Refined Prediction of Strain-Softening Parameters of Marine Soft Clay Using Cyclic T-Bar Penetration Tests

T-bar penetrometers are widely utilized for measurements of strength characteristics of soft clay in offshore field investigations and laboratory tests. To obtain strain-softening parameters of marine soft clay, it is necessary to conduct cyclic T-bar tests below the critical penetration depth. Therefore, this study conducted CEL large-deformation finite-element analyses to simulate the entire penetration process of T-bar in strain-softening soils. An equation for predicting the critical penetration depth is proposed, taking into account both the amount and the rate of strength reduction of clay. The numerical results indicate that the minimum depth for initiating the pull-out phase of the cyclic test is the full-flow penetration depth plus three times the T-bar diameter, which ensures that the surrounding soil remains in a full-flow mechanism throughout the cyclic test. Moreover, a refined resistance degradation model is proposed, which corrects a defect in the existing model, which is that when the number of cycles is equal to N95, the predicted result of the model does not reach 95% degradation in penetration resistance. Then, an estimation method for strain-softening parameters of soft clay is developed. The method based on the refined resistance degradation model is verified by comparing the estimated values with the numerical simulation results in this paper and the available test data.