In 1894, construction began for the Stock Exchange Building, also with a foundation of driven wood piles. During construction, the owners for the neigh- boring Herald Building obtained a court order preventing pile driving alongside their building, fearing the vibrations would affect their newspaper printing presses. An early consultant and now legendary Chicago foundation engineer, General Sooy Smith, was retained to devise a solution, which would eventually become standard practice for the next 60 or so years. General Smith’s design required eight hand dug “wells” to replace the line of piles at the property line. The shaft diameters ranged from 5 to 6.33 ft (1.5 to 1.9 m) and the shaft bases were enlarged to 6.5 and 8.5 ft (2 and 2.6 m) diameters, foreshadowing the future belled hardpan caisson. The Chicago hand-dug caisson gained acceptance, and a new era of building construction began. The method of instal- ling them was one of back breaking labor, common sense and pure ingenuity. The work was performed under the risk of soil collapse, and the potential inflow of water and methane gas. The common procedure involved tongue and groove wood lagging installed and braced by iron rings to line the shaft and support unstable soils. Shovels and air spades were used to dig the clay, and small “derricks” were set up over each caisson, with a tripod and air-driven winch to hoist up the well buckets of spoil and lift the workmen in and out of the shafts. Usually a crew of three men would work as a team to excavate the typical hand-dug caisson, which was a minimum of 48 in (1.2 m) in diameter, allowing at least one man to work efficiently within the shaft. The team consisted of a headman, the dumper and a hand miner. The team was expected to dig 16 ft (4.9 m), or three “sets” of lagging, per 8 hour shift. Additional hand miners would be added if the shaft was large enough for more than one miner. This process was repeated until the required bearing depth on the hardpan was reached. It became common practice to enlarge the base of the caisson on the hardpan by the same hand digging methods to form a “bell” and increase the capacity of the caisson. 78 • DEEP FOUNDATIONS • NOV/DEC 2016 1950s-era bucket rig The first mechanical drill rigs to appear Machine Drilled Caissons The hand-dug caisson era extended well into the 1950s and 1960s, particularly with rock caissons, until mechanical drill rigs could be developed with enough power to penetrate through the hardpan and dense in Chicago during the 1950s were bucket rigs, so named because they had no turn- table and could not swing; instead a side boom was used to pull the Kelly bar and digging bucket off to the side for dumping as it came up out of the hole. The first truck augers came to Chicago in the late 1950s. Developed by Hughes Tool Company, these were modified from oil well drilling tech- nology and were more powerful and versatile than the bucket rigs. The largest of these arrived in the 1960s and were the final stage of truck-mounted rig development in Chicago. These machines were capable of excavating shafts up to 8 ft (2.4 m) diameter to a depth of 120 ft (36.6 m); like the bucket rigs, they were dependent on the skill of the operator and “swamper,” or ground man, for best performance and mechanical reliability. Hughes truck auger till overlying the rock. The 41-story Prudential Building, Chicago’s first modern skyscraper and first major post-war building, was undertaken in the early 1950s on large hand-dug rock caissons, which are visible today after a basement expansion in the 1990s added two parking levels below the building. In the 1960s hand-dug caissons were installed from existing basements during demolition of the structure above to make way for the 60-story First National Bank Building in the heart of the Loop. The next step in the evolution of mechanized rigs was the advent of the crane-mounted drill attachment, which began to appear in the 1960s and culminated late in that decade with very large twin-engine platforms capable of delivering 400,000 ft.-lbs. of torque to 18 in (0.5 m) Kelly bars, mounted on 140 ton (127 tonnes) crawler cranes. Depending on crane size and Kelly length, these machines could be configured to drill shafts up to 20 ft (6.1 m) in diameter, and reaching to
