Abstract

Uniaxial Compressive Strength (UCS) is the most widely used index of rock strength. Strength under confined conditions most often is characterized by a straight line Mohr-Coulomb failure envelope. Although required as input to engineering design, reliable values of one or both of these parameters often are not available to the design engineer because of the exacting requirements, and the cost of preparing specimens and conducting a sufficient number of tests, under uniaxial and confined pressure conditions.

The Scratch Test was developed at the University of Minnesota in the late 1990s, when a remarkably strong correlation was discovered between the specific energy required to cut a shallow groove into the surface of a rock and the UCS (http://www.cefor.umn.edu/facilities/rsd/). The test now has matured into a reliable and relatively inexpensive practical procedure to provide the necessary engineering design information. The procedure has the added advantage that, except for a shallow groove cut along the edge of the core, the test is ‘non-destructive’, leaving the core available for other studies.

This paper discusses the theoretical basis for the equivalence between the specific energy of cutting, ε, and the UCS (q), and indicates that the 1:1 relationship between these values has a sound mechanistic basis.

The Mohr- Coulomb strength envelope also can be obtained by substitution of a blunt cutting tool in the Rock Strength Device (RSD) — or Scratcher — to determine the coefficient of friction for the rock.

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