With the origins of the London Underground celebrating its 150th anniversary this year, Mike Chrimes takes a look at the construction techniques that helped deliver the network Londoners rely on today
If it were not for a catalogue of geotechnical failures on major projects, such as the Thames Tunnel, the London Underground could be celebrating a much larger anniversary than the 150 year milestone it has reached this year. Concerns over the cost and challenges of developing an underground railway delayed the start of construction of the network. However, it should be remembered that the construction was undertaken more than 50 years before Terzaghi and other pioneers initiated the modern discipline of geotechnical engineering.
The mastermind behind the Metropolitan Line, which was the first part of what we now know as the London Underground, was John Fowler and it owes much to his tenacity. As one railway director remarked: “I never met anyone who so often was able to convince me that things which I had all my life regarded as white were really black”.
The Met was intended to link the main passenger terminals north of the Thames with the City, but the inner circle, linking the southern railway termini was always part of Fowler’s grand scheme. Progress was slow, and the “circle” was not completed for nearly 20 years.
The costs were enormous – about £5M for the Met, £6M for the (Metropolitan) District and £3M for the final section, including £1M for street works. The level of local disruption caused by the current Crossrail project gives a hint of what was involved.
Fowler was engineer for almost all of the work and these near surface tunnels were largely built in cut and cover, with some tunnelling.
Benjamin Baker, Fowler’s chief assistant on the District line, noted: “With the utmost precautions tunnelling through a town is a risky operation, and settlements may occur years after the completion of the works. Water mains may be broken in the streets and in the houses, stone staircases fall down, and other unpleasant symptoms of small earthquakes alarm the unsuspecting occupants.”
One way in which methods changed over the course of construction was to build retaining walls for the cuttings and covered way in trenches before excavating the “dumpling” between the walls.
By 1900 Baker could state: “It is now known what precautions are necessary to ensure the safety of valuable buildings near to the excavations; how to timber the cuttings and keep them clear of water without drawing the sand from under the foundations of adjoining houses; how to underpin walls…how to drive tunnels, divert sewers over or under the railway, keep up the numerous gas and water mains, and maintain the road traffic when the railway is being carried underneath; and how to construct the covered way, so that buildings of any height and weight may be erected over the railway without risk of subsequent injury from settlement or vibration.”
Traditional underpinning techniques were used by Baker, Basil Mott and David Hay to support the church of St Mary, Woolnoth, at the corner of King William Street and Lombard Street over the City and South London Line’s new Monument Station. Large steel box girders and stanchions were used to support the fabric designed by Nicholas Hawksmoor.
The disruption and cost of construction in central London led engineers to consider deeper and subterranean solutions.
While working on the cylinder foundations for Lambeth Bridge, Peter William Barlow developed the idea of using horizontal tubes of cast iron segmental linings with a circular shield to create a network of “omnibus subways”.
In the late 1860s he raised enough capital for the Tower Subway – a cable-powered link beneath the Thames near the site of Tower Bridge. Concerns over the ground conditions meant that no established contractors were willing to tender, so James Greathead, Barlow’s South African born assistant, took on the challenge.
The subway was driven with a wrought iron cylindrical shield on a modified design to Barlow’s original patents and was forced forward by six screws as material ahead was excavated. The lining was of bolted cast iron segments, with lime grout forced into the space between the soil and lining. The working opening was towards the top of the shield, although Barlow had modified his patented shield in 1868 with a lower working opening to facilitate the use of compressed air to exclude water in the upper part of the working. Completed in 1869, tunnelling speeds of 9ft per day were achieved.
Greathead went onto develop a system using water to excavate material ahead of the shield and designed a new shield with compressed air lock and hydraulic lifting equipment for the Woolwich Subway, although the project was later abandoned. He later developed hydraulic jacks to advance the shield in place of screws for the London (City) (1884) and Southwark (Subway) in 1884.
Greathead died in 1896 but Baker worked with Basil Mott to bring further mechanisation on the Central Line, where Price’s rotary digger was introduced to improve excavation rates.
The success of the City and South London line led to six new railways being authorised in 1893 and once money was raised there was a frenzy of subterranean activity in the early 20th century. More than 42km of Tube lines were opened between 1903 and 1907, including the
Piccadilly, Bakerloo and the Charing Cross branch of the Northern Line.
The engineer largely responsible for the tunnelling was Dalrymple- Hay, who developed a method of compressed air working beneath the river on the Waterloo and City line. The technique excavated small pockets of earth by hand up to 0.6m ahead of the hooded cutting edge of the shield and packed with clay to create a seal as the shield hood advanced.
Dalrymple-Hay was responsible for extensions of the Northern Line from Golders Green to Edgware, and Highgate to East Finchley; and the Piccadilly line from Finsbury Park to Cockfosters. Between the wars he worked with Mott, Hay and Anderson, or William Halcrow to improve station interchanges at Piccadilly Circus, Leicester Square, Waterloo, King’s Cross, Knightsbridge, Elephant and Castle, and Holborn.
On the subject of Dalrymple- Hay’s work with Mowlem in the 1920s, future ICE president Harold Harding commented: “The reconstruction of Piccadilly Circus provided the first occasion when a large booking hall and circulating area had been dug out entirely by tunnelling methods without disturbing the road surface. The roadway consisted of unreinforced concrete and in places was found to be barely 9 inches thick.”
The growing understanding of soil mechanics in this era helped to add theoretical support for new techniques but the evolution has continued and is still underway today with development of Crossrail and upgrades on the London Underground.