## 4 Liquefaction Mitigation Using Colloidal Silica Grout  
### Dynamic Properties of CS-Soil Mixture  
CS is an excellent additive for liquefiable loose sand to improve its liquefaction mitigation ability. When CS is properly grouted, cementation occurs between sand particles due to the presence of CS, and thus, loose sand layers tend to have a stronger cyclic resistance, avoiding liquefaction during an earthquake event. [PERSON] et al. (1989) investigated the influence of cementation on the dynamic properties of liquefiable sands. They believed that loose, cemented sand tend to have a similar behavior to dense, uncomented sands. Moreover, liquefaction resistance increases as the degree of cementation increases [73]. [PERSON] and [PERSON] (2001) performed a triaxial test on a 5 wt % CS-grouted sand sample and found that the non-deformability of a loose sand sample at 40% relative density is similar to that of the same kind of sand sample at 75% relative density after being mixed with 5% CS [74]. [PERSON] and [PERSON] studied the reduction of liquefaction risk for loose sand treated with CS. They pointed out that grouting with 15 wt % CS would significantly improve the dynamic resistance against liquefaction [75]. Figure 17: Relationship between chloride ions and silica concentration [45]. To evaluate the liquefaction resistance quantitatively, [PERSON] and [PERSON] (2002) performed triaxial tests on pure Monterey sand and Monterey sand treated with CS [32]. The axial strain during the tests was recorded and is shown in Figure 18. Although very little strain was observed prior to liquefaction for the untreated sample, large strains occurred rapidly once liquefaction was motivated. The sample, tested at a cyclic stress ratio CSR of 0.27, could only suffer 13 cycles before it thoroughly collapsed. Furthermore, 5% double amplitude axial strain was produced in 12 cycles. However, the sample treated with 10 wt % CS experienced much less strain during cyclic loading. The treated sample required 276 cycles to reach 5% strain. Since a magnitude 7.5 earthquake was deduced to generate 15 uniform stress cycles [76], the 10% CS-treated sample could be expected to have a sufficient ability to resist liquefaction in such an earthquake event. Apart from the triaxial test, other laboratory element tests, such as the cyclic torsional shear test, resonant column test, and cyclic simple shear test, were also performed to investigate the dynamic properties of the CS-soil mixture. Ann undrained hollow cylinder torsional shear test was performed by [PERSON] et al. (2005) on both CS-treated and untreated soil samples [77]. For the untreated sample, the cyclic stress ratio was set to be 0.23, and large strains occurred rapidly with a few additional cycles after arriving at phase transformation line. However, the CS-treated sample had a cyclic stress ratio of 0.60 and presented much less shear strain during the cyclic loading process (Figure 19). In fact, collapse and liquefaction did not occur in the whole cyclic loading process for the treated sample. Figure 18: Axial strain during cyclic loading for both the untreated and treated samples [32]. [PERSON] et al. (2008) conducted resonant column tests on pure Nevada sand and Nevada sand treated with two different concentrations of CS [41]. They found that the CS-sand mixtures had a slightly higher shear modulus than the untreated sample. Besides, an increase of 6 MPa in the shear modulus was observed for the sample treated with a 5% concentration of CS after a curing period of 28 days. However, the grouting of CS had little effect on the damping ratio for Nevada sand. In addition, [PERSON] et al. (2008) investigated the dynamic behaviors of sand samples grouted with CS in more detail through a simple shear test [17]. The required cycles to reach a 5% double amplitude (DA) shear strain were recorded for both the untreated and CS-treated samples at 40% and 60% relative density (Figure 20).