Stress–strain behaviour of bio-desaturated sand under undrained monotonic and cyclic loading

Abstract

Microbial (or bio-) desaturation is a new method for mitigation of the liquefaction hazard of sand. In this paper, a series of monotonic and cyclic triaxial tests was performed on bio-desaturated sand under undrained conditions to demonstrate that the bio-desaturation method is effective in enhancing the liquefaction resistance of sand under either monotonic or cyclic loading conditions. The test results showed that, under monotonic loading, the undrained shear strength increased by more than two times and the behaviour of soil changed from strain softening to strain hardening when the degree of saturation (Sr) reduced from 100% to 86·4%. The undrained shear strength ratio (su/p′0) has a unique relationship with the peak excess pore water pressure ratio (Δumax/σ′c) regardless of different effective confining stress conditions. Under the applied cyclic load, the liquefaction resistance showed a significant increase when the sample was slightly desaturated. For the same cyclic stress ratio and number of cycles, the accumulated excess pore water pressure (Δu) in the sample with a degree of saturation of 96% was only a quarter of that in the fully saturated sample. The instability behaviour of sand was also studied. The results showed that the slope of the instability line, which is a measurement of the resistance of sand to collapse, also increased with the reduction in degree of saturation under monotonic loading. The failure stresses obtained in cyclic triaxial tests conducted under different cyclic stress ratios coincide with the instability line obtained under monotonic conditions for sand with the same degree of saturation. This provides good support for the use of the instability line to predict the failure stress state of sand for both monotonic and cyclic loading conditions.