POSIVA Report 2017-1



Parametrisation of Fractures - Final Report


Siren, T., Hakala, M., Valli, J., Christiansson, R., Mas Ivars, D., Lam, T., Mattila, J., Suikkanen, J.



Page count:



ISBN 978-951-652-265-7


The POST project was established by Posiva, SKB and NWMO and carried out between 2014 and 2016. The objective of the POST project was to develop a strategy and guidelines for deter­mining the parameters necessary for assessing fracture stability at the deposition tunnel scale for repository design and post closure analysis used for the KBS-3 disposal concept. To meet these objectives tunnel-scale in situ shear tests were planned both at Äspö and ONKALO. The in situ shear experiments would require the development of a fracture characterization methodology suitable for numerical analysis, use a constant stiffness boundary condition that is representative of confined fractures in an underground repository, need a maximum of 50 mm of shear dis­place­ment to simulate shearing needed for canister damage and be as large as practically feasible.

In the beginning of the project, two fractures, P424 and TASQ, were selected for the in situ experiments at ONKALO and Äspö, respectively. The experiments were planned and the candidate fractures were investigated in detail. Numerical modelling for TASQ tunnel fracture indicated that shear stress acting on the fracture were too low and simply releasing that stresses by slot drilling would not result in large enough shear displacement. Also, the displacement induced would occur along several fractures, such that the experiment would be hard to control and interpret. For the P424 fracture at ONKALO, an experiment layout was planned in detail, however, it was realised that the experiment would only have minor in situ components and it would merely be a large scale laboratory test conducted underground.

Several smaller scale in situ tests were planned and executed during the course of the project. A block associated to the BFZ300 structure was sheared in the PUSH Test executed in ONKALO. A low CNS boundary condition was imposed using three rock bolts and the test was sub­sequently back-analysed. The PUSH Test was a valuable source of information to gain insight on the shear strength behaviour of fractures under such loading conditions. Small scale laboratory direct shear testing using CNS and CNL boundary conditions were conducted for eight cores retrieved from TASQ tunnel in Äspö. In addition, a novel wedge testing methodology and testing equipment were developed for fracture testing.

Because of the challenges with in situ testing, the project focused on the possibility of con­ducting large scale laboratory direct shear tests using existing facilities in Canada and Europe. However shear testing facilities that could generate large normal stresses, necessary for the POST objectives, could not be located. A preliminary design for a direct shear testing device with sufficient normal stress capacity was carried out within the project. This preliminary design required significant modification, including downscaling, to reduce costs. At this point, a decision was taken to terminate the project as the original project objectives could not be achieved within project budget.


POST, direct shear test, Olkiluoto, Äspö, PUSH.


Parametrisation of Fractures - Final Report (pdf) (31.9 MB)


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