Improvement of the Conical-ended Borehole Technique for Rock Stress Measurement with a High Accuracy.

Abstract

Knowledge of the rock stress is one of keys to design and construct rock structures. Thein-situstresses, therefore, have to be accurately measured. Since rock stress measurement is generally time consuming task, the new technique development is being expected to determine the complete stress state from a measurement in a single borehole. Thus, the stress relief method should be improved to meet its demand.Many stress relief methods to determine the complete state of stress have been proposed and developed. Among these methods, the CSIRO hollow inclusion stress cell has been most popular in the world, since it enables us to accurately measurein-situstresses. The eight-elements strain gauge, the hemispherical shaped strain cell and the conical shaped strain cell have been used in Japan. In these three methods, the conical-ended borehole technique with sixteen-elements conical shaped strain cell has an advantage to reduce the time, effort and cost for a series of rock stress measurements, combining the compact overcoring, the diameter of which equals to that of the pilot borehole.In the present study, the conical shaped strain cell has been modified and refined to improve the accuracy in rock stress measurement, increasing the number of strain measured on the conical bottom surface of the borehole with a cross typed strain gauge. This paper, firstly, shows how the stress tensor can be determined from the strains on a conical bottom surface of a single borehole. Then the accuracy of the proposed method was compared with that of existing method to confirm that the proposed method gives us more accurate results. Secondly, based on an in-siturock stress measurement, this paper indicates that the proposed method can be more effectively used forin-siturock stress measurements than the existing one. Also, process simulation of the strains and stresses on the conical bottom surface during the stress relieving gave well consistent results with measured data, which indicates that the proposed method is quite reliable forin-siturock stress measurements.