One of the characteristics of the water industry is that many of its most impressive construction achievements are consigned to obscurity and hidden underground.
Only during the construction process itself is it possible to catch a glimpse of some of the massive structures taking shape as contractors build the chambers, shafts and tunnels used to treat and transport clean and foul water to and from our towns and cities.
Right now there is no shortage of impressive below-ground structures to be found as AMP 5 infrastructure projects reach their climax. Undoubtedly the best known and most ambitious water scheme currently is Thames Water’s Thames Tideway Tunnel, of which the Lee Tunnel is a vital part. And this is yielding some impressive statistics of its own.
The four-mile long Lee Tunnel will prevent an annual 16 million tonnes of combined sewage and rainwater from entering the River Lee in east London and will instead carry it from Abbey Mills pumping station to Beckton sewage treatment works, which is being expanded by 60% to enable it to deal with the increased volumes.
Located along the route of this tunnel are five massive shafts, the first of which has just been completed with a reinforced concrete lining believed to be the longest continual slipform pour ever attempted in the UK.
Teams of concrete engineers and operatives from the main contractor MVB – a joint venture between Morgan Sindall, Vinci Construction and Bachy Soletanche – spent 29 days continuously pouring a variety of specially-designed concrete mixes into the slipform shuttering at a rate of 100-150mm of lift height per hour.
A special 50/60 concrete was designed using a number of admixture blends to control setting times and, instead of the traditional steel bar reinforcement, around 500 tonnes of steel fibres were also included.
A very different approach to a similar challenge was adopted by KMI Water, a joint venture between Kier Infrastructure, J Murphy & Sons and Interserve Construction on a £114m sewer improvement scheme near Preston for United Utilities’ North West capital programme.
This project, which requires 4.5km of tunnel, 9km of rising main plus numerous screens and chambers as well as a terminal pumping station and outfall, also calls for the construction of 13 large-diameter shafts. To help out with the formwork and falsework for this, the contracting team turned to specialist supplier Peri. Work started on the scheme in February 2010 and is due to complete next April.
Like the Thames Tideway scheme in London, this project is designed to clean up discharges, specifically to improve the quality of the region’s bathing waters off the Fylde Coast. The existing sewerage system has long since proved incapable of handling combined sewage and surface water volumes with resulting flood-induced overspills of effluent into the River Ribble.
Of the total of 13 shafts, eight will contain up to 40 million litres of storm water to prevent overspills while an additional five sewer overflow shafts will divert flows into the new interceptor tunnel system.
The new terminal pumping station, at Penwortham, will lift flows from the tunnel and transport them to the wastewater treatment works at Clifton Marsh via twin rising mains. For the 26m-diameter, 36m-deep terminal pumping station shaft, Peri supplied a range of equipment including its SB brace frames, Rundflex and Trio panels, SKS climbing platforms and Peri Up scaffold to create a climbing formwork system for the shaft lining.
In addition to the tunnel lining, a 30mhigh, heavily reinforced concrete H-shaped box, creating separate chambers, was constructed in the base of the shaft. The H-shaped box contains massive quantities of reinforcing steel, a total of 14 separate cages each weighing up to four tonnes.
These cages are exceptionally dense, with 40mm bars at 100mm centres vertically and 32mm bars at 100mm centres horizontally: “The rebar in this structure is so dense that your fist couldn’t pass through it,” says Peri field sales manager Ian Higginbottom. Each of the rebar cages was assembled on the surface and lifted into the shaft in one piece.
The task of erecting and deploying the Peri climbing formwork was helped by the presence on site of a Peri site demonstrator. “We employ two full-time demonstrators and their work is very site-specific,” says Higginbottom. “They usually arrive on site and spend a couple of days just getting things up to speed. On this job we had somebody on site two or three days a week for about six weeks.”
One of the most challenging aspects of this job was the sheer amount of single-sided formwork required to line the shaft. The SKS single-sided climbing system is rarely used in the UK – it is most commonly found in developing countries where it is widely used for dam construction, says Higginbottom.
With this system, instead of the formwork being supported with through-bars, as with normal double-sided forms, the shuttering is supported with V-strongbacks and compression braces tied into scaffold anchors cast into the section below.
Hence, high early strengths are required so that the climbing process can continue as soon as possible after each pour. On this project, the concrete had to reach the permissible bearing pressure of 30kN/m2 before the formwork could be climbed for the next pour.
“Without the SKS single-sided system, the only way of fixing formwork would have been to drill and fix each formwork panel into the outer precast concrete skin of the shaft,” says Higginbottom. This would have been extremely slow and would have required the drilling of more than 3,000 holes, plus installation of their fixings.
Choosing yet another concreting method, NMC-Nomenca – main contractor on Severn Trent Water’s £2.8m sewage tertiary treatment works project at Crankley Point in Newark, Nottinghamshire – opted for precast concrete to create a 7,200m2 holding tank for sewage sludge at the site.
Designed to aerate the raw sewage and activate the sludge process, the tank comprises a complex series of chambers within a structure measuring 60m by 20m and 6m in depth.
“Although we have supplied NMC Nomenca with CSOs (combined sewer overflows) and other small precast units in the past, this is the first major structure we’ve designed for them,” says Steve Gainsley, project manager with Kijlstra, the Stratford-upon-Avon precast concrete specialist that designed and manufactured the tank components.
On the strength of previous experience with Kijlstra’s products and design capability, NMC Nomenca asked Kijlstra UK to propose a precast solution for Crankley Point.
“Precast is so much easier to work with, especially on a scheme like this,” says Gainsley. “A lot of water industry projects are located near watercourses and on poor ground which is where the speed of precast really helps.”
The Crankley Point tank was designed and manufactured so that each component could be delivered to site and lifted into position without delay. Apart from grouting joints and fitting seals, site labour was minimal. The site team had good reason to be thankful for this in late November when torrential rains raised the level of the nearby River Trent and groundwater inundated the site.
James Kay, NMC Nomenca’s project manager at Crankley Point, says that using precasting also made site work safer and easier. “Installation was debris-free and the site was a lot tidier than it would have been using insitu concrete.”