ABSTRACT:

A method for measurement of stress change is developed to monitor rock stress using a borehole. Two dimensional state of stress change within rock mass in a plane perpendicular to a borehole axis can be measured by this method, which is named the Cross-sectional Borehole Deformation Method (CBDM). In this paper, the theory of the CBDM is described, as well as the prototype instrument with the laser displacement sensor. Analyzing influence factors on measurement result theoretically, it makes clear that stress change within rock mass can be estimated by the CBDM.

1 INTRODUCTION

Knowledge of rock stress is one of fundamental importance for designing and constructing rock structure, such as underground openings, since the mechanical behavior of rock mass around it is affected by initial stress. Furthermore, the induced stress measurement is performed to estimate the stability of a rock structure under construction and confirm the design of it. In order to measure initial stress, many methods have been suggested. On the other hand, there are a few methods for stress change around an opening under construction. For example, the stress change of an underground power house has been measured by a vibrating wire strain gauge in Japan (Kudo et al. 1998). However, using this gauge, only stress in one direction in a plane perpendicular to a borehole axis can be measured. Furthermore, this gauge has a rigidity which affects on measured results. In this paper, a method for measurement of stress change is developed to monitor rock stress using a borehole. Two dimensional state of stress change within rock mass in a plane perpendicular to a borehole axis can be measured by this method. This method was named the Cross-sectional Borehole Deformation Method (CBDM) by Tanguchi et al. (2003) and Obara et al. (2004).

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