Stresses in soil–bentonite slurry trench cut-off walls

Abstract

The long-term performance of soil–bentonite slurry trench cut-off walls is highly dependent on the hydraulic conductivity of the soil–bentonite backfill, which according to laboratory tests can decrease significantly as consolidation pressure increases due to corresponding reductions in void ratio. Consequently a reliable estimate of the hydraulic conductivity of backfill in the field requires proper calculation of effective stresses. A model is proposed to predict the steady-state horizontal and vertical effective stresses in the backfill after consolidation. The arching effect is considered by way of force equilibrium, and the lateral squeezing effect of inward displacement of the trench side walls is considered by assuming the cut-off wall is surrounded by soil, which is represented by a Winkler idealisation. The proposed model is applied to consider a soil–bentonite slurry trench cut-off wall in New South Wales, Australia, and the predicted stress profile is in good agreement with that calculated from cone penetration test data. Compared to those predicted by geostatics and other alternative models, the proposed method offers a significant improvement in the prediction of stress in soil–bentonite slurry trench walls. The obtained stresses are then used to estimate the hydraulic conductivity in the backfill. It is found that the hydraulic conductivity is relatively high in the shallow region owing to the low state of effective stresses, which requires consideration in cut-off wall design, and decreases slightly with the depth in the deeper region. Finally a parametric study identifies the side wall friction and the modulus of horizontal subgrade reaction of surrounding soil as having the most significant impact on the estimated stresses.