Evaluation of the ratio between uniaxial compressive strength and Schmidt hammer rebound number and its effectiveness in predicting rock strength

Schmidt rebound hammer (SRH) test has been used worldwide as an index test for estimating the compressive strength and deformation characteristics of intact rocks. Although there is a high correlation between the surface hardness and the uniaxial compressive strength (UCS) of intact rocks, the SRH provides only a crude estimate for the UCS of rocks. SRH numbers reflect the outer surface of rocks and a depth of 30-50mm. It is not sensitive to the intrinsic properties of the rocks such as texture, saturation, porosity and micro-fractures controlling the mechanical behaviour of rocks. In order for an empirical equation relating the surface hardness to the UCS to be widely used, the index parameter should characterise the mechanical properties of intact rocks. The ratio of UCS/SRH defined as a function of the UCS is a much better indicator for assessing the mechanical characteristics of rocks because the UCS of rocks defines the strength of the material and the ratio UCS/SRH is strongly affected by the level of the UCS. The ratio of UCS/SRH increases with increasing compressive strength at an increasing rate. A large-scale regression analysis was carried out using experimental data to evaluate the ratio of UCS/SRH for the rocks. The accuracy and reliability of the relationship was assessed by means of the root mean square error. The standard error associated with the empirical relationship is very small and the reliability and accuracy of the relationship to assess the compressive strength indirectly seem to be higher than those of traditional relationships between the UCS and the SRH. The ratio of UCS/SRH was also verified by a large database collected from previous studies. This strong linear relationship is proposed for engineering projects requiring the estimation of the compressive strength for intact rocks.