Design of the Thames Water Ring Main

Abstract

The Thames Water ring main is a 2.5 m dia. tunnel approximately 50 m below ground and 80 km long, which conveys potable water from major treatment works, located west of London, into central London. The scheme was constructed during the late 1980s and early 1990s. In 1985, to meet increasing water demand, a gravityfed system of deep tunnels was devised by Thames Water Utilities Ltd; this involved a full ground investigation and underground obstruction survey to establish the optimum tunnel route, bearing in mind the many and various constraints. A series of shafts was sunk to connect directly into the ring main with the purpose of either introducing water at major water‐treatment works or acting as pumpout shafts at strategic locations around the capital. The tunnels and shafts were designed to be built in two phases. Phase 1 was constructed using conventional wedgeblock techniques, but problems were encountered which resulted in increased costs and delay. Phase 2 therefore necessitated a different approach. The company adopted a completely new strategy by changing the form of contract, purchasing its own tunnel boring machines, and redesigning the tunnel linings. One of the keys to the success of the ring main was the new expanded wedgeblock concrete lining. Design calculations were prepared and finite element analysis work was carried out. The new rings were subjected to build and load tests, both on individual segments and complete rings, using specially designed test rigs.