Direct tension creep behaviors of sandstone and improvement and application of Burgers model

The direct tension creep of red sandstone is tested by means of the rock direct tension device developed by the authors. The test results indicate that the sandstone in direct tension under the low stress condition is characterized by primary creep and steady-state creep while under high stress conditions by a rather short accelerated creep stage. Based on the creep behaviors of the sandstone, the Burgers model is improved by introducing a nonlinear viscoplastic body, and the direct tension creep test results are fitted by using the BFGS algorithm. The improved Burgers creep model agrees with the experimental curve. The introduction of the nonlinear viscoplastic body with the initial viscosity value η_N as large as over 10¹⁹ (MPa.h) implies that the nonlinear viscoplastic body in the sandstone acceleration creep stage plays a leading role. The improved Burgers model can describe the primary creep and steady-state creep of sandstone under direct tension and better describe the process of the accelerated creep. It is concluded that it is important to evaluate the effects of tensile on the stability of geotechnical engineering.