Hoop-Stress Loading - A New Method of Determining the Tensile Strength of Rock

Although it is well known that rock commonly fails in tension under numerous field conditions, a satisfactory standard procedure for determination of the tensile strength of such materials has not as yet been established. With suitable precautions, direct-pull tests may be used; however, precise specimen preparation and extremely uniform and concentric loading must be utilized if meaningful results are to be obtained using this method. Recently, the authors completed a 3-year study in which a variety of methods for determining tensile strength were evaluated. One method, involving the hoop-stress loading of thick-walled hollow cylinders, was found to be particularly attractive. This paper describes the theory, apparatus, and test technique associated with this method. In evaluating this new technique a number of specimens of Barre granite, Lauders and Indiana limestone, and Berea and Tennessee sandstone were tested. Reproducibility for each rock type was fund to be excellent, with the percentage standard deviation ranging from a maximum of 10.9 percent for Lauders limestone down to 0.9 percent for Berea sandstone. Values of tensile strength determined from hoop-stress tests were found to correspond closely to those determined by direct pull, provided the former were analyzed in terms of pull, provided the former were analyzed in terms of an equation based on a suitable elastic-plastic analysis. Under these conditions the ratio of hoop-stress to direct-pull tensile strength values ranged from 0.86 to 1.10 for the five rocks tested. Although studies on additional rock types are required, it appears that the hoop-stress loading method would be a strong candidate as a standard test for determination of the tensile strength of rock. Introduction Although it is well known that tensile stresses play an important role in the behavior of underground play an important role in the behavior of underground openings associated with petroleum, natural gas and mining operations, suitable techniques for the determination of the tensile strength of rock are limited. A detailed study of the literature relating to the measurement of the tensile strength of rock has been presented by Jayaraman. This study indicated that, although rock mechanics literature includes numerous papers dealing with the measurement of the tensile strength of various rock types, relatively little attention has been focused on the development of meaningful techniques for the determination of this parameter. Experimental difficulties and poor results encountered in carrying out direct tensile tests have led a number of workers to investigate a variety of indirect methods. Unfortunately, many of these studies have been carried out on rocklike materials rather than on actual rocks. Furthermore, in many cases an insufficient number of tests have been conducted to evaluate accurately the test procedures employed. procedures employed. Recently the authors completed an extensive study in which a variety of methods for determining the tensile strength of rock were evaluated. During this investigation the apparent tensile strength of a number of rock types was determined using four different methods, namely:direct-pull of solid cylinders,diametric compression of solid-discs,diametric compression of hollow-discs, andhoop-stress loading of hollow cylinders. An outline of this study and details of the experimental results obtained have been presented in a recent paper. One test method, involving the hoop-stress loading of thick-walled hollow cylinders, was found to be particularly attractive. Based on the experimental results obtained to date, it appears that the hoop-stress method would be a strong candidate as a standard for determination of the tensile strength of rock. The present paper describes the theory, apparatus and test technique associated with this method. Tensile strength data are presented for five different rock types; two limestones, two sandstones and a granite. SPEJ P. 246