Creep constitutive model for salt rock considering loading and unloading path based on state variables

Creep behaviour in rocks is a typical mechanical property that is directly linked to the stability of underground engineering. The deformations and rate of rocks creep are not only influenced by time but also by the loading and unloading history. To more accurately predict creep mechanical behaviour of salt rocks, the rocks hardening is described by introducing a state variable. A new three-dimensional creep constitutive model of salt rocks was established to describe the loading and unloading history effect of the rheological properties. In this paper, salt rocks creep tests under various loading and unloading histories were conducted to investigate how different loading routes affect the creep behaviour of salt rocks. The effects of the model state variables were analysed through different indicators. An example verification was carried out with the results of plastic deformation tests performed at different loading paths. The findings indicated the creep rate of stepped loading and the stepped unloading under the same stress level were significantly affected by the loading history. The proposed constitutive model can accurately fit the creep test curves of different loading paths, indicate that it can provided a prediction of the historical effect of the creep behaviour of salt rocks. Different parameters affect the different phases of the creep curve. The parameter k primarily affects the overall shape of the creep curves. Parameters m and c primarily influence the steady-state creep length and creep rates, excluding the initial cycle.