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# INTRODUCTION

It is of basic importance to estimate initial stress states in rock mass in order to design and construct high-performance rock structures. Among several methods developed for the purpose, the stress-relief method by overcoring is the most reliable and most widely applied in field measurements. Several years ago, one of the authors proposed a method to measure strains on a conical-end surface of a borehole induced by overcoring (conical-end borehole strain measurement method, CESM)[1].

In the present paper, authors propose an improved version of the method (ImCESM), in which a small-diameter inspection borehole is drilled in front of the conical-end (thus the end-face is funnel-like) and overcoring is made with the same diameter as that of the conical-end borehole, and they carry out computational simulations of the overcoring process by use of the fast multipole boundary element method (FMBEM). Of course, the measured strains must be converted to stresses using a strain-to-stress conversion matrix obtained beforehand. For computing the matrix the boundary element method (BEM) is most advantageously applied, since we make use of strains data measured only on the surface of the conical-end induced by overcoring in practical applications. Specifically in the ImCESM the shape of the over-cored borehole-end is rather complicated, a precise shape modeling is required, that is, the surface of the model is densely divided in BEM. In such a case, we can advantageously apply the FMBEM.

In what follows, we first describe the principle of the stress relief method, that is, how to estimate stresses using the measured strains induced by overcoring, and then the application of the FMBEM to elastostatic problems, and finally we show some simulation results of induced strains during overcoring in the ImCESM.

Next: PRINCIPLE OF INITIAL STRESS Up: 無題 Previous: 無題
Toru Takahashi 平成11年10月13日