Water & Wastewater Treatment

Project focus: tanks a 1,750mm diameter pipe to Ditton Brook, about 680m downstream of the chamber. There is currently no storage at the site and so flows pass through the CSO chamber and over a spill weir into the watercourse. The aim of the current project is to provide a means of storing these flows temporarily and allowing silt and other contaminants to settle before the clean water is discharged under controlled conditions. Work started with tree removal and the erection of newt fencing in January 2014 and continued with construction of a new site access road in October. The project is set to be completed this month (September 2015). The original scope of the project was to have two CSO screens capable of screening up to 2,800 litres/sec each to 6mm in two dimensions. The new detention tank would have 17,000 cubic metres of storage and an integral flushing system to clean the tank on emptying. The tank itself was envisaged as two 35m diameter diaphragm wall structures each about 20m deep. A set of duty/standby storm return pumps would return flows downstream of the new CSO chamber once storm flows subsided while a smaller "scavenger" pump was to be provided within the detention tank so that the tank could be fully drained. Building such large structures on site is time-consuming and vulnerable to delays due to poor weather. Additionally, there was a potential risk of flotation of the detention tank due to upward ground and water pressure. Various options were considered – including using thicker walls and floors and pinning the structure down with anchor piles. 300mm thick, with an additional 200mm thick layer of insitu concrete on top. Kijlstra also supplied the precast CSO chamber and return pumping station along with the tank itself. "One of the innovations that impressed the client was that we presented to them with the flushing system partially factory-fitted," says Kijlstra's project manager Steve Gainsley. When in use, the tank will start collecting excess flows when the rate exceeds 274 litres/sec. The flow is screened and spills into the pumping station from where it spills in sequence into the flushing chambers at the top of the detention tank. When the flushing chambers are full, the flow spills over the weir in the pumping station and down to the bottom end of the tank which then fills until the storage volume is reached. At this point the spill weir level in the CSO chamber is reached and the flows discharge to the existing overflow pipe into Ditton Brook. The considerable volume of soil excavated to accommodate the tank is being re-used on site for landscaping purposes. For Eric Hepworth, who has 40 years' experience in the water industry, the use of precast concrete for this type of structure continues an ongoing trend in the industry. "I've worked with precast concrete many times before – it's not new," he says. "But the biggest change I've seen in the industry is a massive improvement in health, safety and welfare. Offsite construction is much safer and I think that's driving the trend. Precast has always been there, but this takes it to a new level." But aŸer an extensive value engineering exercise by KMI, an alternative design employing a single detention tank assembled using precast concrete components was chosen. This avoided the flotation problems by using a shallower tank that covered a much larger footprint. "The contractor tendered to three or four companies and chose a solution put forward by Kijlstra," says Eric. Kijlstra, whose headquarters are in the Netherlands, is a relative newcomer to the UK; earlier this year it opened its first UK factory in Henlade, Somerset. As a supplier of precast concrete products to the water industry, it has aimed to change perceptions in the civil engineering and utilities sector with time and cost- saving designs. Off-site solution Kijlstra's off-site solutions ticked all the right boxes for United Utilities, says Hepworth. "We have a 'design for manufacture' strategy at United Utilities and a team that is constantly looking for innovative solutions," he explains. "This includes, but is by no means confined to, precast concrete. Off-site manufacture cuts the amount of work done on site, which is safer and quicker, and quality and finish is also much better because it's made in factory conditions". The tank is truly monumental in scale: buried under 3m of soil, is has internal dimensions of 135m x 42.1m x 3m high. It has an in-situ base set at a fall of 1:200 and is divided internally into eight 5m-wide channels which are easily cleaned by the flushing mechanism. The external walls are assembled using Kijlstra's precast concrete panels measuring 6m wide and 300mm thick. The roof comprises panels measuring 5m x 3m and also 18 | OCTOBER 2015 | WWT | www.wwtonline.co.uk The tank is considerably bigger than a football pitch in size

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