ABSTRACT:

A cyclic shear testing system was established to investigate the mechanical behaviors of rough rock joints under the cyclic loading conditions. Laboratory cyclic shear tests were conducted for two joint types of Hwangdeung granite and Yeosan marble: saw-cut and splitted tensile joints. Prior to test the roughness of each specimen was characterized by measuring the surface topography using a laser profilometer. Several important aspects of cyclic joint behaviors, such as high peak shear strength and nonlinear dilation in the first loading cycle, different frictional resistance for the reversed shear loading direction, and anisotropic shear behavior and its dependence on the normal stress level were identified from the cyclic shear test results. These features and their variations in the subsequent loading cycles are mainly due to the effect of second order asperities and strength of rock material. It was also observed from experimental results that degradation of asperities under cyclic shear loading also followed the exponential degradation laws for asperity angle and the mechanism for asperity degradation would be different depending upon the shearing direction and the type of asperities.

I.
INTRODUCTION

Joints can significantly affect the mechanical behavior of rock masses. Most of the previous laboratory experiments for the mechanical properties of rock joints have been performed to determine the peak shear strength and the stress-displacement relations under unidirectional shear loading. Little work has been conducted to study the comprehensive rock joint behavior under the cyclic loading condition where the direction of shear load is repeatedly reversed. Hutson and Dowding (1990) suggested an exponential wear equation for joint asperity based on the experimental results using artificial joints of sinusoidal shape. Huang et al, (1993) tested molded joints of saw-tooth shape under cyclic shear loading and validated the theoretical degradation law for joint asperities proposed by Plesha (1987). Jing et al. (1993) investigated the cyclic behavior of natural joints using the replica of natural joint surfaces. Lately Homand et al. (1999) reported the cyclic shear behaviors of rock joints having sinusoidal shape of asperities. However, the experimental test results concerning irregular rough joints with matrix of intact rock properties under cyclic shear loading has not been reported. In this study, a direct shear test system was designed to study the mechanical behavior of rock joint of irregular roughness under cyclic shear loading. A series of cyclic shear tests were conducted using both the saw-cut and the splitted tensile joint specimens of Hwangdeung granite and Yeosan marble. Prior to shear tests, surface topography of joint was measured using 3D laser profilorneter and analyzed statistically.

2.
EXPERIMENTAL SETUP AND PROCEDURE
2.1
Testing system and specimens

A servo-Controlled direct shear testing machine was designed And attached to MTS 815 loading frame. Both shear and normal loading systems were controlled by MTS controller (TestStar) Fig. 1 shows the schematic diagram of cyclic shear testing system. The loading capacity of the testing system is 250 kN in both the normal and shear directions.

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