Aerial view of top-down construction including over 1,100 tons (998 tonnes) of structural steel. The shafts were designed to carry tension loads, column eccentricity and compressive loads from 800 to over 8,000 kips (363 to 3,633 tonnes) and were socketed into rock ranging in compressive strength from 2,800 to 16,000 psi (19.3 to 110 MPa). Steel columns dressed with slab connection plates were hung in the shafts, and concrete was placed via tremie method to secure the column in place. Each of the 150 drilled shaft steel formed and jacked, and milled using a hydromill. The two-acre (8,094 sq m) footprint was quickly filled with six slurry containers, an on-site mixing plant, three clam shell digging rigs, a hydromill, three support cranes and essential miscellaneous support equipment, making site logistics challenging. One of the more impressive feats of the project was excavating and installing a slurry wall panel around a live concrete encased electrical duct bank that ran through the project site (and eventually had to be structurally supported). This was done by guiding the clam shell bucket diagonally under the structure on both sides. Drilled Shafts An element critical to the success of the top-down approach was the use of drilled shafts. The shafts were installed from the ground surface using a bentonite slurry and extended over 140 ft (42.7 m) into the underlying bedrock. In total, over 150 drilled shafts were constructed ranging from 5 to 9 ft (1.5 to 2.7 m) in diameter and columns arrived at the project in multiple pieces due to the limited amount of laydown space afforded to the contractors. The steel columns were mechanically spliced together with as many as 80 bolts per connection above the shaft. The longest columns located at the corners of the ballroom required three splices. Splicing the columns required close coordination, as each piece was specifically marked to their assigned location and direction. The prefabricated connections along the length of the columns needed to match perfectly to the embedded steel plates and girders to be installed later. Alignment was critical, as any errors in plan location or verticality would result in costly rework during the construction of the subgrade slabs. After the concrete caisson was placed via tremie method, cement was added to the remaining slurry mix in the shaft to solidify the slurry around the column above the concrete to be uncovered once excavation began. Peak production had four drill rigs with three support cranes working each day. In total, during the slurry wall and drilled shaft scopes of work, there were twelve cranes on site including four drill rigs, three support cranes, two digging cranes, one hydromill, one micropile rig, one 50 ton (45 tonne) rubber tire crane, two tower cranes and an army of excavators and loaders. That averaged out to only about 3,600 sq ft (334 sq m) per piece of equipment. Excavation and Diaphragm Slabs Upon completion of the slurry wall, drilled shafts and localized tiebacks, excavation and subgrade concrete placement commenced. Large equipment was used to excavate the first subgrade level in order to prepare the floor for a 0.3 ft (700 mm) concrete prep slab. This thin slab acted as a solid surface for the building reinforcing to be installed on (including rebar, plate steel and embedded beams), and provided a smooth ceiling for the floor below. Once the first subgrade slab was placed, excavation began at shaft openings or designed block outs progressing with low headroom equipment. Concrete placed for the floor slabs at each basement level was structurally connected to both the internal steel columns embedded in the drilled shafts via floor clips and to the perimeter slurry wall with dowels at the top and Drilled shaft installation DEEP FOUNDATIONS • NOV/DEC 2014 • 59
