Stability of keyblocks under self-weight and surface forces

Stability of keyblocks subject to self-weight and surface forces is examined using linear programming methods. The results are applied to determination of the maximum size of unstable keyblocks in tunnels. We find 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 many cases this upper limit can be much less than the maximum keyblock region predicted by block theory.