THE METHOD FOR ESTIMATING TENSILE STRENGTH BY USING SHEAR STRENGTH PARAMETERS AND DISCUSSIONS

The strength parameters of rock mechanics test are mostly discrete because most rocks have various structural planes/interfaces, therefore the parameters obtained by the rock mechanics tests have the properties of estimation. For a uniaxial compression test the normal stress on failure surface, i.e. failure surface parallel to the axial stress, of the Mohr circle is zero, hence the tensile strength formula is derived for the compression splitting circle. The formula is then used to calculate tensile strength from 14 tests of rock and soil. And the results are compared with those of geotechnical Brazil splitting test and direct tensile test usually with a relative error within 20%.From the perspective of the discrete geotechnical parameters the compression splitting failure circle formula has its practical value. In fact, there is no lateral tension failure with zero compression stress under the uniaxial compression in the negative axial direction of the Mohr-Coulomb criterion, namely no circumferential tensile failure mode happening for a cylindrical specimen. Therefore, the ISRM suggested sigma t=f(3) plotting regression method, and a variety of Mohr stress circle based on geometric relations always have errors for calculating tensile strength. The stress cycle and its stress range of three kinds of failure modes with uniaxial compression are further analyzed.