 11 km. And third, the prefabrication yard is capable of producing two tunnel elements while store six tunnel elements inside the dock, eliminating the risk of tunnel element damage from the frequent typhoon in summer time of each year.

### **2.8 The latest technological developments and the future**

To the author's best knowledge, in Bosporus Strait, the immersed tunnel had been built around 70 m below the water surface. In Busan immersed tunnel, special facilities were invented to position the tunnel element under the water accurately and to make direction correction of the tunnel element automatically. *Design of Immersed Tunnel and How We Research Submerged Floating Tunnel DOI: http://dx.doi.org/10.5772/intechopen.88169*

In HZMB Island-Tunnel Project, 35 times of installations were carried out in 3 years in offshore condition with no major accident. The novel foundation solution of composite foundation layer and underwater surcharge were implemented, and the settlement of the immersed tunnel had been controlled within 5–8 cm. Nearly 100 million cubic metres of concrete were cast for the main structure of the 5.664 km long immersed tunnel element with no cast crack. Further, the tunnel element was deeply buried below the seabed; maximally 22 m thick sediment will cover on the roof of the tunnel element. And this extremely high overload (as for the tunnel element structural design of immersed tunnel longitudinally) is overcame by the structural innovation of semi-rigid tunnel element structure, setting permanent prestressing; the structure can become more robust taking the advantage of both monolithic and segmented tunnel element (**Figure 12**). The memory bearing can prevent concrete cracking at immersion joint [4]. The deployable element [5] is a highly effective and rick-manageable way to build the closure joint of the immersed tunnel.

The technological development has been pushing the boundary of application in immersed tunnelling regarding length and depth. However, to cross much deeper and broader water, all existing solution of bridge and tunnel would fail; in that case, the SFT shows its good reason of existence. The pioneer engineer of immersed tunnel Walter Grantz left one thought: "all immersed tunnels are briefly SFT's while they are being lowered into position."

**Figure 12.**

*The mechanism of the novel semi-rigid tunnel element structure.*

### **3. How we research submerged floating tunnel**

### **3.1 Main threads**

The SFT is, as per the term defined by ITA WG11, a tunnel through water that is not in direct contact with the bed. Moreover, it may be either positively or negatively buoyant and may be suspended from the surface or supported from or tied down to the bed (**Figure 13**). It has been proposed a century ago but has never been realised due to various reasons, such as fear of invasion, fishery problem, or ship collision. Therefore, to realise SFT attention must be paid to safety. The safety is in direct connection with SFT's structural form and environment. The former can be designed and developed as per our will and, hence, should be the main threads of SFT's research. Further, the more details the SFT's structural form being developed, the more risk issues regarding it will be raised. Therefore, the risk should

**Figure 13.** *Image of an SFT with positive buoyant tied down to sea bed.*

### *Design of Immersed Tunnel and How We Research Submerged Floating Tunnel DOI: http://dx.doi.org/10.5772/intechopen.88169*

be regarded as an accompanying thread in addition to the main thread of SFT's research. The study of risk can be carried out both quantitatively and qualitatively.

If the SFT is small, we can hold it in our hand; our research method is to produce hundreds of SFT prototypes, test them, improve them, and perfect them. However, our research resources are so limited compared to the real size of SFT; we can hardly build one SFT prototype, not to mention to agitate and test the real SFT. Therefore, the applicable way to research SFT is to build SFT model instead. The model can be further distinguished by tangible and intangible one, which is, in a researcher's language, the mathematical model and the physical model. The model supports the SFT's thread to gain the true knowledge of SFT.

Another support to the research is the development of the construction method. An SFT design scheme that can satisfy all needs and requirements but cannot be built in realistic work is of little value/use to the engineering knowledge. Sometimes we can hardly resist our temptation to study the details and believe that our work can improve the efficiency of the work; in fact, the parallel study on construction method can help shorten research period and lower the overall cost. Construction experiences from other relevant works such as immersed tunnel (**Figure 14**) and offshore structures may be transferable to SFT.

### **3.2 Structural form**

In order to understand the links between the SFT structure and its risk, we need to discretize the structural form of SFT into elements, part of which needs to be further discretized into sub-elements. By changing a parameter of the element or subelement, we can observe how the change affects the structural behaviour or safety of SFT. In this way, we gradually understand SFT's nature. **Figure 15** shows the author's understanding of the relation of structure and safety. This figure needs to be further expanded to cover the full picture of understanding of SFT before making a real one.

The safety belongs to our feeling and judgement, while the element or sub-element of an SFT is a matter; how do the two interact with each other? Mathematics is the gear of interaction. For example, if we set δ as deflection and t as time, then ∂<sup>2</sup>δ/∂ t 2 stands for the deflection's second derivative to time, that is, acceleration. The deflection not only determines the member force of the structure but also affects our safety feeling if we pass through an SFT. The acceleration will make us have "seasickness", if the value of it is large due to improper design of the structure.

### **Figure 14.**

*Image of immersion works of a typical tunnel element in Hong Kong-Zhuhai-Macao Island-tunnel project, showing immersion rigs, lines, and positioning system.*

**Figure 15.** *Relation of structural element and safety.*

For another example, if we set *T*¯ as the natural vibration period of an SFT and *T* as periodic loading acting on it due to natural waves, then *T*¯/*T* represents their ratio. In structural design, we need to avoid resonance by letting this ratio far away from the value of 1. We can either change the mass of the tube or the stiffness of the lines or introduce more damping into the system.

### **3.3 Model**

The slenderness and the size of an SFT are like a bridge, and the submergence of it is like a ship or submarine, while the slenderness, the size, and the submergence of SFT are comparable to nothing. Moreover, no real SFT exists. Thus, no mathematical model has ever been validated. Therefore, the physical model is essential.

The ultimate purpose of building a model is not to obtain figures but to obtain the tools that can obtain the figures and to see through the figures, beyond them. A scaled (and simplified) prototype of an SFT is a physical model, and a reproduction of a physical model is a mathematical model. Once the mathematical model is calibrated and validated by the physical model, it can then be used to predict the behaviour of the prototype by scaling the model up in the computer. However, one problem in this procedure is the scale effect, which may distort our understanding and judgement. The SFT structure is submerged in the water. Thus, the scaled effect exists in both structure and fluid. In HZMB Island-Tunnel Project, comparison of the current drag force was made to the scaled physical model and the measured data; great discrepancy exists between the two [6]. In the design of the steel-concrete-steel composite structure of closure joint for the immersed tunnel for the same project [7], special measures were made to strengthen the structure since the size is larger than the previous application in Japan.

To deal with the disturbance of scale effect, one countermeasure is to subdivide the physical model test into three stages, the mechanism test, the parameter test, and the validation test. The mechanism test focuses on the understanding of the structural behaviour to guide the direction of structural form; hence, the influence of scale effect may be neglected. In the parameter test, we ensure that the scale of the model is large enough so that the results of the test can be used either directly or by extrapolation to calibrate the mathematical model. The validation test is the last-stage test for validation of the overall designs to ensure the robustness and comprehensiveness of the SFT structural system. Engineers or engineer researchers may find that the mathematical model cannot match with the physical model and that limitation can be effectively solved by asking help from applied mathematicians or physicians.

*Design of Immersed Tunnel and How We Research Submerged Floating Tunnel DOI: http://dx.doi.org/10.5772/intechopen.88169*

To study the overall structural behaviour in water and from current and waves, the author proposed and designed a 1:50 physical model test in 2018; the tube model is 24 m long, with a circular cross-section of 252 mm in diameter, representing a 1.2 km long, two-lane traffic road SFT. **Figure 15** shows the design of the tube model. A steel bar is in the centre of the tube for simulating bending stiffness and covered by the form that simulates the volume. Steel hoops were on the external side of the form simulating weight. Some hoops were welded with eyes for connecting to steel wire lines; the lines were connected with springs to simulate the stiffness of mooring lines. The lines are spaced at 3 m longitudinally. Both ends of the model were fixed. By setting up a reference model and altering structural parameters such as net buoyant, line arrangement (**Figure 16**), and boundary conditions, the change

**Figure 16.** *Schematic design drawings.*

### **Figure 17.**

*Photo of SFT overall structural behaviour test sponsored, designed, and led by CCCC; SFT model and sensors prepared by DUT; and basin built by TIWTE.*

of structural behaviour subjecting to the change of structure can be observed. This test is now being prepared (**Figure 17**) as the first case in the world; results are expected to guide the direction SFT structural form design.

### **4. Conclusions**

The high risk of immersed tunnel construction requires a risk reduction through design. Due to varied location, environment, and construction/operation need, almost each tunnel element design varies from each other. The interior space, structural resistance, and tunnel element weight determine the transverse design of the immersed tunnel, while the structural system, element length, and joint configuration determine the longitudinal design of that. Multifactors of the structure were interlinked and linked to construction, time, and space; hence, a satisfying design requires a spiral-up iteration process including the works of analysis and synthesis. The selection of prefabrication yard for tunnel element, the transportation channel, and the water sealing of tunnel element are detrimental for the project.

SFT can cross broader and deeper waterbody. The main threads of our SFT research are structural form and risk, supported by construction method, mathematical model, and scaled physical model. We will find what is unknown, understand the mechanism, obtain parameters through physical model tests, and understand SFT's behaviour by mathematical measures. When encountering our limit, we need to cooperate with physics and mathematics.

### **Acknowledgements**

The author's writing is largely based on his work experience and reflection in CCCC HZMB Island-Tunnel Project, part of which now is CCCC SFT Technical Joint Research Team.

### **Conflict of interest**

The author declares no conflict of interest.

*Design of Immersed Tunnel and How We Research Submerged Floating Tunnel DOI: http://dx.doi.org/10.5772/intechopen.88169*

### **Author details**

Wei Lin1 \*, Ming Lin2 , Haiqing Yin3 and Xiaodong Liu1

1 CCCC SFT Technical Joint Research Team, CCCC Highway Consultant Co. Ltd., CCCC HZMB Island and Tunnel Project, Zhuhai, China

2 China Constructions Communications Company Ltd., CCCC SFT Technical Joint Research Team, CCCC HZMB Island and Tunnel Project, Zhuhai, China

3 CCCC SFT Technical Joint Research Team, CCCC Third Harbor Engineering Company Ltd., CCCC HZMB Island and Tunnel Project, Zhuhai, China

\*Address all correspondence to: linwei0502@126.com

© 2019 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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Section 4
