The French-born engineer Sir Marc Brunel invented the first tunneling shield, patented it in 1818, and used it to build a tunnel for public traffic under the river Thames in London. The tunnel, originally intended for road traffic, was never used as such and became a pedestrian walkway, until it was taken over by the East London Railway in 1865.
Brunel’s shield was rectangular and consisted of 12 massive frames only 3 ft (0.9 m) wide but 21 ft 4 in. (6.5 m) high. These were lined up side by side at the foot of a shaft. Three workers, on three different levels, were stationed in each frame.
There were strong plates above, below, and on each side of the frames so that the whole formed a box with no back, with the front consisting of removable boards.
The shield worked by each worker digging away, piece by piece, the earth in front of his compartment for a depth of 41/2 in. (114 mm). As each narrow section was completed, the worker supported the earth face with one of the boards, which could be moved 41/2 in. forward and held by poling screws.
When one complete frame had the earth before it cut completely away, the frame itself was jacked forward 41/2 in. Bricklayers followed the shield inserting the brick tunnel lining. In this way never more than 41/2 in. of the newly cut walls were left unsupported by either the shield or brickwork. Isambard Kingdom Brunel, son of Sir Marc, supervised the Thames Tunnel work between 1825 and 1828.