Project Focus: Going with the flow in the Elan Valley
The first of three tunnel diversions has recently been completed as part of the £300M Severn Trent project to renovate the Elan Valley Aqueduct and provide a more resilient water supply to Birmingham
The Elan Valley Aqueduct (EVA) has been bringing water to the city of Birmingham for more than a century.
The 119km-long aqueduct is one of the last great civil engineering achievements of the Victorian era.
Opened in 1904 by Victoria’s son, King Edward VII, the aqueduct today carries 320 million litres of fresh water to Birmingham from mid-Wales every day – roughly twice the volume it carried a century ago.
The EVA is one of Severn Trent’s most significant assets, but after more than 100 years of service, the need for regular maintenance and refurbishment is becoming ever more frequent and so the time has come to provide extra support for the EVA to make sure that it can continue to provide service for another 100 years.
This led to the Birmingham Resilience Project (BRP), a scheme to safeguard the EVA and protect Birmingham’s water supply. At around £300M, the BRP is the biggest infrastructure project in Severn Trent’s history.
One of the first phases in the BRP is to replace three sections, totalling 4.6km, of the EVA where work is needed – a task which could get underway before the new alternative pipe is in place. The three new sections, at Bleddfa, Knighton and Nantmel, are being tunnelled alongside the existing aqueduct and the flow of water diverted into the new sections allowing the old tunnel to be sealed off.
The contract for this phase was awarded to BNM Alliance, a joint venture between Barhale and North Midland Construction which has just completed the 1.8km-long Bleddfa diversion, the first of the three sections to be replaced.
“There was no risk of imminent failure in the aqueduct,” says Richard Holloway, the BNM Alliance site manager at Bleddfa. “These replacements are proactive maintenance.”
The new tunnels are being bored with an earth pressure balance tunnel boring machine (TBM) which requires a large, 15m deep launch pit.
The transition, first from the existing conduit into the new tunnel and then back to the original conduit again, has been carefully designed to avoid slowing the flow of water down the aqueduct. The entire EVA, from mid-Wales to Birmingham, is gravity-fed – there are no pumping stations – and so interruptions and obstacles must be avoided at all costs.
Precast concrete manufacturer Kijlstra has been instrumental in the design and production of the critical tunnel sections that divert the flows into and out of the new tunnel.
“The contract was split between upstream and downstream projects,” explains Kijlstra’s production manager Rupert Treadaway. “Each contained large U-channel elements in which we placed bespoke benching elements with continually changing profiles for a gradual transition from a square section to a rounded one to minimise any effect on the flow.”
After each flat-bottomed culvert section was installed the appropriate benching element was cemented in place and then cover slabs placed on top to close the conduit. Custom-designed diversion blocks were installed to divert the water from the existing stream into the new layout without having to shut down the water flow.
Although the diversion is only about 20m long, its design required painstaking calculation, says Rupert. “We worked very closely with the 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 lifted 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.
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