ABSTRACT

A series of uniaxial and triaxial tests are conducted on core samples of sandstone from Hunan highway region in China. The influence of the inherent anisotropy of sandstone on the mechanical characteristics, including the failure mode, failure strength and residual strength, has been investigated. The experimental results show that the tested sandstone represents the low anisotropy, and failure mode, peak failure strength and residual strength under different confining pressures is directly related to the inclination angle of specimen foliation with respect to the horizontal direction. Analyzing the experimental data on the basis of the Mohr-Coulomb failure criterion, the brittle-ductile transition properties under different loading orientations are brought out, and the relevant brittle-ductile transition confining pressure are predicted by failure criterion. The results show that the existence of oriented bedding planes has a strong influence on the brittle-ductile transition confining pressure.

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 that vary with direction and are said to be inherently anisotropic (Amadei 1996). For example, metamorphic and sedimentary rocks are characterized by inherent or structural anisotropy. Foliated metamorphic rocks (e.g., slate, phyllite, schist, and gneiss) with textures showing the parallel orientation of mineral grains tend to split or break along the foliation or cleavage planes, rather than across the planes or at other orientations. Non-foliated metamorphic rocks such as marble also show some anisotropy. Some stratified or bedded sedimentary rocks (e.g., shale, sandstones) possess closely spaced laminations that appear to have a strong directionality (Tien, et al. 2006). Anisotropy of these rocks is dependent on the sampling orientation with respect to loading directions (Goodman 1993). And the influence of the intrinsic anisotropy on failure strength is one of the basic data required for predicting rock performance for a variety of surface and underground structures. Therefore, the design and stability analysis of underground structures located in anisotropic rocks require estimating the variation of strength and deformation behaviors and analyzing the failure mechanisms of anisotropic rocks with the orientation of stress.

Rock anisotropy is a well-known behavior and is of considerable interest in the field of rock mechanics and engineering over the past several decades. Many scholars have devoted considerable efforts to the study of rock anisotropy. Much research has been carried out, including experimental characterization (Nova 1980; Niandou et al. 1997; Tien et al. 2000; Nasseri et al 2003; etc.), theoretical studies and numerical modelling (Duveau et al. 1998; Pietruszczak et al. 2002; etc.). All the results obtained have shown that the rock strength varies with the loading orientation, and the shape of the curve of compression strength and the orientation angle are the most common representation of the nature of strength anisotropy.

In this paper, anisotropic strength behaviour of the sandstone form Hunan highway area in China has been brought out through the testing of specimens with the oriented weakness planes under uniaxial and triaxial conditions.

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