Geotechnical Hazards Caused by Freezing-Thawing Processes Induced by Borehole Heat Exchangers

In closed-loop Ground Source Heat Pump system (GSHP), the thermal exchange with the underground is provided by a heat-carrier fluid circulating into the probes. In order to improve the heat extraction rate during winter, the heat-carrier fluid temperature is often lowered down to -5 °C; this way, the induced thermal anomaly is more intense and can cause freezing processes in the surrounding ground. In sediments with significant clay fraction, the inner structure and the porosity distribution are irreversibly altered by freezingthawing cycles; therefore, the geotechnical properties (such as deformability, stiffness and permeability) are significantly affected. A wide laboratory program has been performed in order to measure the induced deformations and the permeability variations under different conditions of thermal and mechanical loads and interstitial water salinity. The results suggest that, despite the induced frozen condition is quite confined close to the borehole, the compaction induced in cohesive layers is significant and irreversible and could generate negative friction on the borehole heat exchanger; in addition, the permeability increase in the probe surroundings could lead interconnection among aquifers, with increasing effects with the number of boreholes in the borefield.