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Project Focus: HDPE pipes break new ground with Lee Tunnel outfall

The recently completed Lee Tunnel benefits from the biggest plastic pipe outfall ever installed in the UK

The marine section of the outfall being manoeuvred into placeThe marine section of the outfall being manoeuvred into place

Perspectives

Thames Water chief executive Martin Baggs: “While the Victorian system still works very well, it needs additional capacity to cope with 21st century London. The Lee Tunnel and, when completed and connected, Thames Tideway Tunnel, will together capture the tens of millions of tonnes of sewage that currently overflow untreated into the tidal Thames every year. This will protect the future of the iconic river and capital for future generations.”

Emmanuel Costes, construction manager at MVB: “We were very happy with the Weholite technology employed in the implementation of the Lee Tunnel Outfall pipeline. The specifications of the project were very particular, however the solutions provided by the Asset International/Uponor collaboration were impressive, ticking all of the boxes that this complex, and in many cases unique, project demanded.”

Christian Vestman, Uponor Project Services: “The Uponor Asset Partnership demonstrated the full depth of knowledge and versatility available within our organisation. This unique project really allowed us to showcase the impressive capabilities of Weholite and the results speak for themselves.”

by Dr Vasilios Samaras, Technical Director, Asset International

London's Victorian sewerage system was designed to overflow into the River Thames during extreme weather, when the sewers reached capacity. One hundred and fifty years on, the system has outgrown the demands of an increasingly populated 21st century city, and sewer discharges are now happening much more frequently – around once a week on average, according to Thames Water.

To combat this and in order to future-proof one of the world’s most populous cities, Thames Water has developed three major multi-billion pound engineering schemes to help prevent sewer overflows and improve water quality in the Thames. All five major sewage treatment works in London have undergone extensions and upgrades, the Lee Tunnel is in operation, and construction is underway on the Thames Tideway Tunnel.

Asset International - which operates under license from Uponor Infra of Finland and is the world’s largest manufacturer of Weholite HDPE products - is involved in several of the vast infrastructure projects to bring London’s outmoded and over-burdened water management network up to the standard of the modern day, and beyond. In 2013 Asset International supplied over 5km of Weholite HDPE pipe for the Beckton Sewage Treatment Works upgrades. Weholite pipes were supplied in various sizes; ranging from 400mm to 3000mm in diameter. The pipes were used throughout the extensive project for inter-process pipe work, and all of the associated chamber fabrications.

The Lee Tunnel is one of two tunnels which will collectively, with the under-construction Thames Tideway Tunnel, capture an average of 39 million tonnes of sewage a year from the 35 most polluting combined sewer overflows (CSOs).

The £635M Lee Tunnel will manage discharges from Abbey Mills Pumping Station in Stratford - London’s largest CSO - which accounts for 40% of the total discharge. At four miles long, the tunnel will run beneath the London Borough of Newham and will help prevent more than 16 million tonnes of sewage mixed with rainwater from overflowing into the River Lee each year. The new tunnel will capture the overflow and transfer it to Beckton Sewage Treatment Works, which as outlined above has undergone vast extension work in order to deal with the increased volumes it will be processing.

The construction of the Lee Tunnel brought with it a unique set of demands, the main one being the boring of London’s deepest ever tunnel, no small feat in a city that has one of the most complex subterranean networks in the world, creating something of a navigational and engineering headache. Construction work began in September 2010 and operations were completed in late 2015.

In 2012, main contractors MVB (a joint venture between Morgan-Sindall, Vinci Construction Grands Projets and Bachy-Soletanche) approached Asset International Limited and Uponor Project Services to examine the complex design. The original idea included in-situ building a concrete box culvert in a dry trench by holding back the tidal waters of the Thames, which has a rise and fall of 7 metres.

The Asset and Uponor PS partnership set about using their combined design expertise for land and marine applications to re-engineer the project by creating what is a landmark design, the biggest plastic outfall ever installed in the UK and one of the biggest in terms of diameter worldwide.

The proposal to overcome this enormous challenge included 880 metres of 3000mm diameter Weholite pipes laid as a twin culvert along with twelve large-scale Weholite Modular HDPE boxes, and the provision of installation, supervision, site services and health and safety management.

As well as servicing the Lee Outfall Tunnel, the Weholite culvert will also be the final discharge point for the prestigious £4.2BN Thames Tideway tunnel. Construction of this ‘super sewer’ started this year and is scheduled to complete in 2020.

The project was divided into a land section comprising of 105m of twin culvert laid at 10m depths. This section had the added complication of needing to break through the tidal protection wall that stops the Thames flooding Europe’s largest treatment works at Beckton. This issue was overcome by utilising a giant 7m x 11m x 5m Weholite Modular box to house a 3000mm spool section to complete the installation.

The operation to install the remaining 335 metres of twin culvert section into the black waters of the Thames was carried out by marine contractor Commercial Marine & Piling Limited (CMP) alongside the Asset and Uponor PS partnership. However, this ambitious marine project was complicated still further by the fact the pipes needed to be submerged under an existing jetty structure and sections of the project were often isolated by the tide, with no access from land.

All the Weholite pipes and modular boxes were manufactured and prefabricated at Asset’s south Wales factory. The pipes were delivered to the historic London Docks site, in 18 metre lengths, and were welded into strings of up to 90 metres. The 50 tonne strings were then lifted onto the water using three mobile cranes in tandem and prepared for towing to the submersion site, around 3km up river.

Part of this preparation involved utilising Uponor’s patented grouting process. This innovative methodology eliminates the need for heavy concrete collars to ballast the strings. This traditional way of installing marine pipelines can often be extremely risky during submersion. Filling the hollow Weholite profile with an inexpensive and pumpable grout is much safer and affords for a much quicker preparation time. With no concrete collars a smaller trench is needed, the dredging operation is minimised and the volume of excavation is therefore drastically reduced. Since submarine excavations are much more expensive than on dry land the advantages of Weholite are transparent. By using the grouting system, Weholite submersion is much easier to control and ultimately safer.

Once the pipe strings were ready, they were towed individually up river by tugboat and then submerged. Specialist divers were used to bolt up the innovatively designed quick connect flanges that joined each pipe string.

The project also included installation of over 11,000 m2 of steel sheet piling to allow the dredging of the river bed so the pipes could be laid free from obstruction. Over 28,000 m3 of riverbed materials were dredged and much of the dredged materials were reused to backfill the pipes once installed. This provided an environmental advantage by vastly reducing the amount of materials to be taken off site, thereby saving on the carbon footprint.

The London Tideway improvements are but a few of the most impressive and ambitious engineering projects to be undertaken in the UK in a generation, perhaps since the Victorians themselves built the very sewers that Thames Water are currently upgrading. It is a fantastic example of world leading British engineering at its best, and a project that Asset International is extremely proud to be a part of.

The scale and scope of the installation means that not only has it become the UK’s largest plastic outfall pipeline, but it also represented the UKs first ever sub-sea installation of a sectional installed multi-directional sewage pipeline.

Resilience

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