Slopes in soft clay - management of strength mobilisation

Doctoral thesis, 2015

Many natural and man-made slopes close to buildings or infrastructure do not demonstrate sufficient levels of safety with respect to their stability. Nonetheless, it is a requirement from society that the environment we live in is safe. Many slope stability investigations are therefore performed every year so that the risk of slope failure can be evaluated and corrective action applied, where appropriate. Numerous remedial measures are designed and carried out according to Swedish industry recommendations and practice. In a traditional slope stability analysis where limit equilibrium methods are used, one critical assumption is that a constant degree of shear strength mobilisation applies along the critical shear surface. This assumption implies that the entire shear surface is subject to equal levels of shear strain. It is therefore difficult to isolate the effect of different remedial measures on specific parts of the slope. Also, as the traditional design of remedial measures only accounts for the change in overall factor of safety, it is not possible to state which part of the slope exhibits the least favourable stress condition and which remedial measure is most suitable and will improve the stress condition the best. An optimised design is therefore not achieved. The work presented herein proposes a method for achieving an optimised design of remedial measures, based on evaluation of the degree of shear strength mobilisation within a slope. Using numerical methods and observing the stress changes taking place within the slope allows evaluation of the effect of the remedial work by demonstrating whether the shear strength mobilisation is reduced in the most critical areas. The method presents a feasible engineering approach which may be implemented using existing tools which are widely available in the industry. The selection of optimised remedial measures with a focus on critical unfavourable stress concentrations provides a more sustainable approach to society. In addition to the development of the new methodology, the work summarises different definitions of shear strength mobilisation and evaluates the important challenges when working with slope stability modelling, especially when using numerical tools. Two specific issues addressed are the creation of in situ stress conditions and selecting the relevant strength parameters. The work also describes the potential effects that future climate change may have on the stability of slopes.