Numerical analysis of deformation, failure and energy release mechanisms of fractured coal rock under unloading conditions

Coal rock is naturally comprised of a large number of randomly-distributed discontinuities such as joints/cracks and weak interlayers. The geometry, size, distribution, contact and infillings of the discontinuities essentially govern the physical/mechanical properties and failure mechanism of coal rock. It is intractable to theoretically and accurately characterize the discontinuities and the governing influences on the deformation and failure of coal rock. This paper presented a 3D finite element model of fractured coal rock representing the irregular geometry, complex contact, intersection, and network features of interior joints and cracks by means of high-resolution CT techniques and 3D reconstruction methods. The birth-death element algorithm was employed to characterize and visualize the stress field distribution, spatial distribution of failed areas, energy dissipation and release during the deformation and failure of fractured coal rock when subjected to the varied unloaded conditions. The analysis discloses the determinative influences of the unloading paths and the properties of joints and cracks on the macro mechanical behavior and the failure mechanism of fractured coal rock. The paper presents a promising way to identify and characterize the governing effect of unloading due to deep excavation on real fractured coal rock.