Research into the properties of backfill materials and vertical hydraulic system permeability is crucial for validating the fulfillment of requirements for geothermal energy structures in terms of groundwater protection and damage-free operation - especially at sites with groundwater hazard potential. As part of the joint project "Qualitätssteigerung oberflächennaher Geothermiesysteme" (QEWSplus, FKZ: 03EE4020AH) funded by the German Federal Ministry of Economics and Climate Protection, test rigs and measurement methods were further developed at ZAE Bayern for this purpose, and qualitative measurements were carried out on system samples of different construction material and probe pipe configurations as part of three round robin tests.
Cement-bound backfill materials of different compositions (quartz, phonolite, graphite) as well as clay-based materials were used for sample preparation and the influence of filtration was investigated in detail. In addition, different probe pipe types (double U and corrugated pipe) were used. All samples were subjected to variable temperature (temperature changes, freeze-thaw cycles) and pressure variations in the probe pipe.
The vertical hydraulic system permeability of a geothermal energy structure depends to a large extent on the contact area between the geothermal energy system pipes and the backfill material. Disturbances at this interface could be reproducibly induced and quantitatively investigated with the test rigs at ZAE Bayern. The rheological behavior of the PE probe pipes used in geothermal energy structures has a significant influence on the formation of annular gaps. Particularly critical are temperature drops during normal operation for building heating, as a result of which probe pipes cool down and contract. This can compromise the integrity of the structure to such an extent that regulatory requirements for system permeability can no longer be met. Possible solutions are discussed.