Effect of polyanionic cellulose modification on hydraulic and consolidation performance of sand-calcium bentonite slurry trench cutoff walls

Abstract

Sand-bentonite slurry trench cutoff wall is one of the most common vertical barriers to retard the migration of contaminants. This study aimed to explore the hydraulic and consolidation performance of cutoff-wall materials containing polyanionic cellulose (PAC)-modified calcium bentonite (CaB) through a comprehensive laboratory investigation. First, physicochemical properties tests were conducted to demonstrate the superiority of PAC-CaB over CaB. Subsequently, modified fluid loss tests were conducted to investigate the effects of PAC modification and CaCl2 solution concentration on the chemical compatibility of CaB cake formed during slurry trench protection. Finally, slump, Atterberg limit, oedometer, and rigid/flexible-wall hydraulic conductivity tests were performed to evaluate the effect of PAC modification on the hydraulic conductivity (k) and compressibility of sand-CaB backfill. The results indicate that PAC can significantly enhance the impermeability of CaB cake when permeated with salt solutions. The 4 %PAC-CaB cake in 40 mM CaCl2 solution had the lowest k value (9.3 × 10−10 m/s), and was therefore recommended as the optimum proportion for trench protection. The addition of PAC considerably reduced the consolidation coefficient of CaB backfill and increased its compression index and swell index. Moreover, with the increase of PAC dosage from 0% to 0.2%, 0.4%, and 0.8%, the k values of CaB backfill decreased by 0.75, 0.96, and 1.29 orders of magnitude, respectively. The k values of PAC-CaB materials were lower than those of sodium bentonite materials, indicating that PAC is a promising modifier to achieve a low k value of CaB cake and sand-CaB backfill for slurry cutoff walls.