Numerical implementation and application of an internal state variable model to analyze the time-dependent behavior of mining excavations in rock salt

Abstract

The geomechanical behavior of rock salt has been investigated extensively over the years. Experimental studies have identified distinctive features associated with non-linear inelastic response that shows strong time and loading history dependencies. In this paper, a relatively simple constitutive model is presented and implemented into the numerical code FLAC, and then applied to analyze the time-dependant behavior of excavations in salt mines. The unified creep-plasticity model includes an internal state variable (ISV) with an evolution law that induces progressive strain hardening (SH) until a steady-state is reached. Numerical analyses are performed with the proposed ISV-SH model to evaluate material parameters, based on creep tests results on a natural rock salt, and to simulate the response of a circular opening and of rectangular mining excavations created in sequence. The results are analysed and compared with those obtained with the well-known Norton power law equation, commonly used in salt mines engineering. The time-dependent calculation results illustrate key aspects of rock salt behavior and highlight the major influence of transient inelastic behavior and stress redistribution on the response of underground openings. The proposed modelling approach constitutes an advantageous alternative to analyses based on stationary creep laws, often used in rock salt mining operations.