Effects of high-diluted colloidal silica mixtures on the mechanical behavior of potentially liquefiable sand

Seismic soil liquefaction is one of the greatest hazards of earthquakes of a certain size, and its effects on structures, infrastructures and human lives can be devastating. Liquefaction arises because of the soil shear resistance decrease as a consequence of the pore water pressure build-up in loose saturated cohesionless soil subjected to undrained loading conditions. If a soil is susceptible to liquefaction, several remedial measures can be considered to reduce the liquefaction hazard. Different mitigation techniques have been proposed over the past years; colloidal silica grouting is one of the innovative proposals that have been recently developed. A colloidal silica (CS) mixture is a low-viscosity grout able to provide the soil particles with an artificial cohesion which improves the soil behavior under both static and cyclic loading conditions. Cohesion results from a gelation process, developed within the grout as a consequence of chemical interactions. The amount of cohesion depends on the initial silica content: the higher the silica concentration, the higher the development of silica bonds among the grains. Historically, CS contents no lower than 5% by weight have been considered enough to improve the liquefaction resistance of liquefiable sand. However, the effectiveness of high-diluted CS mixtures has not been exhaustively investigated yet. The present study aims to evaluate the effects of low-content CS grouts (i.e. CS contents lower than 5% by weight) on the behavior of a clean liquefiable sand by means of an extensive laboratory investigations campaign. Laboratory tests were carried out on treated and untreated material; an in-depth analysis of soil response is presented and discussed. The performed tests showed that 2% CS content is enough to improve the soil behavior under both cyclic and monotonic loading conditions; however, the compressibility of treated soil is higher than that of the untreated one, and it increases as CS contents increase. For this reason, 2% CS represents the optimal compromise to enhance sand liquefaction resistance by minimizing undesired effects (i.e. increased soil strain) and economic costs of a potential treatment.