Virgin Rock Stresses at Doornfontein Gold Mine, Carletonville, South Africa

The states of virgin stress at depths of 1,320 m and 2,320 m below surface were determined using a stress-relief technique. Doorstopper cells were used to take a large number of strain-relief measurements in at least three boreholes at each site, and these measurements were then related to the state of stress acting in the rocks. The magnitudes and orientations of the principal strain reliefs tended to become constant away from the mine excavation, but there were often abrupt changes in strain relief near shear zones. The magnitudes of the vertical stresses were in good agreement with the theoretical overburden stresses, and at both sites the ratio of horizontal to vertical stresses was approximately equal to 0.5; that is, the state of stress deviated markedly from a lithostatic state. Moreover, there were significant angular differences between the orientations of the principal stresses at each site. These different orientations are discussed in terms of the superposition of several stress systems, namely, gravitational stresses due to the existing overburden and stresses due to residual strains induced by early gravitational and tectonic forces. A comparison of these different stress systems is carried out by plotting the principal stress directions on a stereographic net. The maximum, intermediate, and minimum stresses all plot in well-defined areas of the net, and the measured virgin principal stresses also fall within these areas.