The Victorian style engineering used on Castlewood reservoir in south London has certainly stood the test of time, standing firm for the best part of a century on fissured London Clay.
With no piling, the structure was essentially supported by balancing the weight of the earth outside against the hydrostatic pressure inside. A particularly tricky feat given that it is situated on a hill and the structure is at an unnatural angle of repose.
However during recent years there has been significant slippage of the mass concrete and brickwork structure, so the reservoir, which is of critical importance to the Eltham area it serves, was in desperate need of replacement.
Thames Water contracted Morrison Construction to provide a new reservoir on the site of the old one, capable of providing for a larger population than the original 4.1Ml structure. The new structure had to service 20,000 homes, and be built on the same site as the old one despite the ground conditions.
Construction manager Rod Smith explains: "Originally the site was going to be on common land, but because it is common land situated in a triplet of sites of special scientific interest we couldn't do that. That was another restraint here to the actual size of the compound. And then we had to create a new access for traffic to the pumping station, which is still operational during the works."
Although the final 14Ml capacity structure is much larger, it still uses around 3,000m3 of concrete which is almost the same amount as its predecessor. With advances in construction techniques since the early 1900s allowing for thinner walls, this reduced the amount of extra space needed - vital on such a cramped site.
Preparation work began on site in June 2006 – once the reservoir was isolated from the rest of the network. The site's war-time history played an important part in the enabling works. Eltham was heavily bombed during the Second World War, with many of those bombs falling on the commons and surrounding areas.
Before work could begin, bomb surveys were conducted to eliminate the chance of one of the 661, 450mm diameter piles hitting anything other than the bedrock. The contiguous piled retaining wall used to support the hill during work was itself supported by 30m long anchors driving into the ground beneath Eltham Forest, making a detailed survey vital. But this was far from the only problem faced on the site.
Morrison project manager Rob Anderson explains: "The great challenge here was to actually get up on the site. We've had a lot of enabling works here. We had heavy machinery going up there – 70 to 80t vehicles – so we had to create a bridge and then strengthen the actual haul road. We were going to be working through the winter so it was ideal to put down a concrete road while we were up there, just so that there were no limitations."
The bridge used was a Second World War Bailey bridge sourced from an Army surplus store in Birmingham. The bridge is made up of two 15m spans, and provided secure access to the site.
"The challenge is keeping everyone working on the size of a 2p coin and keeping it all flowing," continues Anderson.
These difficulties were underlined by the method used to construct the rest of the reservoir once the earthwork and construction of the base slab and walls had been completed. With the final design for the reservoir requiring support columns every few metres in each direction, conventional construction methods would have been greatly hampered by the limited space, making construction painfully slow.
Instead it was decided it would be easier to precast the columns on-site but outside the reservoir, and then crane them into place. The columns contain 500mm dowel pins – half of which protrude from their base – and so, as they were lowered into the floor inside the reservoir, they were lined up with holes marked out and drilled in advance.
Once in place, the roof slab shuttering was brought in and locked into position, its weight helping to secure the columns below. Anderson elaborates: "Traditionally you start with starter bars and then construct the column from the base up. Then you'd put your staging up and build your roof slab. The problem with the starter bars is that it makes your base floor impractical for use; you've got to dodge too many things."
"Between the subbies and ourselves we came up with the idea of precasting the columns outside." He continues: "The big challenge was at the tender stage when we were working through how to support these things. When we first got involved there were elaborate schemes to brace up each column, and you can imagine if all these columns every three or four metres had bracing around them, it would just be impractical."
"The idea was then we swing it into position through the roof, build your staging, drill your holes and then trap it with the (roof ) shutter. So it took away all this bracing idea, increased the speed at which the shutters could be moved around and gave us the freedom to actually construct things."