Thames Tunnel Shield

Patented by Marc Isambard Brunel in 1818 the 'tunnelling shield'allowed the construction of the Thames Tunnel to become a viable undertaking.
The diagram above shows a section through one of the frames of the shield.

Twelve vertical cast iron frames (1) arranged side by side across the whole excavation. Each frame was 3ft wide and 22ft high and contained three vertical cells in which the tunnellers could work.
A series of horizontal board's (2) protected workers from the earth in front of them. These were held in place by a series of screw jacks (3).
Iron plates (4) held the earth at the sides and top in place with the frames being 'jacked' against the completed brick lining (5).

To move forward each poling board was removed and a few inches of earth removed. The board was then replaced and 'jacked' against the new surface.
After excavation of the complete surface, the cast iron frames were moved forward. This was accomplished by each frame, which was pivoted on two feet (6), being lifted up and moved forward and screwed back down. By releasing the poling board screw jacks it was possible to move each frame forward by means of the main jack,(7)located at the top and bottom of the frames.
The iron plates on the side and bottom of the frames were moved separately.