Project Focus: SE Water tunnelling to complete vital new pipeline
South East Water is about to complete a £8.1M pipeline project which has seen it tunnel under the River Medway, a railway line and the M20 motorway.
- A Victorian cast iron water main across Burham Marshes was bursting regularly and in need of replacement
- The new pipeline route needed to avoid the marshland so that it was easier to access when required
- Expanded capacity was required for expected population growth in the South East
by James Brockett
South East Water is set to complete a £8.1M pipeline project which has seen it tunnel under the River Medway, a railway line and the M20 motorway.
The water company has used a 25-tonne Tunnel Boring Machine (TBM) for the two crossings, which form the most crucial part of a new 7km pipeline to secure water supplies to Maidstone, Larkfield, Aylesford, Ditton and Snodland in Kent.
South East Water’s biggest AMP6 project, the pipeline is required to replace a Victorian cast iron water main which crosses Burham Marshes. The main is coming to the end of its design life and has burst nine times in the last four years; its inaccessible location, within an environmentally protected area, made it particularly difficult to locate and mend these bursts leading to disruption to customers.
The route for the replacement pipeline, which is made of ductile iron with a diameter varying between 300mm and 600mm, has been years in the planning. It avoids Burham Marshes but passes through two local wildlife sites, Leybourne Lakes Country Park and Bushey Wood, as well as residential areas such as Ditton and New Hythe Business Park. The first phase of construction work began in January, with the pipeline expected to be finished in January 2016.
In addition to passing under several small streams, the route involves two major crossings – one under the M20 and the other under both the River Medway and a railway line. For these tasks SE Water and its contractor Farrans opted to bring in a £500,000, 4.5m long Herrenknecht Tunnel Boring Machine, specially made in Germany and christened Molly by a member of staff.
Chris Love, South East Water Delivery Manager, says: “Molly is two metres in diameter and a smaller version of the machines that dug the Channel Tunnel in the late 1980s. It’s the first time we’ve used a machine of this size as part of a water main improvement project. We’re using a TBM because we need to link this new pipe into existing water pipes in Ditton and Snodland. As the area is so busy with traffic, we knew that closing various roads would cause an unacceptable level of traffic and community disruption which meant the only way round the problem was to drill under the motorway, river and railway line.”
The first crossing was completed in August, when the machine dug 24 hours a day, seven days a week, to create a one hundred metre tunnel 15 metres beneath the M20 motorway between New Hythe Business Park and Cobdown Sports Ground. The second tunnel, being dug at the time of writing in September, is deeper (20m) and longer (200m) and is set to take 20 days.
“The machine is going through hard sandstone, at up to five revolutions per minute,” says Love. “It will take out about 650 cubic metres of material, and ultimately all that material will be reused in filling the shafts when the pipes come through. We’ll put through each of the twin 600mm diameter pipes and then the gaps between the pipes will be pressure grouted. The excavated material is ground up and then mixed with water into a slurry, and the water is spun out of it by centrifugal action, enabling us to reuse the fine material.”
The earlier stage of the project, which saw the pipeline taken through Leybourne Lakes Country Park and Bushey Wood, involved painstaking environmental work to prepare for the pipe laying. Nightingales, water voles and newts were the species of concern; while doing the work in winter ensured nightingales were unaffected, the pipeline route had to have grass stripped and newt fencing installed to discourage the voles and newts from lingering in the area. South East Water’s environmental team made daily visits to collect any newts found and remove from the fenced-off route. It also liaised with the local council about a number of tree preservation orders in force.
In February, the team also needed to take the pipeline under a stream in Leybourne Lakes Country Park while keeping the stream flowing to ensure minimal disruption to the watercourse and environment.
Love explains how this was achieved: “To ensure there was minimal disruption to the stream’s water flow we laid a pipe along the middle of the stream. Following that we filled two large bags with water and placed them over our temporary pipe to create a dam. We then dug down below the base of the stream and below our temporary pipe which was keeping the stream flowing and fed the new pipe through.”
Leybourne Lakes Country Park is a popular area enjoyed by walkers and local people, but good communications and engagement with residents ensured that the section of pipeline through the park was completed without a single complaint being received.
Archaeologists have had a watching brief during phases of the pipeline work, with pottery fragments from a Roman villa being discovered at one stage.
Farrans Construction were the lead contractors and engineers on the project with South East Water. Ward & Burke were the tunnelling subcontractor, with J Murphy & Sons the pipe laying subcontractor.
The pipeline is around 70% completed at the time of writing, and the project is running on schedule. There are six connections between the newly laid pipeline sections and the existing network, four of which have already been completed. Once the new pipes are commissioned the old main in Burham Marshes will be taken fully out of service.
The new pipeline has a larger capacity than the old main, and will be able to carry nine million litres of water a day. With the South East set for a 20% population increase over the next 25 years, the project will ensure that water supplies will not be a barrier to further housebuilding in the area.
“This is a forward-looking scheme… essentially, we are putting more water into an area that isn’t in deficit at the moment but one in which significant development is planned, particularly around the Snodland area,” says Love. “We are also putting in place provision that will prevent people suffering from bursts in the network, and even where there are problems, we will be able to resolve them more easily so customers are either not interrupted or interrupted for a much shorter period of time.”
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