Buried pressure pipelines: designing for thrust restraint – CIRIA guidance

Updating Pipeline Integrity

Buried pressure pipelines, used extensively for water supply and distribution, generate significant thrust forces at bends, valves, tees, or diameter changes due to internal fluid pressure. Without proper restraint, these axial forces can pull joints apart, leading to leaks or catastrophic bursts.

The previous standard, CIRIA R128 from 1994, focused primarily on concrete thrust blocks for unrestrained jointed pipes common at the time. Over the intervening decades, pipeline technology shifted: molecularly oriented PVC, polyethylene, and ductile iron pipes with restrained or tied joints became prevalent, allowing thrust transfer along the pipe length rather than solely into surrounding soil via blocks. Continuously welded thermoplastic and steel systems also grew in use, often needing no routine restraint at fittings but requiring careful handling at transitions to other materials or terminations.

CIRIA published C816 in July 2025 to address these evolutions, providing step-by-step design methods for thrust blocks, restrained joint systems, anchor blocks, and specialised cases like under-pressure connections, line stops, and interfaces with abandoned pipes. The update reflects changes in materials, installation practices, and supporting standards.

Real-world consequences of poor thrust design include not only immediate water loss but also secondary damage—eroded road bases, property flooding, or environmental contamination from treated water escapes. UK water companies, managing thousands of kilometres of ageing mains, face regulatory pressure to reduce leakage rates while replacing assets; misaligned restraint designs could trigger avoidable incidents, regulatory penalties, or escalated project costs.

Non-obvious angles include the guide's introduction of displacement-limited design factors and thrust reduction considerations for existing blocks near new works—protecting legacy infrastructure from disturbance. There is also a subtle trade-off: while restrained joints can eliminate massive concrete pours (reducing embodied carbon and excavation in urban areas), they demand higher-quality jointing and verification, shifting risk from civil engineering to installation precision. In poor soils, conservative block sizing may still prove more reliable than relying on pipe restraint alone.

Overall, the guidance standardises approaches for routine cases up to 1000 kN thrust while flagging when specialist analysis is needed, aiming to harmonise practice across designers, contractors, and asset owners.