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Technischer Bericht NTB 90-38

Sondierbohrung Siblingen Bau- und Umweltaspekte, Bohrtechnik

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In 1980, Nagra (the Swiss National Cooperative for the Disposal of Radioactive Waste) initiated a comprehensive geological investigation programme covering an area of around 1200 km2 in Northern Switzerland. The aim of this programme was to provide the geological information required for assessing the suitability of the Palaeozoic basement as a potential host rock for a high-level waste repository.

The investigations consisted of a deep drilling programme, surface geophysical reconnaissance of rocks and structures in the region, a hydrogeological programme which studied water flow paths in the deep basement and, finally, a neotectonic programme aimed at identifying active crustal movements in the investigation region.

The aim of the deep boreholes was to investigate the geology of the crystalline basement and the overlying sedimentary formations. Within a wider, regional context, the boreholes were also intended to provide hydrodynamic and geochemical data as input for a mathematical model of the hydrogeological conditions prevailing between the northern slopes of the Alps in central and eastern Switzerland and the Black Forest Massif.

The Siblingen borehole was located 1.5 kilometres to the north-west of the village of Siblingen in the Canton of Schaffhausen, on the 'Siblinger Höhe' in the Schaffhausen Tabular Jura. It was the seventh and final borehole of Phase I of the deep drilling programme in Northern Switzerland and reached a final depth of 1522.0 m.

Following a licensing procedure which lasted several years and also involved the federal courts, preparatory work for constructing the drill site started on 16th May 1988; actual drilling began on 1st September 1988. Drilling was completed on 3rd April 1989 and was followed by a test phase which continued up to 30th April 1989.

The construction of the drill site ran according to schedule. The soil investigations which were carried out during the initial stages of site preparation provided input for the detailed planning of the excavation work; they also provided background information which allowed useful extensions to be made to the drainage network when the site was recultivated. This improved the cultivation potential of the ground and there was thus no negative impact on the productiveness of the soil.

Unfortunately, following completion of the drilling phase and during the dismantling work at the drill site, a tragic accident occurred. One of two engineers of the EKS under contract to remove the electricity supply received fatal injuries following a current surge. The reason for the accident was failure to observe electrotechnical safety regulations.

In order to protect groundwater resources, the working surfaces at the drill site were covered with a layer of asphalt and appropriate drainage systems were installed. The conductor pipe was tightly embedded in the bedrock and in the drill cellar. Together with the cantonal water conservation authorities, the efficiency of these measures was checked as part of a groundwater monitoring programme. No influence of the drilling operations on the existing groundwater deposits could be detected.

Despite the fact that the drill site was relatively far away from the village of Siblingen, the noise levels were measured at the closest single farmhouse and residential quarter. The results showed that at no time was anyone adversely affected by noise emissions from the site.

Disposal of all solid and liquid wastes was based on a concept which was developed in cooperation with, and licensed by, the building department of Canton Schaffhausen. This concept has proved itself in practice, and there have been no grounds for complaint at the disposal locations.

The increase in road traffic due to lorry transport was registered during the entire operational phase and showed a low level of less than 20 journeys per week. However, transportation during the phases of constructing and recultivating the drill site was not registered; this would have shown a much higher frequency, but over a shorter time span.

A streamlined scientific investigation programme was carried out in the Siblingen borehole, some use being made of alternative investigation techniques. It was agreed that the investigations would be restricted to questions which are directly related to disposal, and a new drilling concept with a greatly reduced hole diameter in the crystalline was used; this represented a synthesis of mining exploration technology and deep drilling techniques. In this way it was possible to observe the requirements as to depth and quality of information, scientific data output and minimisation of technical risk and, at the same time, to realise an economically optimised exploration project.

The borehole passed through Jurassic and Triassic sediments. The crystalline basement was encountered at a depth of 348.6 m and was drilled down to the final depth of 1522.0 m.

The entire length of the borehole was drilled using the wire line core drilling technique. For the first time in the deep drilling programme, an all-hydraulic core drilling rig (type Wirth B8A) with top-drive was used. This was operated electrically and was also equipped with a specially developed soundproofed drive unit for optional electric or diesel drive.

The sediments and the top section of the crystalline basement were drilled with 6 1/4 × 4" coring tools and a special 5 1/2" wire line coring stem. 96 mm × 57 mm bits with 3 1/2" wire line coring equipment were used from a depth of 490.9 m.

In addition to a conductor pipe and a 9 5/8" anchor casing which was installed down to a depth of 171.9 m, to facilitate drilling and investigations a 7" intermediate casing string was installed in the Muschelkalk at 338.0 m and a 5" end casing was installed in the upper section of the crystalline at 490.4 m. With the exception of the 5" casing, which was partly removed after the drilling phase, all the casings were cemented in up to ground level.

Even in the sediment zone, which was drilled using a conventional clay-water drilling fluid, various borehole sealing measures became necessary due to often complete loss of drilling fluid as a result of strong fracturing of the rock and the presence of sub-hydrostatic pressure conditions.

Over the 1173.4 m long crystalline section, a leucocratic biotite-cordierite-granite with heterogeneous tectonic stressing was encountered. This occurred alternately in the form of a fresh, massive granite and sections of strongly fractured granitic rock which had been hydrothermally overprinted and penetrated by shear zones.

The alternating structural and textural fabric of the rocks and the steeply-dipping fracture systems placed considerable demands on the selection and development of suitable coring tools.

The situation was also complicated by strongly sub-hydrostatic pressure conditions, combined with the use of clear water drilling fluid without additives. Complete losses of circulation and borehole instability meant that rehabilitation measures such as back cementation and plugging were often necessary.

As a result of the difficult drilling conditions, the tools could often not be used in their optimum parameter range. The strong fracturing of the rock also led to a reduction in terms of maximum core recovery as the average length of the continuous core sections was shorter.

In spite of all these complicating factors, the drilling of the borehole was highly successful both technically and economically and no significant complications or costly special work was involved. Compared with the previous boreholes in the deep drilling campaign, all aspects of performance showed a significant improvement, while the costs per drilled meter decreased considerably.

The scientific investigations in the overlying sediments were carried out as planned. The conditions encountered in the crystalline basement meant that the programme frequently had to be adapted to take account of unexpected factors; the hydraulic characterisation of the rock also required more effort than originally anticipated. However, the duration of the subsequent test phase was greatly reduced.

To summarise, all aspects of the scientific programme were realised with a considerable degree of success.

All the work carried out at Siblingen was monitored by a review commission consisting of representatives of the community of Siblingen, Canton Schaffhausen and the Federal Government.

In the report of this commission to the Federal Council, Nagra was accredited with having carried out the work correctly according to the legal regulations.

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