3. Part of the Hell Gate Bridge Sits on Shallow Bedrock
The Queens-side tower of the Hell Gate Bridge sits on solid bedrock, reaching only 15 to 38 feet below the ground level. In order to construct the bridge. The Wards Island side was a completely different story all together because it lay near the upturned rock strata that had made the Hell Gate so treacherous to navigate by water. Also a gas line that had been run under the river prior to the bridge’s construction revealed a fissure in the bedrock which made creating a uniform bridge foundation nearly impossible.
Lindenthal’s solution for the Wards Island tower was to utilize fifteen 18-foot diameter caissons to provide solid footing for the tower. It took sandhogs several weeks to dig down deep enough to discover the reported fissure and it was much deeper than they had original thought based on initial borings. The solution was one Lindenthal had already used closer to the surface. He used concrete. He secured one of the caissons with a concrete arch where the fissure passed through its center and where the fissure lay at a connection point of two other caissons he bridged the gap with a concrete cantilever. It was the first time this practice was employed in bridge building. In comparison to the Queens tower, the Wards Island tower caissons reach down anywhere from 94 to 123 feet.