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Mysteries of the deep

Sealing and grouting deep boreholes on the controversial Sellafield site 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 finished in October.

Geotechnical contractor Midland Grouting & Drilling has been working since March to backfill the boreholes, which are up to 2,000m deep. While the infilling operation was not unusual, the sheer scale of the project resulted in an innovative use of piling 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 infilling.

Although the deepest boreholes did not have to be infilled to their full depth, Midland still had to deal with depths of up to 980m and angles of 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 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 a high drilling torque was needed for rod jointing.

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. This was because it offered the extraction force needed, its mast could be angled, it had the mast height to deal with long drill rods and had a relatively high rotary head torque.

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 100mm diameter drill rod, lowered into the borehole and inflated with water pumped down the drill string at a pressure of 40bar.

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.

Because the rods were too heavy to handle manually a novel 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 jointed adaptor was 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 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.

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