ABSTRACT:

This paper presents the result of experimental study using model joints, and the numerical study to propose a mechanical/practical model to express reinforcing effect of rock bolt in jointed rock. Numerical model for bolt effect derived from the experiments involves such terms as joint roughness, mechanical properties of rock normal stress, stiffnesses of bolt and its installation angle, etc. Stress-displacement behaviors are well expressed by hyperbolic function.

RÉSUMÉ:

Ce rapport presente le resultat d'une etude experimentale avec des joints modèles et d'une etude numerique pour proposer un modèle mecanique pratique pour presenter les effects des renforcements des blutoirs dans les joints. Le modèle numerique des effects des blutoirs provenant des experiences implique des terms come la regosite du joint, les proprietes mecaniques des roches, les contraintes normales, la durete du blutoir et son angle d'installation. Les deplacements de contraites sont bien exprimes par les fonctions hyperboliques.

ZUSAMMENFASSUNG:

Diese Abhandlung prasentiert die Ergebnisse der experimentellen Forschung unter Verwendung eines Knotenmodells und die Forschungsergebnisse fuer die numerische Analyse fuer die Wahl eines praktischen mechanischen Modells fuer Ausdruck der Verstarkungswirkung von Steinschrauben gegenueber verbundenem Fels. Das experimentell eingefuehrte numerische Analysemodell enthalt Ausdruecke fuer Grobheit der Verbindung, mechanische Eingeschaften des Muttergesteins, Normalspannung, Schrauhensteifheit, Installierungswinkel usw. Das Spannungsversetzungsverhalten wird durch Hyperbelfunktionen gut ausgedrueckt.

INTRODUCTION

Rock bolt plays an important role as one of major support members for underground excavation in rock and rock slope. Some method has been proposed to estimate the reinforcing effect, but the mechanism and effect of bolt action in jointed rock mass are complicated and not enough clarified. So we excute a series of laboratory shear tests to explain the support mechanism of rock bolt in jointed rock mass subject to shear deformation.

Test specimens made of mortar with single or layered joint, as jointed rock model, are 80x40x20cm in size. Regular asperities (or teeth) with dilation angles of 0°/10°/20° and Barton's JRC roughness profiles are used as roughness of joint. Rock bolts, 10~25mm in diameter, are installed with angles of 45°/90°/135° and 30°~90°. Shear tests are performed under plain strain condition with normal stress of 0~60Kgf/cm2 (0~5.88MPa)·on joint surface. Some portion of test method and results are presented in our paper (1986).

JOINT STRENGTH
Strength of regular teeth joint

From the test result it is found that there are two phases in the behavior of regular teeth joint, that is: joint surface slides along the teeth when the relative normal stress an/ac is small, and teeth are sheared off before sliding occur if an/ac become greater. In the former cases, shear strength can be expressed by Patton's equation. As for the joint strength of the latter case, it can be estimated by appling limit equilibrium theory to local stress state of a tooth.

Mortar strength is usually expressed by Fairhurst's equation calculated from the result of splitting tensile test and uniaxial compression test, however, the strength from the results of triaxial compression test were represented by straight line. Fig. 1 shows experimental and calculated results of the joint strength, and the strength curves of mortar written above are

(Figure in full paper)

also included.

The joint strength of the latter case seems to be connected with deformation behavior of teeth and to be divided into two groups. It is asummed for one group that failure of teeth occures under biaxial stress state and little restraining pressure act on back face of teeth, because relatively small displacement of joint arise at failure of teeth. It is also asummed for the other that failure of teeth occures under triaxial stress state and some restraining pressure act on back face of teeth, because relatively large displacement of joint arise at failure of teeth.

Fairhurst's equation may be applied for the former and results of triaxial test for the latter. When we calculate the latter, 38% of normal stress is distributed as restraining pressure on back face of a tooth.

Strength of irregular teeth joint

Comparisons are made in Fig.2 between Barton's equation and test results excuted by using two dimensional joint surface based on Barton's JRC roughness profiles. In cases uniaxial strength of mortar σc >23MPa, test results agree with Barton's equation fairly well when JRC=20, but test results are 15~20% greater than values from the equation when JRC=10. In cases σc<13MPa, test results are 25~30% and 2~25% greater than values from the equation when JRC=10 and JRC=20 respectively. It is similar with the cases of regular teeth joint that relative strength of low σc is greater than that of high σc

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