Seaham is a quiet former coal mining town near Durham on the north east coast of England. Lord Byron (the headline is the title of one of his poems) was married there in 1815 and the town was once an important thriving industrial centre. But since the last of its mines, the Vane Tempest Colliery, was closed in 1992, its fortunes have changed.
With high unemployment and the town's commercial port starved of business in the wake of the closure, solutions had to be sought.
One such plan is the development of business parks to the south of the town and, on the Vane Tempest site at the north end of the promenade, new housing is being built to encourage new businesses to the area.
Groundworks are in full swing, cleaning up the site in preparation for 500 houses.
Government-funded regional development agency One NorthEast is paying for the reclamation, which is being carried out by consultant WS Atkins and main contractor Hall Construction Services.
As well as the deep mine, the site was home to a coke production works which resulted in a variety of contamination. Site investigations revealed arsenic and PAH contamination in the colliery spoil (which is mostly mudstone) and identified several contamination 'hot spots' across the site. Groundwater is very low within the underlying weathered limestone aquifer and does not affect the site.
WS Atkins' David Oliver explains that the original plan was to vibrocompact the site but because of the contamination, it was decided to dig up the spoil, remove any heavily contaminated material to licensed landfill, screen the remaining material and replace it in a borrow pit before capping.
The capping scheme included a crushed rock drainage layer, but Yorkshire-based geosynthetics manufacturer ABG Geosynthetics suggested substituting this for its Pozidrain geocomposite. This consists of a high strength flexible HDPE cuspated sheet with a non-woven geotextile bonded to one side. One of the main advantages in using the geocomposite, says ABG managing director Alan Bamforth, was the substantial reduction of the 17,000 predicted lorry movements needed to move the crushed stone. The bright orange layer will also warn anyone digging in the area of the contamination below, he adds.
The polypropylene geotextile can filter a wide range of materials and is 'glued' to the HDPE core so it does not deform into the drainage passages when backfill is placed on top. The geotextile, typically with 100micro m pore spacing, allows water (or gas) to percolate into the core while supporting the backfill material - the collected water or gas is transported along the core to a discharge point.
Bamforth says an added environmental benefit is that the material is inert - 'it can be cleaned and recycled' - and the polymer used to make the geocomposite is itself made from recycled plastic containers.
At Seaham, 70,000m 2of the geocomposite is being installed to collect and drain water from the clean soil above while providing a separation layer and reinforcement for the backfill. It also acts as a capillary break to prevent contaminated water seeping up from below. However, as there is an impermeable layer of Boulder Clay between the aquifer and the colliery spoil and as a consequence very little groundwater movement, the layer acts more like a warning system, Bamforth says.
'The geocomposite is not a structural element, ' he adds. 'It just has to support the backfill above and site traffic moving over the capping. ' The material does not need any tensile strength either as it is being laid flat.
The clean-up operation begins with excavation of the colliery spoil, which is then screened for metals (mostly demolition material) and then replaced into the borrow pit. Any heavily contaminated material is taken off site to licensed landfill.
At least 2m of st iff Bou lder C lay is lef t above the limestone bedrock. Up to 3m of colliery spoil is placed in the borrow pit, which is then compacted before the Pozidrain layer is put down. This is supplied in 4. 4m wide, 50m long rolls, laid by hanging the roll from an excavator bucket and simply rolling it out. As the material is light (a roll only weighs 300kg) it can be laid by four people, says Bamforth. Because the geocomposite is not acting as a watertight barrier, it is not sealed but simply overlapped and taped together. He adds that up to 5,000m 2ofgeocomposite has been laid in a week.
While the laying operation is not weather dependent, speed is of the essence because the geocomposite is light enough to be lifted by the wind, which can be quite strong at Seaham - the day GE visited the site it felt more like January than mid-August. Because of this, the covering layer of Boulder Clay is placed almost immediately and bulldozed, says David Oliver.
This 500mm thick layer of clay is covered with a 250mm thick crushed concrete capillary layer and finished off with 250mm of topsoil for landscaping.
Dynamic penetrometer testing and platebearing tests check the capping structure and with the boulder clay at 95% compaction value, Oliver says that settlement will not be a problem.
Earthworks will be finished by the end of the year and the site will be ready for construction of the houses next spring.