Geological Aspects of Tunnelling

## **Chapter 1** Engineering Geology and Tunnels

*Vassilis Marinos*

### **Abstract**

Currently, knowledge and understanding of the role of geological material and its implication in tunnel design is reinforced with advances in site investigation methods, the development of geotechnical classification systems and the consequent quantification of rock masses. However, the contribution of engineering geological information in tunnelling cannot be simply presented solely by a rock mass classification value. What is presented in this chapter is that the first step is not to start performing numerous calculations but to define the potential failure mechanisms. After defining the failure mechanism that is most critical, selection of the suitable design parameters is undertaken. This is then followed by the analysis and performance of the temporary support system based on a more realistic model. The specific failure mechanism is controlled and contained by the support system. A tunnel engineer must early assess all the critical engineering geological characteristics of the rock mass and the relevant mode of failure, for the specific factors of influence, and then decide either he or she will rely on a rock mass classification value to characterise all the site-specific conditions. Experiences from the tunnel behaviour of rock masses in different geological environments in Alpine mountain ridges are presented in this chapter.

**Keywords:** weak rocks, ground types, tunnel behaviour, tunnel design, tunnel behaviour chart, geotechnical classifications

### **1. Introduction**

Currently, knowledge and understanding of the role of geological material and its implication in design is reinforced with advances in site investigation methods, the development of geotechnical classification systems and the consequent quantification of rock masses. Rock mass rating (RMR) [1] and Q [2] were developed to provide tunnel support estimates through a rating of rock masses. In addition, the advancement of the geotechnical software that is easier to use led to an increased requirement of data related to mass properties. This kind of data is needed as input for analysis in the numerical solutions for designing tunnels. The onset of numerical tools to handle rock-support interaction and the advancement of concepts related to ground reaction curve permitted issues to be managed well beyond the ultimate extent for application of different tunnel support classification systems such as the RMR and Q systems. Practice picked up from the early application of more modern numerical modelling recommended that there was great correspondence between the rules from these classifications and the displaying outcomes about reality when rock mass behaviour was generally simple; for example, the RMR system does not give good outcomes beyond the range of values between 30 and 70 under moderate stresses. Good results and realistic outcomes may well be produced where the

sliding and rotation of intact rock pieces essentially controlled the overall failure process, comparing to an encounter database on which the early classifications were built. Truly hundreds of kilometres of tunnels were effectively excavated on the sole premise of this application.

Solid appraisals of the strength and deformation characteristics of rock masses are required for nearly any procedure of investigation concerning an underground work. Subsequently, an approach for the estimation of rock mass properties from intact rock properties and joint characteristics is fundamental. The Hoek-Brown failure criterion [3–5] would be of no benefit in the event that it might not be promptly connected with engineering geological perceptions for the nature and fabric of the rock mass. Hoek [6] proposed a methodology for getting estimates of the strength of fractured rock masses based on an appraisal of the interlocking of rock pieces and the condition of the surfaces between these pieces. For such an evaluation, the Geological Strength Index (GSI) was presented. The GSI has been established over a long period [3, 5, 7–13] to meet the desires of practitioners and cases that were not at first realised. The application of the mass properties from the GSI values basically accepts that the rock mass behaves isotopically. It is not factional where there is a clear anisotropic behaviour, e.g. clearly characterised favoured failure surface or discontinuities. The appropriate use of rock mass characterisation systems, notably the GSI (for details, see [14, 24]), allowed the quantification of difficult ground for the evaluation of the geotechnical properties and the selection of the design parameters. An extension of the original GSI application charts for heterogeneous and structurally complex rock masses, such as flysch, was initially introduced by Marinos and Hoek [10] and recently updated and extended by Marinos [11]. Specific GSI charts for molassic formations [12], ophiolites [13], gneiss in its disturbed form [14, 15] and particular cases of limestones [15] and under particularly difficult geological conditions have been developed from experience gained during excavation of 62 tunnels as part of the Egnatia project in Northern Greece, along Alpine mountain belts.

In expansion to the GSI values, it is additionally fundamental to consider the choice of the 'intact rock' properties σci and mi for these rock masses with different mineral composition. The fundamental inputs of the Hoek-Brown failure criterion are assessments or measurements of the uniaxial compressive strength (σci) and the material constant (mi) related to the frictional properties of the rock and of the GSI. Furthermore, to assess the deformation modulus of the rock mass (Em), Hoek and Diedrichs [16] proposed a formula based on the values of the intact rock deformation modulus (Ei) or the modulus ratio (MR).

The role of the ground characteristics and its effect in tunnel design, strengthened with progresses in site investigation techniques, cannot be exclusively based on the advancement of geotechnical classification frameworks and the following quantification of rock masses. Temporary support measures for rock masses with equivalent classification values can be diverse. The engineering geology appraisal displayed in this chapter cannot bypass the geological and/or in situ characteristics managing or affecting the tunnel behaviour compared with a regular classification that might miss the specifics and particularities of and around a tunnel segment. The likely ground types must be assessed, and after that, combined with the components of the tunnel geometry, the primary in situ stresses and the groundwater regime, the possible failure modes must be considered. These classified behaviour modes, followed by the appropriate mechanical properties that are required for sound tunnel design, are the premise for the numerical design of the appropriate primary support measures to achieve stable tunnel conditions.

There has been a serious effort to develop guidelines and procedures for tunnel design in which the observation of rock mass behaviour is incorporated in

### *Engineering Geology and Tunnels DOI: http://dx.doi.org/10.5772/intechopen.90462*

the determination of excavation and support classes [17–21]. The first step of this methodology involves the definition of the possible rock mass type, the second step involves the evaluation of the rock mass behaviour in tunnelling, the third step suggests the type of tunnel excavation-support system, and the final step is the definition of tunnel length with equal support requirements and the appraisal of time and cost for incorporation in the tender documents.

The design methodology discussed here incorporates the assessment of the tunnel behaviour type in the selection of design parameters and the definition of temporary support measures. A flowchart in **Figure 1**, based on Schubert [20] with modifications, presents this design methodology. As shown in this flowchart, the fundamental link between the rock mass model and the excavation and support classes is the definition of the tunnel behaviour type.

Hence, the contribution of engineering geological information for safe and economical tunnelling cannot be simply presented solely by a rock mass classification value (e.g. RMR, Q, GSI, or others). A classification rating, if used, must be accompanied by an understanding of the actual rock mass behaviour in tunnelling [22]. The tunnel behaviour may vary from one rock mass to another, indeed on the off chance that they have the same classification rating within the same stress field and the same groundwater conditions. An illustration of two different ground types with the same classification value but distinctive tunnel failure mode is displayed in **Figure 2** [22]. The two frameworks in **Figure 2** outline that the choice of the immediate support measures cannot be based solely on a classification rating (either GSI or RMR or Q ) but that it moreover requires an understanding of the tunnel failure type.

Attention, therefore, should be given to the evaluation of the failure mechanism that 'fits' the ground type after its excavation. For instance, it is clear that in the process of design, the structure of the rock mass must be considered together with the classification index. Taking after the assessment of the failure mechanism, one can be more certain either in utilising the rating of the associated classification value or in deciding the particular geological or in situ characteristics—'keys' that oversee the tunnel behaviour of the ground type. This procedure assists the designer in the analysis of tunnel behaviour and the selection of support measures and in the establishment of the contractual documents and guidelines for the construction.

After the appraisal of the tunnel failure mode, the appropriate numerical modelling can be performed, the conditions can be more soundly analysed, and the principles of tunnel support can be more precisely considered. The appropriate and critical design parameters can also be chosen in connection with the standards of the failure mode. If the behaviour of the rock mass can be considered isotropic and governed by induced stress, the tunnel engineer must be focused on the rock mass parameters (e.g. GSI in the Hoek-Brown transfer equations relating intact and rock mass properties with respect to the GSI) [3, 4]. On the other hand, in the event that the main failure mode is gravity-induced instability, the practitioner must focus on parameters related to the joints. In the event that the rock mass is weak but moreover anisotropic (e.g. due to schistosity or well-defined bedding planes), both the rock mass parameters and the persistent discontinuity properties must be considered [23]. Being that most tunnel designs presently incorporate numerical analysis, the issue is whether to utilise rock mass parameters (such as shear strength of the rock mass, cmass, φmass and Εmass) when the rock mass behaves isotopically or to incorporate the joint parameters (orientation, distribution, persistence, shear strength cjoint and φjoint) when the behaviour is controlled by the discontinuities or impacted by the resulting anisotropy.

Recent research regarding weak rock masses and their engineering geological behaviour, as well as the experience gained by the recent tunnelling projects in the Greek mountains, offered sound and adequate information for the investigation of

### **Figure 1.**

*Flowchart of the design procedure for tunnelling using conventional drill and blast excavation from the geological model to the definition of rock ground types and from the appreciation of the tunnel behaviour modes to the numerical design. Based on Schubert [21] and modified by the author [22].*

the impact of these conditions on the behaviour of the geological material, as well as on the design and construction methods. To make substantial use of the experience accumulated from the design and construction of these tunnels and to correlate this material, a database was built, i.e. 'Tunnel Data Examination System' (TIAS), which was outlined and created for 62 bored tunnels within the Greek region along

### *Engineering Geology and Tunnels DOI: http://dx.doi.org/10.5772/intechopen.90462*


### **Figure 2.**

*Example of two equally rated rock masses with the GSI or RMR system but with completely different behaviours in tunnelling [22]. The selection of the temporary support measures should not be based only on the classification ratings but also on the understanding of the tunnel failure mechanism, which is greatly dependent on the rock mass structure.*

the Egnatia Highway [25] mainly spanned in Alpine mountain ridges under difficult geological conditions in weak rock masses, excavated with conventional methods, in the concept of top heading and bench excavation. This database is built to 'relate' all available data through all the phases of a tunnel project and premises deep knowledge from the geological and geotechnical investigation to the final design

and construction. The data processed by TIAS came from a variety of sources such as geological mapping, boreholes, laboratory and in situ testing, geotechnical classifications, engineering geological behaviour, groundwater, design parameters, information concerning immediate support measures, construction records and cost. The scope of the system is to provide a tool for the evaluation of anticipated and encountered geological and geotechnical conditions, the evaluation of geotechnical classification and design methods and the relations regarding rock mass conditions and behaviour and immediate support methods and types.

The variety of geological formations under different in situ stress conditions, not only in both mildly and heavily tectonised rock masses but also in altered and/or weathered rock masses, provided a significant amount of information regarding the engineering geological behaviour of several rock mass types. The general geological and engineering geological characteristics and the behaviour in tunnelling of specific rock masses, such as heterogeneous rock masses of flysch and molassic formations (tectonically undisturbed heterogeneous sediments) as well as sound, disturbed and altered ophiolites, are briefly presented in the next paragraphs as examples.

### **2. Tunnel behaviour appraisal**

Engineers can analyse reinforced concrete or steel structures utilising certain checks for a particularly predefined failure mechanism. Particularly, analysis is performed against the bending moment, axial force, shear, penetration and deflection (serviceability limit state). In the case of tunnelling, there is no particular methodology to check against a predefined failure mode.

It is pointed out that the primary step is not to begin performing various calculations but to characterise the potential tunnel behaviour modes. After the evaluation of the ground behaviour in tunnelling, the analysis of the temporary support system can be utilised, in two stages: the choice of the appropriate support elements and their detailed analysis. The selection of support measures should be established equally on experience and geotechnical data and on the analytical solutions but must be confirmed or re-evaluated during construction, supported by the monitoring of the tunnel.

Rock mass behaviour evaluation in tunnelling and its relationship with the design process have been significantly researched. Goricki et al. [18], Schubert [20], Potsch et al. [26] and Poschl and Kleberger [19] studied rock mass behaviour from the design and construction experiences of Alpine tunnels and Palmstrom and Stille [27] from other tunnels.

### **2.1 Tunnel behaviour types**

The term 'failure mechanism-behaviour type', as alluded here, includes all the components that endanger the tunnel segment when the ground has not yet been supported after excavation.

This paragraph presents the tunnel failure modes as they have been designated by Terzaghi [28] and Schubert [21] and also suggested by the author from the tunnel experience of 62 designed and constructed tunnels along Egnatia Motorway and from other cases in Greece. The tunnel failure modes, apart from stable (St) conditions, are separated into gravity-driven failures (wedge and chimney-type failures and ravelling and ravelling ground) and stress-driven failures (failures, squeezing and swelling, anisotropic deformations and brittle failures). The limits and ranges where each behaviour type is connected are briefly depicted and


### **Figure 3.**

*Brief descriptions and schematic presentations of tunnel behaviour types [22] (based on data from Schubert [21], Terzaghi [28] and from the author). Photos from the author except for 'Sq' from E. Hoek (personal communication) and for 'San' from Seingre [29].*

appeared in **Figure 3**. The failure modes are assembled based on the examination of tens of rock mass types, their rock mass and joint quality properties and their actual behaviour below different stress conditions (from 30 m to 500 m overburden).

Stress-driven failures: The advancement of critical strains around a tunnel is characterised by the ratio of σcm/po [30]. Specifically, when σcm/po is between 0.3 and 0.6, shear failures can develop in a shallow zone around the tunnel perimeter (Sh failure mode). Such cases include rock masses with poor to very poor fabric and low intact rock strength (< 10–15 MPa) under medium overburden or with

more competent structure and low intact rock strength below high tunnel cover. Squeezing conditions (Sq failure mode) with severe tunnel strains can be induced when σcm/po < 0.3.

Gravity-driven failures: They are generally differentiated with relation to the rock mass fabric (original conditions and tectonic deformation) and to the conditions of being kept in confinement or not. These gravity-controlled failures occur in rock masses that are clearly characterised by the joints. When the rock masses are just excavated, wedges may fall or slide, depending on the tunnel geometry, the orientation and the shear strength characteristics of the discontinuity planes. Wedge (Wg), chimney (Ch) or ravelling (Rv) failure types can take place in rock masses with poor interlocking of rock blocks due to fracturing degree and/or low confinement. The rock mass cannot arch after the falling, and the crown failure may be significant and irregular. The volume and recurrence of these sorts of tunnel behaviour depend on the structure of the rock mass ('blocky-disturbed' and 'disintegrated'), its relaxation ('open structure') and the tunnel cover/lateral confinement conditions. With an increase in the depth of the tunnel, the rock mass quality is generally improved, and the confinement pressure 'tightens' the structure of the mass.

Of course, there are cases where both stress and gravity-driven failures can be met in a rock mass. In such cases, particular consideration ought to be given to the principal failure mode for the choice of suitable support measures.

A tunnel behaviour chart (TBC) [22], illustrated in **Figure 4**, has been proposed for assessing the rock mass behaviour in tunnelling and covers a wide range of rock mass conditions. This assessment is based on the structure of the rock mass, the strength of the intact rock and the overburden depth.

This classification frame, the TBC, joins the rock mass characteristics straightforwardly with the design and the tunnel support standards and covers a wide extent of conditions. The TBC could be a classification for the estimation of tunnel behaviour and requires three parameters: the rock mass structure, the overburden (H) and the intact strength of the rock (σci). This is an integrated classification based on the TIAS database and the data from the design and construction of 62 tunnels in Greece [25]. The purpose of this chart is to foresee the basic failure modes of several rock mass qualities and conditions. The cases that were investigated elaborated intact rock strengths up to 100 MPa and depths not more than 500 m, while many tunnels were less than 300 m deep. It is noted that the values of the uniaxial compressive strength of the intact rock (σci) and the overburden thickness (H) utilised within the chart are reasonable trends but should only be considered as indicative.

This chart can be applied in a wide range of geological and geotechnical conditions, since numerous geological formations with various tectonic, weathering and alteration intensity, commonly found worldwide, have been excavated and effectively supported, under a large range of tunnel covers (up to 500 m). The chart does not refer to very high overburden (e.g. many hundreds of m or > 1000 m) and very large intact rock strengths, where brittle failures (spalling or rock burst) can be developed. Hence, TBC can be really useful in any mountain formations in a tunnel excavated with the conventional principles within this wide application range.

The rock mass structure is an essential parameter to appraise its prompt reaction in underground opening in the TBC chart. From the structure of the rock mass, one can 'read' the tectonic disturbance, the blockiness of the mass, the probable size of blocks, the shape of rock elements (massive, blocky, foliated or sheared) or the ability of the rock blocks to rotate. Rock mass fabrics were categorised after the GSI system [8].

For gravity-controlled failures, the tunnel depth impacts the degree of a failure since the degree of interlocking between the rock blocks changes and the confinement

### **Figure 4.**

*Tunnel behaviour chart: an assessment of rock mass behaviour in tunnelling [22].*

stress varies with depth. For instance, the rock mass may ravel (Rv) near the ground surface, but under higher overburden, a chimney-type (Ch) failure may be developed.

As far as the stress-driven modes are concerned, tunnel cover H characterises when shear failures and deformations are formed. These limits are appraised in the following manner: 150 m for competent structure (intact and blocky-seamy undisturbed),

100 m for very blocky structure and 70 m for the very poor fabric (seamy-disturbed, disintegrated or laminated-sheared). These values are basically evaluated by back analysis and by the calculated values of the ratio σcm/po, with po, the in situ stress, considered isotropic. For example, in the event that σcm/po < 0.3, squeezing conditions are likely; in case 0.3 < squeezing <0.6, minor to medium strains may happen; and on the off chance that σcm/po > 0.6, minor or no deformations are expected.

The limit of intact rock strength (σci), i.e. 'low' vs. 'high', considered to characterise the rock mass behaviour in tunnelling, in the TBC chart, is based on the value when shear failures and deformations initiate. This limit is assessed at 15 MPa. The σci values that were analysed in the design of the investigated tunnels are extended between 5 and 100 MPa.

### **3. Examples of engineering geological appreciation and behaviour in tunnelling of particular rock masses**

The general geological and engineering geological characteristics and the behaviour in tunnelling of specific rock masses, such as heterogeneous rock masses of flysch (tectonically disturbed heterogeneous formation) and molassic formations (non-tectonically disturbed heterogeneous sediments) as well as sound, disturbed and altered ophiolites, are briefly presented in the next paragraphs as examples. For more details on each of the specific rock types, their engineering geological characteristics, their specific GSI characterisation and their tunnel behaviour, the interested reader is referred to the original publications presenting the individual charts [9–13, 15, 22, 23, 30, 31].

### **3.1 Flysch formations: tectonically disturbed heterogeneous rock masses**

Due to the generally poor characteristics and uncertainties with respect to its geotechnical characterisation, flysch frequently causes problems or challenges towards the design and construction of engineering projects. Flysch is composed of variable alternations of clastic sediments that are related with orogenesis, since it closes the cycle of sedimentation prior to the paroxysm folding process. It is characterised basically by rhythmic alternations of sandstone and pelitic layers (siltstones and silty or clayey shales). The thickness of the sandstone or siltstone beds ranges from centimetres to metres. Conglomerate beds may also be included. Heavy folding and highly shearing with various overthrusts characterize the environment in areas of flysch formations. The main thrust movement is associated with smaller satellite reverse faults within the thrusted body. The overall rock mass is profoundly heterogeneous and anisotropic and moreover may be influenced by extensional faulting creating mylonites. The structural deformation due to tectonism radically debases the quality of the rock mass. Hence, flysch rock mass types are associated with undisturbed, fractured, heavily sheared or even chaotic structures. Such flysch qualities are classified into 11 rock mass types (I to XI) [11] according to the siltstone-sandstone proportion and their tectonic disturbance (**Figure 5**).

The design of tunnels in poor rock masses such as folded and sheared flysch presents a major challenge to geologists and engineers. The complex structure of these materials, coming about from their depositional and structural history, implies that they cannot effectively be classified in terms of the broadly used characterisation systems. The range of geological conditions under varied in situ stresses, in both mild and heavy tectonism investigated here, offered valuable data with respect to the engineering geological conditions and geotechnical behaviour of several flysch rock mass types.


### **Figure 5.**

*Rock mass types in tectonically disturbed heterogeneous formations such as flysch [11].*

A classification of flysch rock masses depending on their geotechnical behaviour (strain due to overstressing, overbreaks or wedge failure, 'chimney' type failure, ravelling and their corresponding scale) is displayed from now on. Depending on its type, flysch can show a range of behaviours: be stable even under significant overburden, display wedge sliding and more extensive chimney type-crown failures, or show large deformations even under low to medium overburden.

In a general sense, the behaviour of flysch arrangements amid tunnelling depends on three major parameters: (i) the structure, (ii) the intact strength of the governing rock type and (iii) the depth of the tunnel. The anticipated behaviour types (stable, wedge failure, chimney type failure, ravelling ground, shear failures and squeezing ground) can be outlined within the tunnel behaviour chart [22]. A detailed introduction of the range of geotechnical behaviours in tunnelling for each flysch rock mass type (I–IX), which is based on engineering geological characteristics, is displayed in **Figure 6**.

The rock mass is frequently taken as a 'mean isotropic geomaterial', in the case that rock mass properties are quantified through a classification system. This presumption is normally accepted in conditions of a uniformly jointed, highly tectonised rock mass without persistent joints of certain unfavourable orientation. This condition can be quite true for types VII to IX. Where bedded rock masses are involved, at a scale of the tunnel segment, the engineering geological behaviour during tunnel construction is controlled by the properties of the bedding planes. This case may be applied to the flysch rock mass type III to VI.

A reliable first estimate of potential problems of tunnel strain can be given by the ratio of the rock mass strength to the in situ stress σcm/po [30]. This is usually followed by a detailed numerical analysis of the tunnel's response to sequential excavation and support stages. Minor squeezing (1–2.5%) can be developed in the very poor flysch rock mass types X and XI from 50 to 100 m tunnel cover, while severe (2.5–5.0%) to very severe squeezing (5–10%) can be developed from 100 m to 200 m cover. Undisturbed rock mass types of sandstone or conglomerate (types I and III) do not exhibit significant deformations under 500 m.

Regarding the rheological characteristics of flysch formations, the creep potential of sandstone formations is considered to be negligible. However, in the case of tunnel excavation in siltstone or shale formations, especially under high overburden, time-dependent displacements or loads may be developed.

### **Figure 6.**

*Tunnel behaviour chart with projections of the principal failure mechanisms for the rock mass types of flysch (I–XI) [31].*

The influence of groundwater on the rock mass behaviour in tunnelling is very important and has to be taken into great consideration in the estimation of potential tunnelling problems. The most basic impact of groundwater is on the mechanical properties of the intact rock components, particularly on shales and siltstones that are susceptible to changes in moisture content.

### *Engineering Geology and Tunnels DOI: http://dx.doi.org/10.5772/intechopen.90462*

The evaluation of tunnel behaviour and the conceptual assessment of the support measures must be also based on detailed ground characterisation. This detailed characterisation cannot bypass the geological and/or in situ characteristics managing or affecting the tunnel behaviour compared with a standardised classification. This classification, named 'Ground Characterization, Behaviour and Support for Tunnels' [22], urge the practitioner to assess the information in detail, to appraise the tunnel behaviour and to select the appropriate design parameters and the suitable support measures. An illustration of this characterisation in tunnelling through tectonically disturbed flysch type is displayed in **Figure 7** [31].



### **Figure 7.**

*(a) Modified example of the ground characterisation, behaviour and support for tunnels illustrated [31], in light characters by an example of tunnelling in a tectonically deformed intensively folded siltstone (flysch rock mass type X) [for page 2 from 2 see (b)].*

Apart from a few cases of simple tunnelling conditions in areas of good rock mass types of flysch (sorts I–V), most of the investigated tunnels were excavated under challenging geological conditions (sorts VII–XI). These tunnels have been excavated utilising top heading and bench methods. Particular measures were applied to stabilise the face, such as forepoling or/and establishment of long

### *Engineering Geology and Tunnels DOI: http://dx.doi.org/10.5772/intechopen.90462*

grouted fibreglass dowels in the face. Furthermore, immediate shotcreting and face buttressing have been utilised in several combinations for face stabilisation. After the stabilisation of the tunnel face, the application of the immediate support shell, consisting of shotcrete layers, rockbolts and steel sets embedded within the shotcrete in different combinations, was essential to ensure the stability of the tunnel. Elephant's foot and, in uncommon cases, micropiles were utilised to help the establishment of the top heading foundation zone and to secure stability when benching. Temporary and final invert closure was applied to meet the squeezing conditions.

Under severe squeezing, the application of yielding systems is an alternative solution [e.g. in Schubert, 1996, 20]. In the case of tectonically sheared siltstone rock masses under high cover (e.g. up to 250 m), where tunnel squeezing is a significant problem, the pillar stability in these twin tunnels requires careful evaluation.

### **3.2 Molassic formations (non-tectonically disturbed heterogeneous rockmasses)**

The term molasse comes from a Swiss local title at first allotted to soft sandstones related to marls and conglomerates belonging to the tertiary that had an extraordinary advancement within the lowland parts of Switzerland. They are as a result of debris of weathering and erosion of the Alpine mountains. The term is currently used to describe the deposits from the erosion of a mountain belt after the ultimate stage of orogenesis behind the mountain building zone. Molasse comprises of a sequence of tectonically undisturbed sediments of sandstones, conglomerates, siltstones and marls. Molassic rock masses may have exceptionally distinctive structures near to the surface compared to those restricted at depth, where bedding strata do not show up as clearly characterised joint surfaces that separate the rock mass into blocks [12].

Tunnelling through molassic rocks is based on the experience picked up from the design and construction of 12 tunnels along the Egnatia Highway in northern Greece. A context is displayed here concerning the distinctive rock masses of molassic rocks, the geotechnical behaviour of each type in tunnelling and the temporary support philosophy, both for underground construction and portal zones. The major characteristics of the investigated geomaterial that cause its specific tunnel behaviour are (a) the lithological heterogeneity, as the series comprises of a nearly continuous units of sandstones, siltstones, marls or claystones and conglomerates, with alternations of layers from some centimetres to some metres thick; (b) the low to moderate strength of the intact rock of these units; (c) the compact, nearly intact structure at depth, indeed when sandstone strata alternate with siltstones; and (d) the problematic behaviour of the siltstone-marly units near to the surface due to slaking and weathering.

Molassic rocks display noteworthy differences between the surface and at depth. These contrasts lie within the rock mass fabric, weathering and permeability and thus are exceptionally critical for the rock mass quality and behaviour in tunnelling. Molassic rocks, especially sandstone and well-cemented conglomerates, tend to be profoundly frictional. Due to the narrow deformation to which they have been subjected in deposition, the discontinuity in these rocks is by and large free from the impacts of shear development (slickensides).

Siltstone or claystone beds, being restricted shortly beneath the surface, are compact enough to create a nearly unbroken medium. Their presence may, be that as it may, diminish the quality of the whole rock mass, due to its nature. In any case, there are occasions where siltstones are fairly competent and below low stress; their behaviour does not essentially contrast from that of sandstones. The bedding is the basic joint set in a molassic rock mass but is only communicated on and close to the surface. At depth, the bedding is mostly concealed. For the cases examined in this chapter, rock quality designation (RQD) values close the surface run from 0 to 50%. At low depths (∼5 m), the rock masses ended up medium broken and weathered, whereas bedding planes are still apparent. At depths greater than 10–15 m, the rock masses are as a rule homogeneous in structure and continuous, with RQD values >60%.

Weathering usually transforms the rock mass strength. Siltstone (or marly) members are susceptible to weathering, and fissility may be built parallel to the bedding when these rocks are uncovered to the surface or are close to it. Siltstone (or marly) members in outcrops show up thin layered or even schistosed, and when they alternate with sandstones, the appearance of the rock mass takes after that of flysch. This appearance in outcrops can be deceiving when considering the behaviour of molassic rocks in a limited underground environment, in which the slaking is confined and the rock mass is massive. There are conditions where sandstones are loose and may be treated as dense sands. In such poor molasses, clays and silts also present, and the fabric can be treated like a soil. It is not the goal of this chapter to address these soil-like molasses that have constrained spatial dissemination. In any case, it ought to be underlined that molasses close to the surface may make a cover with such soil characteristics.

Based on the outcomes of numerous in situ permeability tests (Marinos et al. [9]), the overall permeability of the molassic series is rapidly reduced with depth. Though, the permeability of the sandstone members within the molasse is altogether higher than that of the siltstone ones. Within the case of variations of the two types of rocks, the permeability approaches the value of the siltstone since the siltstone layers do not permit the water flow through the rock mass and decrease the overall permeability. Besides, the frequent horizontal transitions do not allow the development of a uniform aquifer. Fault zones, in spite of the fact that they are more permeable, are neither frequent nor extensive. Thus, in spite of the fact that the water table will ordinarily be over the tunnel, only minor water inflows are expected, in spite of the fact that in a few circumstances it may be essential to relieve water pressures by drilling.

The high strength of the molassic rock mass in relation to the in situ stresses at shallow to medium depths does not qualify stress-driven failures. The prevailing failure mode in tunnels is the gravity-induced falls and slides of rock blocks and wedges characterised by intersecting joints and bedding planes. It ought to be stressed, however, that this behaviour has been confirmed with tunnel construction in depths up to 110 m and should not be reflected for much larger depths.

These types of behaviours are differentiated in two regions (see **Figure 4**):


less wedge sliding events (St-Wg) (range #8 in **Figure 4**). A marginally different failure mode can be presented within the case of thin-bedded series with nearly horizontal bedding planes. Failure of rock blocks due to self-weight from the crown section may be occasional and extensive once their base is exposed due to deconfinement which might cause subvertical tension joints. Such unfavourable conditions must be controlled to face systematic crown failures.


With regard to water inflows, minor occurrence of water has been met along the 12 tunnel projects, which develop basically within the form of increased moisture to drips. In a few uncommon cases, periodic or continuous low flow at different areas, primarily in sandstone-siltstone contact layers and along major discontinuities, has been experienced. However, this presence degrades the characteristics of the discontinuities and ought to be taken under consideration when evaluating the geotechnical characteristics of the rock mass types. The low geotechnical properties of the molassic formations, near to the surface, have driven to numerous failures in the portal areas. These instabilities were not directed by pre-existing discontinuities, such as the bedding planes, but they were related to the advancement of a new circular-shaped failure surface across the weathered poor rock mass.

The tunnel support concept in molassic rock mass types must take into consideration the rock mass fabric and the expected failure modes in connection to the depth as depicted above. These approaches for the philosophy of temporary support measures have been formed based on the geotechnical behaviour of molassic series as well as on construction data. Along the tunnels of the Egnatia Highway through molassic formations, 54% of the whole length was excavated employing a support category with shotcrete shells, anchors, steel sets and light spilling. A really light support category containing a thin shotcrete shell and a sparse pattern of bolts was received for 38% of the entire length. At long last, a heavy support category with thick shotcrete shells, steel sets, forepoling and fibreglass nails was executed in only 6.5% of the full length and basically within the region of the portals. Hence, absent from the ground surface, where the rock mass is subjected to surface weathering conditions and any credible fault zones, there are two basic types of immediate support systems that could be implemented.

The first type concerns stable conditions with solely gravity-controlled failures and minor to zero deformation. This is often the foremost common case for all molasses at depth and ought to be connected for low to medium overburden or

indeed under higher overburden in cases of sandstone or conglomerate mastery. The immediate support comprises of a thin shotcrete layer and a pattern of rock bolts, while the advance step can be 3–4 m. The primary 3–5-cm-thick layer of shotcrete, implemented, as soon as possible, on the uncovered rock mass surfaces, seals and secures the siltstone layers from slaking. The rock bolt design reinforces the rock mass, keeps it restricted and prevents likely gravity-controlled falls of loose, fundamentally defined blocks or wedges due to decompression of the otherwise sealed bedding planes. The introduction of another layer of shotcrete, strengthened either by wire mesh or by fibres, makes a complementary shell, engaging the heads of the rock bolts and guaranteeing the stability of the tunnel. The next type of support system for competent molasses absent from portals or faults alludes (a) to conditions with frequent wedge failures due to the geometry of major joint systems and the conditions already displayed (horizontal bedding planes) and/or (b) to cases of weak rock (e.g. siltstone) governed by molassic formations below considerable to large overburden. In expansion to the shotcrete and the rock bolts, light steel sets may be necessary, while the advance step must be restricted (around 2 m) to prevent any wedge formation or critical strains in the case of large depths.

For weathered molassic series near to the tunnel portals or intensely jointed and poor molassic rock masses along fault zones, stiffer support is required by the use of heavier steel sets and a thicker shotcrete shell. Consideration ought to be given to limiting disturbance to the encompassing geomaterial by reducing the excavation step (∼1 m). Furthermore, it may be essential to stabilise the tunnel face utilising face support measures (e.g. fibreglass nails) or face protection schemes (e.g. spiles or forepole umbrella) to avoid progressive detachment, deconfinement and creation of chimney-type failures.

### **3.3 Ophiolitic complex**

The term ophiolite was at first given to a series of basic and ultrabasic rocks, more or less serpentinised and transformed, appearing within the Alpine chains. Ophiolites are presently considered as pieces of the oceanic crust produced at an oceanic ridge and the upper mantle of an ancient ocean, thrust up on the continental crust during mountain building [32].

The ophiolitic complex is in a general sense characterised by underlying peridotitic rocks that are overlied by gabbroic/peridotitic rocks, which, in turn, are covered by basalts or spilites. The basal peridotites are laminated ('tectonites'). The subsequent alternations of peridotites and gabbros frequently have a layered structure of cumulates and are taken after by enormous gabbros, norites or other basic rocks richer in SiO2. The overlying basalts are either continuous or within the frame of pillow lavas. In between these rocks, sedimentary rocks of deep sea may be stored. This geometry is exceedingly exasperated since the ophiolitic complexes happen primarily in tectonic zones with superposition of various overthrusts. Metamorphism, which is additionally displayed, changes the initial nature of the materials. The high degree of serpentinisation and the intensity of shearing can make it difficult to distinguish any lattice mineral of either fibrous or laminar shape. This unordinary alteration is a phenomenon of autohydration that occurs amid the final phases of the crystallisation of magma where there's an abundance of water. In other scenarios, serpentinisation compares to a low initial cumulate texture [33].

Serpentinisation is the change of ferromagnesian minerals, specifically olivine, to serpentine—a grade metamorphism of peridotites. In all these cases, the peridotites can be changed into serpentinite. This new rock is initially compact, moderately soft and more naturally sheared by tectonic processes. Serpentinisation can moreover be created due to exogenic conditions with meteoric water under regular

### *Engineering Geology and Tunnels DOI: http://dx.doi.org/10.5772/intechopen.90462*

weathering conditions. In this case, the alteration deteriorates the parent peridotite to a schistosed mass and later to clayey soil-like mass. The development at depth of weathered peridotites is less generalised up and clearly restricted compared with the endogenic serpentinisation portrayed already [13].

Rock masses in an ophiolitic complex display a wide variety of engineering behaviour in tunnelling. Typically, this is true due to their petrographic variety and structural complexity. An advanced degree of serpentinisation together with the increased shearing may result in a mass in which it is hard to recognise any initial surface or texture. Thus, behaviour can change from stable to severe squeezing conditions in cases where ophiolites are related with overthrusts. The main rock mass types are peridotites, gabbros, pillow lavas, peridotites that are more or less serpentinised, serpentinites, schisto-serpentinites, sheared serpentinites and chaotic masses in melanges. Peridotites are sound and behave as typical brittle materials. Serpentinisation can be found along the discontinuity surfaces, and the conditions of the joints are significantly reduced to very poor with coatings of 'slippery' minerals such as serpentine or talc. In a disturbed ophiolitic mass, the serpentinisation procedure regularly loosens and disintegrates parts of the rock matrix itself, contributing to lower GSI values and reducing the intact strength [13].

The extraordinary assortment of numerous rock mass types, the unpredictable changes and the alteration mark the ophiolites a formation where great care is required in the tunnel design. This is often true for tunnel projects due to their linearity and their depth that increase the possibility of experiencing the unfavourable zones related with the ophiolites, whereas the uncertainty as to their exact location and extent impairs the difficulty.

In sound and competent rock masses of peridotite, simple and straightforward tunnelling conditions can be anticipated, where consideration has got to be concentrated on maintaining wedge failures. Within the case of a more broken peridotite, schistose or great serpentinite, the behaviour is controlled by sliding and rotation on joint surfaces with generally little failure of the intact rock pieces. In this case, the control of stability can be amended during tunnel excavation by keeping the rock mass confined. In poor quality serpentinite, due to alteration or shearing, blockiness may be totally missing, and clayey areas with swelling materials may be present. Tunnel instability will at that point be due to stress-dependent rock mass failure with severe squeezing at depths [13].

Peridotites: In great quality masses of peridotite, straightforward tunnelling conditions can be anticipated. Consideration must be concentrated on controlling gravity-driven instabilities from wedges. For these failures comprising some joints, the issue is basically one of three-dimensional geometry and stereographic tools or numerical analyses such as UnWedge (see http://www.rocscience.com) ought to be utilised for an investigation of design of support measures.

However, compared with other rock masses of comparative structures, the peridotites in a general sense have smoother joints with poor frictional properties. As clarified previously, it's due to the existence of serpentinised material, which is regularly present even if the serpentinisation has not affected the rock itself. This makes the gravitydriven failures more challenging and for the most part requests heavier rock bolting patterns and/or thicker shotcrete (zones #2, #4, #6 and #8 in **Figure 4** depending on the depth and intact rock strength). In exceptionally hard rock masses at large depths, spalling, slabbing and rockbursting are the mechanisms of failure which will be developed and controlled by brittle fracture propagation in the intact rock with the joints having as it were a minor influence. In these cases, the utilisation of the brittle rock failure models must be considered, such as that proposed by Kaiser et al. [34].

Disturbed peridotites or schistose serpentinites: Within the case of a more disturbed peridotite, schistose or weaker serpentinite, the behaviour is controlled by sliding and rotation on joint surfaces with generally minor failure of the intact rock fragments (ranges #10, #12, #14 and #16 in **Figure 4** depending on the depth and intact rock strength). In this extent of GSI values, the RQD values can be exceptionally low. This is typical, given the structure of the rock masses, but some of the frictional behaviour of the unaltered fragments of the mass is reserved. In such cases, the control of the stability can be effectively achieved during tunnel excavation by maintaining the rock mass confined.

Sheared serpentinite, squeezing behaviour: In low-quality serpentinite, as a result of alteration or shearing, blockiness may be nearly totally lost, and clayey areas with swelling materials may be available. Tunnel stability will at that point be controlled by stress-dependent rock mass failure with significant squeezing at depths (regions #21 to #24 in **Figure 4** depending on the depth and intact rock strength). In these cases, a detailed numerical analysis must be performed that permits progressive failure and support interaction analysis to be demonstrated. In any case, it is exceptionally instructive to carry out a closed form analysis of the tunnel behaviour to get an indication of the significance and value of deformation [13]. The 'strain' can be evaluated from the proportion of the rock mass strength to the in situ stress [30]. This plot is valid to single circular-shaped tunnels.

### **4. Conclusions**

In general, the application of well-known classification systems has the drawback of not necessarily displaying information concerning rock mass behaviour in tunnels. Consequently, there are many cases in which the geological 'identity' of the geomaterial is lost since it is not involved in the analysis, and in that way, it is possible that its special characteristics are mislaid. Despite the capabilities offered by the rapid advance of the numerical tools in the geotechnical design, the outcomes can still include uncertainties when parameters are utilised straightforwardly without considering the real failure mode of the rock mass in tunnelling. This chapter points out that the assessment of the principle tunnel failure mode is an essential information for the temporary support measure definition. The work presented in this chapter was based on a large set of data, incorporated in a tunnel information and analysis system (TIAS), from the design and construction of 62 tunnels through a wide variety of geological conditions.

Two classifications and characterisation schemes have been presented to assess tunnel behaviour based on the engineering geological identity of the rock masses. The primary, called the tunnel behaviour chart, is a classification framework for predicting the rock mass behaviour in tunnelling and covers a wide extent of rock mass conditions. This evaluation is based on the fabric of the rock mass, the strength of the intact rock and the tunnel cover. The moment, called Ground Characterisation, Behaviour and Support for Tunnels, is a step-by step appreciation of a rock mass quality, with detailed engineering geological and geotechnical characteristics, towards the evaluation of the foremost tunnel behaviour and its support requirements.

After defining the most possible failure types for every kind of the predicted rock mass, the most appropriate design parameters are identified, either of the rock mass, if it displays an isotropical behaviour, or characteristics of discontinuities if it behaves in an anisotropic manner. These proposals allow an early assessment of the principles for the choice of appropriate support measures and their basic dimensioning, as dictated by the ground behaviour and the associated mode of failure. The accuracy of the classifications and the support system can be managed directly from direct tunnel observation and monitoring.

### **Acknowledgements**

All the experience and constructive comments, provided, all these years, by Dr. Evert Hoek, Canada, and Emeritus Professor Paul Marinos, Greece, are gratefully acknowledged. The author would like to thank Egnatia Odos S.A. for its support and the data provided. I am also thankful for the helpful discussions and deep collaboration with Mr. Nikos Kazilis, Mr. Nikos Rachaniotis and Mr. Giorgos Aggistalis, Greece, throughout the Egnatia Odos S.A. tunnelling construction. Ms. D. Papouli, Geologist, M.Sc., gave valuable assistance in editing the figures.

### **Author details**

Vassilis Marinos School of Geology, Faculty of Sciences, Aristotle University of Thessaloniki, Thessaloniki, Greece

\*Address all correspondence to: marinosv@geo.auth.gr

© 2020 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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[6] Hoek E. Strength of rock and rock masses. ISRM News Journal. 1994;**2**(2):4-16

[7] Hoek E, Marinos P, Benissi M. Applicability of the geological strength index (GSI) classification for weak and sheared rock masses—The case of the Athens schist formation. Bulletin of Engineering Geology and the Environment. 1998;**57**(2):151-160

[8] Marinos P, Hoek E. GSI A geologically friendly tool for rock mass strength estimation. In: Proceedings of the International Conference on Geotechnical and Geological

Engineering (GeoEng 2000). Lancaster: Technomic Publishers; 2000. pp. 1422-1446

[9] Marinos V, Fortsakis P, Prountzopoulos G, Marinos P. Permeability in flysch— Distribution decrease with depth and grout curtains under dams. Journal of Mountain Science. 2011b;**8**(2):234-238

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(in Greek), Doctoral Thesis. Athens, Greece: School of Civil Engineering, Geotechnical Engineering Department, National Technical University of Athens (NTUA); 2007

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[17] Austrian Society for Geomechanics. Guideline for the Geotechnical Design of Underground Structures with Conventional Excavation. Translated from version 2.1, 29 p, 7-page Appendix; 2010

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[19] Poschl I, Kleberger J. Geotechnical risks in rock mass characterization. Tunnels and Tunnelling International. 2004. Part 1—September 2004, 37-9. Part 2—October 2004, 36-8

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[21] Schubert W. Basics and application of the austrian guideline for the geomechanical design of underground structures. In: EUROCK2004 and 53th Geomechanics Colloquium. VGE; 2004

[22] Marinos V. Assessing rock mass behaviour for tunneling. Environmental and Engineering Geoscience. 2012;**18**(4):327-341

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### **Chapter 2**

## Advanced Geological Prediction

*Shaoshuai Shi, Xiaokun Xie, Siming Tian, Zhijie Wen, Lin Bu, Zongqing Zhou, Shuguang Song and Ruijie Zhao*

### **Abstract**

Due to the particularity of the tunnel project, it is difficult to find out the exact geological conditions of the tunnel body during the survey stage. Once it encounters unfavorable geological bodies such as faults, fracture zones, and karst, it will bring great challenges to the construction and will easily cause major problems, economic losses, and casualties. Therefore, it is necessary to carry out geological forecast work in the tunnel construction process, which is of great significance for tunnel safety construction and avoiding major disaster accident losses. This lecture mainly introduces the commonly used methods of geological forecast in tunnel construction, the design principles, and contents of geological forecast and combines typical cases to show the implementation process of comprehensive geological forecast. Finally, the development direction of geological forecast theory, method, and technology is carried out. Prospects provide a useful reference for promoting the development of geological forecast of tunnels.

**Keywords:** advanced geological prediction design, content of advanced geological prediction, method of advanced geological prediction, geological hazard detection, engineering application

### **1. Introduction**

### **1.1 Main contents and common methods of geological forecast**

The advanced geological prediction of the tunnel includes geological analysis and macroscopic geological forecast of the tunnel area, advanced prediction of tunnel geological disasters, and warning of major construction geological disasters [1, 2]. The main forecast content [3–6] is as follows:


In response to the abovementioned exploration targets, the researchers have developed geological forecast techniques for various types of construction tunnels. The common methods for geological forecast are geological researching, advance drilling geological prediction, geophysical prospecting prediction in tunnel, advance heading, etc. [7–13], as shown in **Figure 1**.

### *1.1.1 Hydrogeological survey*

The hydrogeological survey method mainly includes the supplementary geological survey of the tunnel surface, the geological sketch of the working face in the tunnel and the geological sketch of the tunnel wall, the underground and surface correlation analysis of the stratigraphic boundary line and the structural line, and the geological mapping.

The hydrogeological survey method is the earliest and most basic method used in various tunnel geological advance prediction methods. The interpretation and use of other tunnel advance prediction methods are based on geological data analysis and judgment. The hydrogeological survey method is based on the existing survey data, the geological survey data supplemented by the surface, and the geological sketch in the tunnel, through the sequence comparison of the geological layers, the stratigraphic boundary line, and the correlation analysis of the subsurface and surface of the stratigraphic tectonic line, the fault elements, and the tunnel geometry. Correlation analysis of parameters, possible precursor analysis of adjacent geological bodies in the tunnel, etc. use conventional geological theory, geological mapping, and geological development trend analysis, etc. to speculate the possible geological conditions ahead of the excavation face. The method has high accuracy in the case where the tunnel has a shallow depth and the structure is not too complicated, but in the case of deep depth and complicated structure, the method is difficult, and the accuracy is poor.

### *1.1.2 Probe drilling*

**Probe** drilling methods mainly include advanced geological drilling, deepened shot hole detection, and borehole photography.

The **probe** horizontal drilling method is a kind of advanced geological prediction method for obtaining geological information by drilling with drilling equipment or directly using blasting holes to drill ahead in the tunnel excavation working face. The method can directly reveal the lithology, rock structure, groundwater, karst cave and its properties, rock integrity degree, etc., from tens of meters to hundreds of meters in front of the tunnel working face, and can also

### **Figure 1.** *Common methods for geology forecast of tunnels.*

### *Advanced Geological Prediction DOI: http://dx.doi.org/10.5772/intechopen.88406*

be obtained through core test. Quantitative indicators such as rock strength are applicable to the main unfavorable geological sections that have been basically identified. For undetermined unfavorable geological sections, the unsatisfactory leakage of unfavorable geological bodies is often caused by the problem of "one hole seeing."

### *1.1.3 Geophysical prospecting*

Geophysical methods mainly include elastic wave reflection method (seismic wave reflection, horizontal acoustic wave profile method, negative-vision velocity method, and very small offset high-frequency reflection continuous profile method), electromagnetic wave method (geological radar, transient electromagnetic), and electrical method (high-resolution DC method, induced polarization method, etc.).

The geophysical prospecting is based on the physical difference between the target geological body and the surrounding medium, such as electrical, magnetic, density, wave velocity, temperature, radioactivity, etc., and the spatial distribution of the underground geological body is determined by observing changes in natural or artificial physics. The scope is a physical exploration technology that solves geological problems. The method is fast, comprehensive, accurate, and economical. It is a nondestructive testing method, which mainly includes seismic wave reflection method, electromagnetic wave method, and electric method.


### *1.1.4 Advance heading method*

The advance heading prediction method mainly includes the parallel advance heading method, the positive hole advance heading method, etc.

The advance heading prediction method is used to excavate a parallel heading in the tunnel or on the side of the tunnel, through geological conditions revealed in advance heading, geological theory, and mapping method to predict the geological conditions of the main tunnel. The advance heading prediction method includes parallel advance heading method and positive hole advance heading method. The two tunnels with small line spacing can be parallel heading to each other; the tunnel excavated first can predict the geological conditions of the tunnel excavated after. Because of its large cross section, the advance heading method can reveal the geological conditions in front of the positive hole more comprehensively and accurately, but it takes a long time and has high economic cost.

### **2. Geology forecast design**

### **2.1 Geology forecast design**

The tunnel engineering should carry out corresponding geology forecast design at each design stage, and the selection of prediction methods should be compatible with the construction method. The geology forecast design can be implemented by referring to the following steps, as shown in **Figure 2**:

1.The geological survey method is adopted to investigate the engineering geological and hydrological conditions in the area and obtain unfavorable geological structures, special geotechnical areas, and possible geological problems.

**Figure 2.**

*Flow chart of tunnel geology forecast design.*


### **2.2 Classification of geological factors by intricacy (hazard)**

Considering comprehensively the geological and hydrogeological conditions of the tunnel, the influence degree of the possible geological hazards on the tunnel construction and environment classifies the geological factors by intricacy. Tunnel classification of geological factors by intricacy (hazard) is shown in **Table 1** [3]. The purpose of the tunnel classification of geological factors by intricacy (hazard) is to determine the depth (accuracy) of geological prediction and exploration; to select different exploration methods and means (and their combination); determining the relevant technical requirements, workload, and so on; to complete the geology forecast design of tunnel construction; and to realize scientific planning and controllable management of geological prediction in tunnel construction.

### **2.3 Advanced geological prediction method selection**

The advanced geological prediction can generally adopt long-distance forecast, medium and long-distance forecast, and short-distance forecast. The choice of forecast length and forecast method should meet the following requirements:


### **2.4 Forecast method in typical bad geological body prediction**

### *2.4.1 Fault*



**Table 1.**

*Classification of geological factors by intricacy (hazard).*

*Tunnel Engineering - Selected Topics*


### *2.4.2 Karst*


### *2.4.3 Coal seam gas*


### **2.5 Advanced geological prediction design content**

The advanced geological prediction design of the tunnel firstly evaluates the complexity of the geological complex (hazard) of the tunnel and clarifies the risk events and risk levels. The assessment of the complexity of the geological complexity of the tunnel can initially determine the causes, possibilities, and consequences of various risks. Secondly, according to the complexity of the tunnel and the risk assessment after the prediction plan is made, the prediction grading can be reasonably determined. In the case of complex high-level sections, several geophysical methods with complementary physical parameters should be comprehensively implemented. Targeted advance drilling can be carried out purposefully according to the results of geophysical exploration, and we should give full play to the advantages of geophysical exploration and drilling so that we can achieve accurate and resource-saving objectives. Finally, the forecasters were requested to adopt advanced data collection and processing methods, strive to improve the level and accuracy of the prediction, further investigate the engineering geological and hydrogeological conditions in front of the tunnel excavation face, guide the smooth progress of the project construction, and reduce the probability of a geological disaster occurring. The advanced geological prediction design relies on the assessment of the complexity of the tunnel geological (hazard) and can be divided into the following four levels, as shown below:

A-level forecast: Based on the geological survey method, integrated seismic wave reflection method (TSP, TRT, etc.), electromagnetic wave method (GPR, GPR, etc.), electrical method (high-resolution direct current method), and other methods conduct comprehensive prediction. According to the comprehensive prediction conclusion, the advanced prediction method is used to verify the comprehensive prediction conclusion. For water-rich layer, detection methods such as transient electromagnetic method (TEM) and tunnel-induced polarization (TIP) should be added to qualitatively locate and estimate the water-bearing structure and supplemented with information such as targeted drilling to detect water pressure to guide the design and construction.

B-level forecast: Based on the geological survey method, mainly seismic wave reflection method (TSP, TRT, etc.), and supplemented by electromagnetic wave method (GPR), electric method (high-resolution direct current method), etc., conduct comprehensive prediction. Adopting **probe** drilling method verifies the forecast conclusions. When it is found that the engineering geological conditions of the local section are complicated and rich in water, it is implemented according to the requirements of class A.

C-level forecast: Based on the geological survey method, the seismic wave reflection method is mainly used to detect the important geological (layer) interface, fault fracture zone, karst cave, or geophysical anomaly section by electromagnetic wave method and high-resolution direct current method. The drilling method is adopted to verify the prediction conclusion.

D-level forecast: Geological survey method is the main method, supplemented by seismic wave reflection method, and if necessary, geological radar and highresolution direct current method can be used for detection, and **probe** drilling method can be used to verify the prediction conclusion.

In addition, for the coal seam gas, hydrogen sulfide and other harmful gas forecasts according to the tunnel situation select special equipment to carry out the forecast work.

Each type of advance forecasting technology has different characteristics in terms of scope of application, detection distance, and recognition accuracy. According to the geological and geophysical characteristics of the unfavorable geological bodies, the comprehensive advanced geological prediction technical system of the whole process of bad geology can be adopted, which is guided by geological analysis, combined with geology and geophysical exploration, drilling, combining inside and outside the cave, and combining different geophysical methods [8, 9].

The design documents for advanced geological forecast shall be prepared and shall include the following main contents [3]:


of the different methods, technical requirements (the same kind of forecast method or overlap between the different forecast method length, hole angle and length, etc.), and in advance, when need should be of meteorological springs, important points and main water hole (flow rate is >1 L/s water point), such as rivers flow observations plan, technical requirement, etc.


### **3. Comprehensive advanced geological prediction design example**

### **3.1 Project overview and hydrogeological analysis**

The typical geological disaster prone area of Chenglan railway in western Sichuan is an important part of China's railway "five vertical and five horizontal" planning network. The Yuelongmen tunnel, one of the landmark projects of the Chenglan railway, spans two areas of Anxian and Maoxian. The geological structure at the Longmenshan fault is particularly complex. The maximum depth of the tunnel is 1445 m, the length of the right line is 20,042 m, and the length of the left line is 19,981 m [14–16].

The Yuelongmen tunnel crosses the Longmenshan Central Fault Zone. The tunnel crossing section intersects the mountain range at about 60°. The Yingxiu-Beichuan fault develops multiple secondary faults in the tunnel area. The Guangtongba fault and Gaochuanping fault that the tunnel passes through belong

**Figure 3.** *Hydrogeological plan of the tunnel area of Yuelongmen tunnel.*

to its secondary faults. The tunnel also passes through the Qianfoshan fault, the Qianfoshan No. 1 fault, and the Tujiamiao fault. A number of intermountain rivers are developed in the survey area, which is crossing three watersheds. The main surface water in the survey area is intermountain trench water, which is mainly replenished by atmospheric precipitation and partly by bedrock fracture water. The hydrogeological plan of the tunnel area is shown in **Figure 3**.

### **3.2 Tunnel advanced prediction design**

According to the geological survey analysis and comparison of the geological hazard degree classification (**Table 1**), it can be determined that the XJ3K0 + 000– XJ3K0 + 396 segment of the Yuelongmen tunnel should be detected by the class A method of the advanced forecast design, and the seismic wave reflection method (TSP, TRT, etc.) and electromagnetic waves are comprehensively adopted. Methods such as geological radar (GPR, etc.) and electric method (high-resolution direct current method) are used for prediction, and **leading** target drilling is performed to verify the geophysical results.

### **3.3 Implementation of comprehensive geological prediction in typical water-rich section**

### *3.3.1 Geological analysis of 3# inclined shaft in Yuelongmen tunnel*

The 3# inclined shaft of Yuelongmen tunnel is located at the interface of D2K97 + 700 on the left side of Yuelongmen tunnel. The whole field is 2025 m, and the maximum buried depth is 872 m. The location is a more weathered hilly landform, overlying the Quaternary Holocene alluvial pebble soil, silt-splitting layer silty clay, slope residual coarse breccia, breccia, and block stone. In the lower Devonian system, the Wushan group is a dolomitic limestone, the first subgroup of the Silurian group of the Silurian, the Carboniferous phyllite limestone, and the black carbonaceous slab of the lower Longmaxi group. There are also interbedded with thin-layer siliceous rocks, Ordovician Zhongtong Baota Formation marl, crystalline limestone, Cambrian Qingqing Formation limestone, siltstone, apatite, Sinian Lower Juejiahe Formation Siliceous rocks, shale, carbonaceous shale, limestone, dolomite, and fault breccia. Among them, the XJ3K0 + 396–XJ3K0 + 273 segment is dominated by the Cambrian Qingping Formation limestone, and the rock is hard, but the karst is moderately strong, the joints are developed, and the surrounding rock is broken. Therefore, when the 3# inclined shaft of Yuelongmen tunnel digs into XJ3K0 + 396, water inrush occurs, and the amount of water inflow is about 1000 m3 /h.

### *3.3.2 Comprehensive advanced geological prediction analysis*

In order to understand the geological conditions in front of the tunnel face, the TSP method is used to make a large-scale preliminary judgment on the geological body in front of XJ3K0 + 393. The prediction conclusion is that the whole section of XJ3K0 + 393– + 273 is hard but is formed by karst development. The rock is relatively broken, and there is more water between the cracks of XJ3K0 + 393– + 379, XJ3K0 + 374– + 354, and XJ3K0 + 344– + 330. Based on the TSP prediction conclusion and the geological analysis (**Figure 4**), the 3# inclined well fracture of Yuelongmen tunnel is developed and is water-rich. The geological radar method is used to focus on the location, scale, and development of the fracture. The induced polarization method and transient electromagnetic are used. The method carries out detailed detection of the water-rich position and scale of the rock formation.

### *Advanced Geological Prediction DOI: http://dx.doi.org/10.5772/intechopen.88406*

Comprehensive analysis of TSP, geological radar, transient electromagnetic, and tunnel-induced polarization detection results combined with the geological conditions of the tunnel can be an accurate quantitative judgment of the geologic body in front of the XJ3K0 + 393 face: in general, the front of the face. The surrounding rock within 47 m is generally poor, and the fracture is developed and rich in water, but the water-rich area is uneven. The full section of 0–15 m in front of the face is almost rich in water, while the main middle left side of 16–30 m is rich in water; the 31–47 m side is rich in water in the middle right side. Using the different data of the induced polarization half-life, the estimated static water reserve within 70 m in front of the face is 700 m3 . In order to further verify the geological and water-rich conditions in front of the face, further advance drilling operations were carried out. Comparing the three detection results, it is found that the three detection methods are consistent with the prediction results of the water body. In particular, the polarization method not only determines the spatial position of the water body but also determines its distribution pattern, which can guide the drilling operation well [17–19].

### *3.3.3 Advanced targeted drilling detection*

According to the comprehensive prediction conclusion, four **probe** drill holes were applied at the face of the face to verify the comprehensive geophysical findings. The **probe** drilling position is shown in **Figure 5**.

### **Figure 4.**

*Comparison of detection results. (1) Geological radar detection results; (2) transient electromagnetic detection results; (3) excited excitation detection result (hole body range extraction map).*

**Figure 5.** *Position of the lead drilling hole.*

The verification of the on-site lead drilling indicates that the forecast results are in good agreement with the drilling results and the water-bearing structures predicted in the forecast conclusions are also verified by the disclosure. The forecast results also optimize the location and quantity of targeted drilling, which not only reduces the number of boreholes for the construction unit but also effectively covers the detection of unfavorable geology.

### *3.3.4 Excavation result verification*

During the excavation of the tunnel, XJ3K0 + 393–XJ3K0 + 378 paragraph, the water surge appeared in the middle and lower part of the face, and a certain depth of water appeared in the bottom plate. In the XJ3K0 + 377–XJ3K0 + 368 paragraph, there is water in the left and middle of the face, and there is water in the XJ3K0 + 367–XJ3K0 + 363 section. The water inflow has an increasing trend (**Figures 6** and **7**). The distribution of water content in rock mass is almost always consistent with comprehensive advanced geological prediction, which confirms the accuracy of comprehensive advanced geological prediction.

### **3.4 Tunnel construction measures**

The results of comprehensive geophysical exploration combined with geological analysis and advanced targeted drilling show that there are no large caves or karst pipelines within the effective geophysical range and the main unfavorable

**Figure 6.** *Water inrush in the tunnel after the advanced drilling.*

**Figure 7.** *Water inrush after tunnel excavation.*

### *Advanced Geological Prediction DOI: http://dx.doi.org/10.5772/intechopen.88406*

geology is dominated by bedrock fissure water. The geological integrity of the surrounding rock in front of the face is poor; the bedrock fissure is relatively developed and rich in water. Therefore, according to the requirements of tunnel construction safety, quality, schedule, and environmental protection, the Yuelongmen tunnel 3#, the sloped limestone water-rich section changed the original "full-section curtain grouting" to "advanced peripheral grouting" treatment measures, which not only saves time and effort but also speeds up the progress of the project.

Through the implementation of **leading** peripheral grouting measures for the 3# inclined well water-rich section of Yuelongmen tunnel, the risk of inrush water was effectively reduced, and remarkable results were obtained, ensuring the safe and orderly construction of the tunnel.

### **4. Development and prospect of advanced geological prediction technology**

At present, the location, scale, and distribution of most unfavorable geological bodies have been detected by corresponding technologies, but it is still impossible to determine the rock mechanic properties of unfavorable geological bodies, the detection range is mostly limited to two-dimensional plane, the detection effect is not ideal in complex environment with large disturbance, the construction area is occupied and the time-consuming is longer, etc., which are still urgent problems to be solved in the field of advanced forecasting and detection. In addition, the development direction of future advanced geological prediction technology is the ultra-long **leading** horizontal drilling technology, the advanced geological prediction method while drilling, the fine imaging technology of multi-physical field information joint inversion, the quantitative technology of advanced prediction, the real-time advanced prediction technology, and the intelligent interpretation technology of advanced prediction [20–23]. At the same time, in order to obtain more accurate quantitative information in the tunnel, it is necessary to combine advanced geological prediction and surface exploration technology; use aviation electromagnetic exploration technology and system and intelligent geologic deep drilling; adapt to complex environmental geophysical techniques as breakthrough points; and form the comprehensive exploration and prediction technology that includes space-borne remote sensing, airborne exploration, and surface geophysical exploration and intelligent drilling. These could improve the ability of obtaining geological information and provide guidance for safe and efficient tunnel construction.

### **Acknowledgements**

This research was funded by the National Natural Science Foundation of China(Grant No.51609129,51709159,51709160); the Key Research and Development Project of Shandong Province (Grant No. 2019GSF111018); the State Key Laboratory for Mine Disaster Prevention and Control, cultivation base co-built by the province and the Ministry of Shandong University of Science and Technology (Grant No.MDPC201707); and Open Fund of State Key Laboratory of Water Resource Protection and Utilization in Coal Mining (KFJJ2018089).

*Tunnel Engineering - Selected Topics*

### **Author details**

Shaoshuai Shi1,2,3,4\*, Xiaokun Xie1 , Siming Tian<sup>2</sup> , Zhijie Wen3 , Lin Bu1 , Zongqing Zhou1 , Shuguang Song<sup>5</sup> and Ruijie Zhao1

1 School of Qilu Transportation, Shandong University, Jinan, China

2 China Railway Economic and Planning Research Institute, Beijing, China

3 State Key Laboratory of Mining Disaster Prevention and Control Co-founded by Shandong Province and the Ministry of Science and Technology, Shandong University of Science and Technology, Qingdao, China

4 State Key Laboratory of Water Resource Protection and Utilization in Coal Mining, Beijing, China

5 School of Transportation Engineering, Shandong Jianzhu University, Jinan, China

\*Address all correspondence to: shishaoshuai@sdu.edu.cn

© 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 2
