Ladybower Dam has been settling almost continually since it was built, says Severn Trent Water project manager Stewart Harwood.
'This is one of the steepest earth dams in the UK and the knowledge of soil mechanics wasn't as good in the 1930s when it was designed as it is today,' he explains.
The last major works on the dam were in 1987 when a 2m high reinforced concrete flood wall was added to the crest. But the latest scheme is designed to blitz every aspect of the dam's maintenance.
'We have got to the stage now when we can't just keep on topping up the dam. We are having to significantly reinforce the downstream side as well as raising the crest,' says Harwood.
Around 200,000m3 of sandstone infill is being added to the dam, increasing its vertical depth by 10m and the crest height by 3m at the centre. This should give it enough additional freeboard to last well into the next century, plus an allowance for further settlement.
The waterproof puddle clay core of the dam will be extended up to the new crest height using a 2mm thick HDPE membrane. Main contractor Alfred McAlpine site agent Duncan Elliott explains: 'We will excavate and install a bentonite slurry trench into the existing core and then insert the geomembrane. When the bentonite goes off we will have a perfect key into the clay.'
The sandstone infill is being added in 500mm layers and compacted with a self propelled vibrating compactor. The face of the dam will be covered with crushed sandstone and then reinstated with topsoil and turf. As Elliott says: 'It is basically a case of getting the rock down here as quickly as we can now.'
In addition to the dam strengthening works Severn Trent is refurbishing two pumping stations, including all electrical and mechanical equipment, and constructing a new pumping main to suit the new dam profile. It is also having to raise the height of the adjacent road, and to strengthen and repair two unusual viaducts under a seperate contract at a cost of £12M.
The viaducts are interesting in that they are riveted steel lattice structures, like the Forth Bridge, but encased in concrete.
'The concrete provides some stiffness but we think it was mainly added to provide protection to the steel and for aesthetic reasons,' says resident engineer Alan Towers of Charles Haswell and Partners.
Contractor Norwest Holst discovered that the extent of repairs needed was worse than expected due to ettringite attack on the concrete and corrosion of the structural steel members from road salts. It also found concrete reinforcement bars to be missing or inadequate in some places.
'The extent of the problem became evident when we started receiving complaints from fishermen,' says Towers. 'They were having to dodge pieces of concrete which kept dropping off the viaducts.'
Repair work has meant stripping off the damaged concrete and inspecting the structural steel members underneath. The concrete road decks of each viaduct have been beefed up to cope with the new 40t weight limit and infill walls at the end of the lattice arches have been replaced with structural walls.