:
This paper presents the results of preliminary experiments that were carried out with the Rock Strength Device (RSD), to investigate the feasibility of determining rock strength parameters from cutting tests. The approach used to determine the strength parameters of the rocks with the RSD is novel. It is based on an interpretation of the forces required to [scratch] a rock. This approach has many advantages over the conventional method for measuring strength parameters: low cost and portability of the RSD, operational simplicity and rapidity of measurement, ability to make many measurements, ability to provide a log of strength from cores, and relatively non-destructive testing of the rock core.
Research in the understanding of rock drilling and cutting processes is usually motivated by the need to optimize the performance of drilling/cutting equipment, see for example Fowell (1993) and Nishimatsu (1993) for recent reviews of rock cutting models. In contrast, this paper describes a methodology to extract information on the strength of rock, directly from results of cutting tests. The approach is based on a phenomenological model of continuous rock cutting with a blunt cutter proposed by Detournay and Defourny (1992), which will be referred to as the DD model in the following. The DD model is based on the assumptions that the total force acting on a cutter is the sum of forces that act on the cutting face and on the wear flat (which often exists), and that these two contributions are uncoupled. By assuming simple relations for these two forces, the model introduces three parameters which are: (i) the intrinsic specific energy, e, (ii) the ratio of the vertical to horizontal forces acting on the cutting face, ?, and (iii) a friction coefficient µ characterizing the rock-cutter flat interface. The DD model was developed to describe the rock cutting process, under conditions when the mode of rock failure induced by the cutter can be described as plastic. This is typically the failure mode observed in sedimentary rocks at small depth of cut, of order of 1 mm. The predictions of this model are indeed consistent with published results of cutting experiments performed at Sandia National Laboratories (Glowka, 1987), at Imperial College, London (Almenara, 1992; Almenara and Detournay, 1992; Samiselo, 1992), and at the University of Minnesota (Lasserre, 1994; Detournay et al, 1995). Furthermore, these experimental results have provided strong indications that ì can be regarded as the coefficient of internal friction of the rock in a linear Mohr-Coulomb failure criterion (i.e., µ = tan µ, where µ can be regarded as the internal friction angle of the rock), and that the intrinsic specific energy e of a given rock can be considered as a measure of its uniaxial compressive strength q. (These cutters are made of a thin layer of polycrystalline diamond material supported by a tungsten-carbide substrate). This paper presents the methodology to assess the strength of rock using cutting tests and discuss the experimental evidence that supports this claim.
