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William Girling Reservoir
between Enfield in London and Chingford, Essex, UK
associated engineer
Sir Jonathan Roberts Davidson
Sir Alec Westley Skempton
Sir Robert Meredydd Wynne-Edwards
Karl von Terzaghi
date  July 1935 - 4th September 1951
era  Modern  |  category  Dam/Reservoir  |  reference  TQ365939
ICE reference number  HEW 2221
The collapse during construction of part of the William Girling Reservoir’s embankment dam, and its subsequent repair, led to the birth of modern soil mechanics in Britain. Built by the Metropolitan Water Board and known originally as the Lee Valley Reservoir at Chingford, it was renamed after the board’s Chairman William Girling on opening. It remains in use.
Contractor John Mowlem's tender of £682,156 was accepted in July 1935 and work began straight away. The non-impounding storage reservoir was designed by the Metropolitan Water Board’s Chief Engineer Jonathan Davidson (1874-1961, knighted in 1942), who also held the Presidency of the ICE in 1948-9. However, the construction project was interrupted by World War II (1939-45).
The work attracted widespread technical interest on 19th July 1937 when a major slip occurred in the north west corner of the partly formed embankment. The dam's fill had reached a height of 7-8m, or 2.5m below crest level, and a 20m wide section dropped 700mm and moved forward 4m. Fortunately the failure occurred before any water was stored, or the consequences of flooding could have been catastrophic.
For the first time the movements of the slip circle (failure line) were measured and monitored — by Alec Skempton (1914-2001, knighted in 2000), then leading British research into soil behaviour at the Building Research Station near Watford. The Soil Mechanics department there concluded that the alluvial clay — in layers of soft clay over sandy gravel above London clay — beneath the dam had not consolidated completely.
The embankment around the neighbouring King George V Reservoir (TQ372964) had been built on similar ground over a five year period (1908-13), which allowed some of the pore pressure to dissipate and the clay to consolidate. This dam was constructed much more quickly, using American earth-moving equipment rather than manual labour, and became unstable in areas where the underlying clay was weak.
The contractor’s agent Robert Wynne-Edwards (1897-1974, knighted in 1965), future President of the ICE (1964-5), advocated removing the weak clay but this was not done and a second slip occurred in December 1937. Wynne-Edwards sought advice from Professor Karl Terzaghi (1883-1963), who agreed with the Building Research Station’s evaluation. Dr Herbert Chatley (1885-1955) was also asked for his recommendations.
The subsequent investigations into these landslips can be regarded as the birth of modern soil mechanics in Britain. In 1939 Terzaghi delivered the 45th James Forrest Lecture at the ICE, entitled Soil Mechanics – a New Chapter in Engineering Science, which heightened awareness of the subject’s importance. John Mowlem established Britain’s first commercial soil mechanics laboratory — at first an in-house facility — that became Soil Mechanics Ltd in 1943.
In July 1938, the Metropolitan Water Board approved important modifications by Terzaghi to the original design, based on the results of tests on the clay around the site. The reservoir was redesigned to increase its capacity by 11.3% to 16,000 million litres. The dam was reconstructed with a berm (step) at mid height on the outer face and concrete slabs on the inner face. The completed reservoir has water 12.5m deep inside an embankment 14.3m high, with a perimeter of 5.7km and a water area of 135ha.
The reservoir opened on 4th September 1951, by which time the cost of its construction was around £2.3m.
The Metropolitan Water Board was abolished in 1974 and Thames Water Authority (now just Thames Water) took over. The reservoir forms part of the Lea Valley chain of reservoirs supplying the north London area. Raw water from these reservoirs is treated at the Coppermill Water Treatment Works (TQ354882). This one is unique among the Lee Valley reservoirs — it has jetted inlets and siphons that can be used to aid mixing of the water body, minimising algal blooms (which can clog filters).
This reservoir and the earlier King George V Reservoir (TQ372964) to the north are known collectively as the Chingford Reservoirs. They are separated by the A110 trunk road (Lea Valley Road) and are designated jointly as a Site of Special Scientific Interest for their varied bird life. Public access to the William Girling Reservoir is not allowed.
Contractor: Mowlem
Research: ECPK
"Obituary: Colonel Sir Jonathan Roberts Davidson, CMG, 1874-1961”, in ICE Proceedings, Vol.22, pp.339-340, London, July 1962
"Obituary: Professor Karl Terzaghi, 1883-1963”, in ICE Proceedings, Vol.32, pp.698-699, London, December 1965
"Sir Robert Meredydd Wynne-Edwards (1897–1974)” by Robert Sharp, in Oxford Dictionary of National Biography, Oxford University Press, 2004, online edition October 2008
"Sir Alec Westley Skempton (1914–2001)” by Richard J. Chandler, in Oxford Dictionary of National Biography, Oxford University Press, January 2005, online edition January 2009
reference sources   CEH London

William Girling Reservoir