Investigation of Stress Relaxation Characteristics of Fine-Grained Soils Based on CPT and Viscoelastic Constitutive Modeling

Abstract Stress relaxation is a common phenomenon in fine-grained soils. However, most existing studies rely on laboratory triaxial tests, with limited attention to in situ methods. In particular, the relaxation behavior under cone penetration test (CPT) conditions has not been systematically investigated. Through triaxial relaxation tests and CPT calibration chamber experiments, this study adopts a five-element viscoelastic model to analyze the stress–time response of fine-grained soils. Both testing approaches reveal distinct stress relaxation characterized by rapid, decelerated, and residual stages, with cone resistance and sleeve friction exhibiting S-shaped attenuation and eventually stabilizing. Dissipation tests further showed that cone and sleeve resistances decayed exponentially with time. Model analyses indicate that the five-element viscoelastic model provides improved accuracy in describing residual stresses and relaxation rates. In triaxial tests, the primary elastic modulus increased with strain before stabilizing, and grew linearly with confining pressure, whereas dashpot parameters followed an exponential growth trend. CPT-based results suggest that initial strain ranged from 3.5% to 5.2%, with a primary elastic modulus of 200–300 kPa and instantaneous stiffness exceeding 400 kPa. Elastic parameters increased approximately linearly with cone resistance, while dashpot parameters increased exponentially. A CPT-based framework for predicting initial strain and quantifying viscoelastic parameters is proposed, providing a feasible approach for in situ characterization of stress relaxation in fine-grained soils.