Can We Trust Numerical Collapse Load Simulations Using Non- associated Flow Rules?

ABSTRACT: Numerical simulations using continuum mechanics and elasto-plasticity are often used in geotechnical design. Determination of collapse loads may be part of such simulations. However, non-associated flow rules applied to frictional materials normally provide lower collapse loads than stated by classical methods. Should we stick to the classical solutions for design or should we rather trust the numerically obtained solutions? Non-associated plasticity is attractive when observed soil behaviour in conventional laboratory tests shall be reproduced. In this paper example simulations are made by the computer program PLAXIS showing lower failure loads for non-associated plasticity than the classical solutions. Well accepted theories for material instability using non-associated plasticity may explain the discrepancy, Rudnicki and Rice (1975). The instability implies bifurcations, zero stiffness modes and localized deformation in shear bands. The illustrations in this paper are focusing more on the physical collapse mechanisms than the mathematical singularities. The paper encourages a discussion on the practical consequences of using non-associated flow rules. The problem is of growing relevance since failure margins increasingly often are estimated using commercial finite element codes.