Influence of asperity degradation on the mechanical behavior of rough rock joints under cyclic shear loading

Abstract

A cyclic shear testing system was established to investigate the mechanical behavior of rough rock joints under cyclic loading conditions. Laboratory cyclic shear tests were conducted for two joint types of Hwangdeung granite and Yeosan marble: saw-cut and split 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 behavior, such as high peak shear strength and non-linear 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. Based on the experimental results an elasto-plastic constitutive model, which can consider the degradation of second order asperities, was proposed. Numerical simulations for the monotonic and cyclic shear loading indicated agreement with the laboratory test results.

Introduction

Joints can significantly affect the mechanical behavior of rock masses. More specifically, joint properties such as roughness, strength of asperities, separation, matedness, gouge and even the spatial distributions make the behavior of jointed rock masses more complicated. Most of the previous laboratory experiments for the mechanical properties of rock joints have been focused on determining the peak shear strength and the stress–displacement relations under unidirectional shear loading [1]. The comprehensive behavior of rock joints under the cyclic loading condition where the direction of shear load is repeatedly reversed has been rarely reported. Since the large-size underground rock structures such as energy storage cavern and radioactive waste repository require the higher standard of stability against blasting and earthquakes, the cyclic shear [2] and dynamic shear [3] behavior of rock joints become more important for providing the fundamental ideas to understand the natural phenomena of higher loading frequencies.

Hutson and Dowding [4] suggested an exponential wear equation for joint asperity based on the experimental results using artificial joints of sinusoidal shape. Huang et al. [5] tested molded joints of saw-tooth shape under cyclic shear loading and validated the theoretical degradation law for joint asperities proposed by Plesha [6]. Jing et al. [7] investigated the cyclic behavior of natural joints using the replicas of natural joint surfaces. By utilizing these results Jing [8] proposed a conceptual model for the cyclic shear behavior. Kana et al. [9] suggested the interlock-friction model for dynamic shear response, and the importance of the second order asperities on the dynamic shear behavior was explained by Fox et al. [10]. Recently, Homand et al. [11] reported the cyclic shear behaviors of rock joints having sinusoidal shape of asperities. However, the experimental results concerning irregular rough joints with matrix of intact rock properties under cyclic shear loading has rarely been reported.

Algebraic calculation and numerical simulation for the mechanical behavior of jointed rock mass require the constitutive law of rock joints. After Patton [12] proposed bilinear models of saw-tooth joints, several empirical and theoretical constitutive models were developed by Ladanyi and Archambault [13], Barton and Choubey [14], Plesha [6], Amadei and Saeb [15], Jing [7], Qiu et al. [16]. Among these Barton's empirical model [1], [14] has widely been used because it is easy to use and includes several important factors of joint characteristics. However, Barton's model is not formulated incrementally, it is somewhat difficult to implement numerically. Plesha [6] idealized Patton's saw-tooth type asperities and developed a constitutive model based on the classical plastic theory. In his model, the asperity degradation is an exponential function of the exerted work during shearing. Huang et al. [5] verified this exponential degradation law through a series of experiments for joints having saw-tooth type asperities. Though Plesha's model is theoretically complete and highly adaptable to numerical implementation, this model over-predicts the dilation during cyclic shearing and has not been evaluated for the behavior of rough rock joint. Qiu et al. [16] revised Plesha's model by idealizing the sinusoidal asperities, but it would be less practical due to the complexity of constitutive equation.

In this study, a precision direct shear testing system was designed to study the mechanical behavior of rock joint of irregular asperities under cyclic shear loading. A series of cyclic shear tests were conducted using both the saw-cut and the split tensile joint specimens of Hwangdeung granite and Yeosan marble. Prior to shear tests, surface topography of joint was measured using 3D laser profilometer and analyzed statistically. Based on the experimental results, Plesha's [6] plastic constitutive model was revised by considering the second order asperities, and finite element analysis was performed to simulate the laboratory shear tests. The results of numerical analysis for both the monotonic and cyclic shear behavior were compared to the laboratory test results.