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The Domain at Phipps Plaza is a luxury condominium development in the well-traveled and populated urban area of Buckhead in Atlanta, Ga. This 319-unit mid-rise residential complex is within a mixed-use area that incorporates hotels, office space, high-end shopping, theaters and restaurants.
This condominium complex has a 610-foot (185.9 m) long mechanically stabilized earth (MSE) retaining wall that borders the fill section supporting the building structure and an adjacent parking facility. The MSE wall varies in height from 4 to 29 feet (1.2 to 8.8 m) tall. In mid-2017, stress cracks in the blocks were observed at various locations and heights and in a consistent fashion, and visible bulging was evident in the wall.
It quickly became apparent to the complex owner that to keep residents safe and ensure no disruption to this densely populated area, a structural assessment was required. Equally important to the owner was that any necessary structural repair would be performed with minimal disruption to the residents and adjacent businesses while also maintaining the high aesthetics that those in the area appreciate.
The owner engaged a local engineering firm who determined that an immediate structural repair of the entire MSE wall was needed. The owner also hired a structural engineering firm and required third-party owner representation during the design and construction phase to ensure an immediate repair was achieved while ensuring long-term building and site integrity.
After interviewing multiple firms and reviewing multiple design-construct proposals, GeoStabilization International was ultimately entrusted to design and construct this emergency repair project.
Design-build solutions
After multiple site visits and analyses, GeoStabilization’s integrated team of licensed geologists and geotechnical engineers moved into the design-build phase. Design-build solutions are customized to the varying demands, constraints and unique characteristics of a given site. This approach offers many advantages to owners and contractors, including lower total project costs and reduced construction time due to consistent and continual in-office and on-site collaboration with the entire owner-driven team of experts.
During the design phase, the team immediately identified potential challenges and constraints, and coordinated logistics to deploy the appropriate construction crews and equipment. At the same time, failure modes were being confirmed in conjunction with repair methodology to determine the full scope of work for the project.
Soil nail technology
Knowing there was no desire to vacate the building or to inconvenience the residents, GeoStabilization had to find a solution that would allow for safe construction within the significant space constraints while not hindering the construction of the high-rise condominiums only 20 feet (6.1 m) away from the northwestern side of the repair. Due to these challenges, the existing retaining wall had to remain in place while the site was stabilized. Conventional construction methods, therefore, were not a viable solution. Because of these factors, GeoStabilization embraced soil nail technology as the primary construction method.
The repair involved installing a matrix of grouted soil nails varying from 15 to 40 feet (4.6 to 12.2 m) long in a pattern along the many faces of the 610-foot(185.9 m) long wall. The existing wall retains foundation material for the building and the adjacent parking structure, and the wall repair extends the entire length of the southwestern face along Longleaf Drive Northeast in Atlanta. The repair along the northwestern side within the alleyway adjacent to the new high-rise was 160 feet (48.8 m) long with approximately 30 feet (9.1 m) of repair needed along the southeastern side of the property. Once the soil nails were in place along these three sides, a steel-reinforced shotcrete facing was installed.
Soil nails combined with a shotcrete facing countered the forces associated with the retained earth wanting to push out into the surrounding streets. The primary action of each nail is to develop a force of friction between the injected grout around the nail and the surrounding soil. This friction develops along the length of the nail, and then all nails are “tied” together through a steel-reinforced shotcrete face.
Because the rough texture and gray color of the standard shotcrete face was not a look conducive to the aesthetics in this area, the owner chose a geological finish for the final treatment of the shotcrete. To achieve this look, the design incorporates an additional thickness of wall beyond the structural requirement. This approach allows for “sculpting” of the shotcrete while still wet such that after it cures and is stained, it resembles natural rock.
Site obstacles
For GeoStabilization, which has worked in virtually every geological setting with the added real-world experience of constructing more than 6,000 projects, the design was the easy part. The challenge for this site, however, was in physically getting the soil nails where they needed to be to provide the needed stabilization. The number and location of the soil nails were critical in order to provide the needed factor of safety to protect the building, its residents, and the surrounding structures and general public from a catastrophic building collapse.
The Domain at Phipps Plaza had significant underground utilities to support the conveniences typical for today’s tenants to live and work at home.
Although these utilities are nicely secure below ground and out of sight, they do pose a challenge when there is a need to introduce stabilizing elements, up to 40 feet (12.2 m) long, within the same earth supporting the building and its inhabitants. It is essential for a designer and contractor not to disrupt or negatively affect any of these buried utilities.
While in the design phase and well into the construction phase, GeoStabilization was able to work with the owner’s team to be guided by the efforts of three different utility location service companies to physically locate conflicting utilities through minor and moderate excavation methods. The results of three different companies, however, were not the same.
To increase the odds of not striking a utility, ground-penetrating radar (GPR) was also utilized. GPR is a minimally invasive method that uses radio waves to capture subsurface images that aid in determining the horizontal and vertical locations of utilities. Unfortunately, these results were less than favorable due to the presence of the layers of geogrid material that is the mechanical element of the existing MSE retaining wall system. The utility location efforts all had their challenges, with varying results. Through the combined effort and collaboration among the owner’s total team of consultants, design-build contractors and various subcontractors, GeoStabilization was able to avert any utility strikes.
However, during the grouting process of the soil nailing—in which a flowing grout encapsulates the soil nail for its full length in the earth—grout entered an opening in a buried conduit. This can happen when there is an existing compromised joint in a pipe or electrical conduit in the same area. Fortunately, there were no electrical issues or loss of power for the residents.
Safety
Because the complex was part of a highly urbanized, well-traveled area with ongoing vertical construction of the adjacent high-rise project, ongoing site and construction safety was paramount. Throughout the duration of this project, many building occupants and visitors were interested in walking around the construction site and inquiring about the work being done. The team was also challenged by vehicular traffic ignoring construction signage as they tried to find a quicker way to reach their final destination.
To ensure safety throughout the entirety of this project, the team activated an on-site communications system, conducted regular equiptment checks and consistently followed protocols outlined in the written safety plan. Crew members were diligent about keeping pedestrians off the property and redirecting vehicular traffic.
Results
There are many construction methods available to achieve the same end result, but sometimes site challenges and constraints limit what can really be considered a viable solution. In this instance, soil nail technology made the most sense to pursue, and to pursue immediately. If left unrepaired or delayed by conventional construction, the wall was at risk of “blowing out” in the bulging section, necessitating a more significant repair. Leveraging soil nail wall technology avoided the additional construction time and costs associated with conventional repair methods that would have been inappropriate for this site. Equally important, this soil nail repair allowed for minimal impact on residents while minimizing potential displaced income of the adjacent businesses dealing with road closures around a critical area construction footprint.
The project was completed in less than eight weeks, with minimal disruption. None of the condominium residents were displaced during the repair process, and local businesses at the adjacent Phipps Plaza Mall had only minor disruption during this significant emergency repair project.
GeoStabilization utilized soil nail technology as the stabilization component to rehabilitate this MSE wall. Drilled and grouted holes effectively created a matrix of reinforced concrete tension elements that keep the wall stable through the interface of the soil nails with the reinforced shotcrete wall face. This is a different technology and design methodology than the typical MSE wall system, which uses geotextiles/geosynthetics or geogrid connected to a facing element that retains compacted engineered fill. Traditional MSE design follows tieback wedge methodology where each reinforcement layer will vary in height as a function of backfill material type and other factors, and the design length of the inclusion is a direct function of the strength of the geosynthetic material.
GeoStabilization does use a considerable amount of geotextiles or geosynthetics in its many repairs all over the United States. In fact, closely spaced geosynthetic inclusions separating layers of compacted aggregate become the primary design components in this highly stable and free-draining wall system that GeoStabilization uses to build back a failed slope, county road or interstate roadway. This system is referred to as a geosynthetically confined soil (GCS) or geosynthetically reinforced soil (GRS) by the Federal Highway Administration (FHWA). The tightly spaced geosynthetics confine the reinforced fill to go from a dilation mode of failure to a through-the-particle mode of failure. These confined soil walls are incredibly strong, flexible and resilient. The confined mass is also internally stable, eliminating or minimizing the forces exerted on the facing elements. For many roadway projects, soil nails are utilized for shoring or to stabilize the failed earth, while a GCS wall with geosynthetics is used to build the earth back to its condition prior to failure.
Steve Poole is a senior engineer at GeoStabilization International (GSI). He has over 25 years of experience with public and private markets providing geohazard assessments and construction repairs along with bridge, culvert, engineered slope, and wall solutions from the concept phase through final plans and construction.
All photos courtesy of GeoStabilization International
Project Highlights
SOIL NAIL TECHNOLOGY FOR MSE RETAINING WALL
OWNER: Phipps Plaza Residential Partners, LP
LOCATION: Atlanta, Ga.
GENERAL CONTRACTOR: Amicus Construction
PRINCIPAL: Robert Shaw
PROJECT DEVELOPMENT ENGINEER: Reid Bailey, P.E.
REGIONAL ENGINEER: Dru Miller
PROJECT MANAGER: Jared McDowell