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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

References: 1. SZWEDZICKI, T. Draft ISRM suggested methods for determining the indentation hardness index of rock materials. International Journal of Rock Mechanics and Mining Sciences & Geomechanics Abstracts, vol. 35, 1998. pp. 831–835. 2. CHEATHAM, J.B. An analytical study of rock penetration by a single bittooth. 8th Annual Drilling and Blasting Symposium, University of Minnesota, 1958. pp. 1–21. 3. GNIRK, P.F. and CHEATHAM, J.B. Indentation experiments on dry rocks under pressure. Journal of Petroleum Technology, September, 1963. pp. 1031–1039. 4. PAUL, B. and SIKARSKIE, D.L. A preliminary theory of static penetration by a rigid wedge into a brittle material. 7th Symposium on Rock Mechanics, Pennsylvania State University, 1965. pp. 119–148. 5. CHEATHAM, J.B. and PITTMAN, R.W. Plastic limit analysis applied to a simplified drilling problem. ISRM, Proceedings of. 1st Congress, Lisbon, vol.2, 1966. pp. 93–97. 6. HARTMAN, H.L. The effectiveness of indexing in percussion and rotary drilling. International Journal of Rock Mechanics and Mining Sciences & Geomechanics Abstracts, vol. 6, 1966. pp. 265–278. 7. GNIRK, P.F. An experimental study of indexed single bit-tooth penetration into dry rock at confining pressures of 0 to 7500psi. ISRM, Proceedings of. 1st Congress, Lisbon, vol. 2, 1966. pp. 121–129. 8. PARISEAU, W.G. and FAIRHURST, C. The force-penetration characteristic for wedge penetration into rock. International Journal of Rock Mechanics and Mining Sciences & Geomechanics Abstracts, vol. 14, 1967. pp. 165–180. 9. MILLER, M.H. and SIKARSKIE, D.L. On the penetration of rock by threedimensional indentors. International Journal of Rock Mechanics and Mining Sciences & Geomechanics Abstracts, vol. 5, 1968. pp. 375–398. 10. BENJUMEA, R. and SIKARSKIE, D.L. A note on the penetration of a rigid wedge into a nonisotropic brittle material. International Journal of Rock Mechanics and Mining Sciences & Geomechanics Abstracts, vol. 6, 1969. pp. 343–352. 11. MORRIS, R.I. Rock drillability related to a roller cone bit. Society of Petroleum Engineers, Paper No. 2389, 1969. pp. 79–83. 12. LUNDBERG, B. Penetration of rock by conical indentors. International Journal of Rock Mechanics and Mining Sciences & Geomechanics Abstracts, vol. 11, 1974. pp. 209-214. 13. PANG, S.S., GOLDSMITH, W., and HOOD, M. A force-indentation model for brittle rocks. Rock Mechanics and Rock Engineering, vol. 22, 1989. pp. 127–148. 14. BILGIN, N., ESKIKAYA, S., and DINCER, T. The performance analysis of large diameter blast hole rotary drills in Turkish Coal Enterprises. 2nd International Symposium on Mine Mechanization and Automation. Rotterdam, Balkema, 1993. pp. 129-135. 15. KAHRAMAN, S., BALCI, C., YAZICI, S., and BILGIN, N. Prediction of the penetration rate of rotary blast hole drills using a new drillability index. International Journal of Rock Mechanics and Mining Sciences & Geomechanics Abstracts, vol. 37, 2000. pp. 729-743. 16. COPUR, H., BILGIN, N., TUNCDEMIR, H., and BALCI, C. A set of indices based on indentation tests for assessment of rock cutting performance and rock properties. Journal of The South African Institute of Mining and Metallurgy, vol. 103, no. 9, 2003. pp. 589–600. 17. KAHRAMAN, S. and GUNAYDIN, O. Indentation hardness test to estimate the sawability prediction of carbonate rocks. Bulletin of Engineering Geology and the Environment, vol. 67, 2008. pp. 507–511. 18. YAGIZ, S. Assessment of brittleness using rock strength and density with punch penetration test. Tunnelling and Underground Space Technology, vol. 24, 2009. pp. 66–74. 19. SZWEDZICKI, T. The indentation hardness testing of rock. International Journal of Rock Mechanics and Mining Sciences & Geomechanics Abstracts, vol. 35, 1998. pp. 825-829. 20. SHEOREY, P.R. Empirical Rock Failure Criteria, Rotterdam, A.A. Balkema, 1997. 176 pp. N VOLUME 112 MAY 2012 T r a n s a c t i o n Figure 22—Measured versus estimated uniaxial compressive strength for Equation [3] indicating the relation between the UCS and IHI for igneous rocks P a p e r Figure 23—Measured versus estimated uniaxial compressive strength for Equation [4] indicating the relation between the UCS and IHI for metamorphic rocks metamorphic, and sedimentary rocks, were collected from the field and tested in the laboratory. The test results were statistically analysed and the UCS and BTS values were correlated with the corresponding IHI values. The data were evaluated for all rock types and for rock classes separately. A strong correlation (R2 = 0.76) between UCS and IHI was found for all data. The correlation between BTS and IHI is also good (R2 = 0.58), but not as strong as the correlation between UCS and IHI. To see the influence of rock classes on the relations, regression analyses were repeated for igneous, metamorphic, and sedimentary rocks separately, and it was shown that the correlation coefficients were generally increased. Confirmation of the derived models was carried out by the t-test, and F-test and the scatter diagrams of the observed and estimated values, and it was concluded that the derived models were valid. The study covers the three rock classes igneous, metamorphic, and sedimentary, and a remarkable number of samples were tested. The samples were collected from almost The Journal of The Southern African Institute of Mining and Metallurgy 339 L