ABSTRACT:

This paper presents several methods for determining the deformability, tensile strength, and fracture toughness of anisotropic rocks by diametral compression (Brazilian loading) of thin discs of rocks. The mechanical and fracture properties of several sandstones and one shale were successfully determined using the proposed methods. The deformability and tensile strength were calculated using the complex variable function method combined with the Generalized Reduced Gradient approach. The mixed-mode fracture toughness was evaluated using a new formulation of the Boundary Element Method (BEM) developed by the authors.

1 INTRODUCTION

Many rocks exposed near the Earth's surface show well defined fabric elements in the form of bedding, stratification, layering, foliation, fissuring or jointing. In general, these rocks have properties (physical, dynamic, thermal, mechanical, hydraulic) that vary with direction and are said to be inherently anisotropic. Anisotropy can be found at different scales in a rock mass ranging from intact specimens to an entire rock mass. Anisotropy is a characteristic of intact foliated metamorphic rocks (slates, gneisses, phyllites, schists) and intact laminated, stratified or bedded sedimentary rocks (shales, sandstones, siltstones, limestones, coal, etc..). At a larger scale, rock mass anisotropy is found in volcanic formations (basalt, tuff), in sedimentary formations consisting of alternating layers or beds of different rock types, and in rock formations cut by one or several regularly spaced joint sets. Anisotropy plays a crucial role in various rock engineering activities. In civil and mining engineering, rock anisotropy controls the stability of underground excavations, surface excavations, and foundations. Rock anisotropy affects drilling, blasting, and rock cutting. In petroleum engineering, rock anisotropy is a critical factor in controlling borehole deviation, stability, deformation and failure. It also impacts fracturing and fracture propagation. Despite its noted importance, rock anisotropy is still poorly understood in particular by the practice. Questions that often arise include: (1) How to characterize rock anisotropy in the laboratory and in- situ? (2) When is rock anisotropy important? and (3) How much of an error is involved in neglecting rock anisotropy by assuming the rock to be isotropic ? This paper does not attempt to answer all those questions. It presents experimental and analytical methods for determining in the laboratory the deformability, tensile strength and fracture properties of intact rocks that are transversely isotropic (i.e. with one dominant direction of planar anisotropy). The methods are used in the interpretation of diametral loading tests of discs of rocks (Brazilian tests). Discs of four different types of clearly bedded sandstones and one shale were tested under diametral loading in the laboratory. The rock elastic properties were determined by measuring strains using 45° strain rosettes glued at the center of the discs. The effect of rock anisotropy on tensile strength was determined by testing several discs with planes of anisotropy inclined at different angles to the loading direction. Tests were also carried out on initially cracked discs of one sandstone and one shale in order to determine the fracture toughness under pure mode I and mode II loading.

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