Pumping concrete in pitch black, freezing water is not to be undertaken lightly, as engineers working on Germany's Ems Storm Dam discovered. Nina Lovelace reports.
The sill foundations of the US-183M Ems Storm Dam in northern Germany are nearing completion, thanks to the endeavours of some hardy individuals.
Diving engineers, clad in drysuits and squeezed through metre wide holes in the roof of precast box foundations, have spent the last few months sucking mud out from the river bed and replacing it with infill insitu concrete to form a bond between foundation and bed.
'Often they can't see a thing, ' says main contractor Philipp Holtzmann principal engineer Ingo Fehrenbach of the divers, as they battle against the swirling tides and close to freezing water temperatures in the Ems.
The work is essential, he adds.
Construction of the storm dam began in 1998 in a bid to protect the Ems area from increasing tides and frequent flooding, Fehrenbach explains. This came to a climax in 1994 when the Papenburg area suffered some of the worst flooding for years, and the German government realised that the system of bankside dykes was nearing the end of its design life.
But the resulting government investigation revealed that to protect the area against future floods, the dykes would have to be raised by several metres.
'Huge amounts of sand and marine clay would have to be dredged, ' says Fehrenbach, making the task onerous, costly and causing considerable environmental disruption.
Instead, the government chose the cheaper, quicker and less environmentally damaging option of building a storm dam across a narrow width of the Ems between the towns of Gandersum and Nendorp. The dam was the preferred option because it offered storage capability that could potentially save the area's main industry - shipbuilding.
Until then, shipbuilding yards in Papenburg had been suffering a decline because the average high tide 7.3m depth of the Ems did not have sufficient 8.5m draft for the new trend of large passengers liners.
So a storm dam that could protect against storm tides 2m higher than normal high tide - plus impound an extra 2.7m depth of water on the upstream face of the dam - was the ideal solution.
The design incorporates six piers and sills. With the sills effectively needed to retain a greater volume of water on their upstream face, steel support trusses were added to downstream faces.
Holtzmann's first priority was to drive the 40m raked steel piles for the three insitu reinforced concrete northern piers.
These needed to be larger than their southern counterparts because they provide the abutments to the two main navigation routes through the dam - a 60m wide upstream passage for passenger liners, and the 50m wide navigation for smaller ships and all downstream traffic.
Such phased construction also allowed ships to continue to pass on the southern side of the river.
Piers for the liner route are 10m wide and 50m long, and will house huge hydraulic rams for the task of rotating the massive 800t steel sill to lie flush with the river bed.
The decision to build insitu using temporary sheet piled walls was forced by the difficulty of casting the large and complex foundation needed for the submersible sill, including walkways within the block for maintenance staff.
Fehrenbach says he would have preferred to use the same precast method as on the other piers, being a quicker and cheaper option.
'But strong tides and water ingress made the underwater concrete laying difficult, ' he says, adding that repair work using epoxy resin is under way to infill the resulting leaks.
Concrete for the piers and the submersible sill foundation was pumped from a Putzmeister unit on a boat moored alongside the site, attached to the mixing plant on the bank via a 310m long supply pipe. 'We 3ofconcrete in this way, ' says Fehrenbach.
To withstand sub-zero temperatures, the concrete was laid at 15°C and insulated during curing to reduce temperature differentials and to prevent surface cracking.
Concrete laying becomes impossible below -10°C, says Fehrenbach, but fortunately temperatures remained above this during construction - although concrete placing for the southerly piers has been delayed by fog and high winds which makes working over water unsafe.
The secondary 50m navigation also has precast foundations but has a curved rather than vertical sill to allow it to be raised to give the necessary 6m headroom for ships.
Most of the time, all seven sills will stand open to allow natural estuarine movement within the Ems. They will be lowered only when the control centre on the northern bank receives warning of an oncoming storm tide, or if a liner needs to pass downstream.
To enable launching of ships, the gates are lowered at high tide and the upstream area filled with water from natural flow and upstream pumping stations.
However, pumps located in each of the dam piers are capable of pumping water upstream at 1,000m 3/s into the impoundment area, raising the Ems by 2.7m and allowing safe passage of the liners.
To date, four of the six piers are complete, with two southerly piers left to pour and their adjacent sill foundations and sills to place. Completion of the dam is scheduled for mid2002.
Work is also ongoing on the river bed which is stabilised against scour by a geotextile mat overlaid with rip rap.
Closer to the dam, cement mortar is poured over the stone for protection against high localised surge velocities when the gates are closed or opened.