Georgios Birmpilis

Unique four-dimensional (4D) deformation data are collected during drained triaxial tests on intact specimens of a natural sensitive clay. This requires the development of a miniature triaxial cell for advanced stress path testing, specifically designed for X-ray computed tomography. Salient features include the omission of a membrane, and a mounting procedure that minimises disturbance by the experimenter. Three distinct drained stress ratios are studied for pseudo-isotropic, K₀, and highly deviatoric loading paths. The results indicate that the K₀path shows the most uniform deformation mechanism, where the measured ratio of deviatoric and volumetric strain increments reach the stress ratio applied at boundary value level for large magnitudes of total strain. The pseudo-isotropic test also reaches a strain ratio close to η at large total strain levels; however, the deformation field is less uniform. Furthermore, the highly deviatoric stress path shows the most heterogeneous deformation fields commensurate with the applied stress ratio, although the ratio of deviatoric and volumetric strain increments falls above the η applied. The mean value of the three-dimensional spatial fields of strain corresponds well with the changes observed at boundary level, supporting prior research on drained stress-probing on clays for which there are no 4D deformation data available.

In the Nordics, engineering of sensitive soils is of vital importance for realising the urban environment and the infrastructure connecting and underpinning it. The hydro-mechanical behaviour of sensitive clays is strongly affected by the geological deposition history in the area and subsequent human activities resulting in changes in stress and environmental loading. Finally, the characteristic physico-chemical identity of the clay-water system is expressed by the sensitivity of the material. A combination of classic geotechnical tests in the laboratory needs to be complemented with state-of-the-art technologies for material analyses, in order to deepen our understanding of the interaction between the colloidal nature of the clay and the observed response at the engineering scale. Until now these activities have been performed separately. The microstructural observations using various microscopy techniques were not directly combined with classic geotechnical tests. In contrast, the work presented herein showcases methodologies for simultaneous monitoring of fabric and mechanical probing under controlled conditions on samples of sensitive clay. In order to enable real-time fabric measurements of samples of sensitive clay two non-invasive techniques are utilised: X-ray Scattering (XS) and X-ray Computed Tomography (XCT). Furthermore, a bespoke apparatus for sample probing is designed and built at Chalmers University of Technology. The design of the apparatus is adapted to the special challenges of very soft samples and addresses issues of sample quality in soft soil testing (i.e., sample mounting, membraneless configuration). The intention of this work is to demonstrate the feasibility of expanding geotechnical testing outside the limits of the traditional geotechnical laboratory, combining geotechnics with state-of-the-art technologies for material analysis. This, in the end, provides a critical view on the perception of the material and its constituents that aims to contribute in improvement in geotechnical laboratory testing and the development of advanced constitutive models for sensitive clays.

The potential of X-ray scattering measurements for monitoring changes on the nano-scale in fine-grained materials in their natural wet state is demonstrated with a series of feasibility tests on well-controlled kaolin samples – that is, water content, pH and loading history. The results indicate that subtle changes on the nanometric scale, especially the particle orientations, can be measured with high fidelity using a standard laboratory small- and wide-angle X-ray scattering instrument. This opens up possibilities for future in situ loading tests with simultaneous monitoring of the evolving changes of the fabric in fine-grained soils.

X-ray scattering is a promising non-invasive technique to study evolving nano- and micromechanics in clays. This study discusses the experimental considerations and a successful method to enable X-ray scattering to study clay samples at two extreme stages of consolidation. It is shown that the proposed sample environment comprising flat capillaries with a hydrophobic coating can be used for a wide range of voids ratios ranging from a clay suspension to consolidated clay samples, that are cut from larger specimens of reconstituted or natural clay. The initial X-ray scattering results using a laboratory instrument indicate that valuable information on, in principal evolving, clay fabric can be measured. Features such as characteristic distance between structural units and particle orientations are obtained for a slurry and a consolidated sample of kaolinite. Combined with other promising measurement techniques from Materials Science the proposed method will help advance the contemporary understanding on the behaviour of dense colloidal systems of clay, as it does not require detrimental sample preparation