13/10/04 Major contract for Consolidate secures housing project

Ground engineering specialist Consolidate has completed a major £1.75m contract to secure old mine workings for a housing development in East Lothian under budget and eight weeks ahead of schedule.

The contract from Persimmon Homes for its new 500-house development at Prestonpans was one of the largest single contracts ever undertaken by Broxburn based Consolidate, which recently changed its name from SeaMark Geotechnical. The project involved the systematic consolidation grouting of seven different coal seams so as to stabilise the ground prior to construction.

The project took place adjacent to the site of the old Prestonlinks Colliery, which was first sunk in 1853 with nine coal seams having been worked on the site from shallow depths down to about 140m. The most notable working was the economically important Great Seam, which was up to 3m thick in places. Work ceased at the colliery around 1915.

The structure of the workings to be consolidated was based on the typical 19th century 'stoop and room' system of mining. In this arrangement, tunnels were driven into the seam in a checkerboard fashion, leaving behind unworked supporting pillars of coal, many of which have subsequently collapsed.

Site investigation concluded that the stabilisation of the mine workings by drilling and grouting would be the most appropriate treatment method. Consolidate's extensive experience in this area combined with the technical expertise of consultants DR Murray & Associates and Wardell Armstrong Laird Menzies led to the contract award.

"There is ever-increasing pressure now to develop marginal land and brownfield sites for construction," states Consolidate director Beaton Sutherland. "Old mine workings are a recognised condition over a large area of central Scotland and this is where our specialist skills and equipment can ensure that previously unsuitable land is made safe to build on."

Consolidate has a large holding of specialist plant specifically designed for drilling small diameter bore holes and for the mixing and injection of cementitious grouts. At Prestonpans, six rotary percussive drilling rigs bored over 6,000 holes in a carefully planned 3.5m grid to ensure every mineworking on the site requiring treatment was covered.

In a strictly controlled sequence, each borehole was pressure injected with a pulverised fuel ash (PFA)/cement grout to ensure complete infilling of the mineworkings. The PFA was conveniently sourced from nearby holding lagoons in Musselburgh that store the ash from Cockenzie Power Station, which itself overlooks the construction site.

The seven treated seams varied from six to 33 metres in depth and required the injection of 16,265 tonnes of grout for stability by the time the 35-week long project finished eight weeks ahead of schedule in August 2004.

Paul Burns, Consolidate's contract engineer who has supervised the project on a daily basis, said: "This has been another challenging project for us and one we are pleased to have completed ahead of schedule. It was a difficult area to cover, particularly with the complex geology, but the carefully controlled pattern of bore holes has ensured the site area has been stabilised."

Consolidate has also recently completed contracts of a similar nature, at Falkirk, Newhouse, Kilmarnock, Ayr, Stevenson and Tranent.