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Loaded question

LETTERS

The very successful Stresses and displacements conference (held around 1955) included a paper from the Building Research Station and Metropolitan Water Board showing how tunnel instrumentation saved almost £1M of public money.

Vibrating-wire load cells and extensometers were fitted in the experimental MWB Don-Seg tunnel at Ashford Common; I was closely involved with calibrations and readings.

When lled with water under 140ft head, the tapered flangeless concrete segments hardly moved, and so lost little thrust. The results demonstrated that the proposed steel pressure lining for the planned very long MWB Don-Seg tunnel (from Ashford to the Lea Valley) could safely be omitted.

Regarding errards Cross (NCE 1 Sept & earlier), I assume that with good instrumentation and satisfactory test loadings, the tunnel could be made safe to use almost as planned without excessive extra cost.

But heavier Tesco loads would probably need far stiffer and wider foundations that usual. That would keep contact pressures low and aid their redistribution as settlement proceeds.

Moreover, added security where needed locally could be given by placing, say, 350mm of high-strength concrete well before placing the fill.

Dr Theodore K Chaplin (F), 32 Allnutt Avenue, Basingstoke RG21 4BW

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