Wetland Trail Design and Construction: 2007 Edition
Structures Requiring Foundations, Cont.
Bent Construction
Whether wood end-bearing or friction piles are used, once a pile is in place, the construction is similar. A second pile is placed on the opposite side of the trail centerline so that each is the same distance from the centerline. When both piles are in place, they are connected by one or two ledgers. The combination of ledgers and piles is called a bent.
On a one-ledger bent, the top of each wood pile is cut flat and level with the opposite pile. A 3 by 6 or 3 by 8 timber is placed flat on the top of both piles so that it extends a few inches beyond each of them. This timber, or ledger, is spiked to the top of each pile (figure 31).
When two ledgers are used, one is bolted to the front and one to the back of each pile, spanning the space between the piles. Drill a hole through each pile parallel with the trail centerline. These holes (and ledgers) should be level with each other. A 3- by 6-inch ledger is held in place on one side of the pile, and the hole in the pile is extended through the ledger. This is repeated until each ledger can be bolted to each pile. The ledgers should be level and level with each other.
Figure 31--A bent with one ledger. Spike the ledger to the top of
each pile. The pile and ledger are collectively called a bent.
Another method for the same type of installation is to determine the proper height of the ledgers and clamp the pair of ledgers to each pile of the bent. Drill a hole through the ledger, the pile, and the opposite ledger, all at once. This is faster, but requires two large clamps that can open at least 1 foot (figure 32).
Figure 32--A bent with two ledgers. Trim the tops of the piles at an
angle so they will shed water. The bolts go through the pile and both
ledgers.
After installing a pair of bents, pressure-treated 3- by 12-inch tread planks are nailed to the ledger or ledgers as described for the bog bridge on sleepers. If the planks are more than 2 feet above the ground or water, the tread should be at least two planks wide for trails that do not have to meet accessibility standards. Collect and dispose of treated wood trimmings and sawdust.
Where the deck will be more than 3 feet above the ground, diagonal bracing is needed to connect the piles of a bent. A single diagonal brace is adequate if the deck is just 3 to 4½ feet above the ground (figure 33). If the deck is higher than 4½ feet, two diagonal braces are necessary. These braces should be installed as a cross brace, forming an X between the piles. Diagonal braces are normally wood (figure 34). The angle of the braces should be between 30 to 60 degrees to the horizontal to provide enough support. Angles of 30, 45, or 60 degrees, or a 3-4-5 triangle, make the mathematics of carpentry easier in the field.
Occasionally, the ground is well below the surface of the tread. If the tread is 4 feet or more above the ground and the space between the bents is 6 feet or more, diagonal bracing may be needed to connect consecutive bents. Bracing between bents is done with wood members from the right pile of one bent to the right pile of the next, and the left pile of the bent to the left pile of the next (figure 35). Keep in mind that braces impede waterflow and can contribute to debris and ice jams.
Figure 33--A single diagonal brace is adequate if the deck is no more than
4½ feet above the ground. Alternate braces on successive bents.
Figure 34--Use a cross brace if the deck is higher than 4½ feet above
the ground. Extra-long braces can be bolted together using a
spacer block to increase rigidity.
Figure 35--Bracing between bents is sometimes necessary.
Helical Piles (Screw Piles)
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).
Figure 36--Helical piles are an
alternative to friction piles.
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 (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).
Figure 37--Helical piles installed in Colorado. The saddles are not yet attached.
The ends are covered with temporary plastic caps for safety.
Figure 38--A helical pile was used for this boardwalk. Stringers are attached
directly to the beam saddle, helping to keep the boardwalk
close to the ground.
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.
Figure 39--Screw anchor bracket for helical piles.
In poor soils, longer helical piles are sometimes used to achieve the needed load-bearing capacity. To reach that capacity, the pile is augered 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 augered into the ground until the correct torque is reached.
Helical Pile Assembly
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).
Figure 40--Typical ledger configuration
of a helical pile assembly.
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 (figure 41) or steel angles with diagonal cable attached to them. If bents are 6 feet apart or more, diagonal bracing between bents may also be needed. Consecutive bents may be braced diagonally from the left helical pile of one bent to the right helical pile of the next bent. The procedure is repeated to connect the bents' two remaining helical piles.
