Viscoelastic-Viscoplastic Formulas for Analysis of Cavities in Rock Salt

The material model consists of a viscoelastic Burgers element and an additional viscoplastic element, which includes Bingham viscosity and a viscoplastic Kelvin element. For spherical and cylindrical cavities, expressions are derived for the stress and strain fields and volume convergence. Spherical and cylindrical cavities are treated in a unified formulation. The expressions for the transient phase require a numerical approach. A simple numerical scheme is proposed which solves these expressions efficiently. For constant pressurization stationary stresses, strain rates and convergence rate will eventually arise. Exact, closed-form solutions are derived for these quantities. The magnitude of the Maxwell viscosity is of vital importance for the stationary state. The material model compares favourably with experimental creep data.