Design and Construction of Drilled Shafts in Karst Environments of Florida

Drilled shafts have been widely used as bridge foundation alternatives for more than a decade in Florida. The majority of the drilled shafts are designed to embed into the underlying limestone. However, many unforeseen conditions have been encountered during the construction of drilled shafts due to karst environments, especially in the Tampa Bay area where sinkhole occurrences are common. This paper presents a case history of the design and construction of drilled shaft foundations for the I-4/I-275 Downtown Interchange in Tampa, Florida. A two-phase procedure utilized by the Florida Department of Transportation (FDOT) was adopted to minimize the impact of karst environments on drilled shaft construction and contractors’ claims, while also considering the project schedule and budget. A total of 315 drilled shafts with total lengths of 3,914 meters were installed for this project. Although the estimated total drilled shaft lengths in the preliminary design phase were only underestimated by 10%, high variability of individual shaft lengths between those estimated during the preliminary and final designs were observed with a maximum difference up to 20 m. The evaluation of the impacts of the karst environments on the drilled shaft design, and the comparison and discussion of the drilled shaft lengths determined during design and as-built are presented