ABSTRACT

It is important to understand rock stress state in civil, mining, petroleum, earthquake engineering and energy development, as well as in geophysics and geology. In general, however, estimating the stress spatial distribution in rock is very difficult. The differences between the estimated and the measured stresses can be first attributed to tectonic stresses, as well as other factors such as topography and inhomogeneity of the rock mass. In this research, a study combining strategy and tactics for the determination of in situ rock stress was undertaken to resolve difficulties and improve accuracy of stress estimation. Numerical and experimental tests of a new borehole jack fracturing probe by loading a steel pipe were carried out. On the other hand, an improved program was developed available for inhomogeneous modeling and its applicability was examined by comparing the numerical results with the corresponding strict solution. A non-linear numerical inverse method is presented for evaluating the in situ state of stress in a rock mass.

1 INTRODUCTION

Recently with the planning of underground research repository of high-level radioactive wastes, stress determination for a large volume of rock mass with width of several kilometers and depth of more than one kilometer is necessary (Matsuki et al. 2004). Many endeavors have been made to arrive at a reliable means of measuring rock stress in situ and various techniques have been developed and improved. The state of in situ stresses around boreholes can be measured by borehole pressurization methods, which consist of the hydraulic fracturing method, the double fracturing method (Sleeve fracturing method), the single fracturing method, and plate fracturing method, etc. Hydraulic fracturing method has been widely used for stress measurement deep underground in various rock conditions (Amadei et al. 1997). However, there are uncertainties associated with the interpretation of the resulting data and the fracture cannot be generated arbitrarily at intended direction. In particular, confidence in the calculated maximum principal stress is less than in the minimum principal stress, although the former is often of great importance. Furthermore, the equation established to calculate the maximum principal stress is unclear (Ishida et al. 2003). Double fracturing method has problems such that the second fracture sometimes cannot be generated notably, that the second fracture is not guaranteed to be perpendicular to the first fracture if the tensile strength of the borehole wall is large enough, and that accurate measurement of the reopening pressure of the fracture is theoretically impossible since the curve of pressure and variation of diameter is smooth. For dry single fracturing method, the reopening pressured observed is normally larger than the real reopening pressure of the fracture, and the compression resistance of the oil system for fracturing has to be high enough for practical deep underground application.

On the other hand, for certain geometries, the effect of topography on determination of state of stress can be analyzed by accurate analytical solutions and factors affecting the magnitudes and orientations of in situ stress can be studied.

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