Prediction of coal pillar instability time in room-pillar goaf under dynamic and static load

Aiming at the problem that it is difficult to predict the instability time of large area house-pillar goaf under dynamic and static load,taking the Jinglaoyuan Coal Mine in Ordos City as the research background,the mechanical bearing characteristics of coal pillar under dynamic and static load are analyzed through indoor cyclic loading and unloading tests and uniaxial loading tests.Based on the creep theory and combined with the stress model of roof pillar system,the effective bearing time function of coal pillar in room pillar goaf is deduced.Considering the time factor,a numerical model is established to simulate the action mode of dynamic and static load by applying cyclic loading and unloading load,and the dynamic evolution process of coal pillar instability with time under dynamic and static load is studied.The results show that the ultimate failure strength of coal samples under cyclic loading and unloading is lower than that under fixed load.The effective bearing time of coal pillar under creep is log-positively correlated with the cross-sectional area of coal pillar,and log-negatively correlated with the height of coal pillar and the stiffness of roof.The instability process of coal pillar can be divided into three stages:stability stage,development stage and instability stage.The reliability of effective load function of coal pillar is verified by theoretical model,numerical simulation and field measurement.