Wetland Trail Structures
At least eight types of trail structures are commonly built in wetlands. Some of these are built with no foundation. Others have sleepers (sills), cribbing, or piles as foundations. Most of these structures are built of wood.
The oldest methods for building a wetland trail were corduroy and turnpike, which require no foundation. Turnpike may require constructing timber culverts, which involves building two small timber walls. The walls must rest on a buried timber sill. Planks span the space between the walls.
The various types of puncheon, gadbury, and the simplest form of bog bridge construction may be built on a foundation of sleepers, or on log or timber cribbing. Cribbing is more difficult to construct and is used occasionally where the terrain is hummocky (having small mounds of vegetation interspersed with depressions that hold water).
Bog bridges and boardwalks are often supported on pile foundations. Three types of pile foundations have been used for bog bridges and boardwalks: end-bearing piles, friction piles, and helical piles. Piles are the most labor-intensive foundation. Helical piles and some friction piles require specialized machinery for installation.
Floating trails are another, less common, technique. Where they are used, you need some form of anchorage.
In this manual we describe the structures more or less in historical order. The oldest are early in the list, and the newest or most difficult to construct appear toward the end. The older techniques can be done without machines, although machines make the work go faster. The newer techniques are almost impossible to build without machines.
Sustainable design essentially asks the trail designer or builder:
- Can we use the proposed construction technique and expect the materials and the various processes to be available years from now if we need to replace part of the trail?
- When the item is no longer usable, can any of the materials be recycled?
- Can recycled materials be used in the construction?
- Are recycled materials appropriate for the proposed use?
These criteria should be considered by all agencies, especially conservation agencies.
Corduroy was originally used to provide access through wetlands to areas being logged or mined. Essentially, the technique involved laying a bridge on the ground where the soil would not support a road. Two log stringers or beams were placed on the ground about 8 feet apart. Small- diameter logs or half logs were placed on the stringers, spanning them. The logs became the tread or surface of the road. They were spiked or pinned to the stringers (figure 12).
A variation of corduroy construction was to place the tread logs directly on the ground. No stringers were used, and the logs were not pinned or spiked to the ground or each other. Some excavation was required to ensure the tread logs were level. The tread logs eventually heaved up or sank, creating severe cross slopes in the tread.
Corduroy construction was often used in areas with deep shade and considerable rainfall. The combination of sloping, wet tread resulted in a slippery, hazardous surface. The stringers and tread logs soon rotted. With no support, the cross slope on the tread logs became worse and more hazardous.
When corduroy was laid directly on the ground, it interfered with the normal flow of runoff. Runoff was blocked in some areas and concentrated elsewhere. Erosion and relocation of minor streams resulted. No plants grew underneath the corduroy, further damaging the wetland resource. Many trees needed to be cut to provide the logs for the corduroy. In many cases, these impacts would be unacceptable today. The useful life of corduroy today is only 7 to 10 years. Corduroy is rarely replaced because suitable trees are even farther from where they are needed for the reconstruction job.
Corduroy did not represent sustainable design and required considerable maintenance. Corduroy is rarely used today. We do not recommend it.
Turnpikes are used to elevate the trail above wet ground. The technique uses fill material from parallel side ditches and other areas to build the trail base higher than the surrounding water table. Turnpike construction is used to provide a stable trail base in areas with a high water table and fair- to well-drained soils (figure 13).
A turnpike should be used primarily in flat areas of wet or boggy ground with a 0- to 20-percent side slope. The most important consideration is to lower the water level below the trail base and to carry the water under and away from the trail at frequent intervals. Turnpikes require some degree of drainage. When the ground is so wet that grading work cannot be accomplished and drainage is not possible, use puncheon or some other technique. A turnpike is easier and cheaper to build than puncheon and may last longer. A causeway is another alternative where groundwater saturation is not a problem and a hardened tread is needed.
Begin the turnpike by clearing a site wide enough for the trail tread and a ditch and retainer log or rocks on either side of the trail tread. Rocks, stumps, and roots that would protrude above the turnpike tread or rip geotextiles should be removed or at least cut flush below the final base grade.
Ditch both sides of the trail to lower the water table. Install geo-textile or other geosynthetic materials and retainer rocks or logs. Geotextile and geogrid should go under any retainer rocks or logs (figure 14). Lay the geotextile over the ground surface with no excavation, then apply high-quality fill. An alternative method, one that not only provides separation between good fill and clay, but also keeps a layer of soil drier than the muck beneath, is called encapsulation (the sausage technique). Excavate 10 to 12 inches of muck from the middle of the turnpike. Lay a roll of geotextile the length of the turnpike, wide enough to fold back over the top with a 1-foot overlap (figure 15). Place 6 inches of crushed stone, gravel, or broken stone on top of the single layer of geotextile, then fold the geotextile back over the top and continue to fill with tread material.
Rocks or logs can be used for retainers. Rocks last longer. If you use logs, they should be at least 6 inches in diameter, peeled, and preferably treated or naturally rot resistant. Lay retainer logs in a continuous row along each edge of the trail tread.
Anchor the logs with stakes or, better yet, with large rocks along the outside. The fill and trail surface keep the retainer logs from rolling to the inside.
The practices described above work best on turnpikes in mountain bogs or other areas that are not subject to periodic river flooding. In flood-prone wetlands, different techniques work better. One turnpike was flooded to a depth of 6 to 8 feet on two occasions in 1 month. Stones up to 1 cubic foot in an adjacent area of riprap were lost in the flood. The edges of that turnpike were logs pinned to the ground with diagonally driven drift pins that helped to keep the logs from floating up and away. The logs were still in place after the flood, but the fill material between the logs had been swept away. Geotextile fabric that had been installed between the fill and the ground was still in place. In retrospect, if geoweb or geogrid had been placed on the geo-textile fabric and stapled, nailed, or placed underneath the logs, most of the fill material would probably have remained in place (figure 16).
Wood used in turnpike construction should be either a naturally rot-resistant species or treated poles. Pinned as described, the logs or poles should survive some floods.
Firm mineral soil, coarse-grained soils or granular material, or small, well- graded angular rock are needed for fill. Often, gravel or other well-drained material must be hauled in to surface the trail tread. If good soil is excavated from the ditch, it can be used as fill. Fill the trail until the crown of the trail tread is 2 inches or a minimum of 2 percent grade above the retainer. It doesn't hurt to overfill initially, because the fill will settle (figure 17). Compacting the fill with a vibratory plate compactor will help reduce settling.
Fill material imported from outside sources may contain seeds of invasive weeds. Instead, it is standard procedure to dig a borrow pit near the site. The pit and routes to it should be carefully located to avoid resource damage and a construction scar that will be seen for many years. The borrow pit should be dug into a slope so that the floor of the pit can slope out and carry runoff water out of the pit. After the trail has been completed, grade back the sides of the pit and revegetate the disturbed area with native plant material.
Keep water from flowing onto the turnpike by constructing a dip, waterbar, or a drainage structure at each end of the turnpike where necessary. Keep the approaches as straight as possible coming onto a turnpike, to minimize the chance that packstock or motorbike users will cut the corners and end up in the ditches.
Turnpike maintenance, especially recrowning, is particularly important the year after construction; the soil settles the most during the first year.