One of the main parameters that is commonly used for the rock mass characterization is the Uniaxial Compressive Strength (UCS) of the intact rock. This parameter is conventionally measured by diamond drilling to obtain core, and testing core samples in the laboratory, which is an expensive and time consuming process. It would be ideal if the strength of the intact rock could be estimated in the field as an in-situ measurement by running a device inside a borehole. Estimation of the rock strength by scratch tests has been reasonably accurate. Application of this concept for the assessment of rock strength in a borehole has led to development of a borehole strength measurement probe, namely Rock Strength Borehole Probe (RSBP). RSBP is designed to scratch the surface of rock with a miniature disc cutter inside a relatively slim borehole. The cutting forces are measured and used to estimate the rock strength. This paper discusses the background theories and experimental tests conducted for development of the RSBP, along with formulas developed for estimating the compressive and tensile strengths of the intact rocks. The RSBP probe and its components are briefly explained, and the observations from the initial field experiments with this probe are discussed in this paper.

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