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Thames Water Tunnel Ring Main
Ashford Common to Walthamstow, Greater London, UK
associated engineer
Thames Water Utilities Ltd
Mott MacDonald
date  6th March 1986 - 11th November 1994, partly opened 1991, 2007 - 201
UK era  Modern  |  category  Water Supply/Pipes  |  reference  TQ237800
ICE reference number  HEW 2284
The Thames Water Tunnel Ring Main project has transformed water supply and distribution throughout London, securing the capitalís potable water resources. Its infrastructure includes Britainís longest water tunnel, running unnoticed deep beneath the city, with 11 pumping stations and five water treatment works. The original configuration has been extended and is in constant use.
From 1855, owing to pollution and outbreaks of disease, no drinking water could be abstracted from the River Thames ó the source of around 75 percent of Londonís water ó downstream (east) of Teddington Lock, under the provisions of the Metropolis Water Act 1852. Instead, river water was stored in a series of surface reservoirs between Hampton and Wraysbury, to the west of Teddington, and at Walthamstow in north London, before being treated and pumped to consumers in central London.
By 1985, when the Thames Water Tunnel Ring Main (originally called London Water Tunnel Ring Main) was approved, the cityís largely Victorian water supply network was struggling to cope with ever-increasing demand. The high-pressure pumping system required continual maintenance. Mains running close to the surface and under arterial roads leaked or were vulnerable to bursts, with consequent traffic congestion while repairs were effected.
Addressing the ongoing expense and frequent failures required a new approach, a large-scale solution capable of handling half the capitalís drinking water. The Ring Main scheme consists of a subterranean main connected to the surface by 21 vertical shafts. Water enters in west London, flows by gravity to 11 underground pumping stations located around central London and is fed into the distribution pipe network. The aims were to improve water distribution and achieve large-scale economies (estimated at £2m a year) by reducing overland pumping, which is costly.
On 6th March 1986, the Secretary of State for the Environment, the Right Honourable Kenneth Baker MP (b.1934), inaugurated project construction at Stoke Newington. The first shaft was sunk here, 40m into the underlying London Clay. All the access shafts are connected to the tunnel ring main at depths of 40-60m ó well below the London Underground train system. Above ground there is little to see except the shaft caps, five of which are located within existing water treatment works in the Thames and Lee valleys.
The shafts along the northern arm of the ring, west to east, are at Ashford Common water treatment works(TQ085702), Kempton water treatment works (TQ112707), Mogden (TQ151753), Kew Bridge Pumping Station (TQ186779), north bank of the Thames at Hammersmith (TQ222782) and opposite on the south bank at Barnes (TQ222780), Holland Park Avenue roundabout pumping station (mapped at right), Barrow Hill pumping station and New River Head pumping station (TQ312827).
Those along the southern arm, west to east, are at Walton water treatment works (TQ116684), Hampton water treatment works (TQ133695), Surbiton pumping station, Hogsmill, Raynes Park, Merton pumping station, Streatham Vale pumping station, Brixton pumping station, Battersea pumping station and Park Lane pumping station.
A separate branch is located to the north east of the ring, between Stoke Newington pumping station (TQ323869) and Coppermills water treatment works (TQ352880) at Walthamstow.
The 80km long concrete-lined main is generally 2.54m in internal diameter, and is designed to deliver up to 300 million litres per day to a population of 5.5 million. The tunnel forms a complete loop, so water can flow round it in either direction. A section can be closed for maintenance or repair without compromising supply. The whole system is managed and monitored from a control centre at Hampton water treatment works.
The shafts and tunnel ring were constructed in two phases from Ashford Common to Barrow Hill: the southern arm in 1986-91, and the northern arm in 1991-4. Each phase was packaged into stages, let as separate contracts. Originally the whole project had a target completion date of March 1996.
Phase 1 works included refurbishing the existing Southern Tunnel Main from Ashford Common to Merton, establishing the control centre at Hampton, constructing the new main and shafts from Merton to New River Head and from Coppermills to Stoke Newington, plus upgrading the treatment works at Hampton and Coppermills.
The Southern Tunnel Main was completed in 1974 using an expanded wedge block lining to create a tunnel of 2.54m diameter. In the wedge block system, developed by the Metropolitan Water Board in the 1950s and 1960s, precast concrete segments are placed around the excavated bore and then expanded against the ground by driving home a wedge-shaped key section. The main advantages of this is that segments are not bolted together and usually do not require secondary lining or infill grouting.
The Thames Water Tunnel Ring Main passes through varied geology, mostly in London Clay, which provided good tunnelling conditions as it is water impermeable and self-supporting while the tunnel lining was installed. However, some sections had to be driven through abrasive, water-bearing Thanet Sand.
The Phase 1 tunnel boring was carried out initially by open face machines with backhoe excavators and full face machines. Four different methods of lining were used ó wedge block, bolted smooth bore, bolted segmental and bolted trapezoidal. All the bolted linings needed a secondary lining of reinforced concrete, cast in situ, to contain the hoop stresses that would generated by the water flow. The wedge block section between Battersea and Barrow Hill also has a secondary lining, of internal diameter 2.32m.
Tunnelling was disrupted by an unexpected encounter with 50m deep piles beneath a five storey residential building. This resulted in a four month delay while the tunnel shield was diverted.
On 23rd November 1988, the tunnel was being driven deep beneath Tooting Bec Common, through an incline of saturated fine Thanet Sand, 1.4km south of the Streatham geological fault. Catastrophic inundation occurred at the face of the excavation. The incoming water and silt, at 400kN per sq m (4 bar) pressure, easily overcame the 100kN per sq m (1 bar) compressed air environment of the tunnel. Work was delayed for 15 months and tunnelling only restarted after a considerable volume of ground had been stabilised by freezing.
In 1989, the government passed the Water Act, creating the National Rivers Authority (now the Environment Agency) and 10 regional water companies. Thames Water Authority was privatised (Thames Water Utilities Ltd). It is the largest of the water companies, now serving some 14 million people.
The experiences of privatisation and of rectifying difficulties during construction prompted a fresh look at administering piecemeal contracts. It highlighted the requirements for co-ordinated project teams, detailed underground surveys and investigation of subsoil and structures. Taking a then-new approach, Thames Water and its contractors entered into partnering agreements with the aim of addressing potential challenges before they became problems.
From 1990, as well as the traditional tunnelling shields, earth pressure balance machines were deployed to drive the ring main tunnels, marking their first use in Britain. Under the partnering system, the manufacturers, contractors and client collaborated on redesigning the machines, the tunnel linings and the bogies carrying the lining segments. The Phase 1 works were commissioned in 1991, going into service between January and November.
The Phase 2 works on the northern arm of the ring main were divided into two stages, Ashford Common to Barnes and Barnes to Barrow Hill. Tunnelling commenced at Ashford Common on 12th July 1991. The three contracts for Phase 2 used the Institution of Chemical Engineers' Cost Reimbursable Form with a target cost system, setting incentives for maximising productivity.
The works benefitted from the earlier redesign ó the employment of three full face tunnelling shields and erection of expanded wedge block linings, both greatly improved construction time. The design of the new lining segments was developed using finite element analysis and empirical formulae. Each lining ring is 685mm long and 140mm thick, consisting of 12 unreinforced concrete segments ó nine 31 degree pieces, two 29 degree tapered pieces and one 21 degree tapered wedge block, placed at the tunnel crown.
Water flows through the ring main at low pressure, feeding into the existing supply pipes at a lower pressure than before, consequently reducing the number of leaks or bursts. The entire main also carries fibre optic cabling, housed in twin medium density polyethylene ducts.
Periods of high predicted demand on the system can be balanced by filling the appropriate service reservoirs in advance. The system can transfer up to 1,300 million litres of water a day around Londonís supply network.
The project was completed without incident on Phase 2. Despite encountering difficult ground conditions at depths of up to 65m below the surface in Phase 1, and all the associated constraints of construction in a congested city, the ring main was completed two years ahead of schedule. It cost some £250m. On 11th November 1994, it was opened officially by Queen Elizabeth II.
In July 2007, ring main expansion work commenced in order to keep pace with escalating water demand. The main tunnel was extended northwards to bridge the gap between New River Head and Stoke Newington. A new branch was constructed eastwards from Brixton pumping station shaft to the covered reservoir (TQ354746) at Honor Oak.
Work started at Honor Oak, with a 55m deep shaft ó the first of four new shafts, 7.5m and 10.67m in diameter. Tunnelling began in June 2008, using two earth pressure balance tunnelling machines.
In May 2009, the 4.5km tunnel bore was completed from Stoke Newington to New River Head by tunnelling machine Cleo. It has an internal diameter of 2.5m, with a primary lining of 180mm thick steel fibre reinforced precast concrete bolted segments and a secondary lining of reinforced concrete.
The 5km Honor Oak branch was connected to the existing ring main on 5th August 2009, when tunnelling machine Helen broke through into the existing shaft at Brixton. The tunnel is of similar construction to the Stoke Newington link.
The works were assessed under the CEEQUAL scheme for improving sustainability in civil engineering, infrastructure, landscaping and works in public spaces. Initial judgement in 2007 was that both were 'Excellent'.
The two tunnels were commissioned in spring 2010, at a cost of £95m. They increase the ring mainís transfer capacity by 500 million litres of water per day, to 1,800 million litres per day.
On 1st April 2010, it was announced in New Civil Engineer that the Thames Water Tunnel Ring Main project had won an Institution of Civil Engineers London special award.
Contractor: Amec Tunnelling
Contractor: J Murphy & Sons Ltd
Contractor: Costain Civil Engineering
Contractor: Isometal
Contractor: Taylor Woodrow Construction
Contractor: Miller Civil Engineering
Contractor (2007-10): Morgan Est
Contractor (2007-10): Costain
Contractor (2007-10): Black & Veatch
Earth pressure balance machines: Lovat Tunnel Equipment, Canada
Specialist contractor: Tunnelcraft
Research: ECPK
"Innovation and the Rise of the Tunnelling Industry" by Graham West, Cambridge University Press, 2005
"Development and performance of tunnel-boring machines on Phase II of the London Water Ring Main" by R. Remington, presented at the 7th international symposium Tunnelling í94, Chapman & Hall, 1994
reference sources   CEH Lond

Thames Water Tunnel Ring Main