Repository tunnel backfill material tests in-situ

Methods for backfilling nuclear waste repository tunnels have been studied in cooperation between Finland and Sweden.The study concentrates on the backfilling of bentonite - crushed rock mixture (30%/70%) and compaction methods on site.Strict criteria have been set for the backfill in order to maintain the long term properties.For example, the hydraulic conductivity (k) of the backfill material has to be lower than 1·10-10 m/s.This means that the mixture needs to be compacted to a very high dry density.The main objective of the study was to develop compaction methods and to collect data for compaction modelling.Two test structures were constructed: one with horizontal and one with inclined (about 30°) layers with thickness of 200...250 mm.Four types of compaction equipments were used: a vibrating plate, a modified roof compactor, a multipurpose compactor and a heavy roller.With these methods the target density could not be achieved.Yet, reasonably high densities were obtained.The highest average dry densities were achieved with the heavy roller.The target degree of compaction (90%) was achieved with the heavy roller.Horizontal compaction with a vibratory plate proved to be nearly as efficient as the heavy roller.The bentonite - crushed rock mixture proved to be very sensitive to the excess of optimum water content.