Testing sensitive clays through time and length scales

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

Modeling Aging of Displacement Piles in Natural Soft Clay

A multitude of mechanisms will affect the evolution of the pile response over time, each with their respective time scale. It is shown that most of the processes can be linked to the pile installation stage, which alters the soil surrounding the pile. As a result, there is a change in the mechanical properties of the soil that will influence the subsequent pile response over time. These long-term mechanisms include the dissipation of excess pore pressures from pile installation and the creep in the soil. This paper presents a numerical approach that combines the strain-path method, an advanced effective stress-based constitutive model for soft soils, and a multiphase numerical framework that enables the modeling of the pile installation and subsequent change of pile bearing capacity over time. The presented results demonstrate that the degree of remolding of the soil during the pile installation stage is closely linked to the subsequent pile response. For the Ons\uf8y test case studied, the increase in shaft capacity over time, demonstrated to be linked to undrained strength recovery, could be faithfully reproduced during and after dissipation of excess pore pressures. Hence, pile aging of displacement piles installed in clay is strongly linked to installation effects and the creep and relaxation processes in the soil. Further study is required to fully reveal the physicochemical mechanisms that underpin these processes

Grain kinematics during stress relaxation in sand: Not a problem for x-ray imaging

X-ray tomography is a very valuable tool for studying the full-field 3D deformation of granular materials. The requirement to stop loading and scan a given state (assumed to be stationary) used in most approaches implies unavoidable stress relaxation during scanning. Since scanning times on laboratory tomographs are normally in the order of 1 hour, the strength of the assumption of a stationary state cannot be tested, which introduces some potential weakness in the interpretation of the rich micro-mechanics observed. This paper presents the kinematics of relaxation of a dry natural sand in a typical oedometric cell used for X-ray scanning, using a synchrotron X-ray source to provide scanning times of around 3 minutes, at two different magnifications. This allows the relaxation of the cell & sand system for the first time to be quantified. Advanced image correlation tools are used to quantify the rearrangements of the soil skeleton during loading and the subsequent relaxation. The results indicate that the magnitude of grain displacements during relaxation, associated to ≈4% reduction in externally measured axial stress under oedometric loading, falls below 0.01 D50. It can, therefore, be concluded that the relaxation step required prior to an X-ray scan during an in-situ geomechanical experiment on dry sand does not lead to appreciable uncertainties

Viscoplastic cyclic degradation model for soft natural soils

Cyclic loading affects the long-term response of geostructures build on natural soils. Soft soils are particularly susceptible to the development of large deformations, induced by the repetitive nature of loading. A new viscoplastic cyclic accumulation model is presented, which is an hierarchical extension of the Creep-SClay1S model, to model the long-term permanent deformation resulting from undrained cyclic loading of natural soft clays. The cyclic cumulative strains are incorporated by means of an additional viscoplastic multiplier. This cyclic viscoplastic multiplier adds four additional model parameters that are derived from undrained cyclic triaxial tests. The model is calibrated using experimental data from undrained cyclic triaxial tests performed on high quality block samples of natural Ons\uf8y clay, at different average shear stresses, shear stress amplitudes and loading periods. The accuracy of the proposed model is demonstrated by comparing the element level simulations with the experimental data. The applicability of the proposed model is further illustrated with a boundary value problem, where an embankment submitted to cyclic loading is simulated. The use of the new model enables the simulation of the response of cyclic loaded foundations on soft soils, where the serviceability limit state over a long period of time is governing the design

Towards a methodology for the characterisation of the fabric of wet clays using X-ray scattering

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 preparatio