A UNIQUE geotechnical project to encase a shaft filled with radioactive waste with a 'grout boot' gets underway this month at Dounreay nuclear plant in Scotland (GE April 2006).
Edmund Nuttall geotechnical contracting division Ritchies, is preparing to pump up to 400 grout curtain injection boreholes around the shaft in a bid to fill the tiny fissures in the rock and make the shaft as dry as possible.
The grout boot is needed because groundwater entering the shaft and collecting at its base becomes contaminated and will be very costly to treat when the radioactive waste is eventually retrieved.
Under the £16M scheme, the contractor has built a raised concrete working platform on the sloping rocky foreshore by the sea next to the shaft.
This will enable the grout holes to be drilled from the cliff edge at the top of the shaft into the hard Caithness Flagstone and mixture of siltstones, limestone and sandstone around it.
'The weather will be an important factor, ' said Ritchies business development manager, David Gibson, adding that a wall had been built on the platform as a buffer against the wind. 'It's very, very exposed up there and if the wind gets very high you can't work because the site workers are handling drilling equipment.' Gibson added that the main challenge for the team pumping the grout was to ensure the accuracy and verticality of the holes at depths of up to 85m. 'We need to ensure that the ground between the holes is reasonably consistent, ' he said, adding that the work would take more than a year.
The team will monitor the bore holes with extensometers - measuring devices that monitor the rock during grout injection that can show movement similar to thickness of a sheet of paper. Six sets of extensometers will be used around the shaft to give warning of movement.
The team has already completed phase one of the contract which was to infill and seal off a liquid effluent discharge tunnel that spurs off the main shaft, with a grout shield. This will ensure that the grout isolating the shaft does not escape through the tunnel.
After six months work, Ritchies has pumped 416m 3 of grout into the tunnel after using sonar to determine the target volume. Site workers used another pump to ensure the tunnel was kept dry during the works.
The grout comprising Portland cement and pulverised fuel ash also included special additives that gave it thixotropic properties.
This makes it easier to pump down the bore holes when pressure is applied, but allows it to stiffens up when in place.