The state of stress in a large dyke on E.R.P.M., Boksburg, South Africa

Abstract

Strain relief measurements were made at a depth of 3 km in several boreholes drilled into a large, quartz dolerite dyke on the East Rand Proprietary Mines Ltd., Boksburg, South Africa. The dyke was known to have a history of rockburst activity. The stress tensor calculated from the strain relief measurements had a near vertical maximum principal stress of 340 MPa, and near horizontal intermediate (215 MPa) and minimum (177 MPa) principal stresses striking at angles of 50° and 140° to the plane of the dyke. Because the measurements were made in the vicinity of an extensive excavation, the calculated tensor was compared with a theoretical solution computed for the specific mining geometry. It appears that there is good agreement in the orientations of the principal stresses and in the magnitude of the maximum principal stress but the measured, near-horizontal, intermediate and manimum principal stresses are significantly larger that their theoretical counterparts. This difference probably represents the differences between the pre-mining ambient stress states within the dyke and the adjacent country rock. Subtraction of the mining-induced stresses from the measured stress tensor suggests that the pre-mining horizontal stresses within the dyke were large and were approximately equal to the vertical stress due to the weight of the overburden. These high horizontal stresses may be due to the presence of residual tectonic and thermal stresses stored within the dyke as it cooled to the ambient temperature of the surrounding rock.