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Recent publications

Workreport 2018-24



Slope Stability of Installed Backfill Tunnel Pellet Filling


Martikainen, J., Leino, T., Karttunen, P.



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The effects of groundwater inflow on the swelling and slope stability of installed backfill tunnel pellet filling using current Posiva backfill pellet-filling designs were studied in this work. A pellet box (1 x 2 x 0.25 m) test configuration under open conditions was used to approximate an open front face of a naturally formed pellet filling slope, during seven-day long scheduled stops of backfill installation.

Total of eight tests were conducted by pumping a saturating solution (total dissolved solids 10 g/L) for seven days with inflow rates of 0.005, 0.01, 0.05, 0.1, 0.25 and 0.6 L/min for Greek bentonite pellets and 0.1 L/min for Bulgarian bentonite and Italian bentonite pellets from a single pointwise inflow point less than 1 meter from the open pellet filling slope. The tests produced results of density change (volume change and mass loss) and water retention of the pellet volume as well as gave information about distribution of inflowing groundwater into the pellet volume (wetting behavior).

The six tests done with Greek bentonite pellets showed that the total extent of pellet swelling expansion over 7 days was not influenced by inflow rate. At all tested inflow rates with Greek bentonite pellets, the pellet filling slopes were highly resistant to sliding or collapse mass loss. Maximum of 2.4% of the initial pellet filling mass was lost in all Greek bentonite pellet tests. The water retention (void volume filling capacity) was observed to increase with increasing flow rate in tests with Greek bentonite pellets. Greek bentonite pellet tests with inflow rates ≥ 0.05 L/min, the wetting occurred inherently downwards from the inlet point reaching the bottom of the test apparatus. By the end of each Greek bentonite pellet test, the direction of inflowing water was the same at all inflow rates: upwards from the inlet point and after slope breakthrough down along the slope and out of the system.

Strong swelling and expansion ability of the Bulgarian bentonite and Italian bentonite resulted in fast void closing between pellets and therefore initial downwards wetting seen with Greek bentonite pellets did not happen. However, this strong expansion led to severe loss of slope stability with Bulgarian bentonite pellets which caused nearly 20% mass loss from the initial installed pellet mass. Italian bentonite pellets were significantly more stable compared to Bulgarian bentonite pellets and equals to Greek bentonite pellets. With Italian bentonite pellets, nearly the whole pellet filling had wetted at the end of the test. The wetting was also much more uniform across the entire pellet filling than in any other test.


Backfill installation, pellet filling, slope, stability, swelling, density change, wetting behavior, Greek bentonite, Bulgarian bentonite, Italian bentonite


WR 2018-24_rev2_web (pdf) (10.9 MB)


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