Structures Requiring Foundations
- Sleepers (Sills)
- Wooden Piles
- Bent Construction
- Helical Piles/Screw Piles
- Special Site Considerations
- Bog Bridge
Corduroy, turnpikes, causeways, and improved drainage are all constructed directly on the ground and do not require a foundation. The remaining techniques-puncheon, bog bridges, gadbury, and boardwalks-all require some sort of constructed foundation to raise the structure off the ground.
The type of foundation needed varies with the structure being constructed, materials available, and the site-specific soil and water conditions. More than one type of foundation may be appropriate for each structure, so we will discuss foundations first. These foundations include sleepers (sills), cribbing, end-bearing piles, friction piles, and helical piles.
The simplest foundation is to rest the tread plank or stringers on sleepers, also called sills, or mud sills. A log of a naturally rot-resistant wood or a large-diameter treated pole or post is used for the sleepers. Sleepers are used to support puncheon, gadbury, and bog bridge construction. The notching for each type of structure is different and will be discussed later in this chapter.
A sleeper (figure 24) is placed in a shallow trench at a right angle to the trail centerline. A second sleeper is placed in another trench parallel with the first sleeper. The distance between the two sleepers is the span. The span is determined with the help of someone with carpentry or structural engineering experience.
Pinning the sleepers to the ground with 24- to 30-inch drift pins is extra work, but it may reduce future maintenance in wetlands subject to flooding. Pinning is most important near streams or rivers where high water velocities may occur during flooding. Pinning may also reduce maintenance in areas of frequent slack water flooding. The drift pins should be driven in holes drilled at opposing angles. Drift pins installed at these angles will resist flotation and uplift from frost and will also deter vandalism.
Timbers are sometimes used instead of logs for sleepers. Timbers are easier to work with because they do not require notching. However, timbers do not have the same rustic quality as logs. Precast concrete parking bumpers and other precast concrete units have been used for sleepers, but they are far from rustic. Concrete bumpers weighing 150 pounds per cubic foot are difficult to bring to the site. In most wetland soils, they will eventually sink into the ground.
Sometimes the base for the sleeper can be strengthened by excavating deeper, wider, and longer; laying down geotextile; adding several inches of gravel on top of the geotextile; and folding the geotextile back over the top of the gravel to encapsulate it. Lay the sleeper on top of this foundation.
In hummocky terrain or when crossing a wide, low area, log or timber cribbing can be used to support a trail. Usually logs are used to construct cribbing (figure 25). Dig two parallel shallow trenches a few feet apart. Place a sleeper in each trench and diagonally pin it to the ground with three 30-inch driftpins. Drive the outer two pins at opposing angles. Depending on the width of the completed trail, the first layer (or course) may be 3- to 5-foot-long logs. A second course of two more logs is placed on the first course of logs, near their ends. Each course of logs is placed at right angles to the course below and spiked or pinned to it. The cribbing is built up until the proper height is reached. Lay the top course perpendicular to the centerline of the trail. Stringers or plank can either be nailed to each of the top logs or timbers, or a single, large- diameter log can be notched and pinned to the top logs (similar to the sleepers described earlier).
If you use logs, saddle notches may be used in the bottom of all but the sleepers. This will result in a solid wall of logs. A simpler technique is to use a square notch at the ends of each log that contacts another. This technique will leave a 3- to 6-inch gap between the logs (figure 26).
Use spikes or 12-inch-long drift pins at the corners of the cribbing to hold it together. Avoid stacking the joints on top of each other. The joints must be offset or the drift pins from each course will hit the drift pins in the logs of the course below.
Timbers are easier to use than logs because they do not have to be notched. For greater stability and to prevent the cribbing from being washed away in floods, you can fill the open space in the core of the cribbing with stone.
Piles are another foundation technique. Three types of piles have been used for wetland trails. Structural engineers refer to these piles as end-bearing piles, friction piles, and helical piles (figure 27).
End-bearing piles are used at locations where firm earth or solid rock is found 2 to 10 feet below the ground. Although the soil at these sites may support sleepers, piles can be used to support the tread at abrupt changes in grade when the tread must be 1 to 5 feet above the ground or water. Piles can also support handrails.
To place an end-bearing pile, excavate a hole a little wider than the pile to a point below the frostline. If you encounter solid rock in the bottom of the hole before reaching the frostline, the pile can rest on the rock. Power augers help make digging easier and faster. In wilderness areas or where only a few holes are needed, a posthole digger, manually operated auger, or shovel will do the job.
Place the pile upright and plumb in the hole. Place the excavated earth (or imported coarse sand or gravel) in the hole in 6- to 8-inch layers, equally on all sides of the pile, and compact it by tamping. A tamping bar is the best tool for compacting earth (figure 28).
End-bearing piles can be made from naturally rot-resistant or pressure-treated wood, steel, or concrete cast in sonotubes (disposable cylindrical forms). Wood is typically used because it is readily available and easier to use than steel or concrete. Connections to a wood pile are also much easier to make, and the tools needed for the job are lighter and more readily available. Timbers are the first choice for end-bearing piles because their flat, squared sides are easier to connect to than the round surface of logs. Usually, rough-sawn, pressure-treated 6 by 6s are adequate for this work (figure 29).
Friction piles are normally used when the ground is wet and sloppy-areas where you need logs or some kind of a deck to stand on while you work. Friction piles for trail work are usually at least 12 feet long and 10 to 12 inches in diameter. Friction piles are considerably heavier, more awkward to transport, and more difficult to install than end-bearing piles.
A friction pile should be a naturally rot-resistant log or a pressure-treated log or pole. Cut a point on the narrow end and dig a shallow hole where the pile is to go. The pile must be driven with the wide butt end up and the narrow end down. The pile should stand as plumb as possible.
For backcountry wetland trail construction, friction piles are driven by hand. Pile driving is done with a "hammer"-a 2½- foot piece of 12-inch-diameter steel pipe (figure 30). A cap is welded at the top and two ½-inch- diameter holes are cut in the cap to let air out. Two steel handles of ½-inch-diameter reinforcing bars are welded to the sides of the pipe. At one area where friction pile construction was common, two of these hammers were used to drive the piles by hand. One hammer weighed 90 pounds and the other 135 pounds. Using these tools builds strong bodies. Usually two persons work together when operating these manual pile drivers.
The theory of the friction pile is that the surface of the pile develops friction against the sloppy soil. The deeper the pile is driven, the more friction develops, until finally the pile "fetches up" and can be driven only fractions of an inch with each blow of the hammer. When that point is reached, pile driving stops, normally at depths of 6 to 10 feet. Sometimes firm soil or rock will be reached before the pile fetches up. At this point the driving stops and the pile becomes an end- bearing pile.
If the trail is being built progressively, 12- to 16-foot piles can be driven by a small, lightweight machine with a pile driver attachment. Building a wetland trail strong enough to support the pile driver may be worthwhile in coastal areas that are subject to hurricanes, northeasters, or typhoons. If the wetland trail can support the machine, it will probably withstand some severe storms. If longer piles are needed, a much heavier pile driver can be brought to some sites on a barge.