Hydraulic conductivity changes due to freeze-thaw in clays are related to changes in microstructure

What is it about?

The freeze-thaw cycles artificially induced in the ground by Ground Source Heat Pump (GSHP) systems working at sub-zero temperatures or during underwater soil excavation, could cause unwanted permanent effects on the hydro-mechanical behavior of the surrounding deposits. Cohesive deposits present a specific sensitivity to these processes connected to phase changes occurring in the interstitial fluid. The experiments conducted on normal-consolidated cohesive soils point out that freeze-thaw (FT) cycles can cause a hysteretic increase in vertical permeability of about one order of magnitude, despite a significant concurrent reduction in void ratio. Although the induced FT processes are quite constrained close to the BHE (Borehole Heat Exchanger), the vertical permeability increase may lead to the risk of connecting aquifers previously separated by cohesive layers, with an overall effect that increases according to the number of probes in the array. Permeability variation appears to be due to the irreversible alteration of the microstructure provoked during the development of FT processes.

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Why is it important?

The occurrence here higlighted is often not considered when designing new closed-loop ground source heat pump borefiels. The working temperature inside the probes has to be maintained over 0°C.