An 85m by 85m three-sided cellular cofferdam is taking shape in the middle of a main drainage canal near St Germans in Cambridgeshire. The cofferdam, believed to be the biggest in Britain, will create a dry working environment for the construction of a new £38M pumping station.
The station is being built online, in the middle of the canal because the man-made waterway is straight, as far as the eye can see, and building online in the path of the waterway saves re-routing the watercourse permanently, which would bring in troublesome bends.
The north and south sides of the cofferdam extend out from the river bank. They are connected by the third side, which is in the river. The structure has been built to exclude water from the site, but must not block the waterway completely. So, one of the first tasks was to widen the canal to accommodate a bypass channel.
The cofferdam is a twin-wall construction, formed of sheet piles, 10m apart, toed 12m into the riverbed, tied together and filled with graded well-draining fill. A twin wall construction was chosen so that the cofferdam could also be used as a construction platform.
The 10m by 15m (in plan) cells allow for sequential construction. Once one cell has been created, the equipment can move onto the top of it, ready to prepare for the next cell. In this way the north and south side arms have been built, gradually extending out from the bank.
The east arm, which closes the box, stands in the middle of the canal with no part touching the bank so a different construction method was used.
A jetty was created to get workers and equipment across to the inner face of the east bank, allowing construction to take place on all three sides concurrently.
Ten months of the three-year pumping station construction programme is needed for this massive temporary works undertaking. Cofferdam works started in January and are due to finish this month.
The new pumping station will replace its 73-year-old predecessor and is vital for the defence of 70,000m2 of surrounding fenland which contains more than 20,000 residential properties and agricultural land valued at £3.6bn.
Without the defence, the area would return to a wetland bog, flooding at high tide.
Flood defences must be maintained at all times during the construction programme.
"The pumping station is fairly straightfoward," says Atkins project manager Don Lamont. "It's building in the channel while maintaining the systems and the flood defence which is the biggest challenge."
The tops of the canal banks are all set at 7m above sea level, as are the tops of the cofferdam walls. A second level of flood defences has to be built around the land-based edge of the site. Where the high banks have been breached to provide access to the site, a special gate is put in place during high tides, to prevent flooding.
The cofferdam has to be designed for the worst case scenario, which happens twice daily, when the tides come in and the hydrostatic pressure on the toe of the cofferdam increases.
The new structure will consist of bored pile foundations supporting a concrete slab, incorporating spiral depressions which house the pumps.
These snail-shaped indentations are themselves a work of art in terms of formwork.
The tolerances for these pump cradles are tight because even a few millimetres can affect the efficiency of the pump.
A reinforced concrete structure will be built to support the pumps and contain the fuel tanks and generators.
The steel frame, built on top of the concrete works to house the control systems, office and welfare facilities will be glazed along the front to show off the large pumps.
Even once the cofferdam is built, the challenges facing the project team are not over, as the commissioning process poses its own problems.
"We have to think now about how we do it," explains Lamont.
Firstly, the new pumping station has to be run to ensure it works smoothly.
At this stage the cofferdam will still be in place. The cofferdam had to be oversized so that it can be filled with enough water to allow the pumps to run cyclically, sucking in what they've just pumped out.
Once the pumps are running smoothly, the bypass channel can be closed and the cofferdam breached. The presence of the old pumping station presents a hydraulic restriction to flow during draining to the new station and the sluice gates need to be demolished to allow water to drain into the new pumps. After this, the old station can be dismantled.
The project will come online in the summer of 2009 and is scheduled for formal opening in January 2010.
The new pumping station has 40% greater capacity than its predecessor and will be capable of pumping 100m3/s of water, the equivalent of nearly five olympic swimming pools every two minutes. It will be the UK's largest land drainage pumping station, "the Wembley of pumping projects," as Lamont affectionately describes it.
Historically, the British Fens were wetlands in the east of England. They were virtually all drained by the end of the 19th century. Since the 1600s, large straight drains have been used to channel large quantities of river overflow into the sea without flooding the fens.
The fens are sinking as the peaty ground loses water to these drainage systems and to shrinkage. They have sunk up to 4m the past 150 years and some areas are now 3.5m below sea level. This in turn means that the drainage system needs to work harder to keep the land from flooding. St Germans pumping station opened in 1934, after it was decided that gravity drainage alone was no longer enough. The 1934 station now needs replacing due to increasing tidal levels.
Middle Level Commission Client
Atkins Project Manager
Costain Civils contractor
Birse M + E contractor