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Size matters at Sellafield

Sealing and grouting of deep boreholes on the shelved Sellafield deep nuclear waste repository has involved a novel use of piling technology.

WORK TO seal 25 deep site investigation boreholes at the site of the Sellafield deep nuclear waste repository in Cumbria was due to finish last month.

Geotechnical contractor Midland Grouting and Drilling has been working since March to backfill the boreholes, which are up to 2000m deep. While the infilling operation was not unusual, the sheer scale of the project resulted in an innovative use ofpiling technology.

Instead of using conventional drilling and grouting equipment, the firm decided to use two new Casagrande B125 and B130E hydraulic piling rigs to cope with the extreme depths and large equipment required.

The boreholes were drilled in the early 1990s for nuclear waste firm UK Nirex, to investigate the suitability of the underlying geology for a deep underground repository for low- and medium-grade radioactive waste.

Instruments were also installed in the boreholes to monitor local groundwater movement.

The project was shelved in 1998 and UK Nirex had to recover the monitoring equipment and backfill the boreholes. Earlier this year, the firm commissioned contractor VHE Construction to carry out the work and Midland was brought in to carry out the infilling.

Although the deepest boreholes did not have to be infilled to their full depth, Midland still had to deal with depths ofup to 980m. Also, some of the boreholes were angled at up to 27degrees.

Because of the depths involved, the drill string alone weighed up to 25t. The drill rig had to develop an extraction force well in excess of this and had to be capable of guiding the drill string at the required angle. Midland also wanted to use up to 6m long drill rods to speed up the rod handling process, which meant that a high drilling torque was needed for the rod jointing process.

The firm already had a number of Casagrande drilling rigs in its fleet and chose to use a hydraulic piling machine rigged in CFA mode, because it offered the extraction force needed, its mast could be angled, it had the necessary mast height to deal with the long drill rods and had a relatively high rotary head torque.

The rig still had to be modified by Casagrande by fitting a pair of large rod breakout clamps at the base of the mast and a remote control for the banksman.

The first rig to arrive on site was a B125, a new model, and the first of its kind in the UK.

This powerful lightweight rig was joined by a Casagrande B130E as the workload increased.

To infill the boreholes, an inflatable packer was placed at the required depth, a 5m column of sand placed on top and grout pumped in.

Each packer was threaded on to the 101mm diameter drill rod, lowered into the borehole and inflated with water pumped down the drill string at a pressure of40bar.

Pressure in the packer was maintained with a non-return valve and the drill string uncoupled and pulled up about 12m to allow the sand to be placed. The drill string was then withdrawn in stages from the borehole during grouting. An OPC/bentonite grout was used, mixed in a special plant developed by Midland for mine infilling projects.

The rod handling process presented a problem as the rods were too heavy to handle manually. A novel handling process was developed which Midland believes may find applications elsewhere in the piling and drilling industry.

A special storage rack was built, with adjustable legs at the end nearest the rig and rollers to allow the rods to be threaded by hand.

A special jointed adaptor was also fitted below the rotary head which could be turned through 90degrees using the rig's auxiliary winch line, to be offered up to the rod.

For rod loading, each rod was manhandled from the storage section of the rack on to the rollers and the rack's legs adjusted to the correct height to meet the jointed adaptor on the rig. The rod was threaded on to the adaptor and then lifted onto the mast with a small trolley placed under its other end for guidance.

The process was reversed for breaking the drill string on extraction.

This process made a significant contribution to the fast progress of the job.

Work was due to be completed last month.

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