A Comparative Evaluation of Indirect Methods to Estimate the Compressive Strength of Limestone (Chorgali Formation)

Abstract

Evaluation of the strength of rockmass is very important to ensure the safety of the structures. The uniaxial compressive strength test (UCS), point-load index strength test (Is50), Schmidt rebound hammer test (SRHN), tensile strength test (Ts), velocity test (Vp) and density have been commonly used for characterisation of the rockmass. The UCS is comparably expensive and time taking than other strength tests. Therefore, these tests have been correlated by various researchers for ease of evaluating the rock properties such as UCS. This study aims to correlate the UCS with Is50, Vp, SRHN, Ts and density. The core samples were collected from different boreholes locations in the Salt Range, Pakistan. The lithology of the core samples was mainly Limestone of the Chorgali formation. Samples were prepared as per the ISRM standards. Afterwards, the strength tests have been performed in a laboratory followed by correlation of the UCS with Is50, Vp, SRHN, Ts and density. Then estimation capability of these correlations was checked by plotting actual UCS against the UCS estimated by correlation established. Plotted values were checked with reference to 1:1 diagonal. On the basis of which, it was concluded that all correlation were reasonable except UCS versus Is50 axial and UCS versus SRHN axial. The Is50 axial overestimated the UCS because of vertical variation in the lithology from limestone to argillaceous limestone while the SRHN axial underestimated the UCS value because of low rebound due to bedding surfaces encountered. It has been found that the data was scattered and the value of coefficient of variance was considerably high ranging from 29 to 59. These variations in the data were mainly due to mineralogical variation in the Chorgali Formation from limestone to argillaceous limestone.