Weholite proves long lasting asset in flood combat
When a Scottish hospital was suffering persistent flooding problems, a new storm overflow system needed to be built. Claire Smith reports on how the £934,000 project benefited from the world’s largest plastic pipes.
The project is one of seven in Scotland using Asset International's high density polyethylene (HDPE) Weholite pipes - including sections of custom-moulded 3.5m diameter pipes - which are the largest plastic pipes in the world.
Duncan Robertson, project manager for contractor Barhale Construction, says: "The existing system did not have the capacity. They were having storm events on a regular basis."
Although Scotland generally has fewer flood problems than England the rapid development of towns such as Kirkcaldy, which has now become popular as a commuter town for Edinburgh, makes flood control a bigger issue than before.
Stricter regulations from the Scottish Environmental Protection Agency (SEPA) have made it a priority for Scottish Water to ensure that any potential contamination of fresh water is kept to a minimum.
Robertson says: "When this system was laid down they estimated there would be one flood event in 200 years. Now they expect one every ten to 15 years. But in this part of Kirkcaldy there have been 200-300 more houses built over the last two to three years. Victoria Hospital had been having problems for some time and they approached Scottish Water who decided they would have to upgrade."
The new system will carry a maximum of 120l of effluent, road waste and rainwater a second towards the water treatment works at Castlebrae. In the event of severe flooding the combined storm overflow system will trigger an additional screening system that will prevent solid waste from overflowing into the Den Burn.
For Scottish Water the need to improve water quality in the Den Burn, which takes the overflow in the event of storm conditions was one of the big drivers of the project.
It was Robertson who suggested using HDPE pipes to replace the existing concrete system. Lightness, durability and ease of installation were the key factors behind his recommendation.
He explains: "The original plan was to put down 2.2m concrete pipes. It was my suggestion to use Weholite. Concrete pipes weigh eight tonnes each, whereas the extruded high density polyethylene pipes weigh a tonne and three quarters.
"It means they are easier to get into the ground and they are far safer for health and safety reasons.
"They are far quicker and far easier to move about. You don't have to use such large machines and therefore it is cost effective. Although the pipes themselves are slightly more expensive than concrete you save time and money putting them in the ground.
"And it should last. Concrete has a shelf life of about 50 years, whereas Weholite should last for 125 years."
Work on the new 230m section began in November. The work is in three sections of 2.2m pipe, each of which ends in a chamber separated by a weir wall with a penstock gate housed in a manhole made of a section of 3.5m pipe.
The final chamber houses a combined storm overflow chamber that uses a float valve to detect whether storm water is about to breach the final weir wall. If this happens an electrical lead activates a mechanical Hydrok screening system that ensures the wastewater which spills into the Dean Burn is free from solid matter.
But using the new lightweight pipes has made replacing the old system easier. Robertson says: "I expect us to finish ahead of schedule."
Asset International, which manufactures the HDPE pipes, has two workers on site who weld the sections of pipe together. And the company has also trained workers from Barhale and sub-contractor Thomas Menzies to fit the pipes into the trenches in a way that minimises the gaps.
"The idea is to minimise the gaps so you have less welding. You are aiming for 25mm maximum. But the one-day installation programme was very useful. There was a point where we were behind schedule but the training helped us do things quicker - so we were about to fit five 3m sections of pipe a day instead of three."
Welders seal the gaps between the pipes using continuous extrusion hot weld HDPE - a kind of giant glue gun that uses the same material the pipes are made of. Unlike concrete, which contains metal, polyethylene is not subject to corrosion. Weholite pipes use a patented structured wall process and with sizes up to 35,000mm diameter is billed as the world's largest plastic pipe.
UK licence holder Asset International has exclusive UK and Ireland rights to manufacture the Weholite range at its plant in South Wales, and says the new lightweight pipes are increasingly in demand as water authorities respond to the Governnment's new flood regulation requirements.
Asset International product manager Vasilios Samaras says Weholite was perfect for the Kirkcaldy project. "This particular job is next to the road and they decided to go for Weholite because it is very much easier to install."
The company was able to bespoke manufacture the 3.5m manholes, with the weir walls already in place - preventing the need for concrete manholes to be built or assembled on site.
David Anderson, Scottish Water project manager on the Kirkcaldy UID programme, says: "This is a significant programme of work, which will improve the Kirkcaldy environment for many years. It was not only important that the construction of this wastewater solution led to tangible benefits for the local community, but that there was as little disruption as possible."
Anderson adds: "The turnaround time and ease of construction was an important factor in choosing this pipe. It is possible to weld prefabricated manholes into place, where required, thus helping to greatly reduce the construction time.
"The project was designed to improve the quality of the Den Burn, to meet waste water quality drivers. We were aware of volumes of ground water entering into the network and this pipe will help us to address such ingress. This problem also took us down the route of storage, which at 3.5m in diameter this pipe offers a simple, yet effective solution."