An Improved Nishihara Model Considering the Influence of Moisture Content on the Whole Shear Creep Process of Shale

The moisture content is closely related to the shear creep deformation behavior of soft rock, and the linear creep deformation behavior of soft rock can be described by the classical Nishihara model. However, its accuracy in describing accelerated nonlinear creep characteristics and the effects of moisture content still needs to be improved. The innovation of this paper is to propose an improved Nishihara model that can describe the whole creep process of shale with different moisture content. The model uses a strain-triggered nonlinear sticky pot to describe the process of accelerated creep of rock, and proposes a damage factor to reflect the effect of moisture content on the creep characteristics of rock. The relationship between the moisture content and damage factor is an exponential function, and the damage factor and related model parameters are determined by the shear creep test results under moisture conditions (0%, 0.46%, 0.87%, 1.24%). The shear creep tests were carried out by a self-developed rock shear apparatus. The experimental results show that the shear creep rate decreases first and then increases. The higher the moisture content of shale, the greater the initial shear displacement and stable creep displacement, and the longer it takes to enter the stable creep stage. The improved Nishihara model proposed in this paper can accurately fit the shear creep curves of four groups of shale samples with different moisture contents, and the correlation coefficients all reach 0.99. The fitting effect is better than that of the traditional model, which has good accuracy and practicability.