## 4 Liquefaction Mitigation Using Colloidal Silica Grout
### Centrifuge Model Behaviors of CS-Grouted Soil  
Laboratory element experiments have indicated that CS-grouted soil has a sufficient ability to restrain DA strain and inhibit pore pressure generation, and, as such, it can mitigate liquefaction risk. However, the element test can only indicate the material properties of CS-soil mixtures. The structural behavior still needs to be investigated through model tests. [PERSON] (1995) reported that significantly lower strains (\(\sim\)0.5-1%) were observed for the treated centrifuge models compared with the strains (\(\sim\)3-6%) of untreated model in the same kind of test [78]. Another centrifuge experiment was performed to verify the enhancing effect of CS grout to mitigate liquefaction risk for loose sand [7]. A laminar box composed of a stack of aluminum rings was used to provide a flexible-wall container. Grout supply ports were set at the bottom of the container to provide CS under a vacuum. The sand drain was pluviated around the grouting ports to permit the grouting material to permeate evenly into the sand, as shown in Figure 22. The CS was delivered through the soil with a saturation period of 13 h. Then, a firm, resonating gel formed after 56 h of gelling and a 240 h curing time. The prepared model was motivated by two shaking events, with 20 cycles of a 2 Hz sinusoidal input for each. The uniform peak accelerations were 0.2 g and 0.25 g, respectively. Lateral displacement and vertical settlement were measured by the linear variable differential transformers (LVDTs) mounted on the container rings at various depths. It was found that shear strains of 0.5% and 1% were observed during the first and second shake. Furthermore, only 30 mm settlement (0.3% strain) and 10 mm settlement (0.1% strain) were measured during the first and second episodes of shaking, respectively. The results demonstrate that the treated soil layer did not liquify during both shaking events, which provides evidence of the contribution of CS in mitigating liquefaction risk. Figure 21: Shear strains for treated samples at various CS concentrations [17]. [PERSON] et al. (2012) performed a well-designed centrifuge test to discuss the effect of CS concentration on the magnitude of cyclic shear strain for CS-treated models [40]. In the test, the container was mainly constructed with a 4.8 m-thick liquefiable loose sand layer and two slopes made up of loam that sloped 3\({}^{\circ}\) towards a 3 m-wide central channel. In test 1, one slope was grouted with 9% by weight CS (labelled CS-9), while the other was left untreated. In test 2, the two slopes were treated with 4% and 5% CS, labelled CS-4 and CS-5, respectively. Eight shaking events were applied to the model, with a centrifugal acceleration of 15 g. Immediately after test 1, large deformations occurred at the untreated side due to the liquefaction of the loose sand layer, and the upper loam slope was submerged in water. However, the treated side remain intact, as can be seen in Figure 23.  
Figure 22: Model set for grouting: (**a**) Schematic for CS grouting in laminar box; (**b**) photograph of CS grouting [7]. In addition, they also calculated the shear strains at different depths by double integration acceleration in time to obtain the transient displacement, and then differentiating in space. It was observed that the maximum shear strain \(\gamma_{\text{max}}\) in the untreated side reached up to 1% in shake 3, while \(\gamma_{\text{max}}\) was only 0.3% and 0.6% for CS-9 and CS-5, respectively. Similar reductions can be observed in shakes 4 and 5 (Figure 24). This experiment demonstrates that surface settlement and lateral spreading can be effectively controlled with CS treatment. CS grouting offers significant advantages for liquefiable sand layers overlain by existing structures that may be otherwise be difficult to treat by other methods. The results of the centrifuge model test are in agreement with those of the laboratory element test in demonstrating the mitigation effect of CS-grouted soil in terms of reducing liquefaction risk.