Triaxial Creep Characteristics and Constitutive Model of Argillaceous Siltstone Under Water-Induced Deterioration in the Lower Minjiang River

To investigate the time-dependent deformation behavior of argillaceous siltstone under water-induced deterioration and confining pressure, step-loading triaxial creep tests were conducted on specimens from the lower Minjiang River under air-dried and saturated conditions at confining pressures of 4, 6, and 8 MPa. The effects of water-wetting on creep deformation, the steady-state creep rate, and long-term strength were analyzed. A three-dimensional nonlinear viscoelastic–plastic creep model was developed by introducing a moisture–stress coupled damage factor and a nonlinear viscoplastic component into the Burgers model. The results show that water-wetting leads to a substantial reduction in long-term strength by 28.4–37.5% and increases the steady-state creep rate. Increasing the confining pressure enhances the long-term bearing capacity, with the failure axial stress rising from 56.02 MPa to 81.09 MPa for air-dried specimens and from 37.10 MPa to 56.38 MPa for saturated specimens. The reduction in shear strength parameters is mainly reflected in cohesion decreasing from 6.72 MPa to 2.66 MPa, while the internal friction angle changes minimally. The proposed model effectively captures the full creep process under various stress levels, with the coefficients of determination mostly exceeding 0.90, demonstrating its applicability for describing the triaxial creep behavior of siltstone in the study area.