Applicability of in-situ direct tension test and proposal of evaluation method of tensile strength on rocks

A new method of direct in-situ tension test was developed by Central Research Institute of Electric Power Industry (CRIEPI) and some verification tests were conducted on artificial continuous rock mass in the laboratory. In this study, an attempt for this test method was to be applied on a natural discontinuous rock mass in the field. As a result, four in-situ tension tests were successfully conducted while modifying testing procedure and the tensile strength of shale was successfully obtained. The tensile strength of large rock mass (60 cm in diameter) was about ten percent of that of small core sample (5 cm in diameter). Furthermore, a new evaluation method of tensile strength of rock mass was proposed taking the scale effect into account. The tensile strength was analyzed by geometric information of discontinuities in the rock mass and the variation of tensile strengths of core samples obtained by laboratory direct tension tests. The evaluated tensile strength was approximately equal to those obtained by the in-situ tension tests. (author)