On a cold and frosty morning in February 2006 NCE joined British Waterways engineers aboard a speedboat at Putney and headed east along the Thames. Soon after passing the Millennium Dome the boat headed up one of the many tributary rivers on the north bank and chugged towards what will be the London 2012 Olympic Park.
It weaved its way past dumped shopping trolleys, fridges and cars, with the steep concrete river walls on either side providing only fleeting glimpses of the post-industrial wasteland that lay above. As the boat continued its grim journey British Waterways senior project engineer Mark Stephens revealed his dream of transforming this network of rivers and canals in the Lower Lea Valley into a vibrant water city, with pleasure boats and freight passing along its waterways.
But for this dream to come true, a lock would need to be built at the mouth of the Prescott Channel, Stephens said. This would impound the water to keep it at a steady level all the way up to the Olympic Park, replacing the erratic tidal ebb and flow. All that was needed was the money to fund it.
Fast forward to a far warmer May in 2007 and piling work has already begun. The protracted negotiations over who would pay for the £18.9M Prescott Lock was finally concluded at the end of February this year with four organisations sharing the cost with British Waterways (NCE 1 March). The environmental and logistical benefits of removing the equivalent of 500,000 lorries from the capital's roads during construction of the Olympic Park were enough to persuade the Olympic Delivery Authority (ODA), Transport for London and the Department for Transport to part with their cash. The London Thames Gateway Development Corporation came on board because of the added value that the improved waterways would bring to future developments in the area.
The project involves the construction of a 62m x 8m lock, two flood control 'fish belly' gates (see diagram) and a fish pass on the Prescott Channel, plus a new weir at the mouth of the Three Mills river.
Main contractor Volker Stevin was appointed in March and in turn hired consultant Tony Gee to develop the designs before work started on site in April.
Volker Stevin project manager Windsor Young describes the programme demands as the most challenging aspect of the job. Its pace has been dictated by the ODA's own demanding construction timetable: the lock must be up and running in time for the start of the construction of the main London 2012 venues next summer.
Meanwhile, the project's first major target falls in just six months' time.
'Impoundment of the rivers surrounding the Olympic Park must be achieved by Christmas so that the ODA can start breaking out the river walls, ' says British Waterways project manager Carl Ainley.
A major feature of the construction of the 2012 site will be to replace the existing concrete river walls that run the length of the Olympic Park with sloping river banks.
'It's easier for them to work at a fixed water level (the level it will be during the games), ' adds Ainley.
The Prescott Channel was built as a ood diversion channel in the 1930s by Major Prescott, then chairman of the Lee Conservancy Board. While there are canals and other impounded rivers in the area, these are only capable of taking 120t barges.
To make the transportation of freight to the Olympic Park nancially viable, Peter Brett Associates calculated that the waterways would need to be capable of taking 350t barges.
'Labour is the biggest cost in water transport, ' says Ainley.
'Whatever the size of boat it always has a crew of two. The 350t barges are common and the most cost-effective size - each one is the equivalent of 17 lorries.' The lock is designed to allow two to three barges to pass through every 15-20 minutes.
'We think we can get 3,000 to 4,000t of construction materials passing through it per day, ' says Ainley.
The Prescott Channel was the route most suited to impoundment, but due to its primary function as a ooding outlet, the lock structure had to be accompanied by the two sluice structures (see diagram).
The dual nature of the water ow that makes it necessary to for 'fish belly' gates also makes it necessary for the lock to have radial hydraulic gates.
Traditional gates rely on the force of the water to close them, but here the gates must be capable of closing even when the tide is moving out and away from them.
In addition to the sluices a fish pass will be built on the opposite side of the water to the lock for fish whose route will be blocked by the lock, sluices and weir.
The fish pass design is being carried out in conjunction with the Environment Agency, which has also dictated the alignment of lock due to its installation on a flood channel.
Piles are being driven in to the bank and the earth in front cleared to widen the river to accommodate the permanent concrete structure sitting in the water. The waterway is a flood relief channel and the Environment Agency states that its total cross sectional area must not be reduced.
Ground onditions, programme restraints and cost have all been key factors in the method of piling chosen for this.
After 5m of made ground and 6m of terrace gravels, piles hit London clay. A tied sheet piled wall of 15-18m in length on both banks was chosen instead of a 25-30m cantilever piled wall.
This saved on the amount of steel used and meant less driving, which saved time and minimised the noise nuisance that a large impact hammer banging down sheets of steel would cause to the site's neighbours.
The contractor is making other efforts to keep noise to a minimum.
'We try to avoid impact driving where we can. We go 12m down with a vibrating hammer then tackle the last few metres with an impact hammer, ' says Young.
The contractor also liaises with the nearby Three Mills film studio and timetables pile driving to avoid clashing with filming.
The piles are tied with ground anchors 20m long and 133mm in diameter, similar to normal augured piles but inclined from the horizontal. These are augured via an excavator positioned at ground level and drilled at 10-20infinityC below the horizontal (depending on the designed anchor length and ground conditions).
The contractor then inserts a casing full-length and water flushes the hole clean, grouts up the hole, pushes prefabricated tendons into the grout and pressure grouts the bottom of the hole, removes the casings, forms the anchor head and welds this to the sheet piles.
Once the grout has cured for seven days the strands are stressed and the excess strand is cut and capped. Piling for cofferdams to enable main construction will start this month.
Fish Belly Gate The Prescott Channel was constructed in the 1930s as a ood bypass from Three Mills river to Channelsea. This function made it impossible for British Waterways to propose a straightforward lock on the channel, and instead the structure contains two 'fish belly' sluice gates in addition to the lock, which could both keep high tides from owing up river but allow oodwater to escape downstream.
Water north of the lock must be kept at its impounded level of 2.4m in depth at all times. The hydraulic 'fish belly' gates that enable this are very similar to those on the Thames Barrier. To keep the tide out, at low tide they are xed to keep the water impounded at the correct level and allow for some ooding overflow. At high tide the gate must be raised and sealed. If heavy ood water heads downstream, the gate can be fully lowered to allow water to escape into the River Thames.