Investigation on the effects of freezing-thawing cycles on energy piles

In cold regions, the demand for heating can be particularly high. The use of energy piles in such regions is characterized by excessive heat extraction, which may lead to sub-zero temperatures, inducing repeated freezing-thawing cycles. Furthermore, this phenomenon can be induced as a result of design flaws, system malfunctioning, or even by intentional design if it is proven not to cause adverse effects, as this extends the usage temperature range. However, its thermo-mechanical impact on the pile performance is unclear. This study dives into the impact of heating-cooling and freezing-thawing cycles on energy pile systems through small-scale laboratory tests. The results indicated that heating-cooling cycles could induce pile head settlement of the energy pile in dry and saturated sand. These settlements are higher but limited in saturated sand compared to dry sand. Furthermore, freezing-thawing cycles induced a high ratcheting and eventually exceeded the serviceability limits. This phenomenon can be explained by the unfrozen water migrating towards the interface under the effect of a temperature gradient, filling and enlarging the pores and forming an ice film, which initially increases the cementation and cohesion of frozen soil at the interface. However, during thawing, the ice bond at the interface begins to melt, and the loosening of soils could happen. Therefore, the load would be transferred to the pore water (which had been altered during freezing), causing significant ratcheting.