February 29, 1960: Pile and caisson drivers confer in this photo. In the 26,000 square foot area upon which the 52 story Prudential Tower was built, a total of 144 30-inch diameter steel caissons were being driven down to bedrock about 140 feet below street grade. Once firmly on the bedrock, a core of rock 12 to 25 feet deep was drilled out from beneath each caisson. Then a heavy steel H beam was inserted before the caissons were filled with a laboratory designed concrete mix designed to withstand pressures in excess of 4000 per square inch.