Trump Tower rises on 230 caissons – both rock bearing and hardpan – installed on a narrow footprint along the Chicago River.
State-of-the-art caissons provide support for a 92 story highrise tower. The 57 core caissons utilized a record breaking 500 ksf bearing capacity and relied on innovative techniques to drill rock sockets up to 10’ diameter. At many locations, concrete-filled bells of old hand-dug caissons were removed by coring to allow construction of the new shafts. Over 16,000 cy of concrete and 2,360 tons of steel reinforcement and casing was put in place in only 6 months.
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Rock Caisson Foundations for the World’s Tallest Concrete Building
On September 11, 2001, Donald Trump was meeting in Chicago with his design consultants, making plans to build the world’s tallest building in downtown Chicago. The horrifying news that morning from Trump’s hometown, New York City, put an early end to the meeting. Many years from now, historians may be able to trace the full effect of that day’s event on the lives of people here and all over the world. In our world of the construction business, the effect was almost immediate; owners and designers began to question tall buildings. It would be three more years before Trump’s scaled-back plans finally began to take shape along the Chicago River.
The Site
The Trump Organization chose well when they selected the location for their Trump International Tower and Hotel. Two blocks west of Michigan Avenue, the site hugs a curve in the Chicago River, allowing the building to face east toward the mouth of the River and the Lakefront. The new building will dominate the skyline in this River North neighborhood, giving Tower residents the most spectacular views in the city.
Until it was demolished in the fall of 2004, the site was occupied by the Chicago Sun-Times Building, which was constructed in the 1950’s. When the Sun-Times was built, mechanical drill rigs were replacing the hand-dug Chicago caisson method, but mechanical belling buckets of any size were not yet in use. To support the heavy loads of the presses and paper storage, bells up to 15 ft diameter were hand-dug at the bottom of the machine-drilled shafts. This was usually a 2-shift operation; the day shift would drill shafts and pour concrete, while the night crew would cut bells for the next day pour. Fifty years later these caissons, at the end of their service life, would become obstacles for the designers and builders of the next structure on the site.
Opposite the River, the site was bounded by Wabash Avenue, an old 2-level city owned viaduct, which was supported on hand-dug caissons. Part of the Trump project was the complete removal and replacement of the Wabash viaduct to blend in with the new podium structure around the tower. The new podium and Wabash viaduct are supported on 170 belled hardpan caissons.
The Tower
Trump’s final “scaled-back” design is 92 stories of 4-star accommodations made up of private condominium residences, hotel rooms, and hotel residences. Prices for the condominiums have reached $1000 per square foot and higher. The Architect/Engineer for the project is Skidmore, Owings, and Merrill (SOM) of Chicago. The Chicago office of Bovis Lend Lease is the Construction Manager. The Geotechnical Consultant is STS Consultants, LTD.
Before being eclipsed by new record-setting buildings in Asia, Chicago was home to three of the five world’s tallest buildings: The Hancock Building, Standard Oil Building (now Aon Center), and Sears Tower. Each of these landmarks were built in the late 60’s and early 70’s and are between 100 and 110 stories, and each is steel-framed. Two of these, Hancock and Sears, were also designed by SOM. The Trump Tower began its design life as a composite steel frame/concrete core, but a sharp increase in the cost of steel forced the design team into an all concrete frame. Ninety-two stories of concrete superstructure means enormous gravity loads, much greater than SOM had to deal with at the Hancock or Sears Tower. Their foundation solution was 57 rock caissons, with most of the Tower’s load concentrated in a core mat supported by twenty-two 10 ft diameter caissons, each with a capacity of about 40,000 kips.
The Chicago building code section for rock caissons is based on an empirical formula that allows incremental increases in end bearing pressure for each foot of embedment into solid rock, to a maximum value of 400 ksf. For maximum design efficiency, a code variance was sought and approved to increase bearing pressure to 500 ksf with confirmation load testing by Loadtest on an 8ft rock caisson. The Osterberg cell at the bottom of the rock socket was loaded to its maximum capacity of 1500 ksf and negligible movement was recorded. The city code also permits higher allowable stress in the rock caisson concrete, provided that it is confined in permanent steel casing of a certain wall thickness. At Trump, the design was optimized with 10,000 psi caisson concrete.
Current practice on high-capacity rock caisson projects in Chicago calls for a precore boring at each caisson center, to explore the rock quality and establish the bottom elevation of each caisson before construction. STS Consultants performed these precore borings before and during the site demolition phase.
The Geology
Chicago has some of the youngest geology in the country. The whole Great Lakes landscape was “wiped clean” and replaced with till during the ice age; the most recent glaciation was only 10,000 or so years ago. At the Trump site there is 10 to 20 ft of sand and rubble fill left after demolition, which is underlain by soft Chicago clay. At about 75 ft the clay transitions into Chicago hardpan, which is very hard till that was consolidated by the weight of the glaciers. Below the hardpan the till turns granular and contains cobbles, boulders, and water under pressure. The rock is about 100 ft below the surface and usually has a weathered and broken horizon with fractures and clay seams. The unweathered limestone beneath is sound and fairly hard, in the range of 10,000 to 20,000 psi.
The Equipment
When the Hancock was built in the 1960’s large crane attachments were first being developed that were capable of excavating through the overburden and twisting large diameter casings into the rock. Rock tooling was still fairly primitive and so the rock excavation was turned over to hand crews. Until the development of air circulation tools in the 1980’s, rock socket excavation in large diameter shafts still relied on hand labor at the bottom of the caissons. During this period, mechanical coring with drag-tooth core buckets was the most common method of rock socket excavation, but recovery of cored plugs larger than 5ft in diameter was rare. Since the late 1980’s when the industry in Chicago banned entry into caisson excavations, the typical method of rock socket excavation has been coring with air circulation roller core buckets, followed by chiseling and mechanical cleanout to remove the rock plug.
The Schedule
Rock jobs in Chicago are historically long-term projects, with many shifts required to excavate the overburden, install the full-length casing and advance it through the broken rock, and finally complete the rock socket. In the Loop, the business center of the city, it has been common to work a night shift, usually concentrating on drilling rock. In the River North neighborhood, night work is prohibited, yet the construction schedule set by Bovis and Trump was not achievable, even with three drill rigs operating full time.
The Solution
To meet the project schedule, Case elected to turn to an entirely new method to remove the roughly 1200 cy of rock socket required by the final design. Utilizing downhole hammers housed in canisters, the rock sockets were drilled in stages. The largest caissons, with 10 ft shafts, were designed with 9’-6” diameter rock sockets. The initial socket was first drilled with a 58” cluster drill cannister, operating inside a 59” ID conductor casing to centralize this initial pilot hole. Additional runs were then made with openers to make 90” diameter cut and then the final 114” diameter cut. Each donut-shaped opener contains 12 to 14 hammers, and each is equipped with a calyx basket above the canister to collect the rock cuttings. Direct air circulation to actuate the hammer bits and clear the cuttings comes from four 1600 cfm air compressors, with a fifth in reserve. The hammer drills, turning at very low rpm, delivered steady production and penetration rates much greater than “core and chop” methods, unless discontinuities or broken rock was encountered. In these few occasions, air coring and chiseling techniques were used to complete the run. The air tools were turned by a Hughes CLLDH SuperDuty crane attachment, mounted on a Manitowoc 3900W crawler with 190 ft of boom. This rig did nothing but drill rock, so it was equipped with an 11” by 140 ft single air Kelly. Two more crane-mounted drill rigs handled the chores of drilling the shafts to rock and twisting the permanent casings into place: a Calweld 200 CH and a Hughes CLLDH Titan drill attachment. The Titan is the largest and most powerful crane attachment in the Case fleet; because of its weight and torque it’s normally mounted on a Manitowoc 4100W. Each of the machines is either pushing or past 40 years old, but they’re still state-of-the art for this kind of work.
After the rock sockets were completed, reinforcing and concrete was placed. Pressure grouting around the permanent casings greatly reduced water inflow into the caisson shafts, allowing approximately 50% of the rock caissons to be concreted in the dry by freefall methods. The remaining caissons with measured water inflow rates greater than 5 GPM, were flooded and concreted by tremie methods.
Old fashioned methods and equipment were still the best choice when obstructions were encountered. At 43 new caisson locations, the designers were unable to adjust locations to clear the old caisson bells. The worst of these interferences beneath the former Sun-Times building meant coring a large chunk from a 15 ft bell with a 10’-6” core bucket – considering the scale, somewhat of a “delicate operation,” making a vertical cut on a 60° bell slope.
Like all downtown Chicago jobsites, logistics and careful sequencing controlled production to a great extent. In addition to the three drill rigs, a large service crane was also needed; we selected a new Manitowoc 999 (275-ton) crawler, our first experience with a large hydraulic crawler crane. With 180 ft of boom, its long reach was a big advantage, and it still had plenty of capacity for the heavy casing lifts. All 230 caissons, 16,000 cy of concrete, and 2360 tons of reinforcing and casing were safely in place by August 2005. Acceptance testing included CSL, impulse response, and even City – mandated full-depth coring of completed caissons. All testing showed excellent results.
In order to bring this job in early, we had a few advantages: a little experience, reliable equipment, new rock drilling technology, and most importantly, the Case team.
