| abstract |
Rock stress in a borehole wall is determined in situ by obtaining an initial strain reading of the wall, relieving the strain by fracturing the wall from within the borehole with a hydraulic jack, obtaining a releived strain reading, and using the two strain readings to determine stress. The jack comprises a pair of diametrically opposed loading members for applying a self-equilibrating pair of forces to opposite quadrants of the wall, and a pair of diametrically opposed piston members for impressing a set of friction strain gauges against the remaining quadrants. The piston members are pivotally mounted to the jack to allow the strain gauges to conform to the shape of the wall. The jack is first located at the preselected region of the borehole and the loading members are separated until they contact the wall to stabilize the jack in the borehole. The pistons are operated to move the strain gauges into firm abutment with the wall to obtain the initial strain reading. The loading members are then further separated to fracture the wall for strain relief, and the relieved strain reading is obtained. Finally, the loading members and pistons are withdrawn from the wall for relocating the jack to another region of the borehole. The initial and relieved strain readings are used to determine the initial state of stress of the rock and the strain of the rock measured as a function of the loading of the borehole wall is used to determine the value of Young's Modulus of the rock for an assumed value of Poisson's Ratio. |