Water & Wastewater Treatment Magazine
Issue link: https://fhpublishing.uberflip.com/i/814050
Project focus: pipes and pipelines client's hydraulic engineer to refine profiles and dimensions to optimise flow through the diversion. It's far from being a standard design – it is highly bespoke." During the early planning stage, various options had been discussed, including an in-situ concrete diversion. However, this was ruled out as impractical. "Reinforced in-situ concrete was considered during the early stages of design but it quickly became clear that the handling and installation of the formwork would have been far too complicated for installation on site," says Richard Holloway. Bespoke precast components In the confined space of the 15m-deep excavation, it was easier to crane in precast components and install them one at a time than to construct a complex formwork and erect reinforcement, he explains. The bespoke precast units, weighing up to 27 tonnes each, were all manufactured at Kijlstra's factory in Henlade, Somerset, and delivered direct to site on low-loaders. "All the products were bespoke, although the U-channels were cast from our flat tables with special formwork produced by our in-house carpenter," says Rupert. "First we cast the walls and then stood them up and cast the base in between them." These were 3.9m high x 2.3m long x 4.3m wide with 400mm thick sections. "The benching blocks' moulds needed to be highly elaborate to enable us to manufacture a transitional shape from square to rounded over a length of 20m," he adds. Another major advantage of using precast concrete was that the units could be manufactured to close tolerances and a high standard of quality off-site while on-site operations continued. Actual installation of the upstream and downstream diversions took little more than a month, the entire programme commencing on 12 December 2016 and completing before the end of February 2017. Water continued to flow uninterrupted through the existing EVA while BNM Alliance built the new tunnel. However, under normal conditions the EVA is shut down temporarily twice a year for routine maintenance and inspection and in October one of these shutdowns took place, allowing the contractor to demolish the roof and wall of the existing EVA ready to divert the flow into the new tunnel. Stop-logs were used to seal the junction until the diversion had been completed. In March - when the new Bleddfa tunnel, complete with diversions - was ready, BNM Alliance removed the downstream stop-logs, allowing water to flow in from the existing EVA. This allowed the new tunnel to fill steadily with water and the pressure between the old conduit and the new one to equalise. "When the pressure had equalised, we liœed the upstream stop-logs and sealed off the old tunnel so that water now flowed uninterrupted through the new tunnel," says Richard. BNM Alliance has now moved on to the next diversion, 12 miles upstream at Nantmel. The third diversion (at Knighton, 10 miles east of Bleddfa) will be the last to be completed. Kijlstra precast units are already ordered for Nantmel, though supply arrangements have yet to be finalised for Knighton. Severn Trent is currently working on the basis of a three-year construction and commissioning programme and hopes to have the Birmingham Resilience Project completed by the middle of 2019. ● 17 'U' Channels (weight 26 tonnes, total vol 180 m 3 ). Installed by 16th Jan 2017 ● 17 Cover slabs (weight 10 tonnes, total vol 70m 3 ). Installed by 31st Jan 2017 ● 18 Benching blocks (weight 20 tonnes, total vol 120 m 3 ). Installed by 20th Feb 2017 ● 11 Diversion blocks: (weight 17.5 tonnes, total vol 140 m 3 ). Installed by 23rd Feb 2017 Precast units used in the Bleddfa project 18 | MAY 2017 | WWT | www.wwtonline.co.uk The use of concrete precast units enabled the project to progress quickly