Using Indirect Testing Methods to Quickly Acquire the Rock Strength and Rock Mass Classification in Tunnel Engineering

Uniaxial compressive strength (UCS) and tensile strength are essential parameters of rocks, but the direct measurement of these two parameters is time-consuming and requires high-quality samples. As such, academics brought attention to the indirect tests (such as point load test, ultrasonic wave velocity test, and Schmidt hammer test) that could be used to estimate UCS and tensile strength quickly. The empirical equations for estimating UCS and tensile strength of rocks in study cases remain poorly understood as previous equations based on indirect testing methods possess a lithological heterogeneity–dependent nature. Laboratory experiments (e.g., uniaxial compressive test, Brazilian test, point load test) and regression analysis were performed to acquire the correlation equations for calculating UCS or tensile strength of rocks based on indirect tests. Significant linear equations were derived and UCS or tensile strength can be estimated by point load test with acceptable discrepancy. The comparison study showed that using improper empirical equations to predict UCS and tensile strength can yield significant errors. A modified basic quality (BQ) rock mass classification system was also presented to conveniently classify the rock mass in study cases.