Specialist Drainage Systems Helps Preserve A Victorian Aqueduct - continued
Tight space and limited time meant that precast modules from Kijlstra were a better solution that using traditional formwork solutions.
The three new sections are being tunnelled alongside the existing aqueduct and the flow of water diverted into the new sections to allow 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 1.8 km-long Bleddfa diversion, the first of the three sections to be replaced.
The new tunnels are being bored with an earth pressure balance tunnel boring machine (TBM) which requires a large, 15 m-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 transitional sections that divert the flows into and out of the new tunnel.
There was no risk of imminent failure in the aqueduct. These replacements are proactive maintenance
says Richard Holloway, the BNM Alliance site manager at Bleddfa.
Kijlstra’s production manager Rupert Treadaway explains:
“The contract was split between upstream and downstream projects. 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 20 m 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 insitu concrete diversion. However, this was ruled out as impractical.
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