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O'Rourke, with Atkins carrying out the design work. "We were both contracted on the Severn Trent AMP5 frame- work but we had different roles within that process," Tony Heaney, nmcn project manager for phase one, says. "We'd done a lot of major projects for Severn Trent on larger non-infrastruc- ture concrete structures, which is a skill that Laing O'Rourke have as well, but they were taken on really for their work on infrastructure and pipe-laying." Having missed out on the ini- tial tender, nmcn was brought in a„er coming up with a proposal that would cut overall costs, but that did result in a timescale challenge. There was also a significant challenge with the site itself. The original Ambergate Reser- voir was built on a steep slope in ancient woodland, surrounded by Sites of Special Scientific Interest (SSSIs), and space was limited. Some of the woodland needed to be cleared, which involved rehoming bats and badgers. Shire horses also had to be brought in to move the felled trees due to concerns that using heavy plant might cause damage. The team then opted to build bridges to protect the ageing mains underneath. "There is a single inlet main and three outlet mains, all of which needed to be bridged early in the project to be able to move our plant over the top of the main without putting them at risk of damage," Heaney says. Planning conditions were also restrictive, with a relatively short window during the day Inside Ambergate NEED TO KNOW • Ambergate Reservoir feeds customers across the East Midlands, including parts of Derby, Leicester and Nottingham • The original reservoir was commissioned in 1907 and completed in 1910 • Work on the renewal project started in March 2014 and finished in April 2018 VALUE ENGINEERING • The reservoir needed to accommodate a throughput at 2,000l/s, and the pipework was originally intended to be 1,400mm diameter, but the team found that could be reduced to 1,200mm and still deliver at 2,000l/s if the reservoir cells were less than 95 per cent full. When the reservoir does reach maximum capacity, a bypass is available to make up the shortfall • While the original design called for full-bore magnetic flow meters, a wetted ultrasonic flow meter was used, which allows for high accuracy flow measurement but can be maintained or replaced without the need to isolate that section of main, negating the need to add extra bypasses • Butterfly valves were used instead of the more costly gate valves for flow control onsite. While butterfly valves can cause major problems under certain conditions, as a safeguard, KGAL Consulting Engineers was brought in to appraise the use of 16 valves and determined that they were fit for purpose concrete, Laing O'Rourke oper- ates a large precasting facility at Worksop that provided precast concrete structural elements for the columns, beams and roof planks. For the second reservoir, Design for Manufacture and Assembly (DfMA) was fully embraced with the use of Laing O'Rourke's 'Twinwall' solution for precast structural walls. Using two precast skins of rein- forced concrete, the process effectively involves lowering the skins onto the reservoir's base and filling the void between the two with in-situ concrete. "It's slightly more expensive in terms of the construction, but there's a great saving on time," Hope says. "If you can save when heavy vehicles could enter the site, and the local road net- work was poor. With 80,000m³ of material to be excavated, One Severn Trent was understandably eager to reduce the amount taken offsite. Contending with limited space and a sloping site, the team engineered a reinforced earth system involving a series of plastic membranes that allowed material to be stacked and then used for landscaping. "It was an essential measure to comply with the planning conditions," Heaney says. "We also put a concrete batching plant onsite." While the base and walls on the phase one reservoir were constructed using in-situ time, you're saving money on overheads, cranage and the like, and the more you can construct in a factory environment, the higher the confidence you've got in terms of quality control. One other element of it is safety, because you're not swinging large shutter panels around." Hope said the system is "still in its infancy in terms of trial- ling" but a comprehensive workshop was held ahead of construction of the second res- ervoir to establish best practice and review lessons learned from projects that had used similar DfMA approaches. "From my perspective, they took a very responsible approach in using DfMA on some large distribution cham- bers ahead of embarking on construction of the second res- ervoir," he adds. "There's more joints with DfMA and it's imper- ative that you have highly skilled operatives used to work- ing with the shuttering and in particular concrete placing, but they trialled their system on sev- eral chambers on site first of all and so their skills were honed in readiness for the DfMA con- struction. That was extremely successful. It was a very smart end product." The team was also able to part-backfill the reservoir walls with 2,400m³ of polystyrene that had been recovered from the roof of the demolished reservoir, which helped to reduce soil pressure on the walls as well as removing the need to dispose of the material or cause unneces- sary disruption to the public. Managing the impact on the public was an important aspect of the project, with the wwtonline.co.uk | DECEMBER 2018 WET NEWS 17 reservoirs serving the people of Derby, Nottingham and Leices- ter rather than the local residents. "There was a severe lack of trust at the start," Heaney says. "I always remember the first meeting I went to because it was very uncomfortable. "We had to embark on a PR exercise with the locals. We had a liaison group – there were three or four key villages and we were able to invite councillors and school representatives. They became bi-monthly meet- ings, and we were able to over- come that mistrust. They found we were far less of an issue than they'd feared." The most significant achieve- ment, though, was delivering a challenging project on a tight timeframe and at a low cost. "It performs extremely well and will serve future genera- tions to come," Hope says. "It's a real success story."

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