Hydro-mechanical behavior and Cr(VI) containment performance of sand-clay-bentonite backfill

Vertical cutoff walls at heavy metal contaminated sites are subjected to coupled chemo-hydro-mechanical effects. In this study, a series of oedometer compression, permeability and solute transport tests were conducted to investigate the hydro-mechanical behavior and Cr(VI) containment performance of sand-red clay-bentonite backfill. Mercury intrusion porosimeter (MIP) and field emission scanning electron microscope (FESEM) were used to analyze microstructural changes following chemical exposure. Experiment results indicate that the backfill exhibited high compressibility, with a compression index of 0.74, and maintained a hydraulic conductivity below the regulatory threshold (< 10⁻⁹ m/s) under vertical effective stresses exceeding 25 kPa. Under Cr(VI) contamination, the backfill experienced slight shrinkage that correlated linearly with Cr(VI) concentration. This behavior is primarily attributed to the formation of localized sand skeletons, which enhance structural stability and chemical compatibility. Microstructural analyses revealed a reduction in micropore size due to clay aggregate shrinkage and an increase in macropore size caused by the formation of micro fissures at sand–clay interfaces. Consequently, hydraulic conductivity increased moderately (by 11 % to 50 %) with rising Cr(VI) concentration. This change corresponded to a gradual decline in containment performance, as evidenced by a reduced retardation factor and an increased hydraulic dispersion coefficient. Overall, the sand-red clay-bentonite backfill demonstrated low permeability and strong chemical compatibility under Cr(VI) contamination, making it a suitable backfill material for constructing vertical cutoff walls in Cr(VI)-contaminated sites.