**2. Grouting mechanisms**

Hydrofracturing of soil occurs in many important geotechnical engineering issues. It corresponds to the process of initiation and propagation of a crack by injecting water and air as well as chemicals into soils. As the pressure of the fluid injected surpasses a certain threshold value, the hydrofracturing of soil is thus triggered. A typical application of hydrofracturing is fracturing grouting. The fracturing grouting that involves the intentional hydrofracturing of soil with a low viscosity grout to generate a network of interconnected grout lenses has been extensively used to create surface heave and compensate settlements as well as strengthen the soil. To implement an effective field application of the fracturing grouting, a good understanding of its fundamental mechanism is deemed to be necessary. Wong Ron and Alfaro Marolo carried out a field mapping of sand-propped hydraulic fractures at a contamination remedial site where the ground is primarily consisted of silt and clay. The fractures were found to be nearly horizontal, indicating that the ground is overconsolidated, with a K<sup>0</sup> value greater than unity [31]. The sand proppant was thicker at locations where soils were relatively weak, but there was no strong evidence that soil stratigraphy at this worksite controlled the orientation of the fractures. Murdoch and Slack reported similar results in sand-propped hydraulic fractures [32]. Additionally, the horizontal fractures imply that the shallow soil strata should be overconsolidated. Moreover, the elevated volume would be considerably smaller than the injected volume in the event the hydraulic fractures vented to the ground surface. Liu and Yuan established an in situ slurry fracturing apparatus to analyse the slurry fracturing and fracture propagation phenomena [33]. It was observed that the fracturing pressure was highly related to the soil and slurry properties and that slurry with large bulk density and high viscosity was beneficial in preventing slurry fracture propagation. Conducting grouting in clay due to its low permeability rules out any other grouting techniques other than fracturing grouting. The excess porewater pressure generated during grout injection is greater than the in situ effective stress, leading to fractures in the surrounding clay. Existence of fractures accelerates the consolidation process and shortens the strength increase time due to consolidation. During injection, the fractures provide a channel for exchangeable cations, which make the strength increase due to chemical reactions much more quickly than anticipated. Additionally, the compensation efficiency may not be governed by clay type but by the setting time of grout, soil stress history, injection volume of grout and so on. However, the high mobility and low viscosity of the grout can lead to an inability of limiting the travel of grout, thereby resulting in a lower soil-heaved volume to injected volume of grout ratio also known as the grouting efficiency. The grouting efficiency is generally smaller than 1 due to the loss of fluid, resulting from the bleeding effect of grout and escape of the grout from the designated area by migration along fractures, and the ground settlement caused by the dissipation of excess porewater pressure generated in injection [31]. Marchi et al. carried out a comprehensive case study in Venice where a rather unique soil fracturing intervention was implemented to improve the mechanical properties of the soft silty clay underlying the ancient Frari bell tower [34]. For soils with negative values of liquidity index, the gradients from the plots of fracturing pressure against initial confining pressure are approximately 2, which indicated the fracture initiated by tensile failure in these cases, while for soils with positive values of liquidity index, the gradients are approximately 1, indicating that the fracture was triggered by shear failure. Komiya et al. conducted a field trial of shield tunnelling in a deep soft clay deposit to investigate the long-term consolidation effect on grouting efficiency [25]. The grouting programme was consisted of the tail void grouting and grout jacking. In both cases, the monitoring results indicate that the upward displacement owing to grout injection was negated by the consolidation settlement resulting in a net settlement. The considerable consolidation of clay after grout injection due to the dissipation of the excess porewater pressure generated as the grout intruded the sensitive and compressible clay contributed to this phenomenon. This also indicates that the grouting efficiency in soft clay may be negative.

grouting programme. The differential settlement or tilting of building, for instance, can be contributed not only by the soil right below the building foundation but by the successive soils. Thus, a grouting programme aimed to first stabilise the successive soils by 'stabilisation' grouting and then to lift the tilted building by 'jacking-up' grouting in the foundation soil is proved to be effective [24]. In the event that the grouting programme is designed to improve the properties of the foundation soil only, the jacking of the tilted building would not be effectively implemented due to a lack of the sufficient reaction forces given by the successive soils. Additionally, the intrusion of grouts may swell the cohesive soil and generate the positive excess porewater pressure. As long as the porewater pressure dissipates along with time, the associated settlement could override the heave generated during grout intrusion and thus result in a negative final compensation efficiency which is defined as a ratio of the total heaved volume to the injected volume of grouts [25–29]. Since the fracturing grouting due to easy travelling of the low viscosity grouts could generate higher porewater pressure than compaction grouting, regrouting at the same injection point is deemed to be necessary in order to change the stress state of cohesive soil to the overconsolidated state from its ordinary state [24, 30]. Any further grouting activity would only generate negative excess porewater pressure, and resettlement would not be occurred, thereby improving the final compensation efficiency. If both closer spacing between grout injection points and simultaneous injection are introduced, the final compensation efficiency can be further improved [27]. The above indicates that configuration and design parameters of the grouting programme play a leading

104 Current Topics in the Utilization of Clay in Industrial and Medical Applications

The objectives of this study are (i) to present the results of an application of this proposed simultaneous and multiple grouting technique for levelling two tilted buildings seated on soft soil deposits in Taipei basin, (ii) to verify the effectiveness of introducing a grouting programme consisting of the stabilisation grouting of first stage and jacking-up grouting of the second stage by analysing the elevated and settled efficiencies and (iii) to outline the lessons

Hydrofracturing of soil occurs in many important geotechnical engineering issues. It corresponds to the process of initiation and propagation of a crack by injecting water and air as well as chemicals into soils. As the pressure of the fluid injected surpasses a certain threshold value, the hydrofracturing of soil is thus triggered. A typical application of hydrofracturing is fracturing grouting. The fracturing grouting that involves the intentional hydrofracturing of soil with a low viscosity grout to generate a network of interconnected grout lenses has been extensively used to create surface heave and compensate settlements as well as strengthen the soil. To implement an effective field application of the fracturing grouting, a good understanding of its fundamental mechanism is deemed to be necessary. Wong Ron and Alfaro Marolo carried out a field mapping of sand-propped hydraulic fractures at a contamination remedial site where the ground is primarily consisted of silt and clay. The fractures

role in the success of project.

learnt from the case studies.

**2. Grouting mechanisms**

It is reported by Au et al. that for normally consolidated or slightly overconsolidated clays, the significant decrease in the grouting efficiency with time was due to the dissipation of the positive excess porewater pressure generated during grout injection [26]. However, for heavily overconsolidated clays, the excess porewater pressure was positive at the injection boundary, but it was negative at the outer boundary due to dilative behaviour of the clay. The compression around the injection point induced by the dissipation of the positive excess porewater pressure and swelling some distance away from the injection point caused by the dissipation of the negative excess porewater pressure led to a negligible consolidation effect. As discussed, the bleeding effect of grout has been deemed to be one of the factors affecting the grouting efficiency. Au et al. conducted additional injection tests with grouts that were prepared with the water-to-cement (w/c) ratios of 0.5, 1 and 3, respectively, using the 50-mm diameter oedometer [26]. The final grouting efficiency was reduced from about 20% for the grout with the w/c ratio of 0.5 to around −30% for the grout with the w/c ratio of 3. The higher the solid content of grout, the lesser the bleeding effect of grout, and the higher the final grouting efficiency. The effect of boundary condition was also examined by injecting 5 ml of grout into the modified oedometers with the diameters of 50 mm and 100 mm, respectively [26]. The results show that the reduction in the radial boundary size enabled the final grouting efficiency to be improved dramatically as the overconsolidation ratio was within a range of 1–2. In other words, the smaller the spacing of injection point, the smaller the magnitude and extent of excess porewater pressure, and the higher the final grouting efficiency. It is common practice to introduce the tube-a-manchette (TAM) while performing compensation grouting, which allows grout to be injected many times from the same injection port. In the case that a given volume of grout is injected over a fixed area, it is possible to either regrout many times at the same port with smaller injection volumes or, conversely, conduct a small number of regrouting but with larger injection volumes. A series of injection tests comprising the regrouting injection and single injection tests were undertaken in normally consolidated clay specimens to investigate the effect of waiting period on the long-term grouting efficiency after injection [26]. An injection of 5 ml for each injection was made four times for the regrouting injection tests. The test results were compared with the result of a single injection test which was undertaken by injecting 20 ml at once. The results show that in the stage of consolidation, more excess porewater pressure was generated in the subsequent injections for the regrouting injection test, thereby leading to a lower grouting efficiency than that from the single injection test. Additionally, the efficiency of compensation grouting defined as the ratio of building settled volume to total injected volume of grout may be further reduced as only the grout beneath the mat foundation can contribute to the effective lift of titled building. Moreover, injection of extra quick setting grout can only be achieved by introducing the two-shot grout hose system [35]. The shorter the grout gel time, the lesser the excess porewater pressure generated, and the higher the final compensation efficiency. To summarise, it is evident that there are many factors (soil stress state, boundary condition, bleeding of grout, regrouting, grout rheological characteristics, grout hose system and so on) affecting the final compensation efficiency. Lifting tilted building in soft clay deposit can be better achieved by introducing a grouting programme that at least takes the previously discussed factors into account. Also, the two-stage grouting consisting of the stabilisation grouting of first-stage and jacking-up grouting of second stage may be used to ensuring the final compensation efficiency.

**3. Case descriptions**

**3.1. Engineering geology**

**3.2. Grouting programme**

Based upon the preliminary geological investigation [36], the ground for Case A is generally consisted of a 1.5-m-thick surface backfill, a 4.5-m-thick low plasticity clay, a 4-m-thick silty sand and a successive very soft silty clay, as shown in **Figure 1**. The blow count N value for the clays varies from 1 to 4, whereas for the sands it varies from 5 to 10. The groundwater level is close to the ground surface. The soft soil deposits can thus easily get softened once

Clay Grouting Mechanisms and Applications http://dx.doi.org/10.5772/intechopen.74091 107

The geological profile at the worksite of Case B is consisted of a 4-m-thick alluvial loam, a 7-m-thick very soft silty clay and an underlying soft silty clay, as shown in **Figure 2**. The static groundwater level in the vicinity of the worksite is some 2 m below the surface. The N value varies from 1 to 2 for the very soft silty clay, whereas for the soft silty clay, it varies from 2 to 3. The soft clays thus can behave as a fluid once subjected to construction disturbances.

In the Case A, the eight-storey reinforced concrete building with one-storey basement was seated on the silty sand. The nonuniform consolidation of the successive silty clay led to the tilting of the eight-storey building afterwards. Since the tilting of the eight-storey building was amplified along with time, jacking the tilted eight-storey building back to the acceptable range of tilting was urgently needed. After considering all the possible alternatives to level up the tilted eight-storey building, grouting method was chosen due to the two reasons, that

disturbed or even washed away as subjected to significant hydraulic gradients.

**Figure 1.** Geological profile of worksite and properties of soft soil deposits (Case A).
