Friction Gages for In-Situ Rock-Mass Deformability and Stress Measurements

The in-situ deformability of rocks and the state-of-stress must be known for the analysis and design of surface and underground structures. This paper presents a method for determining the in-situ deformability of rock-masses using friction gages. Friction gages utilize the friction between the gage and the rock surface for detecting the strain changes. The method involves impressing friction gages on two opposite quadrants of the borehole wall, then radially loading the other two quadrants over 45-deg contact with a self-equilibrating pair of forces of sufficient magnitude to initiate and propagate tensile fractures. While loading, the friction gages detect the tangential strains on the borehole wall before, during and after fracturing of the rock. From the linear portion of the tangential stress-strain data, the elastic properties of the rock can be determined using the appropriate relationships. Furthermore, from the failure and post-failure portions of the tangential stress-strain data the tensile strength of the rock-mass and the in-situ state-of-stress can also be estimated. The theoretical basis of the method, and the fabrication, calibration and testing of the friction gage system, are described. Furthermore, practical field applications of the method are given.