Numerical and theoretical study of the large-scale failure of strata overlying sublevel caving mines with steeply-dipping discontinuities

The deformation and fracture evolution mechanisms of the strata overlying such mines mined using sublevel caving were studied via numerical simulations. Moreover, an expression for the normal force acting on the side face of a steeply-dipping superimposed cantilever beam in the surrounding rock was deduced based on limit equilibrium theory. The results show that surface displacement above metal mines with steeply-dipping discontinuities shows significant step characteristics and the behavior of the strata as they fail exhibits superimposition characteristics. Generally, failure first occurs in certain superimposed strata slightly far from the goaf. Then, with the constant downward excavation of the orebody, the superimposed strata become damaged upwards away from, and downwards towards, the goaf. This continues until the deep part of the steeply-dipping superimposed strata form a large-scale deep fracture plane that connects with the goaf. The deep fracture plane generally makes an angle of 12–20° with the normal to the steeply-dipping discontinuities. The effect of the constant outward transfer of strata movement due to the constant outward failure of the superimposed strata in the metal mines with steeply-dipping discontinuities causes the scope of the strata movement in these metal mines to be larger than expected. The strata in the metal mines with steeply-dipping discontinuities mainly show flexural toppling failure. However, the steeply-dipping structural strata near the goaf mainly exhibit shear slipping failure, in which case the mechanical model used to describe them can be simplified by treating them as steeply-dipping superimposed cantilever beams. By taking the steeply-dipping superimposed cantilever beam that first experiences failure to be the key stratum, the failure scope of the strata (and criteria for the stability of metal mines with steeply-dipping discontinuities mined using sublevel caving) can be obtained via iterative computations from the key stratum, moving downward towards and upwards away from the goaf.