Estimation of rock mechanical properties by macro indentation test with a conical indenter

The mechanical properties of rock, especially the uniaxial compressive strength (UCS), the elastic modulus (E), and the Brazilian tensile strength (BTS) are critically important for rock engineering. Since standard laboratory tests have some limitations, various indirect methods have been developed for estimating these mechanical properties, e.g., the indentation test. In the indentation test, an indenter is forced into the rock to obtain the load-displacement curve. Most estimation models of rock mechanical properties by indentation test are appropriate for fine-grained rock or soft rock, and there is little estimation model for coarse-grained rock and hard rock. Therefore, it is important to establish an estimation model appropriate for rock with large grain sizes and high strength. Since the specimen size may affect the test result, specimens with different sizes were tested first to study the specimen size effect. The result showed that both the specimen diameter and height affect the specimen failure pattern, the load-displacement curve, and the indentation indices. The proper specimen size is suggested using in the indentation test with a 7-mm diameter indenter. Then, 8 types of rock were prepared for indentation tests, uniaxial compression tests, and Brazilian tests. The UCS, BTS, and E all increase with indentation indices, e.g., the energy per unit (W) and the indentation modulus of the first peak (IM1). There are linear correlations between the UCS, E, BTS, and indentation indices (W and IM1) with high determination coefficients.