Development of Stress Measuring System by Overcoring Method Suitable for Soft Rocks

Abstract

Most in-situ data on rock stresses are from hard or moderately hard rock. It is necessary to develop a stress measurement method for soft rock, toward obtaining the in-situ stress state in such rock. We developed a stress meter that measures seven components of diametrical deformation and axial deformation in a pilot borehole during overcoring and records the measurements on a small data logger installed within it. From these deformation data, three-dimensional stress states in rock are determined using the observation equation derived from the analytical elastic solution. The stress meter is inserted into a 40-mm pilot borehole that can range in water content from dry to completely wet. The advantages of this stress meter are its smallness, ease of use, reusability, ability to measure at any borehole depth, and efficiency, since very little time is required for insertion and withdrawal and no time is needed to shift from insertion to overcoring. Moreover, elastic properties of the rock can be measured using the core in which the stress meter sits. Although the system is particularly suited to soft rock, it is applicable to any type of rock since the stress meter is highly sensitive.Laboratory experiments simulated the functioning of the stress measuring system by using three welded tuff blocks (40 × 40 × 40 cm) with boreholes 40 mm in diameter at different orientations. The stress meter was inserted into the borehole and two cycles of uniaxial loading and unloading were carried out to simulate the stress relief. At same time, elastic properties of the rock were measured using the above-mentioned method. The calculated stresses were found to correlate closely with the applied stresses in all three cases, indicating that the stress meter functions well.