Variation of rock stresses with depth at a Canadian iron mine

Abstract

The three-dimensional stress field was determined at seven locations in an iron mine in the Lake Superior region. The results show that the minimum principal compressive stress is directed close to vertical in all cases and that the horizontal stress field can be described in general as isotropic with local variations. It appears that the stiffer members in the rock mass are subjected to higher stresses, whereas the elastic strain recovery is the same. The vertical stress gradient can be attributed to the increase of the overburden weight, whereas the gradient of the average horizontal stress component is 1·85 psi/ft depth (0·426 kg/cm 2/m) with an extrapolation value of about 910 psi (63 kg/cm 2) at the surface. A comparison of many of the data previously published on stress measurements shows that the here obtained vertical and horizontal stresses are not unique to the test area but can be found in many parts of the earth's crust. From the published dat, the following regression equation was obtained for the vertical stress components with depth: σ v = (272 ± 178) psi + (1·154 ± 0·123) psi/ft = (19 ± 12·6) kg/cm 2 + (0·266 ± 0·028) kg/cm 2/m. 75 per cent of the availbale data on average horizontal components yielded the following equation for the average horizontal component with depth: σ H = (1184 ± 78) psi + (1·842 ± 0·101) psi/ft = (83 ± 5) kg/cm 2 + (0·407 ± 0·023) kg/cm 2m. This relationship is proven to a depth of 3000 ft and the increase of the horizontal stress with depth can be attributed to a failure condition reached by these rocks, which is probably caused by unloading due to erosion. The failure condition is not fully understood, however the found relationship should be considered when making load assumptions for underground or surface excavations.