An experimental investigation has been completed which evaluates transient creep of natural rock salt under conditions of stress anticipated in situ for the purposes of incorporation of the results into finite element computer codes. Initially, creep data is formulated into a power law which accurately models the transient creep of all tests. The data is then used to derive parameters for a new formulation of an inelastic creep law based on internal thermodynamic displacement variables. The inelastic creep law is shown to model the experimental data with equal accuracy as the power law over the transient region. This represents the first stage of incorporation of emperically derived parameters into the inelastic formulation.
Deformational properties of natural rock salt are of particular interest in the field of rock mechanics today as the need for an underground repository for terminal storage of nuclear waste materials becomes iminent. Considerable attention has been focused on experimental determination of the constitutive relationships of salt. Although an attempt has not been made to survey the relevant material on this subject, work by liawersik (1) is a notable example. The present directional effort of thermomechanical analysis of proposed repository geometries includes the incorporation of mechanical properties of salt into the analysis procedures. Some case studies have compared laboratory data extrapolated to field measurements and achieved excellent correlation of observed versus predicted deformations; Project Salt Vault (1) is a landmark example. Other well documented field measurements on a large scale are generally lacking. In a scoping sense, large scale field modeling is in a relatively early stage of development. The function of the laboratory testing program in the overall picture is one of support to the analysis efforts. The purpose of the testing effort is to define and evaluate deformational parameters of rock, especially rock salt, by means of specific tests for input to the finite element codes. This approach provides a basis for constitutive formulation of rock behavior under conditions of confining stress and elevated temperature. The observed deformational behavior of salt pillars requires an extensive analysis of materials properties as determined in the laboratory. In conjunction with the various modeling efforts, ongoing quasi-static and creep testing of salt over a large range of pressures and temperatures is being conducted on dome salt from Jefferson Island, LA, and bedded salt from Lyons, KS, and near Carlsbad, NM (3,4,5) for purposes of formulating general strength and deformation properties. In particular, the goals of the current programs involve obtaining data for the development of a realistic material model which couples elastic and inelastic behavior of salt. Generally, the laboratory test parameters are prescribed to encompass the range of anticipated pre-and post-mining stress states, with elevated temperature introduced to include the aspect of burial of nuclear waste. Each testing program is undertaken to evaluate site specific rock salt properties. Besides evaluation of stress-strain behavior and principal strain ratio (which are nonlinear), the singularly most difficult task common to both experimental and analytical studies is determining the time-dependent deformational properties of rock salt.