**3. Case descriptions**

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

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

ensuring the final compensation efficiency.
