Mechanical Behaviour of Rocks under True Triaxial Compression Conditions – Volumetric Strain and Dil

Mechanical Behaviour of Rocks under True Triaxial Compression Conditions – Volumetric Strain and Dilatancy Author: M. Kwaśniewski It has long been accepted that dilatancy is a phenomenon that can be treated as an important precursor of the brittle failure of rocks. Dilatancy is understood as an inelastic increase in volume during deformation under applied differential stress and is interpreted as being due to microcracking within the rock with a concomitant increase in void space. Dilatancy in rocks is of considerable practical importance since it is associated with possible premonitory signs of earthquakes, mining-induced rockbursts and mine roof collapses. The fundamentals of dilatancy in rocks are introduced at the beginning of the present paper. These are followed by a brief explanation of what the dilatancy-related precursory phenomena to brittle failure are. The introductory part is completed by a review of the studies of dilatancy under true triaxial compression conditions to-date. The main part of the paper includes a presentation of the results of true triaxial compression tests on samples of a foliated, anisotropic crystalline schist and a Coal-Measure, medium- to coarse-grained sandstone. The main aim of the tests was to reveal the effect of, separately, confi ning pressure and intermediate principal stress on the volumetric deformation mode and the dilatant behaviour of rocks. Results of the experimental studies show that the effect of dilatancy is suppressed or inhibited by the intermediate principal stress. While important negative, dilatant strain corresponds to the peak differential stress under axisymmetric stress conditions, under true triaxial compression, where intermediate principal stress is much higher than the minimum stress, the rock material apparently undergoes microcracking to a much lesser extent and behaves in an increasingly brittle manner. Faulting of rock samples in the post-peak region is much more violent and is accompanied by a strong acoustic effect associated with a release of elastic strain energy.

 

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