Jack fracturing technique of stress measurement

Abstract

Jack Fracturing Technique of Stress Measurement This paper suggests a new approach for determining the initial state of stress in rock masses by strain relief measurements without overcoring. The method involves impressing friction-strain gauges on two opposite quadrants of a borehole wall, then uniaxially loading the other two quadrants with a self-equilibrating pair of forces of sufficient magnitude for initiating and propagating fractures of depth adequate to relieve strain. The initial tangential strains on the borehole wall are relieved by the creation of an open axial fracture and measured by the impressed friction gauges. Strain relief by borehole jack-fracturing was studied with finiteelement analysis and also by simple consideration of boundary conditions and external tractions. The dependence of the initial state of stress on the strain-relief measurements at different orientations is obtained by employing the classical theory of elasticity. The instrumentation unique to the method, a borehole-loading device and a friction-strain-gauge module, were tested in the laboratory, and the results summarized. Field-stress measurements made in a sandstone quarry for the magnitude and orientation of the stresses are in good agreement with those obtained by an independent overcoming technique. Work is progressing to compare results with a number of controlled cases.