Hydraulic fracturing technique is a measurement method of in-situ rock stress. This technique has the highest track record for techniques used in sub 1000 mater. This technique is comparatively simple and easy for in-situ stress measurement, because a pre-existing drilled borehole is available. The predominant advantage of this method is that elastic constants such as Young's modulus and Poisson's' ratio are not required for determining the value and direction of initial stress. The measuring parameters to determine the stress state are the reopening and the shut-in pressures by water injection and the fracture directions. However, with conventional methods, the maximum stress cannot be obtained accurately affected by low stiffness of the pressurization system. We have developed a new hydraulic fracturing device based on a high stiffness pressurization system and applied to a 300 mater deep borehole in granite. The most important improvement of the new system is an employment of a syringe pump which enables an accurate flow control of fluid with less than 1 ml/min up to 30MPa.
Knowledge of in-situ rock stress is fundamental to designing stable underground caverns and to understanding the states of crustal stress. The in-situ stress is obtained only by measuring directly in the fields. Various measuring methods like the stress relief method, the hydraulic fracturing method, and the core method, etc. have been proposed so far as the measuring method of an initial rock stress. Each stress measurement method has advantages and disadvantages of its reliability. In application to the actual site, it is necessary to select a more appropriate method for the measuring conditions and purposes. In the case of deep places from the surface, it is considered that the hydraulic fracturing method is the best way to measure the rock stress.