Colloidal silica transport mechanisms for passive site stabilization of liquefiable soils
Passive site stabilization is a new ground improvement technique to mitigate earthquake-induced liquefaction risk at developed sites. It consists of long-distance permeation grouting in which colloidal silica grout is slowly injected into the soil through wells located at the up gradient edge of the treatment area. The stabilizer is transported to the treatment area by the groundwater. Transport is augmented by extraction wells at the down gradient edge of the treatment area. The practical feasibility of this technique depends on the ability to deliver the grout to the liquefiable soil formation efficiently and in an adequate concentration to stabilize the soil. The purpose of this research was to determine if colloidal silica grout can be delivered uniformly over long distances in an adequate concentration to stabilize the sand, to understand the mechanisms of colloidal silica transport through liquefiable sands, and to evaluate numerical modeling methods for simulating colloidal silica transport in the subsurface. A total of 20 column tests, including fifteen 3-foot, four 10-foot and one 30-foot column tests, were performed to investigate variables affecting colloid transport, including pH and ionic strength of the colloidal mixtures, viscosity and gelling behavior of colloidal silica, flow rate of the fluid and the type of the liquefiable media. Samples of the treated soils recovered from the column tests after the colloidal silica gelled were tested for unconfined compressive strength. Numerical modeling of colloidal silica transport through the soil column using UTCHEM was also evaluated. Column tests showed that viscosity was the single most important factor governing transport of gelling colloidal silica grouts in saturated porous media. Colloidal silica was able to be delivered throughout the 3-foot, 10-foot and 30-foot columns in an adequate concentration as long as the viscosity remained low during injection. The ionic strength and pH affect transport of gelling colloidal silica grouts because they influence the gel time and hence viscosity of the grout. Hydraulic gradient and hydraulic conductivity of the porous media influenced the transport rate of colloidal silica grouts, but not as significantly as viscosity. Column tests also showed that normalized chloride concentration is an excellent indicator of the percentage of colloidal silica in solution. Numerical modeling of variable density and variable viscosity gelling fluids is very complex and challenging. UTCHEM was selected for use because it was reported to have a gelation module for polymers as well as the ability to handle variable density and variable viscosity fluids. However, numerical experiments indicated that UTCHEM is not suitable to model colloidal silica transport. The polymer gelation module is not applicable to the colloidal silica gelation mechanism. Additionally, the results of numerical predictions of flow through columns using variable density and variable viscosity options were unable to reasonably reproduce the experimental results.