The paper presents an experimental investigation into the micro-mechanisms controlling the behaviour of non-active clays. Clay microstructural behaviour was investigated by way of mercury intrusion porosimetry (MIP) accompanied by scanning electron microscope images. To gain insight into the mechanisms underlying reversible and non-reversible compression, samples for MIP testing were taken along both normal compression and unloading–reloading lines. To investigate the nature of inter-particle forces, the response of clay samples prepared with deionised water (characterised by acidic pH) was compared with clay samples prepared with alkaline water. A high pH ‘deactivates’ the edge-to-face contacts that are indeed active in the clay prepared with deionised (acidic) water. The pore size distribution data clearly highlighted that the smaller pores are associated with particles in non-contact configuration, that is only interacting by way of the overlap of the repulsive electrical field generated by the negatively charged faces. On the other hand, larger pores are associated with contact configuration, generated by the attraction between the positively charged edge and the negatively charged face of the clay particle. The pore size distribution data also made it possible to infer that reversible behaviour is mainly associated with the reversible overlap of the repulsive electrical field in contact configuration, whereas the plastic response appears to be associated, at the micro-scale, with the loss of edge-to-face contacts. Finally, an embryonic one-dimensional discrete-element method (DEM) model was developed to show the potential of the micromechanical conceptual model to be implemented into a DEM model.

Over the last three decades, colloidal silica has been investigated and more recently adopted as a low viscosity grouting technology (e.g. for grouting rock fractures within geological disposal facilities nuclear waste). The potential of colloidal silica as a favourable grouting material exists due to: its initial low viscosity; its low hydraulic conductivity after gelling (of the order of 10-7 cm/s); the very low injection pressures required; its controllable set/gel times (from minutes to several days); the fact it is environmentally inert; its small particle size (less than hundreds of nanometres) and its cost-effectiveness. Despite the documented success of colloidal silica based grouts for hydraulic barrier formation, research has not translated into widespread industrial use. A key factor in this limited commercial uptake is the lack of a predictive model for grout gelling which controls grout penetration: whilst data are available to underpin design of a grouting campaign in laboratory conditions, little research has been done to underpin applications in natural environments. Here we develop and validate an analytical model of colloidal silica gelling in groundwaters with varying pH and background electrolyte concentrations. This paper presents an analytical model that accounts for changes in pH, electrolyte concentration, cation valency and molar mass, silica particle size and silica concentration giving predictive capability without the need for site-specific calibration. The model is validated against experimental observations for gel times of 32 minutes to 766 minutes, the model accurately predicts the log(gel time) with an average error of 4% which corresponds to an R2 value of 0.96 The model is then applied to a hypothetical case study to demonstrate its use in grout design, based on published in-situ groundwater data from the Olkiluoto area of Finland. The model successfully predicts the required accelerator concentration to achieve a grout gel time of approximately 50 minutes, taking into account the cations already present within the synthetic groundwater

Reconstituted and compacted soils are commonly assumed to exhibit a fundamentally different behaviour due to different microstructure. However, inspection of pore size distribution of the same soil in compacted and reconstituted states suggests that the boundary between these two states is more blurred. This paper explores the continuity between the microstructure of reconstituted and compacted states of kaolin clay and formulates a conceptual constitutive model unifying these states. Clay samples were prepared by saturating the pore space with different fluids (water, acetone and air) and the effect of pore-fluid fraction on the micro- and macroscale response of the clay was investigated experimentally. A conceptual constitutive model for unsaturated clays for quasi-isotropic stress states was therefore formulated, which allows the modelling of various unsaturated hydro-mechanical paths based on constitutive parameters only derived from the compression behaviour of clay under dry and saturated conditions (testing on samples formed from dry powder and slurry, respectively).