1.Introduction Rock masses are initially stressed in their natural state and understanding of this state of stress is essential for design calculations and prediction of rock mass behavior with excavations 1) . Over the past 30 years, many endeavors have been made to find a reliable technique for measuring in situ stresses 2) . Although there are many kinds of in-situ stress measuring methods which can be carried out routinely, only hydraulic fracturing and sleeve fracturing appear most

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... for deep stress measurements because of their simple maneuverability 3) . These two techniques, however, have some serious difficulties in finding real re-opening pressure. The followings are the reasons for that in hydraulic fracturing: • We do not know actual fluid pressure distribution in the fracture • Apparent re-opening pressure is affected by the stiffness of the hydraulic system and fluid injection rate. • The fracture is already opened before pressurization in case that the ratio of the maximum principal stress and the minimum principal stress is greater than 3, because the direction of the fracture produced is always perpendicular to the direction of the minimum principal stress direction. On the other hand, the reason for that in sleeve fracturing is that fracture re-opening pressure is to be found from pressurediametral deformation curve, and diametral deformation is insensitive to detect the moment of fracture re-opening 4, 5) . In this paper, a newly developed prototype probe for stress determination by borehole jack fracturing is introduced to solve the above problems 6) . The new borehole jack single-fracture probe consists of eight borehole jacks for loading instead of fluid pressure in hydraulic fracturing or sleeve fracturing techniques. Borehole jacks are adopted here considering their efficiency, sturdiness, and compactness 7) . Two new specific tangential strain sensors (TSS) are installed in the middle of the probe to detect the opening displacement of the fracture on the borehole surface directly. The final aim of this research is to demonstrate potentiality of the borehole jack single-fracture probe newly developed for precise stress determination. Although the stress distribution and a stress concentration factor at a point of fracture formation on the borehole wall surface are essential, it is difficult to find them theoretically due to complexity of the loading mechanism. Thus numerical and experimental tests of the probe by loading a steel pipe are carried out. In this paper, by comparing the numerical results with the experimental ones, the stress distribution and the stress concentration factor are clarified. Compressive stresses are expressed negative in this paper. 2.Borehole jack single-fracture method 2・1 Probe for the borehole jack single-fracture method The borehole jack single-fracture probe mainly consists of eight loading jacks, upper and lower load platens, two pairs of friction shells, and two tangential strain sensors. The eight borehole jack cylinders are grouped into upper section and lower section corresponding to the respective loading platens and friction shells, and thus each section consists of four borehole jack cylinders. Fig.1 shows the photo of the borehole jack probe used for the borehole whose diameter is 98 mm. The probe is 676 mm in length and 96.6 mm in diameter. The cross section view of the probe is shown in Fig.2 . Semi-cylindrical load platens of the probe are covered by two pairs of half-pipe shaped friction shells. Before loading as A prototype probe for in-situ stress determination by borehole jack fracturing has been newly developed. The authors carried out a loading test in a steel pipe to study stress distribution on borehole surface induced by loading with the probe. Strain gages were glued on the external cylindrical surface of the steel pipe to know the actual stress distribution induced by probe loading. In order to obtain the stress distribution along the internal surface, a numerical simulation of the loading test was also carried out. Since the numerical simulation results agreed with the results measured with the strain gauges, reliability of the simulation was elucidated. Thus the stress distribution on borehole surface and a stress concentration factor at a point of fracture formation were obtained by this research. Through laboratory and field tests from now on, reliability and performance of the stress measurement using this probe will be checked.