approach and connector structures. The Phase 1 eastbound bridge opened in April 2017 and the Phase 2 westbound bridge opened in September 2019. The Brooklyn and Queens approaches encompass westbound piers WB-1 through WB-8 and WB-12 through WB- 17, respectively. Piers WB-9 to WB-11 are part of the main bridge span. Except for the tower of the main span, all piers for the two connectors were initially planned to be founded on driven piles. However, due to interference with the old bridge foun- dations, driven piles were not a feasible solution for Pier WB-13. Thus, two drilled shafts were used to support this pier. This article describes the subsurface conditions, design approach, construction methods and axial load testing for the drilled shafts supporting pier WB-13. complex, boundaries between strata are not clearly defined and considerable inter- layering of the strata is observed, particularly in the glacial deposits. The groundwater was encountered at a depth of about 17 ft (5.2 m) at pier WB-13. Foundation Design Each pier, except WB-13, comprises two columns and are bearing on a pile cap supported by driven piles. At WB-13, however, each column is supported directly onto and by a drilled shaft. In New York City, it is common to extend the drilled shafts into rock to develop the required resistance. However, the use of these high capacity shafts founded entirely in soil was an opportunity to provide an innovative, technically sound and cost- effective solution for Phase 2 of the project. Due to the depth of rock at WB-13, the drilled shafts were d e s i g n e d t o b e entirely in soil, with a design length of 150 ft (45.7 m). The diameters of all of the columns and the two drilled shafts are 6 ft (1.8 m) and 7 ft (2.1 m), respectively. The axial resis- Generalized subsurface conditions at pier WB-13 Subsurface Conditions More than 200 borings were drilled for the entire project, five of which were located near the WB-13 foundations. At WB-13, the subsurface conditions consist of six strata: a surficial man-made fill, clay or silt with some organic matter, glacial deposits, Raritan clay, decomposed rock and bedrock. Stratification is generally 94 • DEEP FOUNDATIONS • NOV/DEC 2019 Stratum Designation Man-made Fill Soft Clay/Silt Glacial - upper cohesive Glacial - cohesionless Glacial - lower cohesive Raritan Clay tance of the drilled shafts is derived from both frictional re- sistance and end bearing resistance. Side resistance to a depth of 29 ft (8.8 m) was ignored (i.e., within the man-made fill and underlying soft clay/silt). The calculated geotech- Unit Weight (pcf) [kN/cu m] 130 [20.4] 90 [14.2] 120 [18.8] 130 [20.4] 120 [18.9] 120 [18.9] nical resistance of the drilled shafts was estimated using the Beta method as described in the AASHTO LRFD Bridge Design Specifications (2014 edition with interim revisions through 2016) and in the FHWA Drilled Shafts: Construction Proce- dures and LRFD Design Methods Manual (2010). With the top of the shaft at El. +12 ft (El. +3.7 m) and the minimum shaft tip elevation at El. -138 ft (El. -42.1 m), the nominal resistance was estimated as 8,400 kips (37.4 MN), which was achieved from 2,700 kips (12.0 MN) in end bearing and 5,700 kips (25.4 MN) in side resistance. The controlling case for lateral load was the Extreme Load Case, which applied a maximum shear force of 115 kips (511.5 kN) and a maximum bending moment in the longitudinal direction of 8,000 kip-ft (10,847 kN-m). The pier column reinforcement cage includes 28 #11 bars that are embedded 12.5 ft (3.8 m) into the drilled shaft, and the reinforcement cage for the drilled shaft includes 28 #18 bars that extend to the bottom of the shaft. No permanent steel casing was left in place, as the total capacity of the drilled shaft considered and relied on the side friction resistance between the concrete and the adjacent soil. The design required a minimum 28-day compressive strength for the concrete of 6,000 psi (41.4 MPa) along with a minimum cover of 5.5 in (140 mm). Design of the Load Test To meet the accelerated project schedule, a bi-directional static load test was performed using an Osterberg Cell (O-Cell) on a production shaft. During a bi-directional load test, the shaft is loaded in two directions simultaneously: (a) above the device – the upper portion of the shaft is loaded upward whereby the movement is resisted by side shear (only) of the upper Depth to Bottom of Stratum (ft) [m] 20 [6.1] 29 [8.8] 40 [12.2] 90 [27.4] 125 [38.1] 170 [51.8] Soil properties for design of drilled shafts at pier WB-13 Avg Friction Angle (deg) ignored N/A N/A 39 N/A N/A Avg Undrained Shear Strength (psf) [kPa] N/A ignored 2,900 [138.9] N/A 5,100 [244.2] 7,400 [354.3]
