information:classification
Caissons

The towers rest on underwater caissons made of southern yellow pine and filled with cement. Inside both caissons were spaces for construction workers. The Manhattan side's caisson is slightly larger, measuring 172 by 102 feet (52 by 31 m) and located 78.5 feet (23.9 m) below high water, while the Brooklyn side's caisson measures 168 by 102 feet (51 by 31 m) and is located 44.5 feet (13.6 m) below high water. The caissons were designed to hold at least the weight of the towers which would exert a pressure of 5 short tons per square foot (49 t/m) when fully built, but the caissons were over-engineered for safety. During an accident on the Brooklyn side, when air pressure was lost and the partially-built towers dropped full-force down, the caisson sustained an estimated pressure of 23 short tons per square foot (220 t/m) with only minor damage. Most of the timber used in the bridge's construction, including in the caissons, came from mills at Gascoigne Bluff on St. Simons Island, Georgia.

The Brooklyn side's caisson, which was built first, originally had a height of 9.5 feet (2.9 m) and a ceiling composed of five layers of timber, each layer 1 foot (0.30 m) tall. Ten more layers of timber were later added atop the ceiling, and the entire caisson was wrapped in tin and wood for further protection against flooding. The thickness of the caisson's sides was 8 feet (2.4 m) at both the bottom and the top. The caisson had six chambers: two each for dredging, supply shafts, and airlocks.

The caisson on the Manhattan side was slightly different because it had to be installed at a greater depth. To protect against the increased air pressure at that depth, the Manhattan caisson had 22 layers of timber on its roof, seven more than its Brooklyn counterpart had. The Manhattan caisson also had fifty 4-inch-diameter (10 cm) pipes for sand removal, a fireproof iron-boilerplate interior, and different airlocks and communication systems.