Modified compressibility of cohesive sediments induced by thermal anomalies due to a borehole heat exchanger

In Ground Source Heat Pump systems (GSHP) a continuous circulation of a fluid inside the exchangers installed in the ground transfers heat between the ground and the building's conditioning system. Often the carrier fluids are brines consisting of a mixture of water and anti-freezing solutions, which lower their working temperature in order to improve the heat extraction from the ground during the cold season. The international community has already pointed out the importance of assessing a minimum temperature threshold for the brines inside the probes, in order to constrain the thermal anomalies induced in the soil. This paper analyzes how a borehole heat exchanger could affect the natural consolidation processes of surrounding cohesive layers, if the thermal anomaly induces freezing and subsequent heating processes. The study case of Venice (Italy) is considered, where GSHP systems could be a very interesting solution for the issues related to the particular configuration of the city center, the density of historical buildings and the local regulations. Venice represents an example of a densely urbanized area with a subsoil characterized by a continuous alternation between cohesive and coarser layers, as inmost coastal lowland plains. The induced thermal settlement and the increased stiffness are measured in a silty-clay sampled in the area by means of a special device consisting in a thermostatically controlled oedometer. The obtained results contribute to the definition of the environmental hazards connected with the use of GSHP systems.