POSIVA Report 2016-14



Outcome of Geological Mapping and Prediction/Outcome Studies of ONKALO


Nordbäck, N., Engström, J.



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ONKALO has been constructed as part of plans for building a repository for spent nuclear fuel in Olkiluoto. Prior to excavation, information about the rock conditions has been obtained by drilling pilot, characterisation and probe holes. This information together with deep drill holes and surface investigations is used for modelling and prediction purposes.

ONKALO has been mapped and described in a total of 1 169 separate sections. A total of 55 976 structural observations have been made. In addition to the tunnel mappings, scanline mapping has been performed in the three shafts. The bedrock in ONKALO is composed of different types of migmatitic rocks. Veined gneiss is the most common rock type but diatexitic gneiss and pegmatitic granite also regularly occur. Principally in ONKALO, the foliation dips moderately towards SE but varies between E- to SSE-dipping and variations can be either gradual or zonal. Measured fold axes are mostly moderately dipping and NE-SW trending or NNW-SSE trending. A total of 54 908 fractures have been mapped and from chainage 0–3116 m all individual fractures were investigated, while from chainage 3116 m onwards only the fractures with trace length over 25 cm. Two main and three minor fracture sets are distinguished from the measured orientations. A total of 333 tunnel-crosscutting or  20 m diameter large fractures have been recorded in ONKALO. Slickenside fracture surfaces and ≥ 2 mm thick fracture fillings are the most typical properties for fractures over 50 m in diameter. The ONKALO tunnel is also cut by 99 deformation zone intersections, of which 74 are brittle intersections and 25 are high-grade ductile intersections.

The tunnel mapping data has proven to be an important source of geological information because it enables relatively large-scale and three-dimensional observations, compared to i.e. outcrops which are limited in size and roughly two-dimensional and to the more or less one-dimensional and strongly orientation biased drill holes. Due to the restricted time, large mapping areas and other limiting factors, the level of detail is however, usually lower for tunnel mappings compared to drill holes.

By comparing the predictions to the outcome of the geological tunnel mapping, in the so called prediction/outcome method, the predictive capability of models have been tested and modelling procedures have been improved. The site-scale models have shown to be effective for predicting large features but insufficient when it comes to smaller ones. Due to the requirements of the Rock Suitability Classification, predictions have been replaced by the so called detailed-scale modelling procedure.


Final disposal of spent nuclear fuel, Olkiluoto, ONKALO access tunnel, geological and geotechnical mapping, modelling, outcome, prediction.


Outcome of Geological Mapping and Prediction/Outcome Studies of ONKALO (pdf) (17.2 MB)


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