The role of rock joint frictional strength in the containment of fracture propagation

The fracturing phenomenon within the reservoir environment is a complex process that is controlled by several factors and may occur either naturally or by artificial drivers. Even when deliberately induced, the fracturing behaviour is greatly influenced by the subsurface architecture and existing features. The presence of discontinuities such as joints, artificial and naturally occurring faults and interfaces between rock layers and microfractures plays an important role in the fracturing process and has been known to significantly alter the course of fracture growth. In this paper, an important property (joint friction) that governs the shear behaviour of discontinuities is considered. The applied numerical procedure entails the implementation of the discrete element method to enable a more dynamic monitoring of the fracturing process, where the joint frictional property is considered in isolation. Whereas fracture propagation is constrained by joints of low frictional resistance, in non-frictional joints, the unrestricted sliding of the joint plane increases the tendency for reinitiation and proliferation of fractures at other locations. The ability of a frictional joint to suppress fracture growth decreases as the frictional resistance increases; however, this phenomenon exacerbates the influence of other factors including in situ stresses and overburden conditions. The effect of the joint frictional property is not limited to the strength of rock formations; it also impacts on fracturing processes, which could be particularly evident in jointed rock masses or formations with prominent faults and/or discontinuities

Initiation mechanism of extension strain of rock mine slopes

Slopes in open pits exhibit fracturing around excavations, often initiated by extension strain which results from a combination of principal stresses adjacent to the slope boundaries. This extension strain is commonly described using minimum principal strain or minimum principal stress equations. These equations show that the extension strain can expand and fracturing occurs if the extension strain exceeds a critical value. Anisotropic rock masses with multiple and complex structures increase the potential of the development and coalesce of cracks with pre-existing discontinuities for further potential failure. This paper presents finite element analysis to model the extensions strain implementing the criterion of Stacey (1981). The distributions of extension strain are predicted around slope of Handlebar Hill open pit mine at Mt Isa, Queensland, Australia. Around the pit wall, fracturing near the excavation boundary is often the result of extension strain of the rock. Through the mining activities, fractures in the slope face can manifest into slabbing and spalling. Extension strain may develop circumferential fractures close to the slope surface, the closer to the excavation perimeter, the more open the cracks. The result of the extension strain distribution simulated in this paper is in accord with failure events observed on site. The numerical modelling and the discussion of this study focused on the prediction of potential fracturing zones within the critical values of the extension strain around the slope. © Springer Nature Switzerland AG 2020