and average about 250 kips (1,112 kN), while the column loads for the towers range between 730 and 2,800 kips (3,247 and 12,455 kN) and average about 1,280 kips (5,695 kN). In addition to the significantly different column loads, the towers are subject to uplift loads while the podium- garage is not; therefore, the foundation design for the podium-garage and two towers had significantly different design criteria, requiring different solutions. Subsurface Investigations and Conditions During the concept design phase, a desktop study evaluated records from 11 soil borings advanced to depths ranging from 27 to 32 ft (8.2 to 9.8 m) below grade that were performed across the site. These shallow borings identified miscellaneous fill above medium-dense to dense sands. Although compressible materials were not observed in these records, local knowledge of the geology predicted that compressible Coney Island Clay layers should be expected and could be found at significant depths below grade. In 2016, Langan performed a subsurface investigation to investigate the presence of the compressible layers and to achieve the minimum requirements of the New York City Building Code (NYCBC). The subsur- face program included 16 soil borings with standard penetration testing (SPT), 8 cone penetrometer test (CPT) probe soundings, and the installation and monitoring of two groundwater observation wells. Exploratory probes were advanced to various depths, with a maximum depth of about 200 ft (61 m) below the ground surface. The subsurface investigation revealed deep compressible layers with a very soft clay layer located at about 65 to 100 ft (20 to 31 m) below grade. The subsurface conditions were found to be generally uni form, consist ing of a loose to medium-dense uncontrolled fill, underlain by interchanging layers of sand and silt- clay. A broad laboratory testing program (water content, mechanical grain size, hydrometer, organic content, Atterberg limits, incremental consolidation tests and UU triaxial tests) was performed to define the soil parameters. The site is located about 500 ft (152 m) north of the Coney Island shoreline and is outside the limits of moderate wave action. As such, the groundwater level is moder- ately influenced by tidal fluctuations and varies between about 7 and 9 ft (2.1 and 2.7 m) below ground surface, as observed with continuous data loggers placed within the observation wells. Foundation Design and Testing Shallow foundations incorporating spread footings beneath the podium-garage and a mat foundation below the towers were initially considered. However, this option was abandoned when the design team recognized that differential load demand and the presence of the compressible layer would result in excessive total and differential settlements; that is, the estimated total settlement below the towers exceeded 6 in (15 cm). Therefore, a hybrid foundation system consisting of deep pile foundations beneath the towers and spread footings to support the podium-garage was selected. Summary of soil strata thickness and strength Layer Misc. Fill Medium to fine Sand (Upper) Organic Clayey-Silt Medium to fine Sand (Lower) Silty-Clay Medium to fine Sand Medium to fine Gravel Medium Sand Approx. Elev. (NAVD88) Top ft [m] GS 5 [+1.5] -60 [-18.3] -98 [-29.9] -114 [-34.8] -133 [-40.5] -169 [-51.5] -178 [-54.3] Bottom ft [m] +5 [+1.5] -60 [-18.3] -98 [-29.9] -114 [-34.8] -133 [-40.5] -169 [-51.5] -178 [-54.3] Avg Layer Thickness ft [m] 2 – 10 55 – 65 35 – 40 15 – 25 15 – 20 30 – 40 10 – 20 > 15 GS = ground surface; WOH = weight of hammer; WOR = weight of rods 90 • DEEP FOUNDATIONS • SEPT/OCT 2018 Avg SPT N-Value (blow/ft) 12 25 WOH 23 WOR 26 >100 40 Pile Design The tower’s pile foundations were designed to accommodate the relatively high tower loads as well as considering down-drag from the fill surcharge and influence from adjacent footings on nearby piles. This was done because the development plan involved locally raising grades around the towers to be above the design flood elevation and because the podium-garage was to be supported on shallow foun- dations. A three-dimensional finite ele- ment analysis was performed to determine the influence of the footings on the adjacent foundation piles. In addition, the results from dynamic pile testing data (e.g., output from the pile-driving analyzer (PDA) and Case Pile Wave Analysis Program [CAPWAP]) were used to deter- mine skin friction parameters and estimate pile downdrag. To understand and evaluate the increase and dissipation of excess pore water pressure in the compressible upper clay layers during pile driving, CPTs were performed adjacent (within 3 ft or 0.9 m) to the test piles prior to driving, within 24 hours of driving and about three weeks after driving. The results from the CPTs did not show evidence of material strength increase and dissipation of excess pore water pressure induced by the pile driving. Avg. Shear Strength, Su (tsf) 0.8 1.6 Pile Load Test Program Langan recommended a load test program, which exceeded the minimum NYCBC requirements, to optimize the design of the piles and to achieve a constructible solution. The load test program consisted of both static and dynamic tests, performed in two phases. The first phase was performed during the design dev- elopment to evaluate the drivability and geotechnical capacity of several pile options and to provide in-situ data required to finalize the pile design. The second phase was performed prior to the driving of production piles to evaluate the contractor’s proposed modifications to the pi le thickness and splicing details.
