Building a house on sand is deemed to be the definition of foolishness. But for United Utilities it is proving a wise decision, as Margo Cole reports.
Back in the mid-1980s, the Mersey was the UK’s most polluted estuary, with industrial and domestic sewage being discharged directly into the river. That all changed with construction of the Mersey Estuary Pollution Alleviation Scheme (MEPAS), which included a 29km long interceptor sewer to pick up untreated discharge from 26 sewer outfalls and channel it through a new treatment works built inside an old dock.
Since then, the river’s fortunes have changed, and the Mersey is now home to salmon, trout, lamprey and dace, as well as porpoises and grey seals.
“We were very careful with the dock walls. They were continually monitored during silt removal, but we didn’t detect any movement”
John Redford, GCA
The original treatment works, built on Sandon Dock in the late 1980s and commissioned in 1991, is the second largest in United Utilities’ (UU’s) portfolio in the North West, but it is no longer capable of handling all the effluent that goes through it. “It’s struggling to perform,” explains UU principal project manager Lorne Large. “It can only treat about 40% of what it should”.
The problem is not capacity - the works is more than big enough to deal with the 950M.l of sewage a day that reaches the works from a catchment of more than 1M people. The big issue is the biological aerated flooded filter (BAFF) plant that provides the secondary treatment. “It is sized for 100% of the flow, but it just can’t do it day in day out,”says Large.
After looking at all the performance improvement options, UU concluded that little could be done to the BAFF plant itself, and that a completely new secondary treatment process would have to be added. But where could it go?
The answer was next door - in the adjacent disused Wellington Dock. It and Sandon Dock were built in 1848, and were used by the Mersey Docks and Harbour Company until they were leased to United Utilities in the early 1980s. Wellington Dock has sat empty. Now it is the perfect location for a massive new sequential batch reactor (SBR) that will provide secondary treatment.
“We put together a solution package that included work in Sandon Dock on the existing assets that we could improve, but the key bit for us was going into Wellington Dock and adding new secondary treatment as a replacement for the BAFF,” explains Large.
The result is a £200M upgrade that includes around £44M of improvements on the existing site, £11M for a new outfall and £145M to infill Wellington Dock and build the new SBR. The contract for the Wellington Dock infill and SBR construction has gone to GCA, a joint venture of Galliford Try, Costain and Atkins that is currently on its third successive AMP framework with UU.
The JV joined forces with the client in May 2010 to help develop the options and to value engineer costs out of the project as much as possible, with the result that the scheme is currently forecast to cost around £7M lower than originally projected. The insitu concrete SBR is currently rising rapidly within the old dock, but before construction could begin, over 30,000m3 of silt had to be removed from the Victorian brick structure. Before the removal began, divers went down to survey the condition of the dock walls, and the contractor set up equipment to measure movement at various points.
“We were very careful with the dock walls,” explains GCA chief construction manager John Redford.
“They were continually monitored during silt removal, but we didn’t detect any movement.”
Safety record to be proud of
With the project more than 25% through, the liverpool wastewater treatment works project has an exceptional safety record.
There have been no reportable accidents, and contractor GCA has introduced a range of initiatives to keep all accidents (including non-reportable events that often happen in the site offices) to a minimum. These include behaviour-based safety initiatives aimed at everyone who interacts with site, according to GCA senior construction manager Darren Dobson. “We’ve made a conscious effort to get delivery drivers, subcontractors and suppliers using behaviour based safety techniques,” he explains. “Every 100,000 hours without an accident we give £1,000 to charity.”
Once the silt had been removed, the next task was to seal the dock by closing the dock gates and installing two rows of sheet piles across the entrance. One the dock was sealed, it was filled with 110,000m3 of sand dredged from 13km out in the Irish Sea. The dredger collected the sand and then moored at a temporary discharge terminal in the estuary, outside the dock. The sand was then pumped in solution with seawater down an 800m pipeline and via spreader pontoons into the dock.
The entire operation took just 20 days - half the time that had been programmed - filling an area of 29,000m2 to a depth of 8.5m. Once the sand had been placed, it was dewatered, and the water pumped out.
The sand was then compacted hydraulically and, close to the top, by dozers and rollers, before being topped with an 600m thick permanent piling mat made up of 42,000m3 of imported stone. This enabled piling subcontractor Bachy to install 860 continuous flight augered (CFA) piles with 900mm diameters to an average depth of 15m.
The piles, which are socketed up to 5.5m in the underlying sandstone, provide the foundations for the new SBR but, according to Redford, the team did look very closely at the option of building directly on the sand.
“One of the big lessons learned from the construction of the original works was to use good quality sand,” he says. “We went for sand with a very low fines content - just over 1%. It’s a very consistent material, and it was a fine line deciding whether we needed to pile or not - that’s how well it was compacted.”
However, as Large points out, the process equipment needs to be absolutely vertical, so UU could not risk any movement of the foundations.
The good compaction of the sand did save on the amount of stone needed for the piling mat, and the ease with which it could be dewatered helped simplify the piling process. “The CFA piling was initially bid on using cased piles, but part way through we changed to uncased because we weren’t getting any ingress,” explains Redford. “We also introduced compressed air to blast through the rock.”
The piles were installed flush with the top of the piling mat, enabling the pile caps to be cast on top of the piles the day after installation. They were installed on a 5m grid, with each pile sitting directly beneath the columns of the huge SBR structure.
Building Information Modelling
The complexity of the project has led to the team adopting sophisticated building information modelling (BIM) based techniques, according to GCA detailed design manager Manjit Gill.
“We needed a very good integrated approach to design of the civils, M&E and process elements, so we adopted a BIM-compatible 3D model,” he says. “We have been utilising this to work through clash control and critical interfaces, and our process planner, Aecom, is also integrated with this model.
“It has provided dividends, and given us confidence going forward as well as from an operational point of view. And we have been able to utilise the software for value engineering.”
He adds that the model has the capability to be “full BIM”, by incorporating time and cost information.
Construction of the SBR started in September 2012, and is more than half complete. It stands 21m high, and consists of 16 cells on two levels that are effectively chambers through which water passes during the various stages of secondary treatment. Each cell measures 40m x 49.5m, giving the SBR a total footprint of 110m x 165m, which includes a 9m wide gallery down the centre of the structure.
“One of the big lessons learned from the construction of the original works was to use good quality sand,”
John Redford, GCA
The SBR is being constructed using insitu concrete, based on a network of tall, slim, 500mm to 600mm square columns and 600mm thick wall panels. The contractor is simplifying the concreting process as much as possible by using system formwork for the repetitive structural elements - Efco (check) for the columns and Peri for the walls. There is very little scaffolding, with workers using mobile elevating work platforms (MEWPs) to avoid dangerous working at height.
Since the new Year GCA has been pouring the 750mm thick base of the SBR, which is formed in 26 pours, each requiring 600m3 of concrete. The sheer scale of the structure is apparent when Redford explains that 50,000m3 of concrete and 10,000t of reinforcement are going into its construction. A further 10,000m3 of concrete has gone into the piles, and another 10,000m3 will be used for assorted small structures, including a new pumping station being built inside a 45m by 16m by 4.5m deep cofferdam.
The civils element of the project accounts for about 60% of the contract value, and with construction progressing well, it is only two months before process equipment starts arriving on site - ready for UU to commission the SBR in 2014. The entire scheme is set to be fully operational by April 2016.