Induced thermal compaction in cohesive sediments around a borehole heat exchanger: laboratory tests on the effect of pore water salinity

In closed-loop geothermal systems, the operation of a Borehole Heat Exchanger (BHE) affects the thermal equilibrium of the surrounding subsoil. This study was part of a large laboratory program undertaken to understand how the cyclic thermal stress induced by a BHE in the subsoil can change sediments’ mechanical properties, if the BHE works in extreme running conditions which induce both freeze–thaw cycles and heating processes in the ground. This paper presents the results from an experimental laboratory investigation on the effects of different pore water salinity concentrations on the vertical strain induced in a thermally loaded silty clay. The considered study case is the historical center of Venice (Italy), which provides an example of typical conditions of densely urbanized areas inserted in coastal or transitional environments, with abundance of cohesive layers and brackish conditions in the subsoil. Results show that the irreversible compaction effect induced on sediments increases with higher salinity concentration, despite the fact that increasing salt content lowers the sediment freezing point, thereby protecting the soil from freezing processes.