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Filling the gaps in Brunel tunnels

ICE NEWS

RAILWAY ENGINEERS descended 'en masse' on the Institution last week to find out whether Brunel would have approved of refurbishments carried out on some of his most famous tunnels. They went away thinking they had seen 'the best thing since sliced bread'.

At least that is what they were told during a presentation to the Railway Civil Engineers' Association by Haswell technical manager Jack Knight.

In his talk 'New applications for traditional materials - would Brunel approve?' Knight described the consultant's experiences refurbishing 13 Victorian brick tunnels west of Swindon. These included four Brunel tunnels.

The initial brief from Railtrack was to assess the structural condition of the tunnels. But after finding deformations in some of the tunnel arches and evidence of possible sink holes on the surface, the consultant was asked to refurbish the structures. The remedial works included sealing against leaks and water ingress, brickwork repairs and additional structural support.

Knight said the problems stemmed from the construction methods used by the Victorian engineers. He explained that tunnellers in Brunel's day started with a lead drive at soffit level. This was advanced about 4m and shored, first with timber and finally with brick. The lead drive would meet up with vertical shafts driven from the surface prior to tunnelling and would establish the line of the tunnel.

The full tunnel was then driven the same length as the lead drive in two halves - first one side then the other. Each half was supported by up to four layers of brickwork lining. The process would then begin again on the next 4m length.

Because of the difficulty of meshing the brickwork on the new drives with the completed brickwork of the previous drive, many of the joins between sections were in bad condition. Large cavities around the tunnel arch marked the join between adjoining sections of brickwork.

Knight said these cracks in the lining every 4m meant the tunnels acted like sliced bread removed from its wrapper - wanting to flop and pull apart. The cavities also meant water could leak into the tunnel and cause further damage. He concluded this was the cause of the structural problems.

His solution was to support the brickwork with sprayed concrete, using 1.5m wide steel fibre reinforcement spanning the areas of weakness. This would be bolted to the brickwork with fibreglass rock anchors and anchored to a concrete block, usually at soffit level. Natural cement was used for rapid setting and meant work could be carried out in tight possession times.

The brickwork behind the membrane was not grouted so water would continue on its existing path. Knight explained this meant the water could be managed rather than diverted down a new course, risking further damage. The Haswell engineer felt Brunel would have approved of this attitude because he had adopted the same approach.

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