ABSTRACT
Many boreholes enlarge in cross section from circular to elliptical shape. Such changes are defined as breakouts when the shorter diameter remains equal to the drill-bit diameter. Mastin studied borehole breakouts under uniaxial loading. He explained that breakouts are the result of an association of tensile rupture in a direction parallel to the borehole wall and buckling of thin plates of rock limited by the tensile fractures and the borehole wall. The material in the vicinity of borehole may undergo plasticity and damage at microscopic scale but remains continuous for an observer until the formation of macroscopic fractures which mark the begenning of the spalling process. Gauges were used to measure the local strain at several points of the block surfaces, particularly at the points of maximum stress on the borehole wall. Blocks were compressed between two platens of a 180 tonnes capacity compressive machine, failure initiation and propagation were observed.
