Creep properties of intact and fractured muddy siltstone.

Abstract

Time-dependent characterisation of rocks for the entire strain range (i.e. up to and beyond the yield point, where rocks are expected to be fractured) have received considerable attention for improving the long-term stability of deep underground openings. Although extensive experimental studies have been carried out on creep of different types of rocks, very limited studies exist which investigate intact as well as fractured rock samples taken from the same type of rock. In this paper, the time-dependent behaviour of muddy siltstone was investigated to determine and compare creep properties of intact and fractured rock samples. A series of multistage uniaxial and triaxial creep tests were conducted on the rock samples at room temperature. In addition, multistage triaxial testing was conducted on the rock (intact and fractured) to determine the instantaneous (short-term) stiffness and explore its correlation with creep properties. All stain curves showed an initial instantaneous strain followed by two phases of time-dependent strain including transient creep phase (particularly for the first loading stage) and a steady state creep phase. The results indicate that both the instantaneous and creep strain are proportional to the deviatoric stress and confining pressure. This is clearly evident in the fractured rock samples, where larger deviatoric stress resulted in an increased creep strain and strain rate. The relationship between axial strain and time was successfully fitted to Burgers creep model. In comparison with the intact rock, creep parameters (of the Burgers model) for the fractured rock were found to be significantly smaller, corresponding to the larger creep deformation and steady state creep rate experienced by the fractured rock samples. Despite this difference between the intact and fractured rock samples, the study showed a considerable correlation between the creep parameters of both types of rock samples and their instantaneous elastic modulus (obtained at typical confining pressures). Regression analysis revealed that creep parameters could be reasonably estimated from instantaneous elastic modulus using an exponential function. Furthermore, based on the experimental findings, an improved characterisation of time-dependent properties was proposed. We believe this approach provides a good basis for future research to enhance geotechnical modelling of long-term stability of abandoned mines as well as for the application of underground disposal of radioactive waste and oil and gas storage.