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New River Aqueduct, M25
M25 west of J25, Enfield, London, UK
associated engineer
Greater London Council
date  1985
era  Modern  |  category  Aqueduct  |  reference  TL344000
ICE reference number  HEW 2214
An enclosed concrete aqueduct built to carry the 17th century New River over the M25 motorway. The New River is an early canal, the construction of which was begun in 1605. It was and remains an important source of drinking water for London.
The course of the New River (1608-13) has been altered in the last 400 years, and it now covers some 45km north-south between Hertfordshire and London, approximately following the same route as the A10 trunk road. It provided an interesting challenge for the engineers constructing the M25 (1973-86).
The river at this point was flowing in a conduit on a high clay embankment, so an aqueduct was required to take it over the motorway. The aqueduct was constructed by Greater London Council (now dissolved), acting as agents for the government's Department of Transport.
Steel sheet piling was used to redirect the river. In plan, the watercourse has a dog-leg away from the direct line towards the west on either side of the crossing. The aqueduct is a post-tensioned concrete structure, cast in situ. It carries the river in two rectangular boxes, 90m long, over the then dual three-lane carriageways that run east-west.
The concrete box elements are each 4.25m wide and 2m deep, giving an overall width of 10.5m. They are lined with epoxy panels to prevent contamination of the water.
The double box design enabled the necessary temporary diversion of the river to be done in two stages. It also enables maintenance work to be done on one side at a time without interrupting the vital water supply of 200 million litres per day.
The top slab of the aqueduct boxes is used as an access road for maintenance (by Thames Water) and has metal railings at its edges.
Contractor: Sir Alfred McAlpine
Research: ECPK
reference sources   CEH Lond

New River Aqueduct, M25