Effects of soil structure on the mechanical properties of kaolinite clay

Abstract

Behaviour of kaolinite clay is intimately related to the pore-fluid properties, such as the pH value, electrolyte concentration, ion type, and presence of organic compounds among others, because of its charged surfaces and associated interparticle electrical forces. The first part of this research focuses on the fabric formations and characterizations. The effects of pH and electrolyte concentrations on the interparticle forces and associated fabric formations were studied. Experimental results, including those from sedimentation tests, pore-size measurements, SEM images, and liquid limit tests reveal that the Coulombian attraction force dominates at low pH (pH below the isoelectric point of edge surfaces, IEPedge) and low electrolyte concentrations, which favors the formation of an opened face-to-edge fabric. At high pH (pH > IEPedge) and low electrolyte concentrations, the prevailing double layer repulsion leads to a deflocculated and dispersed fabric which tends to form a denser soil packing with particles in parallel alignment after deposition or consolidation. At high electrolyte concentrations, van der Waals' attraction overwhelms the repulsion due to a shrunk double layer, and therefore soil aggregates and a relatively opened soil packing are formed accordingly. The second part highlights the structure effects on the dynamic properties. The shear modulus and damping ratio of kaolinite with different structures, selected based on the results of fabric characterizations, were assessed by means of the Energy-Injecting Virtual Mass (EIVM) Resonant Column System. The experimental results demonstrated that the dynamic properties of kaolinite closely interrelate with interparticle forces and associated fabric arrangements. Stronger interparticle forces or higher degrees of flocculated structure lead to a greater shear modulus Gmax and lower damping ratio Dmin. The structure influence on the volumetric change and strain response under isotropic compression is apparent. Specimens with different structures have their individual consolidation lines and the merging trend is not readily seen as the isotropic confinement is up to 250 kPa. Moreover, the compressibility increases with increasing degree of flocculated structure. Anisotropic strain responses due to fabric effects can be found as well. The fabric effects on the critical-state and stress-strain behaviour are discussed in the last part based on the experimental observations of undrained shear tests. The soil structure has no apparent influence on the critical state friction angle (∅c'=27.5°), which suggests the irrelevance between the critical-stress ratio and the interparticle forces. The undrained shear strength is seemly controlled by the initial packing density rather than the interparticle forces. Differences in the effective stress path are evident. Specimens with flocculated structure show a contractive tendency until reaching the critical state, while more complicate behaviour can be observed for the specimens with dispersed structure: showing a contractive tendency initially, passing a phase-transformation state afterwards, and performing dilative shearing towards the critical state in the end. Apart from the substantial influence on the isotropic normal compression line, the soil structure also has a noticeable effect on the volumetric behaviour at critical states for the specimens under confining pressure of 100kPa