INTRODUCTION

Recently, rockbolt support has been one of the most essential and practical method for tunneling in rock. The authors have been discussing the fundamental mechanism of tunnel driving from the point of view that, when advancing the face, it is necessary to place artificial Supports to substitute for half-dome action within a span length (D) from the face, so as to mobilize the bearing capacity of the rock mass as fully as possible. Also, it is proposed that the supporting effect can be expressed by an equivalent inner pressure (pi) acting on the inner wall (Tanimoto & Hata 1980). However, the interaction between rockbolts and the ground occurs not only on the wall, but is distributed in the region outside the wall face. Therefore, axial force in and shear force along bolts fully bonded were analyzed by the finite element method for the case of weak rock showing strain softening behavior, and the optimum length of rockbolts through plasticized zone, and how to evaluate the effect of rockbolt sup- Ports, were discussed (Tanimoto, Hata & Kariya 1981). It is also concluded that an appropriate length of bolts can be determined from a ratio of supporting (Pick-up) length to anchor length which is observed at the construction site, and that an equivalent inner pressure given by bolts can be calculated from the ob- Served reduction of displacement. From the practical point of view, this paper describes relation among Competence Factor (Cf), non-elastic zone and Support Intensity (Si), influence of separation (Slip) between~ a bolt and ground to axial force distribution along a bolt, and effect of bearing plate. Furthermore, based On several field measurements carried out in various kinds of rock, practical behavior of bolts and resultant equivalent inner pressure (as a support effect) are discussed.

RELATION AMONG PLASTIC ZONE, INNER PRESSURE AND COMPETENCE FACTOR

Though many people apply an elasto-plastic model, established with some bilinear stress-strain relations, because of its easy application, actual ground is apt to show strain softening behavior in many cases. Specially in the range of low Competence Factor (Cf=l. O), there exists a big difference between ‘elasto-plastic’ and ‘strain-softening’ behavior. Therefore, strain softening analysis has become very important in designing of support in tunneling. Tanimoto introduced a parameter of pi, as an inner pressure acting onto tunnel wall, to constitutive equations which are derived from strain energy theorem by employing Mohr- Coulomb's yielding criteria to strain softening state and flow state (Tanimoto 1980; Tanimoto & Hata 1980). In this assumption, stress-strain relation is shown by the three segmented straight lines, corresponding to ‘elastic’, ‘strain softening’ (negative value of deformation coefficient), and 1 flow' (residual state at a constant stress level). when 10 pieces of input data are given, namely radius of an opening (a), Young's modulus (E), Poisson's ratio (v), unconfined compressive strength (qu), angle of internal friction (ø), negative gradient of deformation coefficient (w), primary stress field (Po), unconfined compressive strength at residual state (qu'), angle of internal friction at residual strength (ø'),

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