The foundation for the tower was excavated to a depth of using pneumatic caissons. They were used to extract the underlying soil, then filled with concrete to create piers. Each caisson pier was designed to carry . A gridiron of steel girders was placed atop the caisson piers. Because of the design of the tower addition's wind-bracing superstructure, the upward pull on some of the piers was greater than the dead load these piers carried. As a result, numerous eyebars of different lengths were embedded in ten of the caissons, with the concrete being poured onto the eyebars. The rods were embedded into the caisson piers. The system, devised in house by Flagg's office, was more than twice as expensive as a conventional foundation would have cost for a building of the Singer Tower's size. The original plan was for the caissons to be sunk only deep, but the builders changed plans midway through the excavations, so that the caissons would go to hardpan.