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Thu July 02 2020

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Demountable tunnel shield solves Barhale's challenge

18 Mar Civil engineering contractor Barhale has developed a collapsible tunnelling shield to solve problems encountered on a project in Surrey.

View down the access well at Hersham showing the pipes being installed to replace the sewer tunnel
View down the access well at Hersham showing the pipes being installed to replace the sewer tunnel

Barhale says that its new approach to tunnel shielding is allowing it to tackle challenging emergency works despite restrictive access, adverse weather conditions and a tight timeframe.

Barhale has been brought in by Skanska, MWH and Balfour Beatty – SMB, the delivery partner for Thames Water’s eight2O alliance – to repair a collapsed sewer in Hersham alongside the London-Woking railway line.

The £6.1m programme of works includes over-pumping of the sewer line from a wet well to a shaft beyond the collapse; excavating and constructing a cofferdam on top of the collapsed sewer; handjacking 50 metres of new larger 1500mm-diameter concrete sewer pipe to replace the existing 1340mm sewer; tying into the downstream shaft; and the construction of a new manhole inside the cofferdam and associated works.

The job was complicated by the accessibility of the reception shaft and the loading considerations on the existing structure. This led to Barhale developing a collapsible shield. 

“Normally, faced with this sort of challenge we would remove the shield as a whole after breaking into the new shaft,” said project manager Gunter Erhardt.

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“In this case, that wasn’t possible so to solve the problem we developed a shield made up of individual 500mm rings which could be mounted and demounted. The rings are moved from the back to the front as tunnelling progresses all the time providing the same protection as a traditional shield at the point where the pipe was being replaced – but without the size and hence removal problem.”

Barhale’s regional director Phil Cull believes the innovative tunnel shield is playing a significant part in the delivery of the job.

“Work on this site is constrained in several ways,” he said. “It is a tight schedule and, physically, there is a small site footprint and an even smaller working area. What’s more the team has to work under sewer isolations at night when entering the shaft and wet well and the proximity to the railway line plus the need to meet Network Rail’s requirements are all having an impact.”

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