This invention relates to a method for the determination of a complete state of tectonic stresses.
Stress state of underground rock provides basic data to be used for earthquake prediction, design of tunnel construction, etc. Stress-relief techniques and hydraulic fracturing technique are now generally adopted for the determination of in situ stresses.
The hydraulic-fracturing method includes measuring the breakdown pressure and the direction of the fracture created by hydrostatic pressurization in a borehole at a deep point. This method, however, cannot give three-dimensional stresses in one borehole. Further, because of the micro-discontinuities on the borehole wall, reliable data cannot be obtained unless the measurement is conducted repeatedly.
One known stress-relief method includes measuring changes in diameter of a borehole during overcoring to calculate the stresses in the plane perpendicular to the axis of the borehole. With this method, however, it is not possible to determine three-dimensional stresses in one measurement.
Another well known stress-relief method includes measuring the strains which occur in the wall surfaces of a borehole when the stresses are relieved by overcoring. Strain gauges are used to measure the strains. This method, however, cannot give reliable data because of the heterogeneity of the wall surfaces of the borehole. That is, since the underground rock is composed of mineral particles with different sizes and different physical properties and has pre-existing microcracks, measured strains vary according to the positions of the wall surfaces to which the strain gauges are attached. Therefore, reliable data cannot be obtained unless the measurement is repeated a number of times or carried out using sufficiently long strain gauges. However, formation of a borehole and an overcore requires much time and money. This is especially so in the case of a large diameter borehole. Accordingly, from the standpoint of economy, it is practically impossible to conduct a number of measurements or use large sized strain gauges to the extent that reliable data are obtainable.
There is also known a method, generally called differential crack strain analysis, for the determination of rock stresses. By coring operation, the rock is released from the in situ stresses so that microcracks open up as stress relief cracks. The differential crack strain analysis measures the crack strains from which in situ stresses are calculated. More particularly, a cubic sample is cut from the core and three strain gauges are mounted on three perpendicular faces, respectively. Then, the sample is jacketed and hydrostatically pressurized while measuring the strains to obtain a strain-pressure curve. By projecting the asymptotic slope of the strain-pressure curve back to zero pressure, there is obtainable the crack strain. Since this method measures the surface strains, rather than the actual deformation, caused by the hydrostatic pressure, the rock stresses determined by this method lack reliability.