Effect of Bio-Cementation on Drained Instability of Poorly Graded Sand with Sub-Angular Particle Shapes

Case studies of slopes experiencing unstable behavior under drained conditions below the stress level that would cause a traditional failure have been reported in the literature. Here, unstable behavior refers to a sudden collapse of soil resulting from a small change in stress conditions. This response, which is known as drained (diffuse) instability, is quite different from the widely recognized static liquefaction mechanism of failure for sand under undrained conditions. Microbially induced carbonate precipitation (MICP) as a soil improvement technique has a promising effect on the shear strength of soils and has potential to be used to reduce the potential of instability in granular slopes. To provide a better understanding of this collapse mechanism, a series of monotonic triaxial tests on treated and untreated specimens have been carried out to evaluate the effect of bio-cementation improvement on the instability under drained conditions. The experiments were conducted in two stages. Different stress conditions, including stress-induced anisotropy, were used for the first stage of loading. After that, a constant shear-drained (CSD) stress path was used in the second stage of the test as the mean effective stress was continuously reduced by decreasing confining pressure with a constant rate until instability was reached. The onset of instability in treated and untreated samples was investigated in this study.