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Summary:
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A number of 6 tests were performed in this test series to compare the behaviour of two candidate backfill materials for the use in nuclear waste deposition tunnel in a quarter-scale test tunnel. Both backfill materials were tested with two short-term and one long-term test. During the wetting tests the development of the saturation process was monitored using the resistivity method.
The amount of eroded clay in Friedland clay block tests was 5…70 times higher than in mixture block test. The erosion rate of mixture backfill was observed to decrease towards ending the test, but respective behaviour was not observed during the Friedland clay tests.
The swelling pressure induced by water was observed with 6 soil pressure gauges located in the ceiling and on the base of the test tunnel. The swelling pressure produced by the Friedland clay backfill increased even up to 190 kPa, when the swelling pressure produced by the mixture backfill remained under 100 kPa.
After each test the backfill was manually removed in sections of 300 mm. Photographs and infrared photographs were shot in each section and water content samples were collected. The preferred flow paths of water and locations exposed to erosion were determined with the swelling pressure recordings, water content distributions and infrared photographs. The locations most exposed to erosion were considered to be on top of the backfill where the water most likely flows along the interface of the pellets and the ceiling.
In wetting tests the wetting did not proceed to the survey lines thus the result of saturation process were not obtained.
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Keywords:
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Friedland clay, bentonite, backfill, deposition tunnel, electrical resistivity sounding, swelling pressure, erosion, piping
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