We have conducted about 850 hydraulic fracturing in-situ stress measurements at 170 test boreholes in the various parts of Korea for the last 10 years. All of the tests were performed using engine driven type wire line system. And almost of all the fracture tracing works were carried out using borehole scanning device to obtain more accurate and visualized information on induced fractures rather than conventional oriented impression packer method. In this paper, the overall characteristics of initial rock stress state in Korea were studied. For the depth less than 400 m, the stress ratio (K) had a tendency to decrease and stabilize with the depth, however, for some areas, the excessive horizontal stresses with the stress ratio greater than 3.0 were observed. Highly predominant orientation of the maximum horizontal stress components was not appeared but azimuths in the range of 70°~90° (N70°E~EW) were superior in frequency numbers. Additionally, from the database of initial rock stress and rock strength, the regions of excessive horizontal stress were mapped, where it has potential possibility of stress induced brittle failure around an excavated opening. The results from the investigation of excessive horizontal stress showed that there exist several regions where the localized excessive horizontal stress was big enough to potentially induce brittle failures around openings at less than 400 m in depth.
The rock stress measurement and its applications have been regarded as one of the key branches in the geo-science fields including rock mechanics, structural geology, civil engineering, etc. And it is well known that it is nearly impossible to estimate the exact state of initial rock stress of interest site by the theoretical and physical approaches except some specific geological situations. Being different from a surface structure, the stability of an underground structure excavated in rock mass is largely influenced by many factors such as geological condition, mechanical properties of in-situ rock mass, initial rock stress state and geometry of an excavated opening. Among these factors, the characteristics of in-situ rock stress have potent influence on the failure and deformation behavior around openings. Since early in the 90s, as the axis of land development in Korea has been turned toward East-West, in which high and rugged mountains locate, from North-South direction, the need for underground rock structures in great depth has been rapidly increased. Therefore, for the optimum design and construction of rock structure, it has been highly required to obtain reliable engineering information on the in-situ rock stress state through detailed field investigation Since the 30s, various testing methods for rock stress evaluation had been proposed and used in the field and the laboratory. These methods are divided into six major groups by key theories, physical properties for stress calculation and testing procedures etc.: hydraulic methods, relief methods, jacking methods, strain recovery methods, borehole breakout and others. Among them, having some merits in the feasibility of testing and the reliability of results than other methods, overcoring and hydraulic fracturing method have been popularly used.
