Liquefaction Mitigation Using Bentonite Suspensions

Abstract

Ottawa sand specimens premixed with 0, 3, and 5% bentonite by dry mass of sand were tested under undrained static and cyclic loading to investigate the effects of bentonite on the static and cyclic shear strength of the sand. The results show that allowing the bentonite to hydrate within the sand pore space increases the cyclic resistance of the sand. For the same skeleton relative density and cyclic stress ratio, cyclic tests on specimens with sufficient hydration times showed a significant increase in the number of cycles required for liquefaction compared with clean sand. When the specimens were allowed an extended postconsolidation aging period, the cyclic resistance increased further. Resonant column and cyclic triaxial tests showed that this is a result of the delay in the generation of excess pore pressure in the presence of the bentonite suspension in the pore space. The improvement in cyclic behavior does not occur at the expense of the static resistance of the soil under working loads because undrained static triaxial tests on specimens with bentonite showed only a minor decrease in the small-strain internal friction angle compared with the clean sand, while the critical-state internal friction angle remained unchanged. To address the need to deliver the bentonite suspension in a sand deposit, a method for engineering the rheology of concentrated bentonite suspensions through the addition of sodium pyrophosphate (SPP) was developed. With the addition of 0.5% SPP by mass of clay, the viscosity of concentrated (10%) bentonite dispersions dropped to a value that allowed delivery of the bentonite into the sand matrix through permeation. Over time, the bentonite suspension recovered the thixotropic properties that ensured its effectiveness in mitigating liquefaction. As a result, sand specimens permeated with 10% bentonite suspensions showed a large increase in liquefaction resistance. Changes in the rheological properties of the pore fluid with time also explain the increase with aging of the cyclic resistance of the sand-bentonite specimens.