The choice of Bran Sands for the location of the effluent treatment works and regional sludge centre was made with confidence that tough geotechnical challenges could be overcome.
Bran Sands was a landfill site on land reclaimed from the Tees estuary. Initially the 52-acre site was capped by placing a geotextile on the tipped material. This was followed by a drainage layer, a gas membrane, drainage medium, a geotextile and finally a further drainage layer of crushed slag. Vent stacks were built to allow controlled release of residual methane gas.
All the main structures were constructed above ground, avoiding the contamination issues associated with excavations and dewatering. Ground conditions comprised made ground (some 8 to 10m thick), overlying alluvium and glacial clays. Beneath the superficial deposits, 18m of Redcar Mudstone Formation bedrock was encountered.
Precast concrete driven piles were preferred to bored piles to ensure that problems associated with contaminated material were minimised. Bachy began driving the first of 7000 piles to bedrock in July 1996. A jetty for receiving ship loads of sludge was constructed by using tubular steel piles. A road embankment and access bridge were built at the entrance to Bran Sands. The embankment was constructed to a maximum height of 9m using staged construction and band drains.
Pipelines were installed on both banks of the Tees Estuary to transfer flows from refurbished sewage treatment works at Portrack and Cargo Fleet. Most of the work took place on land reclaimed from the Tees estuary.
The North Bank Transfer pipeline is 13km long and was completed in three contracts. Ground conditions varied along the route and comprised made ground, alluvium (soft clays, silts and peat) and glacial clays. The 900mm pipeline was installed using open cut excavations and tunnelling.
Excavations through areas of silty sands with groundwater almost at ground level required well point dewatering and/or overdriven interlocking sheet piles. These difficult ground conditions required tunnelling with full face slurry type machines to control groundwater and groundloss.
Shafts were constructed under water using caissons or sheet pile cofferdams. Divers were used to provide fixings to connect shaft base slabs to the sheet piles. Monitoring was undertaken during construction to confirm predicted ground movements at sensitive services to the chemical industries caused by excavations, dewatering and tunnelling.
The pipeline, which crosses the River Tees in a 45m deep 700m long tunnel owned by ICI, was installed within the existing tunnel alongside several 'live' pipelines.
The South Bank Transfer and Interceptor Sewers were constructed partly above ground, and partly in trenches and tunnels to intercept flows and then to divert these from the refurbished Cargo Fleet sewage treatment works to Bran Sands, a distance of 7km.
Discharge from British Steel's coke oven plant was also transferred for treatment. Most of this route was on reclaimed land and ground conditions comprised made ground, alluvial silts and clays, glacial clays/sands and Mercia Mudstone bedrock.
Excavations through water- bearing made ground proved extremely difficult due to fused slag and boulder size fragments of slag from the earlier iron and steel industry, and was sometimes further complicated by the presence of contamination.