An ambitious project to help provide security of water supply to customers across five counties is underway in the West Country.
Although last winter’s extreme weather resulted in large-scale flooding and storm damage, it is fair to say that the West Country suffers more than its fair share of water supply difficulties.
A shifting population that swells enormously during the summer months dramatically increases demand for potable water, just as nature ensures that supplies might be strained.
For water supply company Wessex Water, these difficulties are exacerbated by constraints placed on its business by moves from the Environment Agency aimed at increasing water flow in rivers to improve their ecology under the National Environment Programme. Furthermore, there are occasional incidents of deteriorating water quality - particularly high concentrations of nitrates in some groundwater sources - and, with a legally binding undertaking to meet water quality regulations, the company has to find new ways to ensure it can provide the water needed in the region.
“Dorset is an unconfined aquifer, but we are limited by licence over the amount of water we can abstract. These are being reduced in an effort to improve flow and protect ecology, and so we need to act,” explains Drummond Modley, programme manager at Wessex Engineering and Construction Services (WECS), the in-house contracting arm of Wessex Water.
Now, the water company is investing £225M in a bold strategic plan that will enable water to be moved around its network from areas of surplus to areas of need, improving supply security and quality for its customers. Spread over two asset management plan periods, Wessex Water’s grid project will see more than 50 schemes constructed over an eight-year period, with commissioning of the entire project set for 2018.
“We looked at two different scenarios: localised solutions or establishing an integrated grid, which would allow us to move water around our area, and in the future, link in with neighbouring water companies,” explains Modley.
“After a careful cost/benefit analysis, we went with the integrated grid solution.”
Scheme: Wessex Water supply grid project
Programme Value: £225M
Designers: WECS, Atkins, Aecom, Grontmij
Design contract: PSC Option C Target Cost
Contractors: WECS Civils, WECS Mainlaying, Trant Engineering, Lewis Civil Engineering, Aecom, Clancy Docwra
The project requires more than 200km of pipeline to be laid, with 120 crossings of major trunk roads, rivers and railways. There will be 24 new or refurbished pumping stations and 12 new water storage tanks, with
capacities of up to 8Ml constructed across the scheme. It is a major task for the project team, which includes WECS Civils, WECS Mainlaying, Trant Engineering, Lewis Civil Engineering, Aecom and Clancy Docwra, with design expertise by WECS’ in-house design team backed up by Atkins, Aecom and Grontmij.
The bulk of that new pipeline will be a 74km transfer main that is currently under construction between Sturminster Marshall in Dorset and Camp Hill near Salisbury in Wiltshire. It will provide a north/south link between Wessex Water’s existing infrastructure.
The new main varies in diameter from 450mm to 500mm, with some sections being ductile iron, and others steel or polyethylene, depending on the water pressure expected at that point. It will cross swathes of prime farmland, conservation areas, sites of special scientific interest, scheduled ancient monuments and areas of outstanding natural beauty.
Summerslade Down is at the northern end of the new trunk main layout and will boast one of the new 8Ml capacity storage tanks set to help maintain supply across the Wessex Water network.
This giant new facility is currently under construction, and sits just off line from an existing 450mm diameter main as it connects the Codford water treatment works with the Whitesheet reservoir.
“It will be part of the new transfer and storage system,” explains WECS Civils site manager Adrian MacDonald. “At full demand it will be capable of transferring 25Ml per day.”
The 56m long, 43m wide and 5m deep facility is divided into two main cells designed to give the storage tank a 50%/100% operational split. Each of the main cells has a further dividing baffle wall to help maintain the water circulation and blending.
A 500mm thick reinforced concrete base slab has been cast directly onto the exposed chalk bedrock, while the tank’s internal and external walls are 375mm thick reinforced concrete with 16mm diameter reinforcement at 100mm centres.
“The worst-case loading scenario on the internal walls is when one half is full, one half empty, so it doesn’t need lots of reinforcement,” explains MacDonald.
A 225mm thick suspended reinforced concrete roof slab, with additional thickenings at the column heads, will complete the tank. Precast concrete will be used for the smaller valve house, which sits alongside the reservoir.
More than 1,200m3 of concrete was used to cast the base slab in two separate pours of 700m3 and 500m3 using a C32/40 concrete supplied by Wincanton-based Hopkins Concrete. The larger pour was carried out first to bring the stop end of the slab beyond the vertical wall joint.
“It was probably the largest single structural pour WECS Civils had ever carried out,” says MacDonald, adding that the organisation was keen to use local suppliers where possible.
Two crawler cranes stationed on site helped skip the concrete into position, and have also been used during the wall pours.
Backfilling to help disguise the massive storage tank, deliberately located at the top of a hill to fit in hydraulically with the rest of the scheme, will require 18,000m3 of fill, garnered partly from the excavation work for the tank slab and partly from the excavation of the pipeline itself.
New sections of pipeline will branch off the existing main to serve the Summerslade Down tank. These will be anchored against 2.7m by 2.7m by 1.8m high reinforced concrete thrust boxes before the tank and pipeline is fully commissioned.
“We are on track to complete and hydrostatic test the structure by December 2014, with full commissioning due later,” says MacDonald.
Managing the pipeline’s environmental impact will be key for the project team. The pipeline route runs within a 50m wide planning corridor that can be tweaked to avoid clashes with areas of archaeological significance, but in the main, the new pipelines will be laid using open-cut methods through a 25m wide easement, placing the pipeline in 1.2m wide trenches at depths of between 1.5m and 2m.
“There has to be enough working room for us to load out materials and equipment, store topsoil and install the pipeline,” explains Modley.
The team is currently installing an average of around 100m of pipeline per day.
Open cut methods will be also be used where the pipeline route crosses minor roads, by-ways and footpaths; but where it hits major obstacles such as main roads, railways, rivers or - in this most rural of areas - ecologically sensitive hedgerows, the team will use trenchless techniques such as directional drilling or auger boring.
“Open cut is preferable in operational use. If we have to use directional drilling, for example, it means we have to install two pipes with valves at each side of the obstacle,” explains Modley.
Away from the north/south trunk main are several smaller sections of pipeline that will be installed over the next few years, mostly through rural areas but also through the urban outskirts of Taunton and Shaftesbury. The work is bound to bring disruption to the local population, but Modley is convinced that on completion, the scheme will be worth it.
“For the next three years we are spending around £1M per week on construction. Inevitably there will be some disruption, but so far the public have been very supportive,” he says. “Their main concerns are over traffic management issues, but we know that by the time the project is fully commissioned, we will be able to meet water supply demand for the next 25 years.”