Bachs-1 borehole

The Bachs borehole was the fourth deep borehole to be drilled in the Nördlich Lägern siting region as part of Stage 3 of the site selection process.

With the Bachs-1 borehole, Nagra enhanced its knowledge of the Nördlich Lägern siting region. The subsurface in the eastern part of the siting region differs from that in the western part. Nagra drilled two boreholes in each of the two subareas. The results of the first Bülach borehole in the western section were verified and confirmed with the Bachs borehole.

The Bachs borehole drilled in the Bachsertal was the fourth deep borehole to be drilled in the Nördlich Lägern siting region as part of Stage 3 of the site selection process for a deep geological repository. It reached a depth of 1,306 metres. It was the ninth and final borehole of the deep borehole campaign that began in 2019 and was completed in Bachs in April 2022.

Various tests conducted in the borehole

The new Bülach, Stadel-2, Stadel-3 and Bachs boreholes complete the overview of the geological conditions in the region.

In the Bülach and Stadel-3 boreholes, a fossilised coral reef located above the Opalinus Clay was penetrated while drilling. This reef divides the region into two subareas. The purpose of the Bach borehole was to investigate the subarea without the reef in more detail. In addition, various tests were carried out in the borehole and provided valuable information on the overall picture of the geological subsurface in the region.

Technical problems while drilling

Nagra was very satisfied with the results of the Bachs borehole. It was able to obtain quality rock samples and carry out various measurements. However, this last of the nine deep boreholes drilled within the framework of the deep borehole campaign for Stage 3 posed some technical problems.

In December 2021, a measuring instrument got stuck in the lower part of the borehole. Attempts to retrieve it proved unsuccessful. The measuring instrument was initially cemented in at a depth of around 800 metres. As the plan to continue drilling by deviating the borehole failed, drilling was continued in the original borehole in consultation with the Swiss Federal Nuclear Safety Inspectorate. The jammed measuring device was crushed with a special drill bit.

Despite the technical problems, the Bachs borehole was completed on schedule in April 2022. Nagra was thus able to conclude the deep borehole campaign after three years as planned and considered the campaign a success.

An exhibition in Windisch showed the results of the investigations, which included over 400 metres of drill cores. Today, the drill cores are stored in the drill core archive in Mellingen. The archive is mainly used for research, but also opens its doors to visitor groups.

Earlier boreholes

Data for the Nördlich Lägern region are already available from a deep borehole previously drilled in the community of Weiach in 1983 (see photo on left). The Weiach exploratory borehole reached a final depth of 2,482 metres. As a result, for the first time, it was possible to prove beyond doubt the existence of the so-called Permo-Carboniferous Trough, which is sunk deep into the crystalline bedrock.

Investigations using so-called 3D seismic measurements – similar to probing the subsurface by means of ultrasound – revealed fault zones in the deep subsurface in the northern part of the Nördlich Lägern siting region. For a deep geological repository, the geology has to be undisturbed. Nagra therefore withdrew the other applications for boreholes in Weiach and Glattfelden that had been submitted as part of the 2019 – 2022 deep borehole campaign. No new findings were to be expected.

Find out more about the Weiach exploratory borehole in Technical Report NTB 88-08.

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