Hardness, strength and abrasivity of rocks: Correlations and predictions

Since ancient times, rocks and their geomechanical and mineralogical properties have played a fundamental role for realising construction and infrastructure projects. Workability and excavability of the material itself are still decisive factors controlling tool wear and advancement rates. In engineering geology, standardised tests and analyses related to the strength, hardness, abrasivity and mineralogical composition are commonly conducted in this context. The uniaxial compressive strength (UCS), CERCHAR Abrasivity Index (CAI) and equivalent quartz content are widely used parameters for such an assessment, in order to estimate and predict drillability and associated wear of drill bits, cutting discs or chisels. In this article, the correlations between strength, abrasivity and mineral content of various rock types are investigated. The concept of hardness in geotechnics and engineering geology is elaborated in greater detail, shedding light on hardness definitions, testing methods and how hardness parameters are interrelated. Under the aspect that the CAI shows a good correlation with the Mohs hardness commonly used in mineralogy, a novel approach for estimating the CAI is presented. It is suggested that the CAI of a rock can be estimated within 50 % of the actual value, if its UCS exceeds »60 MPa. On the data basis of various rock types analysed from national and international construction projects, the potential and limitations of this method are discussed.