Collier County planned the construction of two pedestrian bridges on the west side of Vanderbilt Drive. The north pedestrian bridge would be constructed alongside an existing vehicular bridge and the south pedestrian bridge would be constructed alongside an existing box culvert. A utility pipeline also existed between the existing bridge structures and the planned construction. The pedestrian bridges were prefabricated metal structures with concrete decks to be supported on either shallow spread footings or a deep foundation system. The soil profiles in the areas of the bridges generally consisted of loose sands with weathered limestone layers. Weathered limestone was encountered at depths of 32 feet and 68 feet near the proposed north bridge, and 17 to 18 feet near the proposed south bridge. Peat layers were encountered in three of the four test borings at depths of three, five and 13 feet, and were typically estimated at three feet thick. Groundwater was observed at depths ranging from two to four feet.
The bridges could not be supported on shallow spread footings without high risk of differential settlements between the bridge deck and the slab-on-grade trail. These differential settlements would create abrupt changes in surface elevations resulting in trip hazards. Driven piles and helical piles were then considered as deep foundation alternatives that would penetrate the upper weak soils for bearing within the weathered limestone layers. Helical piles were selected over driven piles for two main reasons. First, helical piles could be installed with smaller equipment in the limited areas adjacent to the busy road. Second, installation of helical piles does not cause vibrations. The design team was concerned that vibrations from pile driving could damage the existing pipeline.
The north bridge was designed with support from four pile caps with either four or six helical piles per cap. The south bridge was designed with support from two pile caps with four helical piles each. The helical pile design consisted of 3.5-inch OD by 0.300-inch wall hollow round shaft with 12”-14” lead sections to support a design working load of 14 tons (28 kips) per pile with a factor of safety of three. The 20 helical piles for the north bridge were installed to an average depth of 39 feet below bottom of pile cap elevation. The eight helical piles for the south bridge extended to an average depth of 18 feet below bottom of pile cap elevation. All 28 piles were advanced to torque-correlated ultimate capacities of at least 42 tons (84 kips). The 28 helical piles were completed in three days.