Locked in stresses, creep and dilatancy of rocks, and constitutive equations

Abstract

Locked in Stresses, Creep and Dilatancy of Rocks, and Constitutive Equations A micro rheological analysis is presented of the deformation characteristics of rocks based on the multi mineral and polycrystalline structure of igneous metamorphic and some sedimentary rocks; in addition the polygranular structure of sandstone-type rocks is also considered. Due account is taken with the history of rock formation and tectonics. Torsional creep test results on Ichang sandstone are presented and analysed. A new method of testing is introduced whereby the sample is subjected to a step wise loading function and the deformation measured as a function of the time. In this manner the creep as a function of stress and time can be obtained very easily from tests on only one sample. A hypothesis is presented on the origin and formation of “locked in” stresses the release of internal strain energy is studied at the hand of some typical test results. The practical importance of these stress pockets is discussed and it is stressed that creep and “locked in” stresses are fundamental factors in the behaviour of rocks which must be carefully studied in practice. On the basis of experimental results and physical reasoning constitutive equations are set up, which are three dimensional generalisations of the experimental Griggs equation:γ =a +b logt +ct, whereby the material parameters are scalar functions of the stress invariants.