Failure analysis of deep coal seam floor considering strain softening of surrounding rock and formation contact in goaf

In order to study the failure mode of the floor in deep coal seam mining, we proposed a FLAC^3D finite difference numerical method considering the strain softening of surrounding rock and the contact of goafs .Through the qualitative analysis of Rankine’s earth pressure theory, the failure of the deep coal seam floor is studied based on the measured water conducting fracture zone of the floor in working face 2023 of Linxi Mine. The results show that the mechanical state of surrounding rock after plastic failure can be described more accurately by using the strain-softening constitutive relationship instead of the common Mohr−Coulomb constitutive relationship. The transformation method of “strain softening-emptiness-elasticity” model is adopted to simulate the stress transmission caused by the collapse of goaf roofs, which makes up for the inherent defect of no contact between the roof and floor after the goaf collapse in the previous simulation of coal seam mining. The stress and displacement when there is contact and no contact between the roof and floor of the goaf is compared, showing that the contact between the roof and the floor has a great impact on the numerical results. Therefore, the necessity of the contact needs to be considered. According to the change of the plastic shear strain rate in the simulation results, the three-dimensional display of the floor slip surface is realized, which is in the shape of half-surrounded surface structure of the inclined goaf. Based on Rankine’s earth pressure theory, the plastic zone of the floor is corresponded to the active zone, transition zone and passive zone. The failure forms of the three zones are shear failure, shear failure and the interactive failure of tension and shear. The proposed FLAC^3D numerical method considering strain softening of the surrounding rock and no contact achieves optimization for coal seam mining simulation and can provide a reference for other engineering simulations that need to consider contact after large deformation.