Rock strength and the coring interface

Boreholes in rock usually involve drilling to practical tungsten carbide bit refusal and then changing the drilling technique to rock coring. Often this change is used as an indication of a change in rock weathering or strength. Before the commencement of rock coring the SPT result would typically be to refusal. After obtaining the rock core, point load index tests are used to classify the rock strength. This paper examines the validity of the usual assumption of a change in rock property at this drilling interface. The results show that a drilling interface does not necessarily translate into a geological interface or change in rock properties. The current definition of SPT refusal level in rock is shown to be inadequate, and the strength interpretation of such tests in rock is dependent on the rock type and defects of the rock.