ABSTRACT

Yet the difficulties in preparation of the test specimens according to test standards or suggested methods, and provisions for expensive testing equipment and testing durations motivated the investigators to develop simpler and index type test methods for determination of the mechanical properties. One of these tests is the Block Punch Index (BPI) test accepted by ISRM to indirectly estimate the uniaxial compressive strength (UCS). While the highest strength is obtained in vertical direction to weakness planes in rocks, in case of application of the load to discontinuities at any angle different than vertical, same strength values in every direction can not be obtained due to anisotropy. This study aims to investigate the strength anisotropy associated with discontinuity orientation by performing BPI and UCS tests, and to develop some empirical relationships for estimating the BPI and UCS in the strongest direction, and the UCS from the BPI determined at any angle between the loading direction and weakness plane. The experimental results obtained from six rock types fall into the moderate-to-low strength anisotropy classes. The comparison between the observed and predicted UCS values indicated that the prediction performances of the equations developed are quite well.

1 INTRODUCTION

Rocks are in general anisotropic with regard to their physical and mechanical properties. Evident anisotropy is especially observed among sedimentary and metamorphic rocks which are divided by bedding or foliation. Although igneous rocks may often seem homogenous and isotropic, test results indicate that their material properties also vary in different directions.

Strength and deformability properties of rock materials are determined in laboratory on core specimens. Yet the difficulties in preparation of the test specimens according to test standards or suggested methods, and provisions for expensive testing equipment and testing durations motivated the investigators to develop simpler and index type test methods and associated devices for determination of these mechanical properties. One of these tests is the Block Punch Index (BPI) test accepted by ISRM (Ulusay et al. 2001; ISRM 2007) to estimate the uniaxial compressive strength (UCS) using disc-shaped specimens. The earliest references to a punch test are the articles by Mazanti and Sowers (1965), Vutukuri et al (1974) and Stacey 1980) where the test was employed to determine the direct shear strength of rock specimens with aid of a simple apparatus. These studies were followed by those of Taselaar (1982) and Schrier (1988) and have led the development of BPI test. Schrier (1988) obtained high correlations between UCS, Brazilian tensile strength and BPI values from a limited number of specimens of sedimentary and metamorphic rocks. Then between 1997 and 1999 Ulusay and Gokceoglu (1997, 1998 and 1999) studied on the size effect on BPI test and its general usefulness. They suggested size-correction factors for the BPI test and an equation for BPI-UCS conversion. Sulukcu and Ulusay (2001) used different types of rocks and performed BPI tests, investigated the stress distribution within the rock discs under BPI test, recommended an empirical relationship to convert the BPI into UCS using both their data.

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