The gradual closure of the stope of a longwall gold reef exploitation is described as a process of interaction between at least two, but possibly three cycles of fracture mechanisms. These mechanisms are repeated with a certain periodicity. Each mechanism appears to have its own time scale and the result of one mechanism may cause the triggering of another. An analysis of the diverse fracture systems is given, based on literature; a.o. Adams et al. (1981), Roberts and Brummer (1988) and Kirsten and Stacey (1989). A crucial point is the Principal Law of primary brittle fracturing; Gramberg (1988). Some basic elements of fracture- and fracture-plane- analysis are discussed. The different types of fracture and failure phenomena around a stope are analyzed. This leads to the recognition of the following five elements:
The primary effect of brittle fracturing consisting of parallel multifracture systems at uni- or quasi uniaxial loading, accompanied by "inelastic" transverse dilatation.
The secondary effect during and after failure by the generation of shear zones at high triaxial loading and "softening" or weakening of the rock-mass.
A third element is the effect of sliding across planar discontinuities, e.g. mining induced extension fractures or bedding planes, resulting in the generation of additional shear stresses and second order fracturing.
The fourth element is formed by the S-shaped deformation of the roof layers of the excavation, as a result of the longwall mining.
The reported elevated horizontal stress, which forms a fifth element, is ascribed to the continuous propagation of this S-shaped deformation.
Five different stress regimes are recognized and three different cycles on mini (Stope-), macro and mega scale could be established. Interaction between the three cycles is related to the propagation of the face. The analysis will be explained by a series of drawings and diagrams.
The main purpose of fracture mechanics is to establish the relationship between the shape of the fractures and the loading in terms of stress theory, i.e., if the shape and the nature of the fracture is known, the loading and the stresses at the moment of fracturing can be reconstructed for the particular condition by means of our "fracture-and-fracture-plane-analysis". In the past the processes of fracturing were considered as chaotic. Today, however, we know that, as a rule, these Processes develop very systematically. The mode of fracturing and failure forms a reliable indicator for the state of stress at the time of fracture development. Fracturing is a dynamic process. Every fracture, be it micro or macro, has a beginning, an extension and an end. Time and periodicity therefore play a role. The latest computer models enable us to follow the dynamic changes of the rock-masses during cataclastic processes in the hanging wall. Such models, however, are not always satisfactory, especially not when fracturing plays a major role. In order to integrate the influence of fracturing, accurate observations are required in the first place.
