Mechanical properties and impact tendency of hard floor rock under multiaxial conditions

In order to study the impact hazard of surrounding rock of roof and floor in deep coal mine under high stress conditions,the hard floor rock at the Yuejin Coal Mine 23092 working face is taken as a test object,and a series of compression mechanical properties tests under multiaxial(conventional and true triaxial) conditions are carried out.In some experiments,acoustic emission system has been employed to measure the fracture signal.For the analysis of the test results,the optical microscope and the field emission scanning electron microscope are used to represent the internal feature.It has been shown that the hard floor rock at the 23092 working face has shown obvious elastic brittle deformation and failure behavior.The brittle degree of the hard floor rock is the highest under uniaxial compression condition,and the intermediate principal stress is negatively related to the brittleness index.The whole stress-strain curve,macroscopic failure mode,fracture angle,brittleness index of the rock sample and the change characteristics of the SEM image of the failure rock sample all show that the brittleness failure characteristics of the hard floor rock are related to the stress state.That is to say,the increase of the intermediate principal stress promotes its brittleness failure behavior when the minimum principal stress is constant and the intermediate principal stress increases.Under the condition of multi axial stress,the peak strength of hard floor rock is discrete,which is related to the difference of rock homogeneity caused by the convergence of weak mineral inclusions in the process of diagenesis.The impact tendency of hard floor rock is affected by true three-dimensional stress state.The hard rock near the surface of roadway floor with minimum principal stress close to zero shows no or weak impact risk,but the deep hard rock with high intermediate principal stress shows strong impact hazard,and increases with the increase of intermediate principal stress.Acoustic emission signals with compression and destruction under multi-axial conditions have a precursory release feature,and the acoustic emission signals are significantly enhanced in the post-peak multi-stage rupture.The research results have some reference values for the further research of deep coal rock deformation and failure mechanism and evaluation of impact tendency in deep coal mines.