A tunneling shield involves a protective structure used in the excavation of tunnels. It is used when soils are too soft or fluid to remain stable during the tie it takes to line the tunnel with a support structure. The shield therefore serves as a temporary support structure while the tunnel is being excavated.
THE TOWER SUBWAY
The first successful tunneling shield was developed in 1818 by Sir Marc Isambard Brunel and used to navigate a tunnel underneath the Thames. This tunnel, known as the Thames Tunnel, was used for foot traffic. Peter W. Barlow then improved this original design during the construction of the Tower Subway underneath the Thames in 1870. The Tower Subway was another tunnel beneath the Thames that at first had a cable-hauled wooden carriage to convey passengers across but was soon converted to pedestrian traffic, reducing the congestion on the bridges. Barlow designed a circular cross-section.
MANUAL SHIELD TUNNELING
In the early stages of shield tunneling used in the deep tunnels (in the late 1800s and early 1900s), the shield protected the labourers who performed the digging. They would move the shield forward and in doing so progressively replace it with the pre-built section of tunnel wall. The shield divided the workface into overlapping portions that each worker could excavate.
THE GREATHEAD METHOD
This was a crucial innovation and made construction much simpler and better able to support the weight of the surrounding soil. James Henry Greathead then enlarged and improved the design for the construction of the City & South London Railway (today part of the London Underground’s Northern Line). This system was used for the tunnels for the Waterloo & City Railway which opened in 1898. The City Station tunnels (now known as Bank Station) were the largest diameter tunneling shields in the world at the time. Most tunneling shields are still loosely based on the Greathead shield.