Concrete Caissons: Going Deeper
Building Forensics International is presently working on a project where the original design called for concrete caissons approximately 5 feet in diameter. The design was changed to spread footings. This facility is experiencing concrete cracking defect issues. Is there a relationship to the cracking defect and the change in design? That will have to be determined.
44 concrete caissons are being used to support the new 800-ft, 55-story high-rise at 181 Fremont Street in San Francisco. The caisson shafts are five to six feet in diameter and are being drilled at an average depth of 262 feet into the ground. The 44 concrete caissons are scheduled to be completed by July 1, 2014. Each concrete caisson shaft takes 3 days to complete.
On the first day, the shaft is drilled by a partially cased polymer drilling method. The second day all debris is removed from the hole to prepare for the rebar cage to be set on the third day.
The six-foot diameter shaft is reinforced with a full length rebar cage and has a concrete strength of six thousand psi. Once all the concrete caissons are placed the excavation for the basement will begin. The project is scheduled to be complete some time in early 2016. This high-rise will be home to 404,000 sq-ft of Class A office space. The top 17 floors will be 67 condos. The project was designed by architectural firm Heller Manus Architects of San Francisco.
BFI Staff Writer -