Surficial Stability of Compacted Clay: Case Study

Abstract

The factor of safety for surficial stability is highly dependent on the effective cohesion. This study shows that even when triaxial tests are performed at effective pressures as low as 13.8 kPa (2.0 psi), the results may still provide effective cohesion intercepts that are too high. The reason is that the compacted sandy clay behaves as an overconsolidated soil, with effective stress paths bending to the right with low to negative A‐values. As demonstrated by this case study of a surficial failure, the calculated factor of safety was 1.33 using the extrapolated value of effective cohesion of 5.3 kPa (110 psf). An unconfined submerged triaxial test resulted in a lower effective strength envelope, which provided a factor of safety of 0.97 for the surficial failure. There are two factors that can increase the surficial stability of a compacted clay slope: aging and root reinforcement. For the surficial failure, the effect of aging was minor and root reinforcement was not a factor in this case, because the roots had not penetrated to a significant depth at the time of failure.