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Compact case

Letters

I was very disappointed and sorry to hear and then read your report on the collapse of the Gerrards Cross tunnel (NCE 7 July).

To my surprise, the zone 1 fill was described as 'compaction with light nonvibrating equipment': What measure of compaction was specified and was it checked and achieved?

Also, was the compaction required deemed to be sufficient to develop the necessary passive pressure to resist the anticipated lateral outward deformation of the lower third of the arch rings?

About 30 years ago I was responsible for the design of a very similar tunnel over a railway line in Cornwall.

The line of thrust in the arch units was checked for every increment of loading. The degree of compaction, by void ratio, was specified to be not less than that in the approach embankments on each side of the tunnel. The maximum permitted out-of-balance backfilling was restricted to 300mm.

On site, everything went like clockwork and very speedily. Costain was the contractor. It should provide a very economical form of construction under 10m plus embankments.

Martin Below (M), 2 Motcombe Court, Beford Avenue, Bexhill-on-Sea, TN40 1NQ

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