by Patricia McDaniel Hamilton
Wood pilings are a time-tested foundation. In fact, they have been discovered intact below major buildings of the Roman Empire, They are also the foundation of choice for most of the homes our company builds along the coast of Delaware.
A home built in any flood zone must be elevated above the base flood level, whether it's on pilings or a masonry foundation. For homes built in a Flood Insurance Rate Map flood zone rated for "velocity wave action" (V zone), piles or columns are required. For many other homes in less exposed zones along the coast, piling foundations are recommended but not required. We prefer to use piling foundations on all coastal homes, and even on inland homes where the soils are sandy or the site marshy.
Although steel and concrete pilings are available, most builders in our area use wood pilings for homes. They come as either round, peeled logs or square-sawn 8x8s and 10x10s. The preference for one shape or the other seems to vary by region. In our area, round poles from tree farms in the Carolinas are most common.
|Well-braced pilings and engineered framing connections protect coastal homes from high water and hurricane winds.
The typical piling we use is a Class B 12-3-8 CCA-treated (1-lb. retention) southern pine, 20 to 60 feet long. The "12-3-8" denotes that the diameter 3 feet from the butt is 12 inches, while the tip diameter is 8 inches. We avoid creosoted pilings because the sawdust can burn exposed skin. Also, the noxious odor of the creosote will persist for years - not acceptable if sections of pilings are enclosed within the living space.
Piling length. Pilings resist loads by surface friction against the soil, not at the base the way a post on a footing does. So the length of a pile is determined by the depth necessary to resist vertical uplift and horizontal loads, as well as the potential for erosion, or scouring, around the pile at grade, the ground elevation, and the base flood elevation. Since soil conditions vary, we try during the design stage to speak to a pile driver who has driven piles at a nearby site to determine the expected bearing capacity. This can range from 10 to 20 tons per piling. In our region, most beachfront homes are designed for 15 tons. Sites with marshy soils along bays are designed for 10 tons.
Embedment. Another criterion that must be satisfied is embedment depth. It's occasionally possible to get the bearing you need without going deep enough to provide the necessary lateral support for the pilings. My engineer usually requires at least 20 to 25 feet of embedment.
Driving. Piles are driven into the ground, tip first, by a crane-mounted diesel- or air-powered hammer (see Figure 1). Our subcontractor uses a double-acting hammer, which automatically rebounds from each blow at the piling. The double-acting hammer is faster and usually more precise than a single-acting hammer, which must be raised by cable after each blow.
The installed cost of the pilings is about $8.50 to $9 per lineal foot of piling (not including the cost of notching pilings or setting girders). The average house we build has forty to fifty 30-foot pilings (around $12,000 total).
Although it is possible to statically test the bearing capacity of each pile after it is driven, this is also expensive. So as a matter of general practice, the bearing capacity is determined by counting the number of hammer blows required to drive the piling a given distance. The accepted formula for this is R = 2(Eg) / (S + 0.1), where R is the bearing in pounds, Eg is the force in psi delivered by the hammer, and S is the pile set per blow in inches. Eg is established by testing the operating rebound pressure at the hammer with a pressure gauge. Operating pressures range from 10 to 25 psi, depending on how fast the diesel motor is run. After the vertical bearing is determined, lateral bearing is figured at one-half the vertical.
We always keep a driving log for each pile, recording the number of blows per foot. Sometimes the building inspector may require that all or part of this log be kept and certified by a licensed engineer.
|By counting the number of blows required to drive the piling a given distance, the builder can determine the pile's bearing capacity. A driving log is usually kept to show the inspector.
Prepping the Site
Before pilings can be driven, the lot must be cleared and leveled. A pile-driver crane runs on tracks, but needs fairly level terrain for maneuvering and stability. Once the lot is cleared, we have a surveyor set a stake for each piling, and for the major corners of the house. If the final tip or butt elevations of the pilings vary, the stakes are color-coded.
The average charge to stake the pilings and set an elevation benchmark is $7 to $15 per piling, plus $75 for the elevation benchmark. Before the project gets its Certificate of Occupancy, the house has to be surveyed again for lot placement, first-floor elevation, and total height. We sometimes negotiate a package deal for all the surveying - usually $500 to $700 per house. Because the typical oceanfront lot is small and we're building within a fraction of an inch of setbacks, the expense of the surveyor is well justified.
A piling is then driven at each stake. The complications start here because pilings are rarely perfectly straight and underground obstructions may divert the piling from a truly vertical path. As the pilings are being driven, we align the tops, which are often 10 or more feet above grade, by eye. Slightly deviant pilings can be realigned with some effort - by digging out the top with the backhoe and pushing it sideways, for example - but wildly crooked pilings have to be redriven. After all the pilings have been driven but before the pile driver leaves, we use string lines to check the alignment of piles. The specs usually require the centers of the tops of all pilings to be within 3 to 6 inches of their staked location.