The stability of slopes and underground excavations is generally affected by the geological structure of the rock mass, and it is important that this factor is adequately assessed. Whether instability will arise from the presence of the structures depends on the tectonic events that resulted in the formation of the structure, the orientation, characteristics (width, infill, strength, ability to dissipate strain energy, etc.), and the effect of mining-induced stress on the structure, which either enhances or reduces stability. Three case studies are presented where the influence of geological structures resulted in major instability in Southern African and Canadian underground mines. One case resulted in changes to the mining layout and support system, one resulted in a support system change, and the third resulted in the collapse of the mine, necessitating redevelopment and sterilization of ore.
A platinum mine in Southern Africa and a diamond mine in North America experienced similar instability associated with a weak (shear) layer in either the pillar foundations or within the pillars themselves. In the one instance, the instability initially manifested as footwall heave, unravelling of the pillar walls, and eventually in the form of a pillar run that resulted in a collapse of the underground workings. In the second instance, the initial instability comprised slabbing of the pillar walls below the weak layer, and footwall heave was observed in another less-frequently accessed roadway. Unfortunately, the long-term effect of this failure mode could not be assessed as the mine has since closed due to economic reasons.
Mechanized mining was taking place on a room and pillar layout, with the only footwall development comprising a conveyor belt decline shaft which was used to convey ore from underground to surface storage bins. The mining height was approximately 2.25 m and pillar dimensions varied according to ground class (typically classed according to the NGI Q-system). Stable pillars, designed to support the load of the overburden, were intended to provide both local and regional stability of the underground workings. Initial instability manifested as footwall heave (Figure 1) adjacent to several pillars in the shallower portion of the mine and unravelling of blocks along joints from the pillar walls (Figure 2).