Water and Effluent Treatment Magazine
Issue link: https://fhpublishing.uberflip.com/i/538666
10 WET NEWS JULY 2015 population there is of national importance, and is currently the fourth largest population in the British Isles. Consultations with Natural England were undertaken in order to minimise any impact from construction. Due to the works, the road and footpath along the crest had to be closed to the public. "We implemented diversion routes to allow local residents to maintain access to their properties, and for members of the public to continue to enjoy walks around the reservoir," says Matt Jenkins, from Yorkshire Water. Supply "For operational reasons, the amount of draw down was restricted during construction to ensure the supply to Headingley WTW was maintained. As such, part of the siphon had to be constructed underwater using a specialist diving team," Mott MacDonald Bentley says. "The siphon itself had to pass through the embankment to avoid blocking off the public highway ONSITE PiPework instaLLation The first part of the diving team's role was to confirm embankment profiles and silt lev- els with those obtained from remote bathymetric surveys car- ried out during the investigation stage. This allowed the design and location of the upstream pipe supports to be finalised. Pinpoint accuracy The divers then worked with the assistance of a long reach excavator to construct the pipe supports, which consisted of stone-filled gabion baskets. Pipe saddles made from concrete-filled sand bags were then added. The divers worked closely with MMB to float the pre-welded section of pipe (that had been constructed in-situ on the upstream face of the reservoir) into position. Although this work took longer than had been originally envisaged due to its complexity and large degree of manual labour required, it was carried out to an excellent standard of construction. The pipework was constructed from both ends of the siphon, met in the middle of the embankment with pinpoint accuracy, well within the required tolerances allowed. The siphon itself was designed using Building Information Modelling (BIM) technology and the earthworks were modelled using Autodesk Civil 3D and imported into Autodesk Revit structure. Due to programme constraints, the siphon had to be built by starting at both the inlet and outlet ends, working towards the middle, meaning that prefabricated bends were required to ensure the upstream and downstream legs were aligned perfectly through the crest. Setting out information was also available for the site team to pull out of the model as and when required. Another advantage of this approach was that once the upstream and downstream legs were in place, the as-built information was pulled back into the model to supply to the pipe manufacturer in order to produce the prefabricated compound bends. Reaction forces The model was also used for the structural analysis of the pipeline. The geometry, layout and arrangement of the siphon pipe were exported from Revit into Autodesk ROBOT where a finite element analysis was undertaken to determine reaction forces at restraint locations along the pipeline. The Eccup siphon project has been a huge technical challenge for all involved throughout design, construction and commissioning phases. This project was the first time that Yorkshire Water has tried constructing a siphon on this scale to improve draw down capacity. Add to this the tight programme and additional constraints. The project threw up numerous challenges. Firstly, the site was situated within the Eccup Reservoir Site of Special Scientific Interest (SSSI), which is designated for its importance to wildfowl in particular for its over wintering birds. Eccup Reservoir holds between 1 to 2 % of the estimated total wintering population of Goosander, so the "the project delivered a complex innovative solution within regulatory timescales, successfully mitigating significant 3rd party issues" Matt Jenkins, Yorkshire Water excavation for the downstream culvert, went through brick pillars the pipework was constructed from both ends of the siphon, met in the middle of the embankment with pin point accuracy, well within the required tolerances allowed the final review of options concluded that a siphon pipe arrangement was the best option to take forward into the detailed design stage of the project running along the crest. "The QCE stipulated that the siphon pipe must be routed through one of the existing brick pillars rather than the clay core. By passing through the brick pillar, a better seal through the waterproof element of the dam was achieved to prevent a leakage pathway developing through the embankment." The company continues: "This approach removed the need to re-puddle clay around the pipe, but did present other complexities: passing through the rigid brick pillar into the embankment either side introduced the potential for significant differential settlement. The brick pillar had to be carefully de-bonded and broken out to avoid any disruption to the embankment it helps support." For Mott MacDonald Bentley, the key to success was the collaborative partnership adopted through all parties at all levels. Regular 'core team' meetings between Mott MacDonald Bentley and Yorkshire Water helped resolve problems and keep a continuous flow of decision making while early subcontract involvement helped to deliver the best solution when it was needed. The compliance date of March 2015 was successfully met and Eccup is now preparing for its next project with spillway improvement works following on from the siphon scheme.