ABSTRACT:

When constructing underground openings at great depths, particular problems resulting from the squeezing characteristics of rock masses are often encountered. Large plastic deformation is one of the most critical problems and it must be considered in design practices. In order to stabilize rock masses, support structures such as steel ribs and rock bolts are commonly installed. However, no reliable design method is presently available for dimensioning these support structures, particularly rock bolts. This may be mainly due to the fact that the function mechanism of the bolts has not yet been fully investigated. In this paper, we shall discuss the effect of rock bolts based on field measurement and laboratory experiment data, and propose an estimation method which is capable of evaluating the effect of rock bolts. 1.

INTRODUCTION

When constructing underground openings at great depths, engineers are confronted with particular problems resulting from the squeezing characteristics of rock masses. In the past, the squeezing phenomenon has caused severe damage to several tunnels in Japan, and the repair work has involved a large consumption of money and time. Squeezing is a fundamental problem encountered when assessing the stability of underground openings at great depths. Its complexity arises from the inter-relationship of in-situ stress, material properties, and the rock supporting system. The first two variables are totally governed by natural conditions, and the third (the rock supporting system) is the only one over which humans can exert control. We hereby define 'some control' as referring to the stabilization of rock masses. Rock bolts have been recognized as one of the most effective supporting measures for stabilizing rock masses around underground openings. As far as the authors are concerned, however, there seems to be no reliable design method presently available for dimensioning a rock bolt support system. This may be mainly due to the fact that the function mechanism of rock bolts installed in rock masses has not yet been fully investigated. In practice, therefore, the design of rock bolts is entirely based on the experience of and judgement by well trained engineers. Engineers always run into difficulties, however, and face questions of how to determine the number and length of the rock bolts necessary to stabilize the rock masses. The purpose of this paper is to give answers to the above questions by proposing a method which is capable of quantitatively evaluating the effectiveness of rock bolts. The effectiveness can be assessed by comparing the strain occurring in the surrounding rock masses with the allowable strain.

2. THE STABILITY OF ROCK MASSES AT GREAT DEPTHS

Many investigators have observed that the vertical initial stress component increases linearly with depth, and that the increase is related to the overburden pressure (see Fig. 1). On the other hand, the strength of rock masses does not drastically increase according to depth. This means that the large deformation/failure of rock masses becomes a serious problem, and should be considered in the construction of underground openings at great depths.

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