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Insight | Thames gas tunnel works underway

Tunnelling has begun on gas firm Cadent’s biggest civil engineering scheme, as part of a major £17.5M gas pipeline project in London.

A 330m long tunnel is being constructed 30m under the Thames, running from Royal Hospital Chelsea on the north bank to Battersea Park on the south. It will house part of a gas pipeline between Fulham and Battersea needed to serve the £15bn Nine Elms development on the South Bank.

The tunnel is being delivered by tRIIO, a joint venture owned by contractors Morrison Utility Services and Skanska in partnership with Cadent (formerly National Grid Gas Distribution). The tunnel forms part of a wider £1bn scheme to replace the capital’s 2,897km of ageing Victorian gas infrastructure.

The 1.8m diameter tunnel boring machine (TBM), named by Royal Hospital Chelsea Infirmary pensioners after the hospital’s business manager Amanda Lord, will reach Battersea Park in the first week of December.

Contractor Barhale will then install the pipeline infrastructure by the end of March 2018. A 450mm diameter steel pipe will be inserted through the tunnel, resulting in an intermediate pressure gas pipeline at 5.5 bar.

Engineering has proved to be one of the less complex parts of the project, as tunnelling under the Thames has meant the job of getting the necessary permits has been the biggest challenge.

“An effective stakeholder mapping exercise is worth its weight in gold, because having stakeholders come in part way through the process can potentially unravel what you’ve done so far, or just add in a load more work that you hadn’t anticipated,” said Cadent engineering support manager Andrew Hejdner.

Discussions were held with 15 different organisations and 20 permissions needed to be secured, including from the Crown Estate. In addition, planning and construction has had to revolve around events such as the Chelsea Flower show, which takes place on the grounds of the construction site.

Construction of the Tiideway mega-sewer has also put pressure on the team to deliver the scheme faster.

As a result the team has had to share its design documentation with Tideway, and to carry out ground settlement assessments to understand whether their work would affect Tideway’s project.

“We’ve had to go through a big technical review with [Tideway], and it’s issued us with a letter to confirm that it accepts our proposals and doesn’t object to our works,” said Hejdner.

“I don’t think we could’ve started doing the tunnelling if we didn’t have that piece of paper in hand.”

In April construction finished on a 30m deep and 7.5m diameter shaft at Royal Hospital Chelsea, down which the TBM was lowered in preparation for its launch.

Once tunnelling finishes in December the TBM will be lifted by crane out of a 6m diameter shaft in Battersea Park. In total, 618 concrete segments willl have been used to construct both shafts.

Work on the pipeline project is expected to finish in December 2018.

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