Stress relaxation of silt under excess pore water pressure impact from cone penetration test in Yellow River Delta

Abstract

The generation of negative excess pore water pressure (u₂) during cone penetration test (CPT) in a given environment represents a deviation from the actual situation, thereby affecting the accuracy of the parameter inversion. Dissipation tests have been conducted to ascertain the dissipation of the u₂ over time, which in turn allows for the parameters to be corrected. However, the tip resistance (q_c) and sleeve friction resistance (f_s) in dissipation process also vary with time, despite its potential impact on the inversion process. In this paper, the evolution of q_c and negative u₂ with time is successfully obtained through the utilization of indoor CPTs on silt soils. In conjunction with a viscoelastic model, the existence of stress relaxation of q_c is demonstrated and the causes of q_c decay are analyzed. The detailed conclusions are as follows: (1) The CPT parameters obtained from the dissipation test can be employed to rectify the discrepancy in negative u₂ that arises during soil classification. (2) The q_c undergoes a gradual decrease, reaching a final equilibrium state during the dissipation process. The stress-time relationship is consistent with the Three-element viscoelasticity model, which represents a stress relaxation phenomenon. The relaxation process can be divided into three distinct phases: fast relaxation, decelerating relaxation, and residual relaxation. The residual stress is found to be correlated with the depth of the soil layer. (3) During residual phase, the loss rate of q_c is observed to decrease in a linear fashion with the rate of u₂, prior to which the relationship is exponential. As the penetration rate increases, the rate of u₂ also increases.