Structures Requiring Foundations - (continued)
Helical piles, or screw piles, are more accurate terms for a recent adaptation of an old construction technique using screw anchors. Screw anchors were originally used in poor soils, often with cable guy lines. The design of the screw anchor was modified to be used as a helical pile. Although technically incorrect, the term screw anchor is still used (figure 36).
Helical piles are now used to support anything from utility poles to large buildings built on poor wetland soils. They require special equipment and techniques to install. Many certified contractors are located throughout the country to allow for competitive bidding. Sometimes certified contractors will train volunteers to do the work. Helical piles are an excellent alternative to friction piles. They weigh less, are easier to install with portable equipment, and result in less ground disturbance. Their overall cost may be much less than friction piles. We recommend them (figure 37).
A helical pile includes a helical lead section and a beam saddle. The lead section is solid high-strength steel 3½, 5, or 7 feet long, pointed at the bottom. One, two, or three solid steel helices 8, 10, or 12 inches in diameter and spaced 2½ to 3 feet apart, are welded around a solid steel shaft. The diameter and number of helices depend on the loads to be carried and the soil conditions at the site. The helices are attached to the steel shaft with one edge of the slit lower than the other, creating a leading edge and a trailing edge. All the elements of a helical pile are hot- dipped galvanized. Bolt holes are provided at the end of each lead section for bolting another helical section to the lead section (figure 38).
A 12-inch-long, L-shaped beam saddle fits into the end of the steel shaft of the helical pile where the sections are bolted together. The beam saddle consists of a steel angle welded to a pipe sleeve. Two bolt holes in the vertical leg of the steel angle are opposite two bolt holes in a steel side lockplate (figure 39). The side lockplate is held in place by two bolts through the steel angle, through a wooden ledger or stringer, and through the side lockplate. The beam bracket can be adjusted up to 3½ inches by tightening the nuts on the bolt. A custom saddle is often used to accommodate larger wood or steel ledgers.
In poor soils, longer helical piles are sometimes used to achieve the needed load- bearing capacity. To reach that capacity, the pile is augured into the ground until a predetermined torque (the force needed to twist the lead section into the ground) is reached. Extensions can be bolted on to the lead section and augured into the ground until the correct torque is reached.
Helical pile assemblies for wetland trails usually consist of two helical piles opposite each other, one on each side of the bog bridge or boardwalk, or they may be located under the boardwalk. The two piles may be tied together with a ledger or a pair of ledgers placed on edge, resting on the beam saddle of each pile. The ledgers are usually solid wood-3 by 6, 8, 10, or 12 inch, or two or three pieces of 2 by 4s or 2 by 6s nailed together. Ledgers may also be glulaminated. The ledgers are bolted to each beam saddle (figure 40).
If the deck is 3 to 4½ feet above the ground, a diagonal brace is needed between the piles. If the deck is more than 4½ feet above the ground, two diagonal braces are needed, installed as cross braces. Diagonal braces may be additional helical piles or steel angles with diagonal cable attached to them. If pile assemblies are 6 feet apart or more, diagonal bracing between bents may also be needed. Consecutive bents may be braced diagonally from the left helical bent of one pier to the right helical pile of the next bent. The procedure is repeated to connect the bents' two remaining helical piles (figure 41).