Modified borehole jack method for elastic property determination in rocks

Abstract

Modified Borehole Jack Method for Elastic Property Determination in Rocks This paper describes a method of determining thein-situ elastic properties of rock masses with friction-strain gauges and a lengthened borehole jack. The method involves impressing tangentially-oriented friction gauges on opposite quadrants of a borehole wall, while the remaining quadrants are loaded by the borehole jack with a unidirectional self-equilibrating pair of forces. The tangential normal strains generated at the borehole wall are thus measured concurrently with the loading. To determine the modulus of the rock mass, equations were developed relating the measured tangential normal strains and the applied ram pressures. Several factors which may influence the interpretation of data were thoroughly analyzed. It was shown that variations in the pressure distribution and area of plate-wall contact have negligible effects on the accuracy of data interpretation. End effects were minimized by lengthening the jack and carrying out strain measurements at the central region only where stresses and strains are homogeneous and plane strain conditions exist. The new method has the same desirable mechanical and operational features as the well-known Goodman-Jack method. Most important, however, its theoretical basis is exact with no estimated coefficients required in the reduction of observed data. While the method requires an ideal elastic material, it has a number of other applications for in-situ measurements besides modulus determination. The modified method could be utilized for determining linearity, anisotropy, inhomogeneity, tensile strength and, with an additional operation, the in-situ stress field.