Impact of grain size or anisotropy on correlations between rock tensile strength and some rock index properties

Brazilian tensile strength (BTS) is a critical mechanical parameter of rock; and the measurement of BTS performed on core samples is a cumbersome procedure. Thus, rock index properties including point load, P-wave velocity and Schmidt hammer tests have been widely used to estimate BTS. The correlations between BTS and index properties are rock-type, grain size and anisotropy dependent, but, how the correlations related to the variation of grain size or anisotropy remain unexplained. In this study, the impact of grain size or anisotropy on those correlations is respectively examined using sandstone (fine or coarse grain size) and gneiss (0o, 45o, 90o inclined anisotropy) samples. Several significant equations for predicting BTS through index properties were established for different types of samples. The finding implies that either grain size variation or multidirectional anisotropy reduces not only the correlated degree between BTS and index properties, but also the BTS estimation reliability of those empirical equations. All three index properties should be used with much care for coarse-grained rock and respectively performed on samples with unidirectional anisotropy. Using an empirical equation between BTS and index properties ignoring grain size or anisotropy can yield considerable discrepancies of estimated BTS. Among three index properties, point load test is the first choice for predicting BTS as the small discrepancies of estimated results. As the invalid correlation, P-wave velocity test should not be performed at 45o angle to the anisotropy in the BTS estimation; and this recommendation is also appropriate for Schmidt hammer test conducted parallel to anisotropy.