ABSTRACT:
A new method is suggested herein to measure the virgin earth stresses by means of a borehole. The destressing of the borehole is achieved by means of inducing thermal tensile stresses at the borehole periphery by using a cryogenic fluid such as Liquid Nitrogen (LN2). The borehole wall eventually develops fractures when the induced thermal stresses exceed the existing compressive stresses at the borehole periphery in addition to the tensile strength of the rock. The above concept is theoretically analyzed for its potential applicability to interpret insitu stress levels from the tensile fracture stresses and the corresponding borehole wall temperatures. Coupled thermo-mechanical numerical simulations are also conducted using FLAC3D, with thermal option, to check the validity of the proposed technique. From the preliminary theoretical and numerical analysis, the method suggested for the measurement of insitu stresses appears to be capable of accurate estimation of the virgin stresses by monitoring tensile crack formation at a borehole wall and recording the wall temperatures at the time of crack initiation.
1 INTRODUCTION
Knowledge of virgin earth stresses is now-a-days considered to be an essential pre-requisite for construction of important underground and open-pit excavations such as mines, tunnels, caverns etc. Orientation and magnitude of earth stresses are vital input parameters in numerical modelling studies for stability analysis and design and are also important in studying tectonic stresses for the prediction of earth quakes and seismic events. In situ stress measurement techniques currently in use include borehole fracturing methods such as hydro- fracturing (Fairhurst 1964), sleeve fracturing (Stephanson 1983) and borehole jacks (Mayer et al. 1951); stress relieving techniques such as overcoring (Leeman & Hayes 1966, Merrill 1967) and borehole slotting (Bock & Foruria 1983); borehole breakout measurements (Zoback et al. 1986); indirect methods such as acoustic emission measurements (Kaiser 1950), etc.
