Neutron imaging and digital volume correlation to analyse the coupled hydro-mechanics of geomaterials

A new approach to characterise the evolution and coupling of deformation and fluid flow in geomaterials is presented. The method exploits some key features of neutrons, namely penetration of dense materials used for triaxial pressure cells, sensitivity to hydrogen and the possibility to distinguish hydrogen from its isotope deuterium (in normal water, H₂O, and heavy water, D₂O, respectively). Illustration of the approach is provided with results from a combined fluid flow/triaxial compression test on a cemented sand specimen performed in-situ (i.e., acquiring images during loading) at a neutron imaging station. Quantitative analysis of neutron tomography images acquired at different stages of deformation is made by Digital Volume Correlation to provide full 3D strain fields that highlight the evolution of localised deformation features. Spatio-temporal tracking of the effect of the evolution of the permeability in the sample was possible by neutron radiographies acquired during pressure driven flow of H₂O into the sample saturated with D₂O. By exploiting the H₂O/D₂O neutron transmission contrast and similarities of their flow behaviour, the tracking of the H₂O/D₂O front can be considered as an indicator of the permeability of the sample that is correlated to the measured evolution of the deformation.