Freeze-Thaw-Behaviour: Observations in grouted BHEs

Abstract

In Europe the construction of shallow geothermal heat-pump systems is expanding. The most favourable system is the borehole heat exchanger (BHE). Due to hydrogeological legal constraints, most BHEs have to be grouted and boreholes must remain sealed under all operating conditions. BHEs running with fluid temperatures below zero degrees Celsius are required to be sealed off with a grouting material that guarantees adequate sealing also under freezing-stresses. Even though requirements regarding the frost resistance of grouting material are laid down in the directives, there is no standardized and valid testing procedure to prove the required properties. A testing device was invented, that measures the hydraulic conductivity of grout specimens after a freely selectable number of cyclic freeze-thaw-stresses. Simulating radial earth-pressure (Sigma 2 = Sigma 3) and following the freezing directions as in-situ, results differ from earlier investigations substantially. It was observed and modeled that the progradation of the frost line and the fabric disintegration processes are correlated. Mass transfer from outside the grout specimens into the inner structure was evident and could be quantified. Numerical coupled modelling was applied to verify the results. More than 450 executed freeze-thaw-tests on different common grouting materials prove the characteristic influence of freezing-stresses on the hydraulic conductivity of grouted BHEs. As an outlook the implications for field practise will be supplied. The investigation and its results are presented in this paper.