POSIVA Report 2018-1



Rock Mechanics Parametrization of Fractures and Brittle Deformation Zones of ONKALO Based on the Mapping Data – Synthesis


Salminen, P.



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This report summarises the results of the working report 2016-23 (Salminen et al. 2018). In that working report, the rock mechanical parameters of fractures and brittle deformation zones were estimated in the ONKALO facility and its surroundings. Parameters were derived from the mapping data of ONKALO tunnels and shafts and from the drill core data.

ONKALO is located in Olkiluoto, in the western Finland. It is an underground rock characterisation facility, which is one element of the site investigations conducted at Olkiluoto. In addition, ONKALO also provides an opportunity to develop excavation and final disposal techniques in realistic conditions and it will ultimately become part of the final disposal facility.

Previously, the parametrization of fractures and brittle deformation zones have been provided in four working reports (Kuula 2010, Mönkkönen et al. 2012, Simelius 2014, Salminen & Simelius 2016). However, the working report of Salminen et al. (2018) is not a continuation to these previous reports. Instead, it provided an entirely new rock mechanics parametrization of the fractures and brittle deformation zones in the entire ONKALO facility (tunnels and shafts). Drill cores in the area of ONKALO and in the vicinity of ONKALO were also investigated to some extent. Drill cores provided a way to investigate brittle deformation zones outside of the tunnels and shafts.

The working report 2016-23 provided a parametrization of the mapped fractures outside of the brittle fault and joint zone intersections. That parametrization of the fractures was mainly based on the mapped Q (Q´) parameters, including RQD, Jn, Jr and Ja. The Q mapping system delivers qualitative presentation of fractures, but for the purposes of the spent nuclear fuel repository and associated modelling of discontinuities, more accurate parameters (e.g. quantitative representations) are necessary. However, as long as the basis of the characterisation is of qualitative nature, some uncertainties are transmitted to the quantified numbers as well.

Fracture orientations, fracture lengths, fracture terminations, fracture undulation and fracture mineralogy were also analysed and JRC1 m and JCS values were estimated for the fractures. In addition, fracture stiffness parameters, instantaneous cohesion, instantaneous friction angle and peak dilation angle were calculated for three fracture length categories and for three effective normal stress levels (representing three different depth levels); within these length categories and effective normal stress levels, all defined fracture sets and fractures outside of the sets were also considered separately.

For parametrization of the fractures, ONKALO tunnels were first divided into 23 different domains. This division was chiefly based on the fracture orientations, fracture intensities and lithology; significant amount of foliation was also taken into account in the domain formation. These domains were further grouped in to three discrete units based on their fracture orientations, RQD values and fracture mechanical properties. Unit I includes Domains 1-8, Unit II Domains 9-15 and Unit III Domains 16-23. Unit I clearly differs from the other two units: compared to Units II-III, fractures in Unit I are more altered, rougher and more undulating, RQD values are commonly lower and fracture orientations are more variable. Differences between Units II and III are smaller: the major differences are in fracture orientations and in Jr values of ≥1 m long fractures.

In total 30 brittle deformation zones were also analysed for their mechanical properties in the working report 2016-23. The investigated zones were, based on their size, classified as either site-scale or repository scale brittle fault zones. The strength and deformability properties of these brittle fault zones were estimated on the basis of the mapped Q' parameters obtained from the fault core zones.


Nuclear waste disposal, Olkiluoto, ONKALO, rock mechanics, fracture, brittle deformation zone, mechanical properties, Q mapping


POSIVA 2018-01_web (pdf) (23 MB)


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