Freeze-Thaw-Cycles on Borehole Heat Exchanger Grouts: Impact on the Hydraulic Properties

In this paper a testing device is described that measures the hydraulic conductivity of grout specimens for Borehole Heat Exchangers (BHE). During the operation of closed-loop ground source heat pumps running with antifreeze, freezing of the backfill can occur due to extensive heat extraction. This laboratory device can assess the influence of frost on the hydraulic seals of BHEs. The device is based on a triaxial flexible wall permeameter. A freely selectable number of cyclic freeze-thaw-stresses as well as a confining pressure simulating radial earth pressure (sigma(2) = sigma(3)) can be applied. Specimens are composed of an annular grout body and a polyethylene pipe simulating the BHE system. The freezing direction is perpendicular to the vertical axis of the BHE from the inside to the outside. Numerical coupled modeling was applied to verify the results of the temperature distribution inside the specimens. It was observed and modeled that the propagation of the frost front and the fabric disintegration processes are correlated. Results of three different grouting materials will be presented. With its relative small dimensions the device can be easily implemented into soil mechanical laboratories and thus can contribute to quality control of grouts.