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Working with the tides

SITE VISIT

Riverside views bring premium property prices, but you can't build on land if it is slowly sliding into the Thames.When developer Greenwich Reach 2000 purchased an area of land surrounded by water on three sides, it found that the river walls needed extensive renovation. Joanna Booth went to find out how Dean and Dyball is holding back the tide

IT IS said that time and tide wait for no man, and neither is making any allowances for Dean and Dyball as the contractor negotiates the muddy reaches of the River Thames at Greenwich.

'Because the work is tidal, we only have a very short time span to work in, ' says project manager Jeff Roberts.'At the bottom of the excavations, we only get one and a half hours of working time per tide.'

There are two tides a day on the Thames, each taking approximately five hours to come in and seven to go out.Near to new and full moons - about every two weeks - there are spring tides, where low- and high-tide water levels are at the most extreme.

Every time the works move up another metre, the team gains approximately another hour and a half of working time per tide.

After surveys to the existing walls surrounding the site, it was found that seven separate sections required renovation, five backing on to the Thames and two on to Deptford Creek.

The Thames walls are being repaired using brick-faced, pre-cast concrete panels, designed by Dean and Dyball and manufactured on site. Brick facings are popular with the Environment Agency, as the rough surface makes it easy for aquatic wildlife to attach to the walls.

'The design specified nine-inch brickwork with ties, ' explains Mr Roberts.'We'd have had to use a sealed clutch cofferdam to keep the water out.'

Dean and Dyball won the contract on its concept for the brickwork panelling, which was then detail designed by Hemsley Orrell Partnership.

The idea germinated on a previous contract at the Limehouse basin, where the panels bolted back to the pre-cast wall.This time, the panels form the front shutter of the pour.

Each unit fixes to another with a male-female groove and tie bars at the end. Panels are dropped into position and left until the next day to be concreted in.

The panels are manufactured on site in a pre-casting yard.A movable heated tent allowed brickwork to continue through the winter.

'Each unit is bespoke for its location, ' says Mr Roberts, 'even though the majority are the same size.We need the details of the brickwork to align, otherwise it wouldn't match.'

Initially, a line of temporary steel sheet piles is driven through the water along the front of the existing footing, using a 17-m leader rig.The piles are set as tight to the wall as possible, about half a metre from the face.

The ground is hard and the piles have been tough to drive, especially at the base as the steel moves through gravels. Every row has had to be backdriven with a hydraulic hammer to reach required depths.

Noise levels from these operations are high, but there was no feasible alternative method.'We're using a noise shroud for the hydraulic hammer, which was made for works on the Millennium Stadium in Cardiff, ' says Mr Roberts.'It takes out the high-end ping and leaves you with just the thump!'

Another line of temporary piles is then driven along the back line of the mass concrete fill, which acts both as the back shutter and also provides protection from flood and possible tidal erosion, which could occur if the bank face was unsupported.

Then the existing wall is demolished to depths between 3 m and 4.5 m below ordnance data.A hydraulic shoring frame holds open the excavation, while the team excavates down through the gravels.

'At the bottom it's a nightmare, ' admits Mr Roberts.'The biggest struggle is to get to the correct depth.Once the footing is in, it's not so bad - we can just fire the concrete in with a pump.'

The first 2 m of mass fill use waterproof concrete, where an admixture holds the cement content into the mix.

'Otherwise you'd just get a big pile of sand and gravels, and we've got that at the bottom of the Thames anyway, ' says Mr Roberts.'It works fine here because the tidal rise and fall is slow - if there was a strong current, it would just wash away.'

Over 8,000 cu m of concrete will be used on the job, so it is convenient that there is a batching plant over the road from the site's entrance.

At the base of the excavations, silt has been a major annoyance for the Dean and Dyball team.'The water is easy enough to deal with, ' says Mr Roberts.'But the tide leaves 300 mm of silt behind every time it ebbs and flows - it's such a dirty river.You should see the state of my operatives by the end of the shift.'

The site was a former gasworks and, due to fears of contamination, the silt that gets into the base of the excavations has to be pumped into the site and disposed of.

With the footing sealing the ground, the team then no longer has to worry about contamination in the silt and can push it back into the river, rather than having to pump it out and deal with it.

'We reverse the pumps as the tide goes out, ' explains Mr Roberts, 'and wash the silt out with the tide.'

The pre-cast panels are laid on the footing and strutted prior to the placement of the mass concrete backfill.The sheet piles are sacrificial and are cut off before the pre-cast concrete capping beam finishes off the wall.

Of the two Deptford Creek walls, one is in relatively good repair. Single king posts are being driven along the front of the existing wall, and timber and scour protection removed.A line of 4 m sheet piles has been driven, and it is being attached to the king posts using 12 m macalloy bars, before pre-cast scour protection is fixed to the front of the wall.

Some sections of the final wall were in dangerous states of collapse and part of it needs to be moved back to allow haulage boats space to turn around in the creek.

A row of temporary steel sheet piles down the front is keeping the bank supported while the top of the wall is removed. Rows of permanent piles are being driven down the front and back lines of the wall and the whole area will be excavated, mass filled and topped off with timber-facing panels.

But an existing wooden crane staging is now throwing a spanner into the works.'We anticipated we could just pull it out, ' says Mr Roberts, 'but they did things properly in the old days and it's not budging. It's old green heart timber and laughs openly in the face of a chainsaw.We're going to get it cut up by specialist subcontractors.'

Rather than progressive phasing, works are ongoing on all the walls at once.Negotiating the tides means that the job is best tackled in small, constantly rising increments.The team is small, with only eight directly employed operatives on site.

'The whole challenge of this job is getting it to come together, ' says Mr Roberts.'The engineering isn't difficult. It would be easy to flood the site with plant and labour, but because of the restricted hours it wouldn't take much less time.'

Project information

River wall repair works, Greenwich: £4 million

Client: Greenwich Reach 2000

Project managers: E C Harris

Engineers: Arup

Quantity surveyors: Bucknall Austin

Civil engineering: Dean and Dyball