| Abstract |
Freeze-thaw processes induced in the immediate environment of borehole heat exchangers (BHE) as a result of operating temperatures below 0 °C can significantly damage the compound structure consisting of the BHE pipes, the cement based grout as well as the surrounding subsoil. The hydraulic integrity of such systems is not ensured anymore and its thermal efficiency could be significantly impaired. However, detailed knowledge on freezing and thawing processes in porous media, such as the grout and unconsolidated rock materials, is still incomplete. It is known, that the content of unfrozen water has a strong impact on material properties influencing the overall heat and mass transfer processes. Moreover, freezing strongly depends on various boundary conditions such as the soil type or pore water chemistry. Accordingly, it is essential to have adequate information about the freezing interval for different boundary conditions, which describe the transition from pure liquid water to the ice phase and vice versa. Therefore, a thermo-hydraulic-mechanical (THM) experiment is used to gain a more detailed knowledge of the freezing processes in BHE grouts and unconsolidated rock materials. For this purpose, an ultrasonic measurement device is linked to the freeze-thaw experiment: The wave velocity in solid particles in the observed temperature range is constant and not affected by temperature changes. However, with descending temperature, the ice content increases, which leads to an improved cross-linking of the solid soil particles. As a consequence, the bulk P-wave velocity increases with decreasing unfrozen water content. Hence, this relationship can be used to determine the content of unfrozen water during a freeze-thaw cycle. In addition to ultrasonic measurements, the growth of ice lenses and the resulting mechanical deformation of the system are monitored. The results of the THM experiments are implemented in numerical models, which allows an upscaling of the experimental findings to the field scale. This facilitates an estimation of the significance and the range of the phenomenon. More importantly, the results are used to improve the grouting and pipe materials in order to ensure a permanent integrity of BHE systems and their efficient operation, even over long-term operating periods with multiple freeze-thaw-cycles. |