Vibro-Displacement Puts School on Solid Footing
In a rare application for the New York City market, vibro-displacement technology has created a solid base upon which to build the $75-million Spring Creek School for the New York City School Construction Authority.
Limited greenfield sites exist for new schools, but a former landfill in a growing Brooklyn residential area provided enough space for a structure comprising two middle schools and two high schools. However, the uncompacted landfill, dating back to the 1960s, could not support the four-story, 154,000-sq-ft school’s loads using traditional techniques.
Before its designation as a landfill, the site was part of an estuary. During a subsurface investigation, the architecture and engineer STV Group Inc., New York City, found up to 15 ft of landfill atop marshland, with organic peat and sand extending several hundred feet. It had a half-ton bearing capacity for static conditions, says STV Group geotechnical engineer Nitikhun “Nicky” Nitichaivorrakul.
STV considered various solutions, including steel piers, before settling on vibro-displacement technology to increase the bearing capacity. The technique—adding stone columns below grade and using vibrations to compact the stone—is common in other parts of the country with poor soils, such as Florida.
“Crushed stone is cost-effective compared to conventional piles,” Nitichaivorrakul says.
New York City records indicate the cost of installing the stone columns is at $3.9 million. Harris C. Feinn, STV vice president and project manager for the Spring Creek School project, estimates the technique saved the project “millions of dollars.”
Plaza Construction’s New York City office began the project in 2009 and is more than halfway complete. Both the middle schools and the high schools are scheduled to open in fall 2012. Classrooms are located on the third and fourth floors; on the first and second floors, the two schools will share a cafeteria, gymnasium, library and other common spaces.
Intercoastal Foundations and Shoring, Rockville Centre, N.Y., performed the vibro-displacement work, installing approximately 1,600 three-ft-dia stone columns, seven feet apart center to center, to depths of 50 ft using a dry-bottom feed. For the majority of the columns, the company used a vibratory probe to open the column hole to create a lateral load on the soil adjacent to the column. Crushed stone is dumped into a hopper that is attached to a feeder running the length of the probe, and the stone comes out the bottom.
“You bring the probe up in increments, and the stone will fill the void as you [extract] the probe,” says Peter Ladouceur, Intercoastal’s senior project manager. “Then you plunge the probe back into the stone to compact and widen the columns.”
Passing The Test
All borings and load tests were successful. A vibratory probe provided a horizontal impact, and a variable-moment vibratory hammer with an ABI Mobilram was used when ground conditions proved more difficult.
Intercoastal placed 20,000 cu yd of virgin crushed stone obtained from Tilcon New York, West Nyack, between February and June 2010.
“The idea is not to remove material from the site but to import additional material,” Ladouceur says. “They are trying to input an additional 16% of material into the volume you already have.”
Vibro-displacement stone columns not only improved the soil-bearing capacity at the column locations but also improved the ground-bearing capacity between the columns. It reduced noise and vibration issues in neighboring residential areas as well.
Langan Engineering & Environmental Services, Elmwood Park, N.J., provided controlled inspection of load testing and boring operations after completion of the vibro-displacement. The three load tests and 44 boring tests showed the soil had “densified.” The soil now has a load capacity of three tons per square foot. During a plate load test with six tons, the ground held, Nitichaivorrakul says. The steel-frame building sits on concrete spread footings atop the stone columns.
Other local projects in which vibro-displacement has been used include an airport fire-department building for the Port Authority of New York and New Jersey, a bridge project at Jones Beach and a hospital in New Jersey. However, STV reports this is the first time the New York City School Construction Authority has approved the technique for a project.