Mr. D. E. Thorp: I would like to give some of the main reasons that led to the final solution of the single raft over the whole area of reactor building. The raft is resting on about 30 m (100 ft) of medium sand with a relative density of about 50 to 60 per cent. I understand that little is yet known in the field of soil mechanics about
stresslstrain relations within sand masses, and sophisticated analysis still assumes, I believe, a semi-infinite elastic medium. Hence the results must be treated with some reserve. Conscious of these uncertainties, we were faced with the design of superstructures whose differential settlements had to be strictly limited to avoid damage to finishes and plant. In particular, the goliath crane for the fuelling machine, which operates on a longitudinal rail system supported consecutively on the fuel handling unit, intervening steelwork and concrete pressure vessel, is particularly sensitive to undue differential settlement.