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Tunnel rescue plan forged

Contractors grappling with a trapped tunnel boring machine (TBM) in Lancashire have told NCE that jet-grouting, ground freezing or dewatering are the three most likely methods to be used to free it.

NCErevealed last week that the KMI Water joint venture (JV) of Kier, J Murphy and Interserve had halted work on part of a £114M United Utilities tunnelling project in Preston after silt breached the tunnel lining.

KMI Water said it was looking at several methods to stabilise the ground and allow the TBM to be recovered. “We’re working up different options to get back into the tunnel and repair [the tunnel lining],” said KMI Water contracts manager Andy Parker.

The TBM is stuck 25m below ground in alluvial clay deposits.

“We’re working up different options to get back into the tunnel and repair”

Andy Parker

“We could drill a hole from the surface down to the tunnel and inject grout around the TBM and the tunnel lining [to alleviate pressure around the TBM]”.
KMI Water is also looking at freezing the ground around the TBM. This would be done by inserting pipes around the TBM and then filling them with a coolant such as brine.

The JV is also considering de-watering the area.

“We could reduce the water pressure on the tunnel, but we would need to use something else to make [the tunnel] safe, such as a compressive air lock,” added Parker. KMI is still unsure what caused the silt breach on 24 November last year.

The lining of the 2.85m internal diameter tunnel is made up of 1m long precast concrete rings. Each ring consists of six sections which are bolted together and then sealed with a rubber gasket.

“We couldn’t get to it, or see where it was coming from”

Andy Parker

Tunnelling for the project was going to plan until engineers noticed silt below the ferryplate – a sledge pulled along behind the tunnel boring machine containing the electrical panels, grouting equipment and a conveyor belt to take away the spoil.

“We couldn’t get to it, or see where it was coming from,” said Parker, adding that the ingress was a fine silt that could “get through any gap”.

A stack wall made of timber and sandbags was built to contain the ingress whilst the extent of the problem was assessed. When it was evident that silt was still entering the tunnel, engineers decided to flood the tunnel to make it stable.

A decision on the recovery method will have to be sanctioned by the Environment Agency due to the tunnel’s close proximity to the River Ribble.

The Agency will seek assurances that jet grouting or ground freezing would not lead to pollutants entering the river.

Health & Safety Executive approval will also be needed.

Parker said he could not put a timescale on the recovery attempt but insisted the project team was “working flat out”. The remainder of the project in Preston city centre remains unaffected.

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