Field construction of novel biopolymer-amended soil-bentonite backfill cutoff wall: Evaluation of in situ stresses, hydraulic conductivity, and chemical compatibility

A novel xanthan gum-soil-bentonite (XG-SB) backfill cutoff wall with a depth of 5 m, length of 10 m, and width of 0.6 m was constructed. For comparison, a conventional soil-bentonite (SB) backfill cutoff wall with identical geometry was also constructed. Three-dimensional earth pressures and pore water pressures within the walls were continuously monitored for 340 d. In situ slug tests and laboratory flexible-wall hydraulic conductivity ( k ) tests were conducted to determine the k of XG-SB and SB backfills. Chemical compatibility was evaluated by using 10.6 mM phenol solution and CaCl 2 (20 mM)-phenol (10.6 mM) mixed solution as permeants in flexible-wall k tests conducted on sampled XG-SB and SB backfill cores. A comprehensive comparison was conducted between the mean in situ effective stresses ( σ ′ m ) and the ratios of flexible wall k to slug test k obtained in this study and those reported in previous studies. A simple equation, developed within the framework of the Kozeny-Carman equation, was proposed to describe the relationship between k and the basic geotechnical parameters of various backfills. The results revealed that the vast majority (95%) of the σ ′ m values ranged from 6 kPa to 14 kPa. Based on the slug tests, the field k value of XG-SB backfill was lower than that of the SB backfill (5.5 × 10 -11 m/s versus 1.1 × 10 -10 m/s). The differences in k values exposure to tap water and CaCl 2 -phenol mixed solution were insignificant for the XG-SB backfill (6.0 × 10 -11 m/s versus 2.1 × 10 -10 m/s). In contrast, a significant difference was observed for the SB backfill (1.6 × 10 -10 m/s versus 1.1 × 10 -9 m/s), highlighting the superior chemical compatibility of XG-SB backfill.