Discussion. The Roofing of Cleadon Reservoir.

Abstract

The Chairman opened the discussion by mentioning a few points which interested him.One was the slide showing trees growing out of the puddle trench; he felt it would interest Mr Tattersall, who had had experience of drying puddle at Chingford reservoir. The use ' of the puddle round the reservoir was similar to what had been done in gasholders round London in previous years. The reservoir had ended up by being almost like a gasholder, but the gasholder had a number of timber supports carrying the light steel dome when it subsided. It was interesting to see that at Cleadon a large number of supports had to be erected and a complete dome had been built in timber, as shuttering, only to be replaced by the permanent dome in concrete.Some years previously the Chairman had investigated a reservoir at Easington for Mr Ruffle's Company where there had been some "rat-holes" and subsideuces in the bottom, and there had been a difference of opinion on whether the cause was mining subsidence or cavernous limestone underneath. Mr McLellan had dealt with the matter by using a rubber lining, and the Chairman hoped he would describe his experience in the following discussion, since it was related to the problem of mining subsidence mentioned in the Paper. M r A. G . McLellan (Engineer and General Manager, Sunderland and South ShieldsWater Company) explained that the general picture in the Water Company's area of supply should be painted against a background of mining subsidence. Of the Company's twelve service reservoirs seven had been open reservoirs, and of those seven, six had been constructed in a similar manner to the one a t Cleadon, using puddle clay as the waterretaining medium. They were in fact "Hawkdey" reservoirs. They had done their job remarkably well over the years. The reservoir a t Cleadon was slightly less than 100 years old. The idea had been that the puddle would move and take up any movement caused by mining subsidence, which had in fact occurred.The first decision required had been whether to interfere with the puddle clay or to adopt some form of roof which had no intermediate supports. With a circular reservoir . the second alternative led automatically to a dome, but not necessarily to a concrete dome.A metal dome might have been used. However, from the point of view of cleanliness and freedom from infection the concrete dome had much to commend it.It could not be assumed that because mining subsidence had taken place it would not recur, or that because the coalmining authorities had said "It is extremely unlikely that we shall take coal out of that seam" any likelihood of their doing so was past. It was therefore necessary always to bear in mind the possibility of mining subsidence under structures in a coalfield until it was known that every seam had been completely worked out, or unless one took the expensive step of buying all the seams in the neighbourhood of each structure. That point had to be considered in relation to the expense of covering a reservoir.The reservoir a t Cleadon was important, es...