Abstract

High porosity soft rocks are characterized by a microstructure that collapse under loading exerting relevant volumetric strains in localized tabular zones perpendicular to the loading direction, called compaction bands. A detailed experimental study of the formation and the evolution of compaction bands will be presented. A strain-controlled oedometric apparatus able to detect radial stress has been used for testing artificial and natural soft rocks. Initially, the mechanical response is essentially elastic until bonds are progressively broken. The axial strain-stress curve exhibits an unloading branch and a sort of curl appears in the stress path. Then, the behaviour becomes similar to an unbonded soil. A series of Micro-CT X-rays scansions are performed at different levels of total axial strain. Two compaction bands are formed one at the bottom and one at the top, due to the friction between the soft rock and the loading cups. By increasing the strain, the thickness of the two bands increase towards the center of the specimen, so that the whole specimen is completely destructured. Finally the experimental behaviour is reproduced with an elastoplastic constitutive model and the occurrence of compaction bands is theoretically predicted.

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