Experimental determination of volumetric strains in failed rock

Abstract

Stress-strain and stress-volumetric strain curves are frequently used to study details of rock failure in laboratory compression tests. ‘Stiff testing machines’ have provided a facility for observing the behaviour of rock beyond the peak of the stress-strain curve. Volumetric strains have not been measured throughout the complete range of sample deformation primarily because the conventional method of measuring volumetric strains—using resistance strain gauges attached to the rock—is limited to small strains and also subject to inaccuracy because of the influences of localized failure. In this paper a method for determining average volumetric strains in cylindrical test specimens is presented. Because the method gives direct measurement of the lateral component of volumetric strain, and because axial strain is the independent variable in deformation-controlled tests, the results are given as lateral volumetric strain-axial strain curves, from which, if desired, volumetric strains can easily be found. Results are given for Wombeyan marble, sandstone, norite, quartzite and, for comparison, aluminium. All of the rocks tested exhibited volumetric expansion relative to the deformation as a result of linear, elastic compression. Volumetric expansion of all specimens began at about one-half the maximum axial stress and, except for the specimen of norite in which failure was not controlled, the expansion was accelerated beyond the peak of the axial stress-axial strain curve. A comprehensive series of tests on quartzite revealed that for this rock confining stresses of up to 5000 lb/in 2 do not appreciably affect the axial stress-axial strain behaviour beyond the peak axial stress, but considerably reduce the amount, although not necessarily the maximum rate, of volumetric expansion.