The Monadnock rests on the floating foundation system invented by Root for the Montauk Building that revolutionized the way tall buildings were built on Chicago's spongy soil. A layer of concrete, reinforced with steel beams, forms a spread footing extending out under the surrounding streets, spreading the weight of the building over a large area of earth. The building was designed to settle , but by 1905 had settled that much and "several inches more", necessitating reconstruction of the first floor. By 1948, it had settled , resulting in a step down from the street to the ground floor. The entire east wall is supported on caissons sunk to the hardpan, installed when the subway Blue Line was dug under Dearborn Street in 1940.