Critical state shear strength at concrete–sand–bentonite slurry interfaces: mix proportions and rate effects

This paper describes direct shear testing of a range of sand–bentonite slurry mixes, identified as a gap in the literature. In the absence of a concrete interface, there was an apparent transition zone from sand-governed strength (bentonite content ≤30%) to clay-governed strength (bentonite content ≥80%), which was linked to the sample void ratio. When a concrete interface was introduced, the critical stress ratio (critical state shear stress normalised by the effective normal stress) was found to change linearly with bentonite slurry content, reflecting an interface sliding failure mechanism. The testing also considered the effect of shearing rates ranging from 0·0025 to 10 mm/min on interface shear strength. The mixtures containing greater bentonite slurry content displayed a greater decrease in the critical stress ratio with increasing shear rate. Very few studies have explored the influence of the relative sand–clay mix composition on interface shear resistance; these results provide a better understanding of their mechanical behaviour and forms a basis for future research in this area.