Custom Chicago Caisson Foundations There are no “green” sites left in downtown Chicago, and developers frequently deal with two or even three generations of existing foundations remaining in the ground after demolition to make way for new buildings on prime downtown sites. Existing caissons from the hand-dug Limestone cores recovered during rock socket excavation with the limiting factor being the struc- tural, rather than the geotechnical capacity of the caisson. Exceeding the code requires load testing, and this has been done eco- nomically by unit-load testing with Osterberg cells placed at the bottom of the first production caisson as a confirmation test. Rock caissons for the 90-story Trump Tower in 2005 were designed and installed at 250 tsf (2,441 tonnes/sq m); two years later the 80-story Aqua Building was built with the same design. Both of these struc- tures have 10 ft (3 m) diameter rock caissons grouped beneath their cores; each of which has a capacity of 35,000 kips (15,875 tonnes). Foundations for the proposed 150- story Chicago Spire were installed in 2008 in a very efficient circular layout of thirty- four 10 ft (3 m) diameter rock caissons with a design bearing pressure of 300 tsf (2,929 tonnes/sq m) and a capacity of 42,500 kips (19,278 tonnes); these are believed to be the highest capacity single deep foundation elements ever installed in the U.S. Modern Rock Excavation Tools The latest generation of tall concrete buildings with high-capacity rock caissons required the development of more efficient 80 • DEEP FOUNDATIONS • NOV/DEC 2016 equipment for excavation of deep rock sockets. Rather than carbide tipped drag- tooth coring or augering tools, percussion tools operated by high-pressure air have been developed, using a “cluster drill” arrangement of downhole hammers in a canister housing. For the largest 10 ft (3 m) diameter shafts, three passes are needed; a 58 in (1.5 m) diameter pilot bore, followed by 90 and 114 in (2.3 and 2.9 m) “openers” to reach the full- diameter rock socket. Another efficient method of rock socket excavation is the piletop reverse circulation drill (RCD), which employs roller cutters and water circulation to remove cuttings from the drill face. These machines apply heavy pressure to the roller bits by hydraulic crowd and a weighted drill string, and require large volumes of water (>1,000 gpm) for circulation, and are very effective in hard rock. To date, their use has generally been limited to bridge foun- dations in the U.S. With the development of compact desanding plants they can also be an effective tool on urban highrise foundation projects. era, and the early machine-dug era, are frequently encountered; they are normally not reusable, but must be accounted for when designing a new caisson system. Complete removal of old caissons is prohibitively expensive and is never attempted, but “near miss” caisson instal- lation close to old caissons has become relatively common. New shafts can be placed adjacent to old, but new bells cannot be cut into the concrete of old bells. Instead, the new caisson is extended beneath the bottom of the existing caisson, to allow belling below and within the hardpan stratum. This involves the additional time and expense of coring through the existing bell, but there is a small offset of savings because designers can take advantage of higher bearing pressures available with deeper embedment into the hardpan; up to 60 ksf (292.9 tonnes/sq m), rather than the Cluster drill at Trump Tower (2005) standard 20 to 35 ksf (97.7 to 170.9 tonnes/ sq m) at the surface of the hardpan. This has led to more economical foundation layouts even on unobstructed sites, allowing smaller, high-capacity bells to be grouped closer together directly under the load path, although belling within the hardpan is time- consuming and hard on the belling buckets.
