When confronted with unstable soil conditions, engineers generally respond with a piling system which is either auger-cast, drilled-shaft or hammer-driven. However, when those same soil instabilities exist and restrictions on noise or vibration render those techniques unsuitable, engineers are forced to seek out innovative approaches to get the stability they need.

On a recent project on St. Simons Island off the Georgia coast, with stable soil anywhere from 40 feet to 50 feet below grade, Coastal Foundations, Inc. installed hundreds of 3-1/2-inch helical piers — sometimes referred to as screw piles — to provide that solution. The result is a foundation designed to support loads in the 40-plus-ton range — plus peace of mind that settling, once a major concern, is now a non-issue.

Developments on Georgia's St. Simons Island generally consist of highly upscale dwellings with hefty price tags well into the seven- (and eight-) figure range. Located as it is on Georgia's marshy coastline, it is obviously not well-suited to traditional foundation construction techniques.

Recently, for example, a St. Simons Island resident wanted to build a large multi-structure project on the island and turned to engineers to develop a foundation system that would support the desired structures. The overall project included a main dwelling which by itself is in excess of 14,000 square feet, an adjacent spa, guest houses, a boat house, a sizable in-ground pool, and a sea wall.

With plans in place, general contractor Johnson Partners, LLC, began consulting with subcontractor Coastal Foundations about what approach to take. Coastal's owner Jeff Addison suggested piers from Helical Pier Systems, Kaledon, British Columbia, Canada, in which the couplers are welded to the shaft and offer a significant increase in strength.

Addison notes that the full scope of the undertaking at St. Simons was equaled only by the magnitude of the pier work itself. In total, the project called for 11,755 feet (that's about 2.2 miles) of helical piers to be installed; because of the nature of the soil upon which construction was taking place, every single project element needed pier work to ensure stability.

Consider the sea wall, for example. Designed to protect the site from tide swell and erosion, the sea wall called for in the overall design is a 500-foot-long structure constructed of 33-foot by 4-foot precast panels. Each 7.5-ton panel was jetted into place using a high output diesel-powered pump. Once in place, tie back anchors were installed on 8-foot centers — a total of 63 in all — and cast into a concrete cap.

"On the sea wall, piles are installed at a 13-degree angle except in the pool area where we went to 25 degrees to get additional depth," Addison says. "The same held true for an area where there are some mature oaks and we wanted to avoid getting tangled up in the roots. In addition, at both ends of the structure there is a straight wall meeting a return wall, so the two piles on the return wall went deep while the other two came over the top. The tolerances were extremely tight. That precaution paid off nicely in an uninterrupted workflow."

While the sea wall was challenging in its complexity, sheer numbers made the pier portion for the main house and guest houses a formidable task. Demanding nearly 240 piles drilled to depths of 40 feet, 45 feet and 50 feet, it was this facet of the job that, says Addison, could have been the most problematic.

"Anytime you are dealing with that kind of volume," he says, "any number of things can go wrong. Yet we were averaging 10 piers per day with each pier consisting of at least four 10-foot sections."

According to Addison, the piers themselves are 3-1/2 inches in diameter with a wall thickness of 0.254 inches and a 12-inch, 14-inch helix configuration.

"Adding sections, which could have been a cumbersome process, went as smoothly as I could have ever imagined," he says. "They were definitely designed and constructed for durability and ease of installation."

On this project, Coastal utilized a Bobcat 435 mini track excavator outfitted with a Pengo Model MDS-12K drill head from Pengo Attachments.

Side-loading the machine and drill is a particular concern in this type of application, he says.

"We attach sections under pressure," he explains, adding that this can present a risk of blowing out the seals or harming the bearings. "I specifically purchased the Pengo for this facet of the business," he continues, "and this is the first real test I've put it to."

The drill, he notes, provides 12,000 foot-pounds of torque.

"Until it hits 40 feet," he says, "it holds steady at about 1,000 psi, almost as if doesn't even know it's got a hold of anything. That all adds up to nice productivity on a very demanding part of the job."

The project at Hawkins Island was originally designed for 14-inch auger-cast piles, and Johnson Partners wanted to approximate the same surface area on the 3-1/2-inch piers as those piles would have provided. To meet that requirement, Johnson Partners came up with an innovative solution. Once the piers are in place, surveyors establish an elevation. Excess pipe is cut off, and a 12-by-12 plate is welded onto the pier at that elevation.

With the sea wall in place and all the piers set, foundation construction was started. Workers excavated the area around each pier section to a specific elevation, lowered a rebar cage and set the forms according to design. A concrete pump, located at the center of the job, then poured a radius around the truck, forming the foundation walls and making the project ready for subsequent work.

"This has really been an excellent project, both in our portion and beyond," says Addison. "This was a unique approach to this kind of construction but one that I think will, in the long run, prove very beneficial."