Erkundung und Sanierungsstrategien im Erdwärmesonden-Schadensfall Staufen i. Br.

Investigation and remediation strategies for the damaging event caused by failed borehole heat exchanger drillings in Staufen i. Br.

The effects of failed borehole heat exchanger drillings in the German town Staufen i. Br. show distinctly the hazards, which can arise from borehole heat exchanger drillings spanning multi groundwater storeys with potential difference of the penetrated groundwater storeys. Groundwater flow induced the discharge of annular sealing in the annulus of the borehole heat exchanger drilling. Under the encountered hydrological boundary conditions, this caused the failure of the required separation of the groundwater storeys in one or more annular sealing. Water influx in previously dry, swellable geological layers caused the so-called "Gipskeuperquellen" (swelling of the gypsum-containing Upper Triassic Grabfeld formation). This water influx could be suppressed by damage-limiting measures (post-grouting of the borehole heat exchanger, permanent groundwater lowering). It is not possible to reclaim water that ingressed into swellable geological layers neither by hydraulic nor by other measures. Thus, the ground heave will continue until the ingressed water is completely exhausted by the swelling process. The initiated damage-limiting measures succeed. As a result, the maximum heave rate has decelerated from 11 mm/month at the beginning to currently approximately 3.5 mm/month. Since autumn 2012, the ground heave measurements indicate, that the linear deceleration of the process observed until now does not continue unchanged. By modelling it became clear, that the groundwater pumping has to be continued to avoid more uplift and further swell processes. Experiences from the case Staufen i. Br. show impressively, that a thorough pre-investigation immediately followed by the initiation of defensive/control measures are absolutely necessary in case of damaging events caused by failed borehole heat exchanger drillings.