United Utilities' wastewater treatment plant for the 335,000 people of Wigan and its surroundings is large enough, occupying some 40ha in a rural area to the west of the town. But the inlet works, which is undergoing a £20M rebuild, is squeezed into a small triangle of land at the corner of the site.
'It is the right place for it in logistical terms for the rest of the treatment plant, ' explains Galliford Try's Aled Edwards, who is working as senior project manager for UU under a framework contract.
But the tight location meant that replacing worn out pumps, concrete flow channels, settlement tanks - known as detritors - and the buildings housing screening and waste separation equipment was never going to be simple.
An added complication was that the old inlet could not be taken out of action. Fortunately there was a little remaining space in the inlet area - enough to build a new structure alongside and make the connection to the inlet sewer.
'We've had to wrap the new plant around the old in the access spaces alongside it, ' explains Atkins designer Martin Meadows.
Atkins is part of the GCA joint venture with civils contractors Galliford and Costain, who are doing the mechanical and electrical work. GCA has one of two framework contracts with UU for design and construction of its major assets and treatment plants. Once the new inlet channels are on line, the older inlet will be demolished.
'The wrapping required some innovation and rethinking, ' says Meadows. Usually, channels are constructed in lines. 'But we put them as a cluster, which meant using some unusual flow paths.' There was also to be a relatively large excavation: 10m deep, 20m across and 40m long. This contains the inlet screens and the main wet and dry wells for the pumps, which lift the flow up to the high-level upper screens and channels in the screening plant structure.
For such an intricate design, Atkins made use of 3D software, in particular Bentley's Autoplant program developed specically for process design. It also used standard AutoCAD drafting tools.
With these it is relatively easy to produce 3D renderings with flythrough capability. Several ythrough videos were also made.
'But you can y through the model yourself on a PC, ' says Meadows. 'It really helps, outsiders particularly but seasoned engineers as well, to visualise what is going to be done.' It certainly helped in discussions with the client about solutions for the project he says.
'The 3D was also useful for local public relations with the residents to explain what was happening and also in the planning application purposes, ' adds Meadows. Securing planning approval was part of the project scheme assigned to GCA, which did the optioneering, detail design and construction.
A 4D model was also used; in other words a 3D model with a timeline, which demonstrated the construction sequences during discussions with the client. An additional capability of the software is to create coloured 3D documents as PDFs. 'It is particularly good for discussing potential conicts, ' says Meadows. 'Looking at the paths of pipelines, for example, to ensure there are no clashes.' Models of another kind were also required. Because of the unusual layout of the detritors it was important to verify the hydraulic design, so the team commissioned large-scale fluid modelling work from the BHR Group. The main project was a 1:6 scale model to test a variety of ow levels and inputs.
Meadows says he was pleased that the model veried the hydraulic design work as 95% accurate. Only minor modications were needed to the ends of the ow channel walls.
These included the addition of small ns at various points to even the flow and the installation of inlet cones beneath the nozzles of the pump intakes, to help prevent vortex eddies occurring.
'Construction itself has not been exceptionally difficult', says Galliford site manager Andre Mann. The major tasks have been excavation and concreting of the low-level inlet chambers, neighbouring pump chambers and the high-level concrete structures.
For these, a 15m deep secant pile wall was bored into the predominantly stiff clay ground.
There was some additional bored piling for the high-level structures.
Bachy Soletanche was the groundworks subcontractor.
It reviewed the design with GCA before work began. This helped reduce the amount of propping needed from three to one, for example.
The excavation comprised two halves, one 5m deep, the other 10m. The two are separated by a permanent secant piled wall, which had its top 5m cut off. 'Despite the apparent waste, it was the best way to do it, ' says Mann.
High-level structures were deliberately designed on a flat slab principle to allow the fairly complex channel walls to be made as simply as possible, explains Meadows. Again the GCA team talked things through first with the concrete subcontractor, Offa-fix.
'They were a very proactive firm anyway and helped value engineer the steel needed on the reinforced concreting, by cutting down lapping distances and so on, ' says Mann. Increases in the price of steel were an issue on the job, he adds, and the team saved some money by placing orders early.
'We had some laydown space about 400m away fortunately, ' says Mann.
Civils work began on site in March last year and is now mostly complete. Costain's M&E team is preparing to install the main equipment. The facility is due to open next year.
Who's who Client: United Utilities Civils contractor:
Galliford Try Design engineer: Atkins M&E: Costain Groundworks subcontractor:
Bachy Soletanche Fluid modelling:
BHR Group Client's consultant: MWH Site safety Concrete subcontractors bend back the ends of exposed reinforcement to stop workers catching themselves on it.
Workers receive specialised training in working safely at wastewater treatment works.