Abstract

Inherently anisotropic rocks are modeled with the use of two-dimensional Discrete Element Methods (DEM). In the simulated anisotropic rock sample, the rock matrix is modeled as an assembly of rigid particles and the existence of weak layers is directly represented by imposing individual smooth joint (SJ) contacts with same orientation into the rock matrix. The properties of a SJ contact include normal and shear stiffness, normal strength, cohesion, and friction angle. A systematic study is conducted to investigate the influence of these parameters on the macro behaviors of anisotropic rocks with different anisotropy angles under uniaxial compression condition. The Young’s modulus is found to increase significantly with the SJ normal stiffness when the anisotropy angle is low (0°-30°). The USC increases with the SJ normal strength at high anisotropy angle (ß>60°) while cohesion raises the UCS at medium anisotropy angle (30°-60°). The influence of friction angle is not significant. Understanding the influence of each parameter is of great importance for the calibration of micro parameters to represent certain type of rock. A general process for the calibration of micro parameters to reproduce the strength and deformation behaviors of different types of anisotropic rocks is proposed.

1. INTRODUCTION

Anisotropy is everywhere while isotropy is rare [1]. Many rocks are characterized by a structural inherent anisotropy which is due to the existence of rock fabric elements such as bedding, layering, foliation and lamination planes [2]. Such rocks are said to be inherently anisotropic as their physical, mechanical and hydraulic properties vary with direction. Rock anisotropy affects many rock related projects, e.g., borehole stability [3], propagation of hydraulic fracturing [2], and deviation of drilling. Therefore, a complete understanding of the behaviors of anisotropic rocks under different stress conditions is extremely important.

In the past several decades, many investigators have performed compression tests on various anisotropic rocks, e.g., Niandou et al. [4] on shale, Nasseri et al.[5] on schists, Tien et al. [6] on artificial materials. In general, the variation of failure strength with the anisotropy angle is characterized by a U-shaped curve with the minimum strength obtained when the anisotropy angle (ß) is around 60°. In fact, the geometry of the curves as well as the failure modes with different anisotropy angles vary for different types of rocks [4]. Attempts have also been made aiming to investigate the effect of weak planes orientation on the behaviors of anisotropic rocks on the micro-scale through laboratory testing [7, 8]. However, it is very difficult to explore the micro-scale mechanisms from laboratory testing which leads to a lack of a thorough understanding of the underlying failure mechanisms.

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