The Centre of Experimental Geotechnics, CTU in Prague has recently launched a sprayed backfill technology development programme. Such technology will, most probably, be used during the future construction of deep underground radioactive waste repositories.
The safe disposal of radioactive waste in deep underground geological repositories assumes the construction of a multi-barrier system the aim of which will be to eliminate the potential penetration of hazardous radionuclides into the biosphere over a time span of several hundred thousand years.
With regard to the location in the repository in which the containers will be placed, bentonite will seal the space between the container and the rock mass in the form of a highly compacted bentonite block, while access galleries to the nests will be backfilled with a mixture of natural bentonite and rock. This mixture will be compacted in layers using compacting machinery thus ensuring maximum backfill density during compaction. Following compaction, however, spaces will remain in the gallery vault to which such compacting machines will be unable to gain access. Sprayed clay (sprayed backfill) technology, however, will allow the filling of such voids by injecting backfill so that the material compaction level obtained using this method is at least the same as the backfill compaction level resulting from that using standard compacting machinery (vibration roller, vibration plate compactor etc.).
Sprayed backfill technology is based on shotcrete technology which is a method commonly applied today in the construction of underground structures. Sprayed backfill technology development may involve the use of concrete spraying machines; however the parameter requirements for the spraying of bentonite backfill differ in principal from those which apply to shotcrete.
The article will provide information on the development of sprayed backfill technology, the machinery employed and the sprayed material as well as research results obtained to date.