Effects of Interface Behavior on Fracture Spacing in Layered Rock

To better understand the periodically distributed fractures in rock sequences, the effects of the layer interfaces on the evolution of the stress distribution in a fracture-bound block and on the fracture infilling process are discussed. A three-layer model is developed based on a cohesive zone model (CZM) for layer interfaces and a plastic-damaged model for rock layers. The results indicate that the interface shear strength can greatly influence the stress distribution in a fracture-bound block so that the ratio of the fracture spacing to the layer thickness (S/T-f) is not the dominant factor in the stress state. If interface debonding occurs, the tensile stress in a fracture-bound block can decrease to zero even for a higher S/T-f ratio. If the interface shear strength is high enough, a new fracture can further infill between two adjacent preexisting fractures even if the S/T-f ratio is at a low level. The fracture process implies that there is a critical value for the interfacial shear strength that controls the fracture pattern between the central layer and its neighboring layers to be either interface debonding or layer-parallel fracture. Both interfacial debonding and layer-parallel fracture can separate the central layer from its neighboring layers and thus lead to fracture saturation. Further analysis indicates that the critical shear strength increases linearly with increasing tensile strength of the central layer.