When an earth embankment at Denton service reservoir showed signs of instability the tried and tested engineering solution was to support it by constructing a gabion wall at its toe. Gabions are mesh baskets that have to be labour-intensively handpacked with stone. They cannot be lifted into position when filled as they deform easily and the manual work has to be carried out in situ. With walls standing 4m tall, construction would involve lengthy work at height. We’d already used design for manufacture and assembly (DfMA) to deliver other water industry solutions, achieving time, cost, carbon and safety savings by bringing together large, ready-made components. We saw the potential to apply DfMA to this challenge.
We used precast concrete Legato blocks, which interlock like giant bricks, to form the 100m long wall. As the blocks are heavier than gabion baskets, we were able to reduce the height of the wall from 4m to 2.4m, reducing the excavation and temporary works while still safeguarding the embankment from future slippage. Because the blocks allowed such rapid erection, it was possible to open up unsupported work faces of 20m at a time instead of the restricted 10m of the original design, enabling more efficient site operations. This is believed to be one of the first applications of DfMA to slope stabilisation in the water industry.
Our solution eliminated the risks associated with manual handling of stone and working at height and reduced time on-site from 47 days to 27 days. Total costs were cut by nearly 20%. Compared to gabion walls, the Legato blocks require almost no long-term maintenance, increasing the resilience of the reservoir to maintain reliable supplies of drinking water to thousands of homes.