The effect of transverse isotropy on the creep behavior of bedded salt under confining pressures

Abstract

In this paper, we investigate the role of transverse isotropy on the creep behavior of bedded salt. We conducted a series of triaxial creep tests on prismatic specimens subjected to confining pressures (\(\sigma _{3}\)) of up to 24 MPa and a constant octahedral shear stress (\(\tau _{\mathrm{o}}\)) of 9 MPa. The specimens were oriented with their bedding planes at various angles (\(\beta \)) to the major principal axis to simulate transverse isotropic conditions. Our findings reveal that both instantaneous and creep deformations are most significant when \(\beta = 0^{\circ }\), decreasing progressively to a minimum at \(\beta = 90^{\circ }\) across all confining pressures. The discrepancy in deformations between these intrinsic angles narrows with increasing \(\sigma _{3}\). Creep deformations for intermediate angles (\(0^{\circ} < \beta < 90^{\circ }\)) follow the elliptical equations. Utilizing the Burgers creep model, we observed that the instantaneous, viscoelastic moduli, and viscoplastic coefficients escalate with \(\beta \). The degree of anisotropy declines sharply as confining pressures increase, reaching an isotropic state under \(\tau _{\mathrm{o}} = 9\text{ MPa}\) and \(\sigma _{3}\) around 40 MPa, beyond which transient creep ceases, indicating a transition to Maxwell-material behavior. Employing linear viscoelastic theory, we derived an equation for time-dependent deformation under varying octahedral shear stresses. This enables the formulation of governing equations for Burgers-model parameters, considering bedding plane orientations, loading durations, and the interactions between shear and confining stresses.