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Predicting the compressive and tensile strength of rocks from indentation hardness index by S. Kahraman*, M. Fener†, and E. Kozman‡

Synopsis
The prediction of rock properties from indirect testing methods is important, particularly for preliminary investigations since indirect tests are easier and cheaper than the direct tests. In this study, we investigate the predictability of the uniaxial compressive strength (UCS ) and Brazilian tensile strength (BTS ) of rocks from the indentation hardness index (IHI ) obtained using point load apparatus. Forty-six different rock types, 14 of which were igneous, 15 were metamorphic, and 17 were sedimentary were tested in the laboratory. The UCS and BTS values were correlated with the corresponding IHI values and the results were statistically analysed. The influence of rock classes on the relationships was also investigated. A strong correlation between UCS and IHI was found for all data. The correlation between BTS and IHI is not as strong as the correlation between UCS and IHI. However, it is in the acceptable limits. When the regression analyses were repeated for igneous, metamorphic, and sedimentary rocks, the correlation coefficients were generally increased. The results show the UCS and BTS can be estimated from IHI. In addition, the effect of rock classes on the relationships between IHI and both UCS and BTS is important. Keywords uniaxial compressive strength, Brazilian tensile strength, indentation hardness index

Introduction
Rock engineers have commonly used the uniaxial compressive strength (UCS) and Brazilian tensile strength (BTS) of rock for designing surface and underground structures. Determining these rock strengths is timeconsuming and expensive, particularly for the preliminary studies of projects. For this reason, indirect tests such as Schmidt rebound number and, ultrasonic test are often used for predicting rock strength. Since indirect tests require less or no sample preparation and the testing equipment is less sophisticated, these tests are very easy to carry out. In addition, these tests can usually be performed in the field. The indentation hardness test is a simple and easy test and can be conducted using a point load test apparatus. The test is of particular value when only a limited amount of The Journal of The Southern African Institute of Mining and Metallurgy

rock material, e.g. a thin disc of core or a small lump sample, is available1. The UCS and BTS can easily be predicted from the indentation hardness index (IHI) for the preliminary investigations, if strong predictive correlations are established. Since rock indentation is the basic process in drilling and boring, numerous researchers2–16 have carried out indentation tests to understand the indentation phenomena or to develop prediction models for drilling or boring. Kahraman et al.15 also investigated the relationships between the slope of load-indentation curves and the rock properties. They found good correlations between the slope of load-indentation curves and the rock properties. Kahraman and Gunaydin17 investigated the sawability prediction of carbonate rocks from indentation hardness tests carried out by attaching a dial gauge to the point load apparatus for measuring penetration. They concluded that the indentation hardness test can be used for predicting the sawability of carbonate rocks. Recently, Yagiz18 suggested a new brittleness index and rock brittleness classification based on type, strength, and density of rock together with the results of punch penetration tests. A standard indentation test was recommended by ISRM1 and Equation [1] was suggested for the prediction of UCS from IHI [1] where UCS is the uniaxial compressive strength (MPa) and IHI is the indentation hardness index (kN/mm).

T r a n s a c t i o n

P a p e r

* Mining Engineering Department, Nigde University, Turkey. † Geological Engineering Department, Nigde University, Turkey. ‡ Graduate School of...
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