Predicting Uniaxial Compressive Strength by Point Load Test: Significance of Cone Penetration

摘要

The common standards (ISRM and ASTM) indicate that the cone penetration depth should be considered in determining I_s(50) particularly for weak rocks. However, accurate measurements of cone penetration depth by a laser distance sensor attached to a point load frame revealed that deeper penetration occurs in fresher/stronger rocks and when the penetration depth is considered, the scatter in the data is minimized leading to a significantly better regression coefficient and lower conversion factor for I_s(50)_f in predicting UCS. The UCS vs. I_s(50); correlation curves from the literature confirm that the scatter in the data lie mainly in the higher UCS range. The UCS value below which the depth of penetration has little or practically no influence on the correlations largely depends on the rock type and its microstructure. Therefore, the standards should acknowledge that the cone penetration could be significant particularly for fresher/stronger rocks. In brittle rocks, penetration depth should decrease with decreasing tensile strength but the rate of change in depth may be smaller in fine grained rocks. Thus, specimens with significant differences in grain size should be evaluated separately, despite their compositional similarities, to enhance the efficacy of I_s(50) as a predictive tool for UCS and as an indicator of weathering grades or for objective confirmation of their visual identification.