Thames Water Ring Main
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|Thames Water Ring Main|
|Type||Urban wastewater infrastructure|
|Status||Operational; being extended|
|Construction Period||1988 - 2025|
|Operational||2025 (planned completion)|
|Tunnel length||25 km (16 mi)|
|Capacity||0.3 Gl/d (gigalitre / day)|
|Depth||10 m (33 ft) – 65 m (213 ft)|
|Tunnel diameter||2.54 m (8.3 ft)|
|Cost of construction||£250m|
The Thames Water Ring Main (TWRM) (formerly the London Water Ring Main/LWRM) is a major part of London's water supply infrastructure, approximately 80 km (50 mi) of mostly 2.54 m (100 in) concrete pipelines to transfer potable water from water treatment works (WTWs) in the Thames and River Lea catchments for distribution within London.
The average daily flow is approximately 0.3 gigalitres (0.3 × 109 litres), out of a London average day demand of approximately 2.0 gigalitres (2.0 × 109 litres). 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 10 m (33 ft) to 65 m (213 ft) below ground level and approximately 10 m (33 ft) to 30 m (98 ft) below sea-level. It is connected to water supply zones by some 20 shafts that extend from the main to ground level.
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 oldest dates from 1838), 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.
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 reservoir (albeit with negligible capacity), 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.
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.
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 4 bar (400 kPa). 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 (TBM). 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.
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.
The 21 shafts connecting the main to the surface are divided into:
- 5 water treatment works (WTW), which supply clean water,
- 11 pumping stations (PS), which withdraw water from the main,
- 3 access shafts, where no water transfer occurs, and
- 2 storage locations, where water is supplied or withdrawn as demand fluctuates.
- Ashford Common — WTW
- Kempton Park — WTW
- Mogden, Isleworth — Access
- Kew — PS
- Barnes — balancing storage
- Holland Park Avenue — PS
- Barrow Hill, Primrose Hill — PS
- Ashford Common — WTW
- Walton — WTW
- Hampton — WTW
- Surbiton — PS
- Hogsmill — Access
- Raynes Park — Access
- Merton — PS
- Streatham — PS
- Brixton — PS
- Battersea — PS
- Park Lane — PS
- Barrow Hill — PS
- Brixton — PS
- Honor Oak — underground storage
- Thames Water Ring Main. Proceedings of the Institution of Civil Engineers 102 (special issue 2) (London: Telford). 1994. ISBN 0-7277-2003-1.
- "London Ring Main extensions commence". Thames Water Utilities. July 2007.
- "Thames Water Tunnels". Mott Macdonald, contractor for the current extensions