Stiff clay masses: pore water salinity, geotechnical properties and system energy
R. Pellicani, I. Argentiero, M.D. Fidelibus, S. Fiore, G. Spilotro
Natural soils are characterized by grain size and mineralogical assortments that derive from the history of sediment: original rocks, selective transport phenomena, chemico-physical interactions with the environment and, in particular, with ground- and surface waters. The deformation response to external loads is defined with different and contemporary mechanisms by the solid skeleton and the clay component. In both cases, mineralogy defines the behaviour at the micro and nano scale. The coarse component responds with a work of elastic (conservative) and non-elastic deformation (rearrangement or crushing of soil particles or components). Almost a century of studies of the inner structure of clays allows to assimilate it to a capacitor, whose dielectric is the DDL and the filler between the plates, electrically negatively charged. This kind of system is conservative from the energetic point of view and the inner capacity of energy storage depends from the mineralogical type of the clay, and from the dielectric type, which in turn depends on water having variable concentration of dissolved ions. Clay swelling should be regarded as a typical energy release phenomenon, which depends from the soil type and from the previous energy stored. It is relevant to note that the energy release can be triggered either by load confinement changes, or by changes of border environmental conditions, such as the salinity and chemistry of interacting groundwater, or their concentration by severe sun driven capillarity.
The paper shows results of the behaviour of clayey soils sampled in several contexts in the blue clays from the Bradanic foredeep (southern Italy), emphasizing the role of geomorphological and hydrogeological context on the salinity and chemical composition of clay pore waters and on the geotechnical and swelling behaviour of this very common kind of soils. The pore water salinity has been tested after high pressure squeezing on soil samples and the energy balance has been evaluated by means of one-dimensional confined compression tests. For the tested soils, the physical parameters and swelling capabilities are strictly relate to the pore water salinity (and chemical composition).
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