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GROUND IMPROVEMENT - Two years of grouting trials have led to design changes for sealing off a waste storage shaft at the Dounreay nuclear site. Jon Young reports.

A working platform taking shape in Dounreay this summer is the first stage in the long process of isolating a shaft full of nuclear waste.

Dounreay, near Thurso on the north coast of Scotland, was Britain's centre of fast reactor research and development from 1955 to 1994.

During this time, three nuclear reactors and fuel reprocessing and other associated nuclear facilities were built and operated on the 57ha site.

In 1953, a 4.5m diameter, 65m deep unlined shaft was built as a mucking adit during building of a subsea outfall pipe, excavated through the Caithness Flagstone: a mixture of siltstone, limestone and sandstone.

The shaft was plugged with concrete at its base and from 1959 it was used as a repository for nuclear waste, including plutonium and uranium. But in 1977 an internal explosion, thought to have been caused by hydrogen build-up, blew off the shaft lid, scattering waste around the surface. This put an end to further disposal, although about 750m 3 of radioactive waste had already accumulated.

In 1998 it was decided to empty the shaft. This will need to be done in the dry by remote handling, which means first dewatering the shaft; from the start it was allowed to fill with a mixture of groundwater and seawater which now surrounds the waste.

This is thought to include large blocks of concrete, sealed drums, small fl asks of liquid and scaffolding poles used to poke waste down, all surrounded by a brown gooey sludge of metal turnings and dust, including 10kg each of plutonium and uranium in the shaft corrosion products and organics. Plastic bags, timber and other debris float on the water.

Operator the UK Atomic Energy Authority (UKAEA) has opted to create a low permeability grout curtain around the shaft to cut daily water infl ows to a level that can be handled by existing reprocessing facilities. The curtain will also stop the flow of contaminated groundwater into the highly fractured surrounding rock.

The £16M design and build contract to isolate the shaft was won by main contractor Ritchies - the geotechnical division of Edmund Nuttall - in 2004, as part of the overall £27M clean-up programme. It is supported by its consultant Halcrow.

Grout injection trials began in August 2005 after several months of borehole drilling and hydrotesting (GE September 2005). These trials allowed the team to finalise selection of grouts for full-scale shaft isolation, establish grouting pressures and volumes and ensure optimum filling of fractures in the rock.

In the process the Ritchies/Halcrow team has evaluated and developed a number of techniques new to the grouting world, says Ritchies business development manager David Gibson.

'These include real time monitoring of the fl ow of grout in the bedrock and the use of pH monitoring to track the actual grout flow geometry in the bedrock, ' he explains.

The original tender design involved constructing a full depth grout curtain, shaped like a cup around and below the shaft and the stub tunnel linking it to the liquid effluent discharge tunnel.

But after trials (including drilling 50 boreholes 200m east of the shaft) and 3D numerical modelling of water infl ow through the curtain, the team realised a more efficient shape could be developed by limiting the internal surface area of the grout curtain. This would help minimise the volume of water entering the shaft after construction, says UKAEA project manager Warren Jones.

The redesign was submitted to the client at the end of May, with the cup as a tight-fitting 'boot'. Grouting will be carried out from surface boreholes, with more grouting over the toe of the boot to form a roof.

As the shaft is no longer a licensed intermediate waste repository it is illegal to add any material to it, so the team must prove there is minimal risk of grout entering the shaft during injection.

This will be achieved by forming two concentric grout curtains. The first, inner curtain will be formed at a relatively low pressure using a blocker grout. This is designed to plug the main fractures and prevent more fluid grout, injected at higher pressures from the outer grout curtain, from entering the shaft. The blocker curtain will be formed from 40 boreholes at 3m centres, 90m deep that extend about 25m below the base of the shaft.

'The blocker holes are deep because their lower sections are used to inject grout at high pressure beneath the shaft and stub tunnel to form the base to the grout structure, ' explains Halcrow deputy design manager Graham Garrard. 'These holes are the closest to the shaft and therefore the easiest from which to form the base.' The main outer grout curtain will be built in three phases using 125, 85m deep boreholes and a 'split borehole spacing' technique.

The first boreholes will be drilled and grouted at 8m centres around the shaft and stub tunnel.

In the second phase, boreholes will be drilled and grouted at 8m centres at the halfway point between the phase one injection holes. In phase three, the boreholes will be put down at 4m centres between the phase 1 and 2 boreholes, reducing the distance between boreholes and grout injection to just 2m.

Grouting design team leader Nick Swannell explains: 'During the injection, grout pressures and volumes are closely controlled and monitored using the GIN [grouting intensity number] method and a purpose-designed computerised grouting module.' Swannell says the GIN concept was developed in the mid 1990s as a means of controlling the injection of modern fi ne grained stable grout mixes.

This approach controls the injection of grout by three parameters: the GIN number, which is a dual criterion based on pressure and volume (GIN = pressure x volume) and represents the energy expended during grouting; the maximum volume limit; and the maximum pressure limit.

Grout is injected at a steady, low to moderate rate until the value of one of the three parameters established during the trials is met.

The method provides a rational approach to injecting grout into rocks of different characteristics and helps prevent the risk of hydrojacking as well as allowing real time monitoring of the grouting process.

The team says that by using the right grout mix design and GIN control parameters, grout stage lengths that refl ect the geology and the split borehole spacing, it should be able to install one of the most impermeable grout barriers ever built.

This will prevent most of the 350m 3 daily flow of water into the shaft.

In March this year the Highland Council granted planning permission for construction of a raised working platform and the drilling of up to 400 grout curtain injection boreholes (GE April 06) for the isolation works.

Work is well under way on the 8m high raised platform that will create a stable surface for drilling boreholes and injecting grout. The shaft sits on the edge of a low cliff and an additional working area of about 1150m 2 will be reclaimed from the sea.

Due to be finished this summer, the platform includes 12,000m 3 of mass concrete held within 1,700m 3 of fi bre reinforced structural concrete, surrounded by a rock barrier.

Before grouting can begin, the plug in the stub tunnel linking the shaft to the liquid effluent tunnel needs to be reinforced with additional grout.

This was due to start this month and take three months.

Construction of the grout curtain can then begin, with work expected to take between two and four years.

Once the shaft is stable waste can be removed, although this is not expected to start for some time - the provisional date is 2019. Conceptual designs are being developed for removal, which is expected to take about six years.

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