The average daily flow is approximately 0.3 gigalitres, out of a London average day demand of approximately 2.0 gigalitres . The main currently comprises a major loop linking the Hampton, Walton, Ashford and Kempton WTWs clustered on the Thames upstream of Teddington Weir to central London by a southern branch via Brixton and northern branch via Kew, and a separate unconnected length from Coppermills WTW to Stoke Newington. The main is well below most water mains, at a depth of to below ground level and approximately to below sea-level. It is connected to water supply zones by some 20 shafts that extend from the main to ground level.
Driver
The main serves as a partial alternative transfer system to existing trunk mains suffering increasing frequencies and severities of leaks and bursts. Some of the oldest operational pressure mains in the world, the high-level trunks have weakened with steady corrosion, and there has been both an increased carrying requirement resulting from increasing water demand, and increasing external stresses resulting from higher vehicle weights and frequencies. This has been exacerbated by limited system redundancies which has restricted preventative refurbishment. The main has both extended the operational life of the high-level trunks by reducing the flow demands placed on them, and, by providing a high degree of redundancy, enabling key trunks to be isolated and maintained.
Hydraulics
Flow through the main is by gravity under the driving head of the service reservoirs at the source WTWs. By virtue of its depth, the pipeline is under some pressure; however, the hydraulic grade line rarely exceeds ground level, and, to enter, supply water must be pumped up into the distribution zones at the pump-out shafts. In some respects, therefore, the main can be considered as a minor reservoir, from which supply is drawn as required. An indication of this dynamic variation in demand is that the minimum hydraulic level moves between the Battersea and Park Lane pump-out shafts. The loop is closed to provide the redundancy that allows any segment on the ring to be isolated and drained for maintenance without interrupting the supply to any shaft, not for hydraulic reasons.
Construction
The main was constructed in two phases: the southern leg from Ashford Common WTW to Barrow Hill Shaft in 1988 to 1991, and the northern leg between the same locations in 1991 to 1993. Each phase was divided into a number of 'stages' that were separately contracted and constructed largely simultaneously. The different contractors involved in each stage is to some degree reflected in the variation in tunnelling and tunnel construction techniques used.
Geology
The main lies mostly within London Clay with sections within the overlying alluvium and underlying Lambeth Group and Thanet Sand. The predominance of the London Clay lengths is by design, as being easily excavated, largely impermeable and somewhat self-supporting for short periods it is a near-ideal tunnelling material. Where the hydraulics have required entry into the Lambeth Group and Thanet Sand, tunnelling was considerably more difficult. In particular, the Thanet Sand requires a high boring torque, is highly abrasive and, most challengingly, sufficiently permeable to contain a water table continuous with the underlying Chalk and measured at pressures up to. An unexpected entry into the Thanet Sand while excavating near Tooting Bec Common led to the flooding of the tunnel and the temporary abandonment of a Tunnel Boring Machine. A further consideration of Thanet Sand is the presence of glauconite, which oxidises on contact with air. The resulting de-oxygenated air resulted in two fatalities during the excavation of a pump-out shaft.
Future
The main will be extended in stages over the coming decades. Work is currently under way to bridge the gap between the main loop at New River Head and the isolated segment at Stoke Newington and to extend a branch from the Brixton Shaft to Honor Oak reservoir. Both these projects are being assessed for their environmental sustainability through CEEQUAL.
Shafts
The 21 shafts connecting the main to the surface are divided into:
