Deformation and Vertical Permeability Variations Induced by Freeze-Thaw Cycles in Over-Consolidated Silty Clays

In closed-loop Ground Source Heat Pump system, the thermal exchange between the building and the subsoil is provided by ground heat exchangers inserted into the ground with a circulating heat-carrier fluid. A heat pump manages the system by regulating the fluid working temperature. When a greater heat extraction rate from the subsoil is needed, the fluid working temperature can be lowered down to −5 °C by adding anti-freezing additives. This way, the thermal alteration induced in the subsoil is more intense and can lead to freezing processes in the surrounding ground. The geotechnical properties of sediments with significant clay fraction are irreversibly affected by the occurring alterations in the internal structure and porosity distribution that are caused by the freeze-thaw cycles. A series of experimental tests have been conducted to study vertical deformations and permeability variations induced by freeze-thaw cycles in Over-Consolidated silty clays at different Over-Consolidation Ratios (OCR). Whilst in Normal-Consolidated silty clays an irreversible settlement up to 16% is gathered cycle after cycle depending on sediment plasticity, pore fluid salinity and applied load, the OC silty-clays show an opposite behavior. Experimental results point out that, in case of OC deposits, higher the OCR lower the freeze-thaw induced settlement. In case of OCR > 15, the settlement turns to a slight expansion. Conversely, the observed augment in vertical permeability increases with the OCR degree.