Rhode Island Lighthouse History

  Plum Beach Light  

Pneumatic Caisson
Pneumatic Caisson
Courtesy of United States Coast Guard 

 

The pneumatic caisson process was employed to construct the Plum Beach Lighthouse foundation. This method of construction was used as the bottom where the lighthouse was to be built is sand, into which the foundation would need to penetrate a considerable distance to withstand the overturning effects of wind and ice.

A timber working chamber was built on shore and launched. On this was placed three courses of a cast iron cylinder, 35 feet in diameter, built up of plates bolted together along their flanges. The caisson was towed to the site of the lighthouse and was sunk in position, by admitting water to the cylinder. An air shaft with an air lock, through which the men passed to the working chamber, was erected on the foundation cylinder and braced to shell plating. Sufficient air pressure was maintained in the caisson to keep the water out. The excavation was made in the caisson within the cutting edges, the sand being blown out through a blow pipe with in the air shaft. In this manner the caisson was sunk to the desired depth. The space within the shell plating was filled with concrete as the work progressed. When sunk to the proper depth, the cylinder was encircled by riprap to prevent scour. A tower and dwelling was erected on the cylindrical foundation pier.

Source: "United States Coast Guard Aids to Navigation, 1945"




  Plum Beach Light