A practical method to evaluate failure envelopes of shallow foundations considering soil strain softening and rate effects

© 2016 Elsevier Ltd. This paper presents a practical method to evaluate the development of failure envelopes of coastal and offshore shallow foundations after the action of multi-directional cyclic loading considering soil degradation. This is realised by implementing a soil strain softening and rate model into a finite element package and using a combined approach of load controlled geometrically nonlinear finite element analysis and displacement controlled small strain finite element analysis. The gradual evolution of soil strain softening and rate effects are monitored and utilised to update the shear strength during the analysis under multi-directional cyclic loading. The failure envelope is then investigated based on the updated shear strength field of the soil. After introducing the approach and demonstrating with an example, a systematic study is presented to explore the effects of soil strain softening and rate parameters, soil stiffness index and period of cyclic loading on load carrying capacity of coastal and offshore shallow foundations. The results show that soil degradation should be taken into account in design of coastal and offshore shallow foundations because it is detrimental to the capacity of shallow foundations. Results of the parametric analyses have shown that soil sensitivity, relative ductility and the stiffness index have more influence on capacity of shallow foundations subjected to undrained cyclic preloading than strain rate parameter and the period of cyclic loading.