Galleries under severe stress conditions will show different types of failure mechanisms frequently resulting in a gradual deterioration of the rock ring around the gallery. Stress concentrations cause primary fractures developing into secondary types of fracturing, associated with a softening behaviour and volume increase of the broken rock mass. The result is a serious convergence of the gallery. Basic research is carried out to develop the principle of destressing a rock ring in order to prevent the development of this mechanism. The aim is to create a stable cataclastic equilibrium in the rock ring around the gallery and the surrounding rock mass. Results are shown of several model tests, scale 1:100 and 1:10. These model tests show that in the models a considerable decrease of convergence occurs when a circular zone with a certain width around the gallery is destressed artificially. Elasto-plastic calculations are carried out in order to analyze this technique of destressing. These calculations show the influence of the softening behaviour, the residual strength and the volumetric behaviour of the broken rock mass. An example is given of a design calculation for a gallery with assumed elasto-plastic behaviour of the coal. The results show that destressing in this situation, with these material properties and boundary conditions, could be an effective method for the control of severe convergence of a gallery. After the calculation, conclusions can be drawn for the design of the dimensions of the destressing slots. The potential of destressing seems to be limited by the required slot width. Some field experiments have been carried out with rows of boreholes, but the results were not uniform and difficult to interpret. When large scale destressing is considered, the material behaviour and the boundary conditions have to be studied carefully.
Mine galleries under severe stress conditions show different failure mechanisms frequently resulting in a gradual deterioration of the gallery. Stress concentrations at the periphery of the excavation cause primary fractures, often developing into secondary types of fracturing. This causes intensive structural changes, associated with softening behaviour, resulting in a much lower residual strength of the rock mass. This softening behaviour as a rule is accompanied by a localisation of large strains. The softening also causes an increase of volume of the broken rock mass. The final result is a serious convergence of the gallery walls.
Basic research is carried out in order to develop the method of destressing to limit the development of these failure mechanisms. The intention of destressing is to keep the stresses in the rock mass low enough to remain within the limits of cataclastic stability and to prevent the softening of the rock and the associated volume increase. The aim is to prevent local stress concentrations and secondary types of fracturing in order to create a stable equilibrium in a rock ring around the gallery; this ring must be able to support the surrounding rock mass; the so-called self supporting rock ring.
