Phosphate-amended sand/Ca-bentonite mixtures as slurry trench wall backfills: Assessment of workability, compressibility and hydraulic conductivity

In regions and countries where sodium bentonite is scarce, the resourceful calcium bentonite can be considered as an alternative material for soil-bentonite slurry trench walls. However, the lower swelling capacity of calcium bentonite as compared to Na-bentonite may result in the unacceptable high hydraulic conductivity of the sand/calcium bentonite backfills, and, therefore, amendment of calcium bentonite is necessary. However, studies on the feasibility of using amended calcium bentonite for soil bentonite backfills are limited. This study investigates the feasibility of using sodium hexametaphosphate amended calcium bentonite for the backfill material through a comprehensive laboratory investigation. First, a series of viscosity, density and filtrate loss tests are conducted using bentonite-tap water slurries with different amounts of phosphate to determine the optimal amendment dosage. Subsequently, the Atterberg limits and free swell index are evaluated for the amended/un-amended calcium bentonite and backfills, which consist of sand and amended/un-amended calcium bentonite. Finally, slump, oedometer tests and flexible-wall hydraulic conductivity tests are conducted on the backfills. The results indicate that the slurry with 20% calcium bentonite amended by 2% phosphate results in the target slurry, and the optimum phosphate dosage for amended calcium bentonite is found to be 2%. The phosphate amendment causes an increase of 1.21 times in the liquid limit, and an increase of 1.40 times in the free swell index of the amended bentonite relative to those of the un-amended bentonite. The sand/calcium bentonite backfill changes from non-plasticity to plasticity after the amendment. The amended backfill displays two times higher swell index value and one order of magnitude lower consolidation coefficient value than the un-amended backfill. The hydraulic conductivity of un-amended backfills containing 15% to 25% un-amended bentonite is greater than the commonly targeted value of 10− 9 m/s for slurry trench walls. In contrast, the hydraulic conductivity of the amended backfill with 15% to 25% amended bentonite varies from 8.3 × 10− 10 m/s to 2.7 × 10− 10 m/s, which is lower than 10− 9 m/s. In addition, the amended backfills containing 20% and 25% amended calcium bentonite have nearly the same magnitude of hydraulic conductivity as the conventional sand/sodium bentonite backfill. This study demonstrates that phosphate is a promising amendment for achieving low hydraulic conductivity of sand/calcium bentonite backfills for soil-bentonite slurry trench walls.