ABSTRACT:

This paper presents the result of experimental study using joint models, and that of numerical study to propose a mechanical and practical model to express reinforcing effect of rock bolt in jointed rock. The numerical model can express well, not only the bolt effect but also joint property including dilatancy.

INTRODUCTION

Rock bolts are much used as a major support member for underground excavation in rock. Although some methods have been proposed to estimate the reinforcing effect, many un- Solved problems still remain. It is well known that mechanical behaviour of jointed rock mass depends essentially upon the joints contained during underground excavation in hard rock, but the deformation mechanism is not enough clarified too. So we execute a series of laboratory tests and numerical analysis to clarify the support mechanism of rock bolts in jointed rock mass subject to shear deformation. In this paper bolt effect derived from the tests is introduced to jeneral analytical model RBSM which was proposed by Professor Kawai as an ultimate analitical method.

TEST METHOD

Test specimens consist of a pair of mortar blocks as shown in Fig.l. Copies of natural rock joints are used as a model joint for the surface of blocks facing opposite to each other. There are three types of the joint, comparatively smooth joint A, comparatively undulating joint B, and comparatively rough joint C. Uniaxial compressive strength of mortal including grout was aimed at 500kgf/cm2 (49MPa) in order to express hard rock. Reinforcing bar D13 (SD30)was used as a rock bolt which was Set through the center of joint surface. Test parameters are shown in Table 1. Loading method is as follows: Vertical stress 02 is adjusted in order to keep Plan 800 Test specimens and tipical profiles of natural rock joint Table 1 Test Parameters Joint A.B.C Bolt Setting Angle 45°, 60°, 75° Bolt End Fix Free 427 plain strain condition. Isotropic stress is applied at first, then principal stress 01 and 03 are changed to increase shear stress T, holding normal stress of joint surface on at constant value Skgf/cm2 (O.49MPa). Teflon sheets are used between each loading plate and specimen. in order to decrease friction.

TEST RESULTS

Fig. 2 shows an example of shear stress 'and dilatancy~shear displacement relation curves of cases with joint C. Initial shear modulus (inclination of T-U curves at initial stage of loading) depend on bolt setting angle a, as well as contact condition of joint. The smaller bolt setting angle a is, the larger initial shear modulus and peak shear strength are in the three cases. Shear displacement at peak shear strength as well as that at failure of bolt is small just in the same order of the three cases. There are little differences in dilation between the cases with bolt and the case without it. Fig. 3 shows an example of the relation between axial bolt strain and shear displacement. It is seen from the figure that the smaller bolt setting angle is and the nearer measured point is to the joint.

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