Pile Group Effects and Soil Dilatancy at the Fort Lauderdale International Airport

The Fort Lauderdale airport runway and taxiway expansion involved the lengthening of a 1.6-km-long landing strip into a new 2.5-km-long runway that partially sits on a 15-m-high manmade embankment, and required the construction of two substantial bridges for the runway and taxiway to traverse over US-1, Florida East Coast railway, and other airport roadways. The stringent settlement tolerances coupled with impact loading for the largest commercial aircraft, the Airbus A380, result in a design where the bridge foundations consist of approximately 2,700 driven 24-in. (61 cm) pre-stressed concrete pilings. The project design had various geotechnical considerations including avoiding differential settlement between 15-m-high MSE walls at the bridge abutments, pile group effects, and variable bearing strata. These inherent features were further complicated during construction by the dilation of dense saturated fine sands, an uncommon phenomenon for South Florida, which resulted in temporary high driving resistances and then subsequent resistance decreases after pile driving. Over 1,000 dynamic pile tests, comprising of nearly 300 test piles, and approximately 900 set-checks and/or redrives were required to ensure the project was completed safely and according to plan. The case study will provide an overview of the project and will include specific examples where both refusal conditions and dilative behavior were observed. The marriage of these conditions presented challenges in terms of driving piles to ensure that measured capacities, through dynamic pile testing, would remain higher than the designed capacity over the long term. The technical aspects of dilatancy, its impact on negative excess pore water pressure, and in turn effective stress and ultimately pile capacity, will be reviewed within the data rich environment. These aspects, among other interesting project features will be reviewed, with emphasis on the project challenges, how they were overcome, and lessons learned.