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Special plant relines stricken Kent tunnels

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SPECIALLY DEVELOPED tunnel lining plant was this week due to enter Strood and Higham rail tunnels in Kent as part of a £40M refurbishment project.

The 12 month project is needed to protect trains using the 180 year old tunnels from persistent chalk falls which have disrupted services since December 1999.

Contractor Costain will use Italian designed steel erecting plant with protective full-tunnel section canopy to install steel ribs at 2m centres.

The machine also trims the tunnel profile to ensure the additional lining does not put it inside the kinematic envelope of the track.

A works train will erect the ribs as it progresses down the twin track tunnel.

A bespoke shuttering system will follow and cast 350mm thick self-compacting concrete bays onto the shuttering.

This full-tunnel section shutter can automatically retract from the lining before moving onto the next 10m long bay.

Each 70m 3bay will be struck within four hours.

Higham tunnel is 1.4km long and Strood tunnel is 2.1km. The tunnels are 60m apart. Around 60% of the total length of both tunnels is bricklined, the rest is exposed chalk.

Temporary repairs have traditionally been carried out in reponse to localised chalk falls triggered by vibration and water percolation. In December 1999, such a fall caused a passenger train to derail.

In June 2000 a shaft collapsed, closing the tunnel. Since July 2000 the tunnel has operated with a 20mph speed limit. The tunnel will remain closed for 2004 as passengers endure a replacement bus service.

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