## 3 Colloidal Silica Transport Through Soil Stratum  
### Feasibility of CS Transport Through Porous Media  
The transport issue of CS is the most critical aspect, determining the feasibility of CS when adopted for ground improvement and liquefaction mitigation. Laboratory and field tests have indicated that colloids can potentially mobilize in aquifers [45], which provides direct evidence for CS transport in liquefiable soil layers. [PERSON] et al. (1999) investigated the permeability characteristics of a mixture of bacteriophage PRD1 (62 nm diameter) and CS (107 nm diameter) in sewage-contaminated and uncontaminated areas of an iron oxide-coated sand layer. Although attachment of the mixture to the porous media appeared initially, release occurred by adjusting the pH, adding an anionic surfactant to change the surface charge, and adding a reductant to dissolve the iron oxide coating on the particles. Figure 9: Horizontal displacement versus the ratio of shear stress to vertical stress for a pure sand sample and a sand sample grouped with CS under effective vertical stresses of (**a**) 100 kPa, (**b**) 200 kPa, and (**c**) 300 kPa [50]. The results demonstrate that the physicochemical properties of the porous media could be modified to facilitate the permeability of colloids [59]. Therefore, the transport of colloids in porous media is feasible with or without treatment of the soil stratum. [PERSON] et al. (1993) monitored the permeable properties of CS in a kind of glacial sand by conducting a laboratory column test and field test. In the laboratory test, almost 100% of the silica particles could be transported in the porous media. However, only 70-80% CS particles mobilized over a distance of 1.6 m. The differences in these two tests could be due to the heterogeneity of the sand layer in the field compared with that in the column, but both tests provide evidence that CS particles can be delivered in a liquefiable sand layer to the targeted location with sufficient concentrations in order to stabilize the soil [60]. [PERSON] et al. (1994) proposed that a one-dimensional advection-dispersion equation could be adopted to model CS transport through nonreactive porous media, on the condition that a small amount of CS deposition occurred [61]. In engineering practice, CS will suffer various degrees of physical and chemical deposition due to the geochemical properties of the underground water and porous media. The deposition process can significantly delay the transport of CS [62]. However, [PERSON] et al. (1996) performed a series of short column tests to study the influence of ionic strength on the transportation of CS. He pointed out that CS deposition took place for chemically active sand media and was more obvious at a low ionic strength. Despite deposition occurring, full colloid concentration could be achieved at the desired location [63]. A box model was adopted by [PERSON] et al. (2007) to study the feasibility of CS transport in liquefiable sand. The box was divided into three compartments, namely, a central chamber used for sand placement and two water reservoirs, located at both sides, for groundwater control [7]. The model was filled with Nevada sand, with relative density of 40%, to a height of 200 mm, as shown in Figure 10. Five injection wells constructed from 19-mm PVC pipes were arranged by pushing them into the sand deposit. Two extraction wells were set at the other side of the box model. The CS solution was colored with red food dye, so that the advancement of grout could be visually observed on the top and sides of the model. At the same time, the chloride concentration, which is a good indicator of the CS concentration, was measured in order to reflect the transport of CS grout. After CS transport was completed, the model was cured for 14 days. Then, six block samples were excavated and carved into smaller samples to perform unconfined compression tests with.