Back analysis of the Joss Bay Chalk Cliff Failure using numerical modelling

Use of numerical modelling for back analysis of slope failure can provide a valuable insight into the underlying failure mechanisms and improve our understanding of factors controlling instability and geomorphology of slopes. This paper provides a brief overview of the range of methods that are available for slope instability analyses, and their respective benefits, using the back analysis of the Joss Bay Chalk cliff failure as a case example. A variety of analytical and numerical approaches are used to back analyse the failure to assess the potential impact of both the rock mass strength and discontinuity-controlled behaviour on the potential failure mechanism. Limit equilibrium analysis approaches are used to assess the sensitivity of the simulated model to variation in input parameters. Numerical simulation, constrained by the limit equilibrium models, is then used to evaluate the progressive nature of failure mechanism and directly simulate the trigger mechanism. Importantly, the modelling demonstrates that shear strain initiates at the base of the cliff and subsequently extends upwards resulting in tensile failure in the upper part of the slope. The case example demonstrates the capability of numerical models to simulate the progressive nature of slope failure, and their ability to evaluate the detrimental effects of a wave-cut notch at the base of a cliff.

Research highlights
► Overview of the advantages and capabilities of the various modelling methods that are available.
► Evaluation of the detrimental effects of a wave-cut notch at the base of a cliff.
► Numerical simulation, constrained by the limit equilibrium models.
► Simulation of trigger mechanism and progressive nature of failure.
► Shear strain initiation at the cliff base, and subsequent extension upwards resulting in tensile failure.