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Workreport 2009-129



Deposition Tunnel Backfill Design for a KBS-3V Repository


Hansen, J., Korkiala-Tanttu, L., Keski-Kuha, E. & Keto, P.



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The backfill is a part of the EBS (Engineered Barrier System) which includes also the canisters, buffer bentonite and the host rock. The reference concept for the backfill has previously been presented in 2007 and developed further ever since. The backfill concept consists of foundation bed, pre-compacted blocks and pellets. Mixture of bentonite and crushed rock (40/60) has been chosen to be the foundation bed material. Cebogel QSE bentonite pellets will be used as the pellet material. Friedland clay has been considered as the main candidate for the block material, but also the mixture of bentonite and crushed rock (40/60) and Milos B clay are being studied. The average block filling degree of the deposition tunnel is 73,5 % when the over excavation rate of 10 % is taken into account.

The manufacturing of the blocks includes purchasing the material, transportation, possible mixing of the raw material, restoring before compressing, compressing and packing. The blocks will be produced by compressing with an unaxial pressure of 25 MPa.

The backfilling of the deposition tunnel will be done in 40 m sequences. The backfill will be installed in a tunnel sequence in which the canisters have been installed. Before backfilling, the tunnels will be emptied of all the equipment and the temporary floor will be removed. The installation of the foundation bed will take place in 20 m sequences. The blocks will be assembled in modules on the compacted foundation bed 3 m at a time. The pellets will be sprayed to fill the empty space between the blocks and the tunnel wall in 3 m sequences. When the backfilling is completed and a plug has been constructed at the entrance of the tunnel, the backfill is in an initial state. As the groundwater begins to seep in causing the backfill to swell, a slow change towards the target state starts to take place. The target state will be accomplished once the backfill has been completely saturated with water and the functional requirements as a result of the saturation have been met. The saturation may last 50…1000 years. In the target state the backfill prevents water from flowing freely and creates the required swelling pressure against the rock. Erosion, piping, homogenization process or defects in self sealing process may bring disturbances in achieving the target state.


Backfill, deposition tunnel, Friedland clay, bentonite, pre-compacted blocks


Deposition Tunnel Backfill Design for a KBS-3V Repository (pdf) (3 MB)


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