This article was written by Philip H. Manavi and originally ran in the Deep Foundation’s July-August 2020 issue.
Drilled Shafts for Debris-Laden Sites
The Atlantic Steel Mill opened in 1901 in the heart of what is now Midtown Atlanta. The plant would close in the mid-1970s, but remained nominally operational due mainly to unexpected costs to remediate soil contamination. The Jacoby Group became the property developer in the mid-1990s and originated the idea of redeveloping the 138 acre (0.40 ha) site into what would become Atlantic Station. The original plan would include some 15 million sq ft (approximately 1.4 million sq m) of retail, office and residential space and 11 acres (4.5 ha) of public parks with a “live, work, play” vision. The footprint was so large that it received its own zip code.
Considered the nation’s largest urban brownfield redevelopment in 2005, construction at Atlantic Station continues to this day. This article focuses on development by Atlantic Yards Investors of new office space on a 2.7 acre (1.1 ha) parcel known as Block C. The new Atlantic Yards office building is under construction at the site of a paved parking lot and will include a 3-story garage near existing grades across the entire property. The garage was designed to have a plaza level on its top. The plaza deck provides the foundation for the office building consisting of a 10- and a 5-story tower. The towers will be split by a plaza with plantings and a road entrance from 17 Street (an elevated street), with construction expected to be completed in 2020
Topographic maps of Atlanta from 1928 indicated that a deep swale and creek once crossed most of the southern portion of the site. These features rested approximately 40 ft (12 m) beneath the existing grades prior to construction. While no exact records were available, it was rumored that the swale had been filled in years ago with all sorts of debris in the form of slag masses/boulders, rock and concrete boulders, and miscellaneous steel trash and debris. The most recent borings and test pits performed by NOVA Engineering encountered fill in all the borings to depths of up to 23 ft (7 m). None of the 12 borings nor the 61 offset borings in the geotechnical study, except one, were able to penetrate the fill layer beyond the 23 ft (7 m) depth.
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.
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 subsurface 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) capacities, respectively. Extensive and timely load testing programs for verification testing, compression, tension and lateral 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 penetrate 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 drivehead 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.
• 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 Construction. 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 or depth to reach rock was less than about 25 ft (7.6 m) were determined to be low risk due to assuming that an excavator could assist with removing possible obstructions. High risk areas were any in which the fill was deeper than 25 ft (7.6 m) and where other measures might be necessary in the event that impenetrable debris was encountered. A contingency was also included in the subcontractor’s scope of work for installation of offset shafts with a grade beam if areas were encountered that were undrillable for any reason.
Over the course of several more pricing exercises and meetings with the owner, general contractor, design team and geotechnical engineer, it was determined that the project would be redesigned to be based on drilled piers and ABE’s tooling and drilling capabilities using the pier sizes described above, bearing on 150 ksf (7,182 kPa) rock. The foundation package would eventually include 148 drilled shafts of the aforementioned sizes.
Foundation installation took place during the first two quarters of 2019, and involved BAUER BG30 and BG42 drill rigs equipped with continuous segmental casing. The debris encountered ultimately fit somewhere in the middle of what had been anticipated, as no crane bodies were encountered. However, slag and steel masses the size of automobiles were encountered in several locations. The slag masses were so massive in many cases that they could not be transported or moved and had to be reburied on site. Numerous miscellaneous large steel components such as hooks, engine blocks, steel rails, steel tracks, steel cogs and more were uncovered. Thick layers of slag gravel were often encountered as well. Furthermore, much of the debris was found to be buried just atop the bedrock throughout much of the site. The smaller debris and unusable fill materials were separated and sieved by a grader and removed offsite.
The slag debris was found to be extremely difficult — if not impossible — to drill through. The smaller and/or thinner pieces were penetrable to some degree with the tooling, but not without severe wear and tear over the course of the project. The most successful approach of those recommended during the preplanning process proved to be pre-excavating each pier location prior to drilling. Further challenges included the depth of the debris in some cases. In one case, large rock boulders were encountered beyond the reach of the excavator. The hole was drilled with oversized segmental casing and the rock boulders were cored through. This allowed the design size segmental casing to be installed through the deep obstructions. The contingency plan to offset drilled shafts was not ultimately deemed necessary.
A total of approximately 4,800 lft (1,463m) of earth drilling and 107 lft (32.6m) of rock drilling was completed. The variable depth to rock across the site required that some amount of rock coring took place to reach minimum required pier lengths and to resolve large lateral load requirements in the foundation design.
A drilled shaft foundation was successfully installed despite numerous challenges. Addressing debris-laden site conditions at Block C of Atlantic Station involved the use of segmental-style casings, which provided numerous benefits. They include a decreased need for storage space, the ability to drill deeper and to use less concrete compared to the traditional telescopic casing method, and reduced costs compared to the original micropile approaches considered. This case study confirmed that, with a combination of the right tooling and equipment, and some “tried-and-true” pre-excavation methods, even some of the most difficult subsurface site conditions can be overcome.
Acknowledgements: Hines served as the owner for the project. New South Construction served as the general contractor. Uzun + Case provided the final structural design, and NOVA Engineering provided geotechnical site investigations and drilled pier design parameters.
Philip Manavi, P.E., is a senior geotechnical engineer with ABE Enterprises in Kennesaw, Georgia. He has 10 years of experience in the design and implementation of deep foundations and shoring systems and 19 years of experience in the geotechnical field.