Discussion on Papers 12–14

MR BALMER AND MR MEERS, Paper 14

1. The Paper follows 3| years' work by the Severn-Trent Water Authority. The final survey covers 2% of 11 987 lengths manhole to manhole, CCTV inspection of 3781 lengths, and collation of about 194 000 pieces of information.

2. We are satisfied that the sample is representative. There is scatter between population and sewer length, but Fig. 1 shows that a general trend exists.

3. Prediction of where trouble might lie is not good. Of 1161 lengths 'believed unsound' only 88 (1 in 13) are actually unsound. However, of 2620 lengths 'believed sound', 230 (1 in 11) are unsound.