The Streets of Buckhead Are Supported with Shotcrete
For those not familiar with Atlanta, Georgia, Buckhead is a community that covers approximately 20-percent of the northern metropolitan area. Comprising some of the wealthiest neighborhoods, the area acquired its name from a tavern, long since gone, that prominently displayed a large stuffed buck’s head. Located along the prestigious Peachtree Corridor, a new development called The Streets of Buckhead will comprise nearly a million square feet of Class A office and retail space along with three world class hotels. Using a cost saving shotcrete method, an Atlanta-base geotechnical specialty contractor is shoring up a deep excavation for a below grade parking structure on the project.
ABE Enterprises, Inc. specializes in deep foundation and earth retention services including drilled piers, dewatering and soil nailing. Most recently the fifteen year old firm has added shotcreting to their portfolio in order to provide a wider range of solutions to fit their client’s needs.
Parcel A of the Streets of Buckhead project is larger than a city block and required soil retention along two sides measuring 350 x 450-feet of the 50-foot excavation which will eventually become five levels of a below grade parking lot.
Originally the specs called for a formed and poured wall that would have required temporary wood lagging. After discussions with the general contractor, ABE decided to bid the project using shotcrete – a method they had used on a large scale only once before.
“We have found shotcrete produces a better product with less labor and accompanying savings, “ explained Danny Brahana, ABE principle engineer, “And I think choosing the shotcrete method was influential in getting this project.”
The company’s bid was accepted by general contractors Balfour Beatty Construction US with offices in Dallas and Washington DC.
“The soil conditions in the Piedmont physiographic province of Georgia are igneous rock 50 to 80 feet deep, “according to Brahana, “Weathering has resulted in silty sands.” In order to provide structural walls to retain these soils, ABE decided to use a top down construction method in five-foot lifts. Since December, the company has placed 35,000 square feet of vertical surfaces that measure 10-inches in thickness and result in the finished interior wall of the structure.
Soldier piles were first driven to the full depth of the excavation at eight-foot intervals. After digging out between the piles, a rebar mat was installed in the center of the 5’ x 8’ x 10-inch opening that would become a wall panel. “Building from the top down, requires that you get a good joint between the lifts. We feel shotcreting is easier and provides a better seal than form and pour, “according to Brahana.
An eight man crew – a pump man, nozzle man, a worker to blow out shotcrete rebound, two hosemen and two finishers – conducts the shotcrete operation using a Schwing SP 750-15 all-hydraulic concrete pump. The pump is capable of 50 yards per hour output and can apply 1100 psi on the concrete. The Rock Valve-equipped SP 750-15 is powered by a liquid cooled Deutz 100 horsepower diesel engine. “We have tried several other pumps and the additional horsepower of the Schwing provides the right combination of volume without hose surge, “ Brahana said.
The hose man fills the void with the 4,000 psi shotcrete to a depth of about nine inches in one pass creating a “soft U” between the piles. The finishers screed it to within an inch of the front of the piles and then a finish coat is applied. The finish or flash coat is shot at a 45-degree angle which causes most of the aggregate in the mix to ricochet off the wall leaving a paste that is hand trowelled.
The two-inch slump shotcrete mix has to be just right for, according to Brahana, to allow sufficient build-up and smooth finishing. It is treated with an accelerator in cold weather and a retarder in warmer temperatures. The mix contains an innovative crystalline concrete waterproofing admixture that eliminates the need for am external waterproofing membrane. The mix is being delivered by Thomas Concrete with locations in Georgia and the Carolinas. They are serving the Streets of Buckhead project from a new plant in the area. The pump is placed at the bottom of the excavation in a depression to allow the truck mixers to lower their chutes to the best angle for the low slump mix to flow into the pump’s hopper.
After each lift is shotcreted, excavators hollow out the area under the completed lift, and the process continues downward. The crew has been able to average 1,000 square feet of completed wall in a day.
Adding shotcrete to their repertoire has been an easy move for ABE. The company also has a Schwing 1000 trailer pump that they use to pump auger cast pilings. “Pump ownership with the Schwing’s has been painless, “ states Brahana, “We use Schwing’s line pump manufacturing facility and factory store in nearby Marietta for accessories and service support. We are looking forward to a bright future competing with shotcrete.”
Project: Streets of Buckhead, Atlanta, Georgia
Owner: Ben Carter Properties, LLC, Atlanta, Georgia
Architects: Smallwood, Reynolds, Stewart, Stewart & Associates, Inc. Atlanta, Georgia
General Contractor: Balfour Beatty Construction US, Dallas, Texas
Concrete Contractor: ABE Enterprises, Inc, Atlanta, Georgia
Equipment: Schwing SP 750-15 trailer-mounted concrete/shotcrete pump