Squeezed Interstitial Water and Soil Properties in Pleistocene Blue Clays under Different Natural Environments

Abstract

Studies dating almost a century relate clay properties with the structure of the diffuse double layer (DDL), where the charged surfaces of clay crystal behave like an electric capacitor, whose dielectric is the interstitial fluid. The intensity of the inner electric field relates to the concentration and type of ions in the DDL. Other important implications of the model are less stressed: this part of the clay soil system, energetically speaking, is conservative. External contribution of energy, work of overburden or sun driven capillarity and long exposure to border low salinity waters can modify the concentration of pore-waters, thus affecting the DDL geometry, with electric field and energy storage variations. The study of clay soils coming from various natural geomorphological and hydrogeological contexts, determining a different salinity of interacting groundwater, shows how the clay interaction with freely circulating waters at the boundaries produces alterations in the native pore water salinity, and, at the nano-scale, variations of electric field and stored energy from external work. The swelling and the shrinkage of clay soil with their volumetric and geotechnical implications should be regarded as variations of the electrostatic and mechanical energy of the system. The study is based on tests on natural clay soil samples coming from a formation of stiff blue clays, widespread in southern Italy. Geotechnical identification and oedometer tests have been performed, and pore waters squeezed out from the specimens have been analyzed. Tested samples have similar grain size, clay fraction and plasticity; sorted according to the classified geomorphological/hydrogeological contexts, they highlight good correlations among dry density, mechanical work performed in selected stages of the oedometric test, swelling and non-swelling behaviour, and electrical conductivity of the squeezed pore waters. The work performed for swelling and non-swelling samples shows well-defined differences; this endorse the relevance of pore-water salinity in determining the volumetric state of clay soils under overburden and specific hydrogeological border conditions, which together define a specific energetic state.

Characterisation of the multi-scale fabric features of high plasticity clays

“…It is mainly illitic, but includes a significant amount of smectite, interstratified illite-smectite and carbonatic silt. The Total Dissolved Solids (TDS) content and electrical conductivity of the squeezed pore fluid in the natural clay at Montemesola, in the same geological basin as Pappadai clay, allow to recognize a low salinity pore water (Fidelibus et al, 2018). The history and the macro-behaviour of both the natural and the reconstituted Pappadai clay are discussed by Chandler (1995 and1997).…”

“…the clay achieving its microstructure through a lab history common for all reconstituted clays, was prepared as clay slurry of water content 1.25•LL. Given the low salt concentration in the pore fluid (Fidelibus et al, 2018) of the natural clay, distilled water was used to prepare the corresponding reconstituted clay slurry. Distilled water was also used to preserve saturation of both the natural and the reconstituted clay specimens during the mechanical tests.…”