In situ geomechanics of crystalline and sedimentary rocks; Part IV, continued field testing of the modified U.S. Geological Survey 3-D borehole stress probe

Two modified and calibrated U.S. Geological Survey 3-D borehole probes were successfully tested in the field at a site on South Table Mountain, near Golden, Colo. The probes were installed in separate core holes at depths of 84 and 99 cm in the latite cap rock and subsequently stress relieved with overcoring techniques. The determined stresses from both probes are very low and contain both tensile and compressive components. Magnitudes range from 1196 KPa in tension to 832 KPa in compression. The principal stress orientations are in fair agreement whereas the horizontal secondary principal stress directions are in good agreement; the maximum horizontal compressive stress is oriented N. 76? W.-S. 76? E. for one probe and N. 63? W.-S. 63? E. for the second probe. The greatest determined Young's modulus of the rock is in the N. 89? E. direction, only 15? from the maximum horizontal compressive stress direction.