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Recent publications

Workreport 2007-75



Packfill-Development of in situ Compaction. Test Report for Field Tests November 2005


Korkiala-Tanttu, L., Keto, P., Kuula-Väisänen, P., Vuorimies, N. & Adam, D.



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Methods for backfilling and sealing of disposal tunnels in an underground repository for spent nuclear fuel have been studied in cooperation between Finland (Posiva Oy) and Sweden (Svensk Kärnbränslehantering AB, SKB) in Baclo programme. The considered methods in programme are installation of pre-fabricated blocks and compaction at site and the considered materials are different type of clays with swelling ability and mixture of ballast and bentonite clay. The Baclo project has been divided into work packages. Work package 2 (WP2) concentrates on the in-situ compaction of mixture of crushed rock (70%) and bentonite (30%). In the step 2 in WP2 the compaction methods and the effect of the compactor has been studied. Based on the literature review the Packfill test construction study was launched. The objectives of the packfill test construction study were to find out the acceptability of in-situ backfilling, to acquire data of the material behaviour during compaction, to find solutions for in-situ backfilling and to compare different compactors. Packfill study includes two parts. This study concentrates on the material parameters, comparison of compactors and further development of the compaction method while the other study will concentrate on the modelling. The studies have separate reports.

Certain strict criteria have been set for the backfill in order to maintain the function of engineered barriers in the repository for a very long time period. For example, the hydraulic conductivity (k) of the backfill material shall be lower than 1·10-10 m/s. In practice this means that the mixture needs to be compacted to very high dry density (90-95% from maximum Proctor dry density). Several studies have been made to find optimum composition for the in-situ backfill material. Based on the studies, the optimum water content of the mixture of ballast (70%) and bentonite (30%) is between 11...12.5% and the corresponding dry density is about 1870 kg/m3. The tested material can be described as a low plastic semi-cohesive material, which is quite sensitive to the exceeding of the optimum water content.
The test construction was done in two phases. The first one was on the 10.11.-11.11.2006 and the other one on the 28.11.2006. The test construction was made in Ekokem’s facilities in Riihimäki. Two test structures were constructed in order to produce data for modelling the effect of different compaction equipment parameters. One test structure (size about 2.5 x 3.5 m2) was constructed in 150...250 mm thick horizontal layers and it was compacted with a light vibratory plate. The other test structure was constructed into layers with inclination of about 30°. The inclined structure was first compacted with the modified roof compactor and later with a multipurpose compactor and a heavy roller for comparison. The density of the compacted surface was followed with Troxler and sand volumeter tests. The deformation properties of the surface were measured with LDWT and Loadman. Besides Troxler measurements, the water content of the material was followed with different drying methods. The mixture was in about the same water content in both tests.

The test results showed that with these methods and equipments the high density requirements of repository tunnel could not be achieved. However, relatively good compaction results were achieved though the compaction methods were not optimized for this purpose. The highest average dry densities were achieved with the heavy roller (in average 1744 kg/m3). Horizontal compaction with vibratory plate proved to be nearly as efficient method (in average 1655 kg/m3) as the heavy roller. The target degree of compaction (90%) was achieved only with the heavy roller. The needed modelling data was achieved from the test.
Even though the highest density requirements of the repository tunnel were not achieved, the test gave many positive results and proved that a relatively compact tunnel backfilling can be achieved with these methods. The used methods could be used for example in the compaction of backfilling of other deep excavations. The compaction work is sensitive to the excess of water content and to the thickness of layers. The inclination had a tendency to become more gentile (from 30° to about 24°) during the compaction. The roller compactors could not be used for compaction of inclined layers without additional arrangements.


Baclo, backfilling and sealing, disposal tunnels, Packfill


Packfill-Development of in situ Compaction. Test Report for Field Tests November 2005 (pdf) (4.3 MB)


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