Experimental analysis of the elasto-plastic zone surrounding a borehole in a specimen of rock-like material under multiaxial pressure

Abstract

The determination of elasto-plastic zones in cohesive frictional materials representing those in a rock through experimentation is important for the analysis of structures. Most of the materials used in such tests are not transparent. The propagation of plastic zones can thus not be observed directly, and a comparison between analytical and experimental results is hampered. In the experimental technique proposed here a transparent model material is used. Its pattern of failure corresponds to that following from Mohr-Coulomb's yield criterion. One observes the development of slip lines intersecting each other at an angle predicted by the slipline theory. Theoretical calculations and experimental measurements of the geometry of the plastic zone surrounding the circular borehole are compared under the conditions of plane strain and anisotropic state of initial stress. The results show that there is a broken plastic zone around the hole, which is similar to a compressed ellipse. The observed significant deviation of the plastic zone from an exact elliptical shape is due to internal friction in the plastic zone of the specimen. There are some instabilities in the compressive response of the specimens that can be observed in the experiment. They are comparable to the instabilities occurring in deep mining borehole drilling. Theoretical calculations may indicate that the instabilities are partly due to the development of an instable stress distribution.