Nuttall opens the gateway

Edmund Nuttall is cleaning up one of the major sites for housing development in the Thames Gateway. It is hoped the £32 million scheme will be a model of waste management

It is proposed that up to 100,000 homes will be built in the Thames Gateway but contractors have had to wait a long time for major projects to start since the much ¬vaunted plans were launched by the Government in February 2003.

And just when people were beginning to wonder if it would ever happen at all, an Edmund Nuttall/Van Ord joint venture was given the job of preparing a massive 39 ha site for a major housing scheme in the Medway town of Rochester under a £32 million contract.

The joint venture's task on the Rochester Riverside Development includes cleaning up a contaminated ex-industrial site, building flood defences along 2.5 km of waterfront and raising the ground level by 1 m using sand dredged from the Thames Estuary.

Tasks include various sheet pile wall solutions, the building and operation of temporary bunds where water from the dredged sand is pumped out and the treatment of soil that has become blackened due to contamination from an old gasworks.

All this is being done under the close scrutiny of the Environment Agency, which is overseeing operations as part of a Waste Management Licence one of the first major construction jobs to operate under the WML which will ensure that all the works comply with EU waste management directives.

"This job has a bit of everything," says Steve Horton, Nuttall project manager. "It's like six or seven construction projects all going on at the same time and it's very unusual to carry out a major scheme under a WML

On the day Construction News visited the site, sheet piles were being hammered in at one end of the narrow site, which is bounded by a railway and a waterfront.

Soil, heavily contaminated with hydro carbons, is being bio¬remediated at the other end. In between, a gas works is being demolished, vertical drains are being installed and in the middle of the site thousands of tonnes of sand freshly dredged from the Thames Estuary is being fed into a huge reception pit.

A vessel operated by Nuttall's joint venture partner Van Ord is dredging two locations in the Princes Channel on the Thames Estuary. It brings back two loads of 2,000 cu m a day to the site in a 24 hour operation over a 15 week period.

In all, nearly 400,000 cu m of sand is being discharged into the reception pit where the water is drained off into a neighbouring lagoon, which is later pumped back into the Medway.

Guidelines under the WML are strict. The drained water is only allowed back into the Medway with 800 mg or less of soil per litre and only 16,000 cu m of water a day may be discharged.

Every 2,000 cu m of dredged sand comes with around 8,000 cu m of water, which takes about four hours to drain off. The sand is excavated from the pit every day and used to raise the level of the site.

The sand from the estuary works out cheaper than importing it to the site, says Mr Horton. The Port of London Authority wanted the Princes Channel dredged anyway to deepen it by 3 m, allowing larger vessels to navigate it.

The sand was going cheap at less than £1/cu m. The alternative would have been to import sand from a Crown Estate quarry for around £2.55/cu m and create chaos as dumper trucks struggle through the gridlocked traffic around Rochester town centre.

To ensure that the new ground is firm and even, Van Ord is installing 9 m vertical drains to extract the ground water from the layer of alluvium beneath the made ground and speed up settlement in the ground of around 300 mm. The drains installed at intervals of 800 mm to 2 m are of a geotextile material that absorbs the water.

The raised ground will meet flush with the new sheet piles at the waterfront to raise the ground to 5.8 m above ordnance datum and the houses themselves to 6.1 m to protect against a 1 in 200 year flood event.

The 2.5 km of steel sheet pile walls are around 15 m deep with a concrete capping beam. So far around 500 m of the piles have been vibration hammered into the gravel underneath the made ground and alluvium.

In some areas the sheet piles are placed along the front of the original concrete wharf, which is being demolished, and the space behind is back filled with crushed concrete to increase the development area.

At another point there are two lines of sheet piles connected with tied rods to retain the front sheet pile wall, with the area in between backfilled with crushed concrete.

Most of the sheet piles are retained with diagonal, steel ground anchor beams but where the houses are to be sited near the waterfront, a horizontal tied anchor beam, supported by vertical concrete piles directly underneath, is used to retain the wall. This is because the diagonal ground anchor beams would interrupt the piling for the new houses nearby.

At another wharf, where the sheet piling is yet to start, Nuttall is considering a soil mixing solution to strengthen the ground behind the river frontage.

A fifth sheet pile wall solution is to restore around 200 m of the original sheet piles and raise them to the 5.8 m level.

Meanwhile, site clearance and demolition work is continuing apace ready for the top layer of sand.

This work includes testing the existing soil and separating the soil contaminated with oil for bioremeditation. Under this process the water and air is removed from the 2 m high pile, which enables bacteria to break down the hydro carbons, After four months the soil is retested and it is hoped that 100,000 cu m of soil altogether will be retained in this way and kept away from the landfill site.

The job is one of the first of the really big brownfield sites along the Thames Gateway to be prepared for housing over the next few years.

If it is successful it will be held up as an example of what can achieved. But there's some way to go yet, says Mr Horton. hard to develop solutions that are both

“It's been hard to develop solutions that are both cost effective and give the client what it wants," he says. "Keeping on top of all the licences we are working under in terms of re use of materials and the interlocked programme of land raising and river wall construction is a big challenge. The planning conditions are much more stringent than other projects we have worked on and Nuttall is working under a WML for the first time.

"This is one of the first major housing projects in the Thames Gateway development and a lot of people are looking very carefully at what we are doing here."

Article courtesy of Construction News (31/08/06)

Further Info

For further information, contact Peter Bishop, Head of Public Relations at:
Edmund Nuttall Limited
St James House, Knoll Road, Camberley,
Surrey GU15 3XW
Tel: 01276 63484
Fax:01276 66060
E-Mail: peter.bishop@edmund-nuttall.co.uk