Application of a non-linear viscoelastic-plastic rheological model of soft coal on borehole stability

Time dependent deformation of coal has a significant effect on the stability of underground structures. In this study, taking the visco-elastic-plastic characteristics and the damage effect into account, a non-linear model was proposed to describe the creep behavior of soft coal. Meanwhile, the creep equations of the proposed non-linear model were derived and the three-dimensional creep equations were constructed by generalizing the one-dimensional creep equations. At the same time, the uniaxial creep tests on soft coal specimens under different axial stress conditions were carried out to validate the proposed model. It is found that the present model can not only reproduce the transient creep under low axial stress level, but also can accurately predict the steady and accelerating creep stages under high axial stress level. Finally, borehole stability in soft coal seam was investigated based on the proposed model. The results show that the vertical displacement and plastic zone range around borehole gradually increase while the increasing rate decreases and tends to be stable after 20 days. It can also be obtained that the creep duration tends to be stable when it is about 20 days after drilling excavation under the simulation conditions. The analysis results should be useful for the long-term stability of boreholes in soft coal seam for gas extraction.