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Arch villain?


The Gerrards Cross Tesco railway arch debacle is a poor reflection on our profession (NCE 18/25 August).

A three-pinned arch is a statically determinate structure which is very easy to analyse.

Unlike a deeply covered tunnel, it is very sensitive to the applied horizontal and vertical loading from back-fill round it.

The working drawing must have made a clear statement showing the thickness and disposition of the daily layers of back-fill, its type, moisture content, compacted density and shear angle. In the past, each layer would have been illustrated on the drawing and coloured-coded to avoid confusion.

The pin joint itself appears to be a male/female longitudinal shallow curved socket faced with stainless steel. For the expected maximum shear force at this joint with a maximum central deflection of 200mm, it looks as though one arch could slide down relative to the other.

For total, all-shear security, the joint should be a clutch or a mechanical knuckle.

The Chiltern Railway has become a tremendous success.

Why have rail travellers waited seven weeks for an answer to such a clear problem- If it had been a motorway it would have been cleared in two days!

Christopher Wallis (M), 2, Fern Cottages, Little Marlow, Bucks, SL7 3SE

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