ABSTRACT

ABSTRACT

The Borehole Shear Tester (BST) provides in situ physical property data which can be used with Wilson's pillar design equations to estimate the peak abutment loads and, therefore, the width of the yield zone across production coal pillars, resulting in efficient and economic designs. Developed by Dr. R. L. Handy, and his coworkers at the Iowa State University, the BST was designed as a practical tool for measuring a material's intrinsic shear strength in situ. However, the derived data can also be used to estimate material properties such as the angle of internal friction, uniaxial compressive strength, tensile strength, and a Mohr failure envelope. Because these values do not suffer the limitations imposed by laboratory tests (dehydration, platen effects, size and shape effects, sample preparation, time delays, etc.), it is felt they more fairly describe the intact rock portion of the structure. Problems associated with anisotropic materials and variations in the stress distribution along the test hole are addressed also.

INTRODUCTION

Because of the friable nature of coal and many other rock types, it is difficult, if not impossible, to obtain and prepare undamaged test specimens for use in determining mechanical properties. At best, obtaining and preparing these samples is an expensive and time-consuming task. These facts have tended to discourage the adoption of engineering methods for designing underground coal mines. Recognizing this problem, the Bureau of Mines contracted with the Engineering Research Institute (Handy, l973,1976) of Iowa State University to develop a Borehole Shear Tester (BST) specifically for rock, based on successes with a similar device developed there for testing soils. As a result, U.S. Patent #4075885 was assigned to the Iowa State Research Foundation after being granted to Handy, Pitt, Girard, and Roorda, in February of 1978. The BST is presently available for use in the mining industry from Handy Geotechnical Instruments, Inc.1 Following this development, Dr. L. A. Panek (1979) of the Bureau of Mines, evaluated the performance of the BST. Based on results obtained for both coal and hard rock mines, Dr. Panek concluded the following: "The BST is indicated to be an efficient, practical apparatus for relatively rapid testing of the strength of a rock mass in situ. The test results are consistent and reproducible. Strength estimates formulated in terms of Mohr's Coulomb criterion appear to be lower by BST than TCT (Triaxial Compressive Test), but the BST results are believed to be more realistic values for design purposes." The BST consists of a pair of shear plates which are expanded against the sidewall of an NX (3-in) size borehole under a known normal force (Fig. 1). The force required to then shear the rock from under these plates is measured and converted to the shear stress of the rock for the normal load applied. Excluding use in highly fractured, altered, or weak materials, the BST can be used to obtain from 12 to 20 shear strength readings per shift. This generates sufficient shear versus normal load data to develop a Mohr-Coulomb failure envelope, from which several physical properties, useful in coal mine pillar design, can be estimated. In addition to the intrinsic shear strength (cohesion), the angle of internal friction, uniaxial compressive strength, and uniaxial tensile strength can all be calculated or estimated.

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