Buried pressure pipelines: designing for thrust restraint

Phillip Clisham, Independent consultant and buried pipeline specialist, P.Clisham Consulting Ltd

The water companies of England and Wales own more than 351,000km of water mains , which is equivalent in length to 8.7 x the circumference of the earth. This is before the length of water mains that the water companies don’t own + the pumped sewers, sludge pipes and pipes at treatment works are considered.

Buried pipe networks are truly vast, they have evolved over more than 150 years, are constructed from a range of materials and are installed in varied ground conditions. Water is heavy and the forces generated at bends, valves and other fittings can become very large (100ton-force and greater are not unusual for larger diameter mains). It is important that engineers know how these forces are restrained by different pipe systems. In my opinion this is essential for those involved in designing, constructing and repairing pressure pipelines. However it is also important for those who may be working on building, highway or rail projects that are to be constructed in close proximity to pressure pipelines.

CIRIA Guide to the design of thrust blocks for buried pressure pipelines (R128) was published in January 1994 and, for the last 31 years, has offered clear guidance on how to design thrust blocks to resist forces generated at bends and fittings in buried pressure pipelines. Back in 1994, the large majority of pipe systems installed for the UK water sector were pipes with simple, push-fit spigot and socket joints. These pipes systems needed concrete thrust blocks to transfer loads into the ground at bends, fittings and valves. Today simple push-fit jointed pipe systems are still used, but many pipe systems offer tied joint solutions which transfer loads along the length of the pipes. This means that the forces can be transferred into the ground in other ways. In addition, continuously welded pipe systems are commonly used. Continuously welded pipe systems don’t generally need thrust blocks at bends and other fittings, but there are circumstances where they do. This is a particular issue at points where continuously welded polyethylene or steel pipe systems interface with other materials. The forces generated by continuously welded pipe systems are different in some ways to the forces that the simple spigot and socket jointed pipe systems generate and this is not always understood. 

CIRIA have now published Buried pressure pipelines: designing for thrust restraint (C816). The new guide offers updated guidance to the design of thrust blocks as well as providing guidance to the design of tied jointed systems. The guidance also helps engineers to understand when thrust blocks are needed and how to design thrust restraint of continuously welded pipe systems as well as providing guidance on thrust restraint for under-pressure fittings. I am really pleased that the new guide has now been published and I am grateful to CIRIA, to the authors at Stantec who have done an excellent job and to the CIRIA steering group which was made up of many of the UK’s leading engineers.

In simple terms, the new guide covers the buried pipe systems that are available today and in my opinion, should be considered as essential reading for civil engineers and anyone else that is involved in designing, building , maintaining and repairing buried pressure pipes.