While the layer of bedrock under the Municipal Building was quite close to the surface underneath the southern part of the building, the bedrock dropped to a depth of about under the northern portion of the site, where it would be extremely difficult to dig caissons. A layer of sand was present to a depth of , while the average depth of the bedrock under the building was about . The contract for the foundations was the largest to be awarded for a single building in the United States, with being excavated at a cost of $1.5 million. The foundations incorporated of concrete for the piers, as well as 70,000 barrels of cement. The foundations also included 125 caissons extending to an average depth of . The maximum depth of the caissons was below grade, and for the northern part of the site, the Foundation Company built larger caissons resting on sand at a depth of . While the caissons under the southern two-thirds of the building carry , the larger caissons under the northern third of the building carry only .