On modelling of slope stability in sensitive clay

Doctoral thesis, 2023

The stability of natural slopes is a global problem, with the number of landslides and the associated socioeconomic losses increasing, as a result of the global trend of urbanisation, deforestation and changed weather patterns. The available methods for identifying potential failure modes and assessing the stability, however, overly simplify the modelled soil behaviour within a slope, which might lead to unnecessary mitigation measures or non-conservative results. On the other hand, advanced soil models suitable to capture the response of sensitive clays, have successfully been demonstrated for Serviceability Limit State problems in geotechnics. Yet, those advanced models have not been exploited for effective stress based slope stability analyses.

The aim of this thesis, therefore, is to explore and integrate advanced soil models for analysing the stability of slopes in natural sensitive clay. As such, the evolving hydromechanical processes known to be occurring in a slope, ranging from anisotropy to rate dependency, should be accounted for. The main research effort has consisted of; (i) verification of the use of an anisotropic failure criterion, based on the NGI-ADP model, in the upper bound limit analysis Discontinuity Layout Optimisation (DLO), and (ii) investigation the applicability of the advanced, rate-dependent, Creep-SCLAY1S model for analysing slope stability.

The result findings indicate that DLO is a most satisfactory alternative to the use of the NGI-ADP model in a Finite Element code, and a more robust alternative than Limit Equilibrium
Method. Furthermore, the second part of the study showed that while it is feasible to use a rate-dependent model for slopes, great care needs to be taken to the initialisation of the state variables, most notably the effective stress state in the sloping ground. The best results were obtained by approximately simulating the geological formation processes leading to a natural slope. The gravity increase method, instead of the strength reduction method, was required to quantify the slope stability. Consequently, the rate of increasing the gravity, i.e. the rate of loading, was shown to have a significant impact on the mobilised shear strength, thereby also the calculated stability. Overall, this thesis has contributed to the understanding of the transient coupled hydromechanical processes acting in a natural slope, and have shown that simplifications are necessary for practical applications.