During the backfilling of borehole heat exchangers, the backfill material is in direct contact with the subsurface and interacts with it. Even at a medium hydraulic permeability, the backfill quality is significantly influenced by filtration processes and the properties of the backfill material. To investigate this, real-scale filtration tests were performed and real geothermal probes were constructed in a quarry, which were then extensively dismantled for analysis. Both the filtration tests and the dismantled borehole heat exchangers give plausible conclusions of the processes in borehole heat exchanger boreholes, during and after backfilling. In addition, the recovered borehole heat exchangers provide unique impressions of, for example, the borehole geometry, the positions of the borehole heat exchanger tubes and the potential occurrence of cavities.