The use of compressed-air caissons
Workers dig in the lower pressurized chamber and the metal caisson descends a few feet every day. Once a certain depth is reached the caisson is filled with concrete and left in place.
Jacques Triger’s revolutionary procedure soon gained world-wide fame. Improvements rapidly followed The original cylinder was soon turned into a genuine caisson. The technique was then used for the foundations of many structures (The Kehl bidge, the Forth Bridge, the Eiffel Tower, etc.).
Workers dig in the lower pressurized chamber and the metal caisson descends a few feet every day. Once a certain depth is reached the caisson is filled with concrete and left in place. All that was requred then was to construct conventional piers on top.
The technique naturally adapted to the construction of the Paris metro: huge metal caissons were constructed on the Seine embankment. Placed end to end these formed the tunnel. Initially they were sunk in the river, almost in their final position, and the miners began to dig underneath them, in a hyperbaric chamber. The caisson gradually sank under the river bed.
If you visit Paris, do not hesitate to stop briefly at St Michel or Cité stations on Metro Line 4 where you will be able to see these huge caissons for yourself. Unfortunately, there were many accidents during the construction works, as was observed sadly by Paul Bert at the end of the 19th century, who gave the name "Caisson disease" to the condition caused by the formation of nitrogen bubbles in the blood. Since his seminal work much progress has been made in this area (regulations, dive tables, treatment, etc.).
The most recent major construction project in which compressed air was used for excavation was the piers of Tokyo Harbour Great Bridge in 1989. All the operations conducted in compressed air were automated, using the so-called UES (Unmanned Excavation System).