Atlantic Station, Atlanta, Georgia: A Case Study of Design Build Foundations
CFA Piles, Drilled Shafts, Driven Piles & Concrete Work
The redevelopment of a 27-acre tract of land formerly occupied by the Atlantic Steel Mill operation for over 85 years required a unique approach to foundation design. The new development comprised an extensive area of multi-level parking, with low to mid-rise retail structures extending above the parking deck. In addition, separate mid-rise residential and low-rise retail structures were proposed along with a bridge structure providing access to the site. The presence of layered concrete debris, slag, and other fill materials to depths of 50 feet presented considerable challenges to foundation support.
Morris-Shea proposed a lump sum design-build option that included additional site evaluation, extensive load testing, and the use of four different foundation types to support the proposed development. The Morris-Shea option allowed flexibility to meet variable site conditions and protected the developers from cost increases resulting from such variability.
The Atlantic Station project is located in metropolitan Atlanta, slightly north of Georgia Tech University. Steel operations had been on-going on the site from 1905 through 1990. Wire milling operations continued until 1998. As a result of these activities, slag, coal and ash fill materials are present over most of the site in addition to concrete and steel debris from the building structures and furnaces.
The proposed development consists of several layers of parking with low to mid-rise residential, office and retail structures extending above the parking deck. Column loads are on the order of 1000 to 3000 kips. High lateral, tension and moment loads were associated with elevated roadways.
Separate low and mid-rise structures are proposed for other areas of the site, in addition to a bridge structure to provide site access from across Interstate 75.
The site is in the Piedmont Geologic Region, an area underlain by igneous and metamorphic rocks, typically overlain by weathered residual soils. A transition zone termed “partially weathered rock” is normally found overlying bedrock. Overlying the weathered rock are a sequence of highly variable fill materials, including slag and concrete rubble, alluvial and residual soils.
To supplement the existing project geotechnical study, Morris-Shea completed additional Standard Penetration Test (SPT) borings, cone soundings, probing and instrumented drilling to better assess the subsurface soil conditions.
After completion of the supplemental work, which included drilling at every column location, the existing and new information were compiled to generate a rock-head profile of the entire structure area. In addition, areas of obstructions and debris were identified prior to foundation construction, thereby eliminating delay potential.
Four main types of foundation options were utilized depending on the load conditions (axial compression, tension, and lateral load), rock elevation, and soil profile.
Spread foundations were used where the rock was relatively shallow. Design was based on bearing pressures and minimum dimensions provided by the project geotechnical engineer.
In areas where high column moment loads were present, pile caps and driven H-piles were used. Piles were typically driven to rock. Due to the presence of hard cemented layers within the PWR, some piles refused above “intact” rock. Piles tipped in PWR could easily be identified in the field by pile rebound as a result of higher tip quakes.
A total of three static load tests and 39 dynamic tests, using a Pile Driving Analyzer (PDA), were performed to verify pile capacity across the site and determine installation criteria. Delmag D-19 and D-25 hammers were used for test pile and production pile installation.
Layers of concrete and slag debris were encountered in some areas, requiring several phases of probing and excavation.
As the drilling and probing was completed, the rock-head contour was updated. Test and production drilled shaft information was also used to verify and update the rock-head model.
Where the rock was sufficiently deep, and tension and lateral loads allowed, CFA piles or drilled shafts were used. Two 1000 ton static load tests were performed on 24” CFA piles socketed differing lengths into the PWR. Multi-layered strain gauges were used to allow determination of unit shaft friction in the differing soil strata.
One full scale static load test was also performed on a drilled shaft socketed onto “intact” rock. To provide uplift reaction, a total of 8 rock anchors were installed, allowing end bearing pressures in excess of 600 ksf to be developed.
The load test data was used to generate unit shaft and end bearing values for shafts and CFA piles, and formulate installation criteria.
Approximately 75 rock footings were excavated, formed and poured by Morris-Shea.
Production driven piles comprised HP 12X53 grade 50 H-piles, with allowable loads on the order of 120 tons. A total of approximately 1300 piles ranging between 8 and 60 feet long were driven at an average production rate of 25 piles/day.
Approximately 200 CFA piles were installed to depths of up to 60 feet. Pile diameters ranged from 24 to 35 inches. In addition, 110 drilled shafts were installed with diameters of 46 to 72 inches. Due to the high rock strength, high torque CFA and drilled shaft equipment was used.
Fixed mast Hitachi rigs with hydraulic turntables were used for CFA piles. Hydraulic outriggers on the rig and a front foot provide rig stability enabling up to 35 tons of crowd to be applied during drilling.
A Bauer BG-36 was used for shaft installation. With available drill and casing torque of 36 ton-meters, in addition to 45 tons of crowd, the required penetration through rubble, PWR and into “intact” rock can be achieved, using suitable tooling.