Three-dimensional stability of slurry-supported trenches: End effects

The common procedures for stability analysis of slurry-supported trenches are typically two-dimensional (plane strain), despite the fact that observed failures in the field possess three-dimensional (3D) characteristics. A limit equilibrium (LE) solution for the stability of a slurry-supported trench is presented, where failures are constrained to a finite length. A rotational 3D failure mechanism, derived from variational LE analysis of the slope stability, is adopted and modified to account for the stability of a slurry-supported trench as well as very narrow trenches. The results are presented in the form of stability charts that yield a safety factor for narrow excavation of a given trench. Due to mathematical constraints on the resulting rigid 3D failure geometry, the critical surface of a deep and narrow trench emerges above the toe, termed the shallow failure. Once a shallow failure is initiated, it progressively propagates down to the toe. This above-toe failure mechanism may overestimate the safety of deep excavations in cohesive soils. Consequently, for shallow failures, an alternative approach is introduced to assess the safety of such progressive failures, which ultimately will emerge through the toe. The three-dimensional end effects are investigated to highlight their impact on the stability of a trench, indicating that end effects are most pronounced in purely cohesive soil, diminishing in cohesionless soil. End effects are significant for toe failures in slurry trenches, potentially impacting design.