Impact Piers Solve Water Table

Story by Matthew Phair | September 28, 2010

Saratoga Springs has long been one of the big-money towns of upstate New York. If current trends continue, its place seems well cemented for another 100 years.

Current work in this ever-expanding city includes the Lexington Club, a $40-million project on 4.5 acres. Summer 2008 is the hoped-for completion date for the 80-room hotel. Also in the mix will be a complex that includes condominiums, townhouses, retail and office space, a restaurant/banquets facility, and a two-level garage. The 65,000-square-foot hotel adjacent to 110,000-square-foot building will have 69 condos plus another 45 condos also available to the hotel for rent.

Developer Andre Schmidt says that when the site was purchased it was just empty land with all the approvals in place. Former site owners had struggled with project financing.

According to Bette and Cring principal Pete Bette, his firm began construction in 2006 with pile driving. Sitework has included about 30,000 cubic yards of cuts and fills.

The condominium building and the parking garage are being supported on H-piles. Bette says that one of the most challenging aspects of the job was installing the 300 100-foot-long piles during the December and January winter months.

Because of the site's high water table, the hotel portion of the project is being supported on Geopiers from GeoStructures. The firm is a licensee of Geopiers, who is owned by parent company Tensar. Mike Perlow is the northeast regional manager of East Greenville, Pa.-based GeoStructures.

On the Lexington project, Geopiers were used to provide soil reinforcement and foundation support for the hotel and restaurant as well as the Condo wall foundations.

The GeoPiers foundation is known as a floating foundation. Compared to some other ground-modification systems, "we don't have to be as big in diameter," says Perlow. "We can also put grout/stiffen up our material when we encounter organics or other soft material."

The method was developed in the 1980s and has grown to become a floating foundation system to support lightly and moderately loaded structures.

Floating foundations do not extend completely through soft or compressible soil strata. Instead, these foundations consisting of a stiff composite layer penetrate sufficiently deep to distribute the applied load and reduce foundation settlement contributed by compression and consolidation of the underlying soft soils. Geopier elements are designed to create this stiff zone by increasing the composite stiffness of the subsurface soils within depths in which footing-induced stresses are the highest.

This floating foundation system consisting of very stiff, short rammed aggregate piers is unique with stiffness modulus values of 10 to 55 times greater than unimproved matrix soils. The design methodology does not require Geopier-reinforcing elements to extend to a "better" soil layer.

According to Perlow, a patented beveled tampering head is used for a displacement aggregate pier system. First a disposable bottom plate is set in preparation for installation of a displacement aggregate pier. Next, the hopper and pipe/tamper are raised and then vibrated/pushed to the design depth. Upon reaching the design depth, the hopper is readied for open-graded AASHTO No. 57 stone. The hopper is filled to create a 24-inch-diameter aggregate pier throughout the design depth. The ABI machine pulls the pipe up a distance of three feet, then vibrates/pushes the beveled head downward, compacting the 3 feet of stone to one 1 foot and creating a 24-inch displacement aggregate pier. The process is repeated until the pile is complete.

According to GeoStructures, the same system was used in 2001 on a superstore in the Philippines. The original design called for 6,500 square meters of suspended structural floor slab to be supported by bored piles. By adopting a Geopier floating foundation system, costly bored piling and suspended floor slabs were eliminated. This allowed the heavily loaded floor slabs to be supported by the Geopier soil reinforcement and designed as a slab-on-grade system.

This floating foundation system was designed to control the foundation and floor slab total and differential settlements to meet the project design criteria. A total of 1,900 Geopier elements with lengths of 3 to 3.5 meters were installed in 60 working days, reducing the project completion schedule by 60 days.

As of press time in early September, the hotel foundation is complete. Bette and Cring also self-performed creating the foundations, which required roughly 1,300 cubic yards of concrete. The first floor, concrete-plank deck of the hotel, is days away from being delivered.

Says Schmidt: "We're still waiting on attorney general on our condominium offering plan. If we get started soon, we'd like have it done by racing season. That would be optimal."