With any construction project, time is money. The quicker you can get on site and the less time you spend there, the lower the cost.
This was one of the main reasons why NMC Nomenca, the main contractor on Severn Trent Water’s £2.8 million sewage tertiary treatment works project at Crankley Point in Newark, Nottinghamshire, built the facility’s new activated sludge plant using precast concrete sections.
The tank is a large, complex concrete structure, located below ground and designed to treat sewage effluent. Measuring almost 60 m in length, 15 m in width and approximately 6 m in depth, the tank will incorporate two mixers and a series of lanes and cubicles when completed.
“Although we have supplied NMC Nomenca with CSOs and other small precast units in the past, this is the first major structure we’ve designed for them” says Steve Righton, director with Kijlstra, the Stratford upon Avon-based precast concrete manufacturer which supplied the tank.
Since establishing its UK base in 2009 Kijlstra, which originates in the Netherlands, has been altering perceptions and challenging assumptions about the design and installation of commonplace civil engineering and water industry structures.
Its basic precast manhole units created a stir within the industry for the simple reason that they are square in section, rather than round. While this appeared to fly in the face of custom and practice, the square manhole was quicker, easier and safer to install and contractors soon appreciated the benefits.
Over time, a growing body of contractors and clients has discovered the advantages of precast and Kijlstra’s UK customer base has increased. NMC Nomenca is one of these converts; several of its engineers have even traveled to the Netherlands to see the Kijlstra factory for themselves.
On the strength of previous successes with not only Kijlstra’s products but also its design capability, NMC Nomenca asked Kijlstra UK to propose a precast solution for the Crankley Point installation.
“Precast is so much easier to work with, especially on a scheme like this” explains Mr Righton. “A lot of water industry projects are located near watercourses and on poor ground which is where the speed of precast really helps”.
Constructing an insitu concrete tank below ground often requires extensive sheet piling and propping to support the excavation, and often involves a considerable amount of time within the excavation to erect formwork, fix steelwork, pour concrete and strip the formwork afterwards. There is also the additional delivery and storage of plant, equipment, fasteners and materials to site, that an of-site precast solution avoids the need for.
At Crankley Point, the ground was relatively stable, but the water table was high being sited directly next to the River Trent. “De-watering and working in deep excavations for long periods of time can present real problems with traditional approaches” says Kijlstra Mr. Righton “But with precast you’re usually in and out quickly and you’re handling less materials.
The Crankley Point tank was designed and manufactured so that each component could be delivered to site and lifted into position without delay and reduced site labour.
In this instance 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.
Installation of the tank began in October 2012 and was almost finished when flooding halted site operations during the last week of November. “The job will have taken about 40 working days. And although it’s quicker than using insitu concrete, we think it could have been even quicker still” said Mr Righton
Simple improvements, such as increasing the crane capacity on site, could pay dividends added Mr Righton “That could speed up the installation time; we think a tank of this size could be installed in four weeks or possibly even three”.
Kijlstra’s £670,000.00 NEC3 subcontract has provided valuable lessons for both Kijlstra and the main contractor, which will be taken forward onto future contracts. While installation took longer than was anticipated quality and site safety benefited considerably, according to NMC Nomenca’s supply chain manager Matthew Barney:
“We’re pleased with the decision to choose precast – it’s been a success. It’s accelerated the programme, given us greater control over quality and reduced risks due to things like working at height, working in confined spaces and exposure to hazardous substances like wet concrete.”
“It’s also helped us reduce our carbon footprint by drastically cutting the number of vehicle movements to and from the site” he adds.
James Kay, NMC Nomenca’s project manager at Crankley Point, says that the Kijlstra system 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. There was far less risk of slips, trips and falls.
“The structure was like a big jigsaw and the concrete finish was excellent. It’s ideal for intricate structures (there were about 180 panels on this job) which would be very fiddly if you were having to erect formwork and fix reinforcing steel.”
After erection of the precast panels, the finished tank must undergo tests to ensure that there are no leaks and that the structure has sufficient strength. This involves filling the tank with water to full depth before the excavation is back-filled.
Hence the tank has to be self-supporting and to ensure that it is, a reinforced concrete ring-beam is stitched and bolted around the entire perimeter edge.
Successful testing was carried out on the structure during February 2013. This was carried out to CESWI specification and involved water testing prior to backfill.
With this project under its belt, Kijlstra is now looking to deliver similar large-scale contracts using the same technology, with a number of new schemes being considered.