Experimental observations and modelling of compaction bands in oedometric tests on high porosity rocks

Abstract

Experimental results obtained in a series of displacement controlled oedometric tests on soft rocks are presented. Four different materials characterised by a high void ratio have been examined; three natural soft rocks and an artificial one. The materials under investigation were conchyliates stone, calcarenite, pumice stone and Gasbeton. In order to monitor the stress path as well, a soft oedometer ring was designed and constructed for the measurement of the radial stresses. The observed behaviour can be divided in three phases. After an initial phase in which the mechanical response is essentially elastic, a second phase starts, in which bonds are progressively broken, so that in the axial stress-strain curve a stress decrease is recorded and in some cases, a sort of 'curl'appears in the stress path. This is associated to the occurrence of strain non-homogeneities in the form of compaction bands. In the final part of the experiment the axial stress increases exponentially and the stress path returns to be linear, as the one expected for a cohesionless material. The experimental behaviour is reproduced by means of an elastoplastic strain-hardening/softening constitutive model and the occurrence of compaction bands is theoretically predicted. © 2009 Blackwell Publishing Ltd.