Discussion on Papers 20–23

DR R. C. OWEN, British Gas Corporation, Newcastle-upon-Tyne

Mr Addy's paper raises three matters on which I should like tocomment.

Firstly very briefly the British Gas Corporation (BGC) mainsnetwork separates into high pressure transmission mains ofsteel and low pressure distribution mains, typically of castiron and polyethylene. The high pressure system is subject tovery rigorous inspection both during and after constructionwith regular surveys along the route. The photograph in Paper20 suggests a cast iron main for the low pressure distributionsystem not the transmission system.

Secondly in Fig. 1 of Mr Addy's paper there is no referenceto trench depth. Certainly BGC are aware of the large anddamaging ground movements associated with some deepexcavations made for sewer renewal. It was for this reasonthat an extensive series of joint field trials was carried outby the BGC's Engineering Research Station (ERS) and the WaterResearch Centre. This culminated in the adoption by BGC andthe Water Authorities Association of the Model ConsultativeProcedure for Deep Excavations, which sets out safeproximities and renewal lengths for cast iron mains adjacentto or crossing deep excavations.