Progressive fracture and swelling of anisotropic rock masses around deep tunnels: a new floor heave mechanical mechanism

Abstract

Floor heave disasters easily occur along tunnels in deeply buried, weak, anisotropic rock masses with high in situ stress. The previously presented floor heave mechanical models (buckling failure, shear dislocation failure, and slip line field theory) do not consider the processes of stress release, transfer, and concentration, and only the initial in situ stress state can be analyzed with these models, i.e., they are static mechanical models. Therefore, the combined finite–discrete element numerical method (FDEM) is employed here to study floor heave tunneling in anisotropic rock masses to propose a new floor heave mechanical mechanism based on progressive fracture and swelling processes and the influences of the in situ stress (lateral pressure coefficient) and the layer thickness are also investigated. The simulation results indicate that (1) the essential mechanical mechanism of floor heave is X-shaped conjugate shear failure accompanied by tensile failure caused by the concentrated maximum horizontal stress present after tunneling; the shallow rock fragments cannot bear the enormous concentrated horizontal stress, and thus, this stress is transferred deeper into the intact rock mass, further causing the shear and tensile fractures to propagate downward (a progressive evolution); (2) for anisotropic floor rock masses, in addition to the X-shaped conjugate shear fractures, shear fractures parallel to the bedding plane and tensile fractures perpendicular to the bedding plane form; (3) with the change in the in situ stress lateral pressure coefficient, the failure mode directly under the floor remains almost unchanged, but it will have a significant impact on the rock mass on both sides of the floor; and (4) the layer thickness has a weak influence on the failure mode of the floor rock mass.