Strength parameter selection framework for evaluating the design life of clay cut slopes

The design of engineered earthworks is predominantly conducted through limit equilibrium analysis, requiring strain-independent strength criteria. Previous studies on deep-seated first-time failures within over-consolidated clay cut slopes have proposed the use of fully softened strength parameters for design. A study investigating shallow first-time failures in clay cut slopes due to seasonal stress cycles was undertaken using a validated numerical model capable of capturing seasonal ratcheting and progressive failure. It was found that fully softened strength criteria are inappropriate for the assessment of shallow first-time failures due to seasonal ratcheting and slopes at angles between the material's fully softened and residual friction angle may be at risk of failure in the future due to this behaviour. However, adopting residual strength parameters will likely result in overly conservative solutions considering the required design life of geotechnical assets. It was found that the strain-softening behaviour of clay defines the rate of strength deterioration and the operational life of engineered slopes. While general guidelines for analysis considering shallow first-time failures in clay cut slopes are provided, detailed understanding of a material's strain-softening behaviour, the magnitude and rate of strength reduction with strains, is needed to establish strength criteria for limit equilibrium analysis.