The Modelling of Soil Plasticity

摘要

Crushable agglomerates can be simulated in the distinct element method (DEM) by bonding elementary spheres in 'crystallographic' arrays. The behaviour of an element comprising these numerically-generated soil grains resembles that of a crushable anisotropic sand, in terms of irrecoverable compression, yielding and plastic hardening behaviour. Stress path tests are simulated to capture the response of the simulated soil to various combinations of deviatoric and spherical stress changes,δq and δp, in both loading and unloading. The plastic deformation is comparable with certain features of the Cambridge soil plasticity models, although the definition of a single yield surface separating elastic from plastic behaviour is arbitrary, exactly as it is with actual triaxial tests on real soils. Normality of the plastic strain increment vector to conventionally defined yield surfaces is not observed. It is shown that grain breakage is a crucial component of plastic soil behaviour. Yield surfaces obtained from numerical stress-path tests are best quantified by the percentage of bond-breaking (pbb) contours. These surfaces are compared with contours of yield defined alternatively from inspecting stress-strain curves. An "elastic wall" in voids ratio log mean effective stress (e, In p') space is found, but the normality rule is not followed at yield. Similarity is observed in the shapes of yield surfaces at different degrees of strain-hardening, both in the e-ln p' space and the q-p' space. Their shape resembles the Modified Cam Clay model more than the Original. Lastly, a reduction of internal coefficient of friction, represented by the Cam Clay parameter M, is observed when stress-level is increased; this also resembles real soils.