London doesn’t have a reputation as a clean city. Indeed it regularly features below its European counterparts in ‘world’s cleanest city’ listings and normally a trip to the capital will involve some kind of encounter with a rogue piece of waste.
So it’s a fair challenge that the Olympic Delivery Authority has set itself to become the greenest and cleanest host city in the history of the Games.
Targets include the recycling or reuse of 90 per cent of demolition waste and ensuring that 50 per cent of all construction materials arrives at the Olympic site in east London by barge or rail.
But before barges can go anywhere near the Olympic arena, the canals need to be cleaned, dredged and made navigable.
This is where the Prescott Lock comes in - the first new lock to be built in the capital for almost 20 years.
Client British Waterways drafted in contractor Volker Stevin to help construct a lock that would act as the catalyst to the restoration of waterways in the area, allowing them to be used by transport barges.
Keeping out the Thames tide
This can only be achieved if the canals and river navigations within the Olympic Park retain a constant and controllable water level, explains Volker Stevin project manager Roger Martin.
“The River Thames essentially comes upstream through the Prescott Channel. So what we need is to have a lock structure that will ensure the canal is only tidal to its south,” he says.
This tidal exclusion will ensure that the water level can be controlled throughout the Olympic Park, increasing navigable capacity.
But working with the tides during construction isn’t an easy task. The team had to ensure an area the width of the existing canal remained open at all times, preventing -flooding and allowing fish to pass.
“We widened the whole channel from around 22 m to 41 m and for now have kept the west weir gate and the bypass channel open. Once the lock and the east weir are working we will close the others to work on them,” says Mr Martin.
In all more than 700 steel sheet piles each weighing as much as 11 tonnes have been driven 18 metres into the ground along the banks of the Olympic Park’s canal, replacing the dilapidated 1930s concrete river walls. These existing walls proved more difficult than expected to work with, says Mr Martin.
“We found that they were constructed of a series of toe piles, made of steel and concrete, below the concrete revetment. So we needed to carry out additional removal works on top of those we had originally planned.”
Work started on the lock at the beginning of January, with the site of the lock chamber excavated to a depth of seven metres and a cast in situ reinforced concrete base placed.
Reinforced concrete upstand walls and capping beams, 0.7 m higher than the piles, were then installed to hold the gates. Gravel removed from the chamber was used to build the adjacent steel sheet-piled lock island.
Before the 2.4 m deep lock can be used, two sets of twin gates are due to be installed at either end of the island, between it and the banks of the canal.
“Tidal exclusion is set to happen in May and the lock is scheduled to be operational in October. But we might be delaying the tidal exclusion in order to bring the lock operation forward,” says Mr Martin.
But the lock can’t operate without the east and west weirs which will control the level of water upstream.
Sheet piling and temporary closure walls are installed to create a 5.6 m cofferdam, increasing the height above ordnance datum (the height above mean sea level for the area) on the canal by 0.8 m to -0.8 m.
The reinforced concrete base and walls are formed in staged concrete pours, including the installation of bolt assemblies, pipework and ducts.
Once the concrete has been poured, stainless steel side plates and hinges are attached to the weir walls, which will hold the ‘fishbelly’ gates in place.
Those weir gates - similar to the mammoth gates of the Thames Barrier a few miles further east - will rise up from the bed of the weir channels.
“The weirs need to be done in stages to maintain the channel’s flood conveyance. We’ve just dropped in the gate on the east weir and the next one is due to be dropped in September,” explains Mr Martin,
adding, “The works for the west weir could not commence until the east weir was installed.”
The first 21 tonne gate was lowered into place at the start of April using a 180 tonne crawler crane, allowing the control of tidal flow during the piling operation to form the cofferdam.
When the cofferdam for the construction of the second weir and piling across for the fish pass construction is completed the first weir will be needed to control upstream water levels.
When both are finished they will control the flow of water up and down the canal - allowing a constant level to be maintained north of the lock.
From next year the ODA will use the 62 m long structure to allow barges weighing up to 350 tonnes to pass through into the Olympic Park.
The regenerated canal will also be a backdrop for new infrastructure, including tow paths, bridges and lighting.
Over the next four years the waterways will play an integral part in the construction works in the Olympic Park. But they will also be a significant part of the legacy for East London - one of the driving factors behind London’s Olympic bid.
How does the fish bypass work?
The fish bypass has been integrated into the project to allow wildlife in the canal to have free rein of the waterway.
To keep a controlled water level above the bypass a series of concrete boxes and a tidal tracking gate were fitted.
The gate shuts automatically when high tide floods the bypass and releases in time with the tide, to allow the water back out.
This way the fish bypass works in the same range as the impounded water north of the lock and weirs.
Regenerating the East End canals
The Prescott Channel takes its name from a past chairman of the Lee Conservancy Board, which carried out the last major works to the waterway when it was built between 1930 and 1935.
It was originally used as part of a flood relief scheme under the Flood Alleviation Act.
The canal is part of the Bow Back Rivers system – which runs from the River Thames north up through Bow and Hackney, eventually linking up with the Regent’s Canal.
Barges last transported goods along its waters about 50 years ago and over the years the channel has become neglected, collecting vast amounts of rubbish.
But now with the Olympics coming, property developers have sprung on the chance to build on brownfield sites along its banks, helping breathe new life into some of the most derelict parts of the city.
Aggregate Industries was chosen as the Olympic Delivery Authority’s concrete supplier, partly based on the firm’s emphasis on sustainability.
It will need to meet the following targets during the park’s construction:
• Materials must have at least 20 per cent recycled concrete
• 25 per cent of aggregates used must be recycled
• Half of materials transported (by weight) must arrive on site by either rail or water
• Energy-efficient, low-emissions vehicles must be used on site
Aggregate Industries has its own batching plant at the Olympic site and is responsible for all delivery, off-loading and distribution of concrete across the park.
It has supplied as much as 5,000 cu m of concrete for the Prescott Lock job.