Optimized strain softening model for marine clay from cyclic full-flow penetration and extraction tests

Strain softening is a major feature of marine clay that plays an important role in offshore geotechnical problems. One of the widely used strain softening models is introducing an exponential relation of the undrained strength to the accumulated shear strain into a simple constitutive model. The strain softening model of seabed sediments can be obtained by the resistance degradation with cyclic full-flow penetration tests. However, for structured soils, the normally used exponential relation failed to reflect the brittleness of the soil under small strain. This paper proposed an optimized exponential strain softening model by adding a parameter beta. The fitting results of 20 cyclic penetration tests on different soils show that the proposed model can reflect both the initial brittleness of soil and its fully remolded state well. Based on the LDFE analysis on the process of pipe penetration, a maximum difference of 22.8% on the pipe resistance may be caused by the value of beta. For the estimation of ductility parameter (N-95) in the strain softening model, a new method is proposed by the percentage of strength degradation (P-su,P-n), which can improve the accuracy of N-95 up to 58.8% for the clay from the South China Sea.