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Radboud Universiteit Nijmegen, 27 juni 2011, Promotor : Vrakking, M.J.J., Contains fulltext : 90925.pdf (publisher's version ) (Open Access)

This paper reports the effects of structure on the mechanical responses of kaolinite with known and controlled fabric associations. The dynamic properties and strength were assessed by resonant column tests and undrained triaxial compression tests, respectively. The experimental results demonstrate that interparticle forces and associated fabric arrangements influence the volumetric change under isotropic compression. Soils with different structures have individual consolidation lines, and the merging trend is not readily seen under an isotropic confinement up to 250 kPa. The dynamic properties of kaolinite were found to be intimately related to the soil structure. Stronger interparticle forces or higher degrees of flocculated structure lead to a greater small-strain shear modulus, G(max), and a lower associated damping ratio, D-min. The soil structure has no apparent influence on the critical-state friction angle (phi'(c) = 27.5 degrees), which suggests that the critical stress ratio does not depend on interparticle forces. The undrained shear strength of kaolinite is controlled by its initial packing density rather than by any interparticle attractive forces, and yet the influence of the structure on the effective stress path is obvious.

In this study on the surface charges and structural characterization of kaolinite clay, it is found that the highly pH dependent edge charges are important to the interparticle forces and associated fabric formations. When the pH is below the isoelectrical point of the edge surfaces, IEPedge, the Coulombian attraction between faces and edges dominates, which favors the formation of an opened edge-to-face (EF) association and a higher final sediment volume. At pH > IEPedge, the double-layer repulsion prevails at all surfaces to result in a deflocculated and dispersed fabric, which tends to form denser soil packing in the sediment. Increasing the ionic strength (with NaCl as the electrolyte) leads to the opposite behavior in clay suspensions below and above the IEPedge: the degree of fabric flocculation and the final sediment volume decrease with an increased ionic strength at pH < IEPedge but increase at pH > IEPedge. The measured pore-size distribution reveals a distinct dual-porosity characteristic in the pH 4 with salts (0.15 mol/L NaCl) specimen: it consists of inter-aggregate spaces (similar to 1.10 mu m) and intra-aggregate pores (similar to 0.30 mu m). Soils with a higher degree of EF flocculation due to the prevailing Coulombian or van der Waals attraction can render a higher liquid limit.