Design-life based approach for the design of cutting slopes in stiff clay

Progressive failure of cutting slopes in stiff clay can be analysed by coupled hydro-mechanical analyses. But such analyses are computationally expensive and reserved for research or large-scale projects. In routine practice, limit equilibrium method (LEM) is still commonly used. However, it is beyond LEM’s capability to replicate the mechanisms of progressive failure, or subsequent deterioration under current and future climate scenarios. Analysis is often carried out with conservative assumptions (e.g., using critical state strength) for long-term soil and groundwater conditions, when these vary temporally. In this study, an alternative method that considers soil strength to be a transient parameter allows for a design-life based approach to be developed. Mobilised strength parameters as a function of time and slope geometry have been derived from the outputs of a series of coupled hydro-mechanical analyses, for cuttings in London Clay, using a regression model. It is demonstrated that peak strength parameters should be used to search for the critical slip surface (CSS) during LEM analysis. The CSS is then fixed, and in subsequent analyses mobilised strength parameters are suggested according to the design-life. The design-life based approach permits generation of design charts. A case example is used to demonstrate the application and potential benefits of the proposed approach.