Passive grouting with colloidal silica for secondary containment systems

Secondary containment systems are used to limit the release of contaminated liquids into groundwater following an uncontrolled leak from a primary containment tank. Migration of spilled contaminants is often restricted by a subsurface barrier. This study focuses on the behavior of a subsurface barrier constructed by passive permeation of colloidal silica grout into the surface layer of soil. Colloidal silica grout sets into a semi-solid gel that leads to a decrease in hydraulic conductivity of the surface soils. The specific purpose of this research is to quantify the effect of colloidal silica on the hydraulic properties of granular soils permeated with water. In order to quantify this complex behavior, a series of tests was conducted to characterize the baseline hydraulic and mechanical properties of colloidal silica grout, sand, and grouted sand. Further, physical models of secondary containment systems were constructed in large-scale permeameters to assess the progression of the grout during passive permeation and the subsequent improvement in the hydraulic properties of the secondary containment system. The baseline characterization tests indicate that the hydraulic conductivity of fully grouted sand with colloidal silica grout having 13% by volume or less of accelerator) was reduced by six orders of magnitude (from 10⁻² to 10⁻⁸ cm/sec). The progression of colloidal silica grout into a prototype containment system was found to depend on both the concentration and pH of the grout. The grout viscosity, infiltration depth, and time for it to turn to a gel were the most important parameters affecting the grout layer thickness. Models were developed to estimate the amount of grout needed to reach a target hydraulic conductivity and the infiltration depth of grout during passive permeation. The shape of the grouted layer was affected by fingering, so the target layer thickness was not obtained. Nonetheless, subsequent evaluation of the hydraulic conductivity of the soil-grout system indicates that the system had a hydraulic conductivity slightly higher than the target value. Equivalent hydraulic conductivity values on the order of 10⁻⁶ cm/s were measured in soil-grout system profiles that had only 70% of their voids filled with grout. These results suggest that in secondary containment systems a subsurface barrier with colloidal silica grout can be achieved by passive grouting. They show that passive grouting of colloidal silica is a promising, innovative technology for secondary containment systems.