## 2 Characteristics of Colloidal Silica (CS)
### Mechanical Properties of CS-Soil Mixtures  
Once the gelling and curing process have been accomplished, some experiments can be performed on CS-soil samples to assess their static mechanical properties. [PERSON] et al. (2013) studied the properties of a clay-line mixture by performing CBR (California Bearing Ratio) tests and found that strength of the mixture increased by 2, 7.5, and 8 times when adding 1%, 3%, and 5% CS ([PERSON] and [PERSON] (1993) observed a significant unconfined compressive strength gain for CS-treated sand samples compared to the original ones. They declared that the unconfined compressive strength reached 335 kPa when 32 wt % CS was grouted ([PERSON] et al., 2013). Moreover, the unconfined compressive strength of the treated sample was increased with the increased concentration of the CS additive ([PERSON] et al., 2013; [PERSON] and [PERSON], 2014), as shown in Figure 8. The increase in CS concentration will aid in intensifying the bonds between CS and soil  
Figure 7: One-dimensional compression curves for CS samples: (a) Those cured with different durations; (b) those cured in different evaporation conditions ([PERSON] and [PERSON], 2014).  
particles, thus developing a higher compressive strength. They further reported that the logarithm of hydraulic conductivity of the mixtures decreases almost linearly with the increase of CS concentration. The relationship between CS concentration and unconfined compressive strength, as well as hydraulic conductivity, has been recognized and summarized by [PERSON] et al. (2017) [56]. [PERSON] (1992) also reported the reduced hydraulic conductivity of loose sand when mixed with 5 wt % CS. It was interesting to find that the hydraulic conductivity reduced to \(9\times 10^{-9}\) cm/s for the grouted sand sample [57].  
Since soil samples grouted with CS will improve compressive strength, CS-soil mixtures should also obtain a higher shear strength compared with pure soil samples. [PERSON] and [PERSON] (2015) added 0.5%, 0.7% and 1% CS to soil to discover the mechanical enhancing effect. They found that both the friction angle and cohesion were enhanced for grouted soils [58]. Besides, the relationship between the shear stress ratio (the ratio of shear stress to effective vertical stress) and shear displacement was investigated for a pure sand sample, a 1-week-cured CS-sand sample, and a 4-week-cured CS-sand sample, as shown in Figure 9 [50]. In the conditions where the effective vertical stress equaled 100 kPa, 200 kPa, and 300 kPa, all of the grouted sand samples had a higher peak stress when compared with the untreated samples. Furthermore, the 4-week-cured CS-sand sample had a greater peak shear stress than the 1-week-cured sample when the vertical stress was 200 kPa. At an effective vertical stress of 100 kPa and 300 kPa, although the shear stresses of the grouted samples seem to share a similar peak value regardless of curing time, the sample cured for 4 weeks showed an initial stiffer response than the sample cured for 1 week. In summary, CS grouting can remarkably improve the static mechanical properties of sand samples. In this process, curing time is a main factor that determines the strengthening effect.  
Figure 8: Relationship between unconfined compressive strength and CS concentration (data collected from [PERSON] et al.) [19].