Ashlett Creek - Outfall Extension

‘Visser & Smit Hanab’s overall service and performance was excellent, particularly in communication and response to problems,’ site manager 4D. (VBMS was part of Visser & Smit Hanab until 2012.) Ashlett Creek, near the port of Southampton in Hampshire, is the site of a large water treatment outfall that needed to be lengthened to comply with environmental regulations.

Facts and figures

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‘Its end had to be moved 80 metres away to lie in a current strong enough to ensure proper dispersion,’ explains Edwin Tieman, Visser & Smit Hanab’s (V&SH) project manager. All this had to be done during daylight hours in one of the diving windows provided by the four daily backwater tides, while not interfering with the busy shipping channel. ‘Ferries whistle across to the Isle of Wight and huge container boats and cruise ships come in and out of Southampton,’ says David Lee, V&SH’s client liaison.

the challenge

While the strong current was essential for the efficient functioning of the outfall, it also created some unavoidable problems during the construction process. ‘The strength of the current meant the sand and silt of the seabed kept being moved all the time and tended to refill the holes as we dug them,’ says Tieman. And there was a lot of digging to do. The first job was to prepare the location of the new up-to-date ‘diffuser’, which would efficiently mix the waste water in with the stronger current. This involved excavating a hole 4 metres below the existing seabed, with a diameter of around 8 metres, which meant moving around 450 cubic metres of sand and sediment.

A trench 80-metres long and 4-metres wide also had to be dug from the site of the new diffuser to the end of the existing pipe. This involved shifting another 2,000 cubic metres of seabed material and side-casting it along the north-eastern side of the trench, so it could be used to cover the extension pipe. Another major challenge was to survey, excavate, detach and winch out the massive old diffuser to allow the new pipe to be connected. All of this had to be done with minimum disruption to the outfall itself, which typically discharges once an hour.

Our solution

The only way to overcome the natural backfilling from the strong water currents was to dig as efficiently as possible. ‘We had to dig very quickly, which we did by making sure we had the biggest machinery on-site,’ says Tieman. Consequently, the project took just two months to complete. Constructing the foundations for the new diffuser was also a race against time, with 30 cubic metres of underwater concrete having to be laid before it hardened. This amounts to about six lorry loads, according to Tieman. ‘From the time we received the concrete and put out to sea, we had three or four hours before it hardened,’ he points out, ‘during which time a diver worked in the water directing and emptying the concrete from the bucket.’

Before concreting began, 9-metre-long piles, spaced 1 metre apart, were driven into the seabed to support the construction. After being surveyed, the old diffuser was excavated, cut off and winched clear, allowing the end of the existing steel outfall pipe to be ground so a Viking Johnson adapter could be fitted to connect it to the new PE extension. The steel this process exposed was wrapped with waterproof tape and a special paste coating to prevent it from corroding. Four 14.5-metre piles were driven in to provide support and reduce stresses at the connection point.

 

The final stage of the installation of the PE extension pipe began by partially sinking it into position using ballast weights. The flow from the outfall was stopped for a few hours, with the permission of Southern Water, to allow divers to seal the connection between the old and new pipe. The new section was then anchored to the piles and sunk further into the trench by adding more ballast, with its level fine-tuned using hardwood blocks. ‘Throughout the whole project, the outfall was only put out of use for a few hours,’ says Tieman.

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