The fill generally consisted of silty sand with rock, concrete, brick, scrap metal, slag, organics and/or other deleterious debris. A test pit study consisting of six pits encountered slag masses ranging in size from 2 in to 50 in (50 mm to 1,270 mm) in diameter and concrete boulders of similar size. The test pits were generally terminated in the fill materials due to reach of the excavator and, in one case, refused in the fill. The rock profile across the site was quite variable as well, ranging from being less than 2 ft (0.6 m) in some areas to almost 70 ft (21 m) in the deepest areas. Foundation Recommendation The original foundation recommendations from the geotechnical report included possible options of cut-and-replace across the site, rigid inclusions, micropiles, auger cast piles or drilled shafts. These approaches assumed that pursuing auger cast and drilled shaft options would also involve significant undercutting and replacement due to concerns of raveling in holes during excavation. The report also expressed concern about drilled shafts and/or piles being able to penetrate the debris-laden fill. Micropiles had been recommended as the best opportunity of penetrating sub- surface materials. The initial permit plans for the project included approximately 1,000 micropile foundations of 8 in or 10 in (203 mm or 254 mm) diameter with 150 ton and 200 ton (1,495 kN and 1,993 kN) capa- cities, respectively. Extensive and timely load testing programs for verification testing, compression, tension and lateral Slag “overpour” debris near Fowler St. Atlantic Yards rendering testing were specified. Approximately 46,000 lft (14,020 m) of micropile installation was estimated to be needed. Although micropiles were chosen at that time, many questions and concerns still remained about the success of these foundation elements. That included concerns about the piles’ ability to pene- trate slag masses and numerous other problematic scenarios that could be encountered during micropile installation. Upon review of the plans and subsurface data, deep foundation and excavation bracing subcontractor ABE Enterprises of Kennesaw, Georgia, determined that segmentally cased drilled shaft foundations, which are relatively rare in the Atlanta area, could provide marked cost savings and other benefits. Segmental-style casing uses the same diameter casing for the full depth of a hole down to refusal or design elevation. The casing consists of thick-walled steel that is bolted together in 6.6 ft and 16.4 ft (2 m and 5 m) segments. The casing was extracted in early 2019 with a drill rig that used a drive- head adapter to install and remove the casing. There are numerous benefits to this type of casing, which include: • Less space needed to store tooling/ casing on site. • Casing can be installed and removed with a drill rig, eliminating the need for a crane for pulling casing. 84 • DEEP FOUNDATIONS • JULY/AUG 2020 • Greater drill depths than typical can be achieved easily by adding more casing segments. In comparison, telescopic casing carries the risk of having to start a hole over from the top, as well as of needing to wallow out a larger diameter and use a larger casing diameter to telescope down to the design elevation and/or to bedrock. • Less concrete usage. Traditional telescopic casing also would not have been feasible due to the loose nature and raveling potential of the debris fill. Review of the test pit study data offered the best picture of the potential debris that could be encountered. Construction crews on previous projects in Atlantic Station had encountered buried crane bodies, engine blocks and other large items, so that an assumption was made in the foundation proposal of encountering similar debris. ABE prepared a preliminary take-off and design for drilled shaft foundations using 3.9 ft, 4.9 ft and 5.9 ft (1,180 mm, 1,500 mm and 1,800 mm) diameter shafts bearing on 150 ksf (7,182 kPa) rock. This proposal was presented to the owner and design team by the general contractor, New South Con- struction. The risk factor for the foundation installation was one of the biggest questions. Areas of the site were designated as either high risk or low risk areas based on the anticipated depth to rock and debris-laden fill thickness. Areas where the fill thickness
