Surficial Slope Failure: A Case Study

In a recent surficial slope failure in a compacted silty sand fill slope it was observed that the failure plane had developed just beneath the bottom of the grass roots. Results of drained direct shear tests performed at low normal stresses indicated that root‐permeated fill had a much higher shear strength than the same root‐free soil. The results of surficial stability analyses showed that live roots will significantly increase the factor of safety. The live roots provided mechanical reinforcement of the slope and thus the failure surface developed beneath the bottom of the roots. Although a factor of safety of 1.5 is generally the minimum value for new construction, a lower factor of safety may be acceptable in recognition that deep‐rooting plants will increase the long‐term surficial stability of a slope.