Keyblock stability under self-weight and surface forces

This thesis presets a method to define the unstable regions in tunnels in hard, jointed rock. The method takes account of both the block self-weight and the forces on the block from in situ stresses. This problem is statically indeterminate in general, but bounds are found on the solution using the linear programming method. It is found that there is usually an upper limit to the size of unstable keyblocks, based on in-situ stresses, excavation geometry, fracture orientations and shear strength. In most cases, the unstable regions are much smaller than the maximum keyblock regions predicted by block theory. The results are used to determine the maximum size of unstable keyblocks in tunnels.