Multi-scale fabric evolution during hydro-mechanical probing of fine-grained soils

Doctoral thesis, 2020

The majority of the studies on sensitive clays so far have focused on the hydro-mechanical response at engineering scale, using concepts from continuum mechanics. The fundamental mechanisms at particle scale, underpinning the emerging response, have so far been studied with post mortem analyses. Recent technological advances offer the possibility to monitor the evolving internal material response of clays simultaneously during testing. As opposed to only characterisation of clays at the nanometre scale, in this thesis X-ray techniques were used for a Swedish sensitive clay in-operando, during geomechanical testing. The aim was to quantify the response of sensitive clays, spanning from intraparticle to continuum scale, enabling to link the evolving internal material behaviour to the constitutive response at boundary value level. At submicron length-scale, Wide and Small Angle X-ray Scattering at laboratory and synchrotron facilities were used to track particle rotation and intraparticle spacing. This required the development of a plane strain X-ray trans-parent oedometer cell. X-ray Computed Tomography (XCT) was used to uniquely characterise the undisturbed sensitive clay at submicron scale using a nanotomograph at a synchrotron beam line in 3D. Most importantly, the 4D evolution of internal deformation was quantitatively monitored during drained hydro-mechanical probing. The latter required development of XCLAY, a bespoke miniature Bishop-Wesley triaxial cell enabling advanced stress-path testing compatible with XCT. X-ray scattering provided insight on the nanometre scale, in terms of the integrated response of intraparticle strain and particle orientation. During the 1D compression test two minerals present in the natural clay behaved differently: illite was stable, while for montmorillonite a new spacing was detected. Both minerals continued to align towards the horizontal axis. For the first time, the internal 3D structure within an undisturbed sensitive clay sample was revealed at submicron scale. The evolving internal deformations of the natural sensitive clay were resolved during hydro-mechanical probing in-operando under drained triaxial compression, considering pseudo-isotropic, K0 and highly deviatoric loading. The 4D deformation fields were extracted from the tomography data using Digital Volume Correlation (DVC). The K0 stress path resulted in the most homogeneous full-field strain maps, while the highly deviatoric path resulted in very inhomogeneous strain fields. The mean values of the strain fields compared well with the external measurements, thereby reinforcing the validity of prior experimental data on soft clays. The thesis demonstrates that geomechanical laboratory tests on fine-grained soils can be elegantly combined with non-destructive techniques.