TENSILE STRENGTH OF ROCKS

Abstract

A description of a proposed technique for determining the tensile strength of rock, the direct pull test is given in the most logical way. Results of briquette tests on eight different types of rocks obtained from different parts of India have been compared with the values obtained by other methods of measuring tensile 'strength. It is found that the values of tensile strength obtained by briquette method or the direct pull test are the lowest and those given by beam tests are the highest, while the values obtained from Brazilian test lie in between . It has been further observed that the specimen to specimen variation in strength depends on both the testing method and the specimen size and that the smaller the size, the higher is strength of specimen. All these methods of determining tensile strength need preparation of test specimens in cylindrical, prismatic or briqu-ette shape and require use of diamond saw or diamond drilling equipment, both of which are expensive. Hence the evaluation of tensile strength from irregular lumps would be cheaper and quicker. Study has been done on irregular lumps of rocks and the tensile strength is found by using the formula. d b Correlations between different methods have been obtained. High values of correlation coefficients obtained for these rela-tions indicate that there exist very good correlations. Minimum T - O.9 F,, o values have also been adopted as the true measure of the tensile strength in each case in addition to conventionally used average values Ratio of uniaxial compressive and tensile strengths from Brazilian tests indicate that the failure of rock specimens takes place as predicted by extended Griffith theory while ratios considering briquette tensile strengths are higher and seem to eon', 7m Mohr's theory,