Physical and Numerical Model of Colloidal Silica Injection for Passive Site Stabilization

al., 2002; Gallagher and Koch, 2003). It is based on slow injection of stabilizing materials at the up-gradient edge Passive site stabilization is a new technology proposed for nondisof a site, with subsequent delivery of the stabilizer to ruptive mitigation of liquefaction risk at developed sites susceptible to liquefaction. This technology is based on the concept of slow injecthe target location via groundwater flow (Fig. 1). Lowtion of stabilizing materials at the edge of a site and delivery of the head extraction wells may be used to adjust the groundstabilizer to the target location by using the natural or augmented water flow pattern for targeted delivery of the stabilizer groundwater flow. In this research, a box model was used to investigate to the entire site. Relying on low-gradient stabilizer the ability to uniformly deliver colloidal silica stabilizer to loose sands delivery instead of the traditional high-pressure injecusing low-head injection and extraction wells. Five injection wells tion of grout requires that the stabilizer have a low and two extraction wells were used to deliver stabilizer in a generally initial viscosity, a long induction period (during which uniform pattern to the loose sand formation. Numerical modeling the viscosity stays fairly low), and long gel times (on was used to identify the key parameters affecting stabilizer migration the order of 50–100 d). and to determine their effective values for the box experiment. In our modeling approach, the stabilizer is treated as a miscible fluid, Dilute colloidal silica (5% w/w) was selected as the the viscosity of which is a function of time and the concentration of stabilizer (Gallagher, 2000; Gallagher and Mitchell, stabilizer in the pore water. Inverse modeling techniques are employed 2001). Colloidal silica is an aqueous suspension of microto reproduce data from the laboratory experiment for the determinascopic silica particles (7–22 nm) produced from satution of soil and stabilizer properties. While the details of the stabilizer rated solutions of silicic acid. In dilute solutions, colloidistribution were difficult to reproduce with the simplified conceptual dal silica has a low initial viscosity of about 1.5 10 3 model we used, the overall system behavior was well captured, providPa s (1.5 cP; water 1 cP), a long induction period, ing confidence that numerical simulation is a useful tool for designing and long controllable gel times of up to a few months. centrifuge model tests, pilot tests, and eventually field stabilizer-injecColloidal silica is also nontoxic, biologically and chemition projects. cally inert, and has excellent durability characteristics, making it an excellent candidate for a stabilizer. The factors controlling the gel time of colloidal silica include L is a phenomenon marked by a rapid and the silica solids content, the pH, and the ionic strength dramatic loss of soil strength, which can occur in of the diluted colloidal silica solution. For a given silica loose, saturated sand deposits subjected to earthquake content, the gel time can be altered by lowering the pH motions. Certain types of sand deposits, hydraulic fills, and changing the ionic strength of the dilute colloidal and mine-tailing dams are particularly susceptible to silica solution. In field applications, gel times will be liquefaction. The onset of liquefaction is usually sudden formulated using water and soil obtained from the canand dramatic and can result in large deformations and didate site. If the pore water has a high ionic strength settlements, the floating of buried structures, or loss of or the soil has a large number of exchangeable cations, foundation support. These settlements can disrupt the preflushing may be required (Persoff et al., 1994). ground surface and vadose zone. At sites susceptible to Use of colloidal silica for stabilizing sands has been liquefaction, the simplest way to mitigate the liquefacinvestigated by Yonekura and Kaga (1992), Persoff et tion risk is to densify the soil. If soil densification is al. (1999), Gallagher and Mitchell (2002), and Liao et al. impossible because of site constraints, grouting or un(2003). Yonekura and Kaga (1992) proposed colloidal derpinning is typically used to protect structures against silica as a replacement for the most commonly used the effects of liquefaction. In the case of grouting, the chemical grout, sodium silicate. Persoff et al. (1999) typical method is to inject cement or chemical grout investigated the effect of dilution on the strength and under pressure through closely spaced boreholes. Howhydraulic conductivity of sand treated with colloidal ever, the need for closely spaced boreholes can limit silica. Gallagher and Mitchell (2002) studied the liquethe applicability of typical grouting methods at develfaction resistance of clean, unconsolidated sands treated oped sites. with colloidal silica in percentages that varied from 5 to Passive site stabilization is a new technology proposed 20% (w/w). Liao et al. (2003) also studied the liquefacfor nondisruptive mitigation of liquefaction risk at developed sites susceptible to liquefaction (Gallagher et tion resistance of sand stabilized with colloidal silica. Injection of a water-based stabilizer into the sand P.M. Gallagher, Drexel University, Department of Civil, Architectural formation leads to a system consisting of three separate and Environmental Engineering, 3141 Chestnut Street, Philadelphia, phases: (i) the solid grains, (ii) a noncondensible gas, PA; S. Finsterle, Earth Sciences Division, Lawrence Berkeley National and (iii) an aqueous phase of variable stabilizer concenLaboratory, Berkeley, CA. Received 17 Nov. 2003. Special Section: tration. After some time, the gelation process is initiResearch Advances in Vadose Zone Hydrology through Simulations with the TOUGH Codes. *Corresponding author ([email protected]). ated, increasing the viscosity of the water–stabilizer mixture, which turns into a non-Newtonian, visco-elastic Published in Vadose Zone Journal 3:917–925 (2004). fluid that eventually solidifies. By the time the stabilizer  Soil Science Society of America 677 S. Segoe Rd., Madison, WI 53711 USA is completely gelled, the sand formation has a lower