January 2025
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Bulletin of Engineering Geology and the Environment
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 (qc) and sleeve friction resistance (fs) in dissipation process also vary with time, despite its potential impact on the inversion process. In this paper, the evolution of qc 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 qc is demonstrated and the causes of qc 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 qc 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 qc 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.