Trails in Wet Areas (continued)
Turnpikes are used to elevate the trail above wet ground. The technique uses fill material from parallel side ditches and from offsite to build up the trail base higher than the surrounding water table. Turnpike construction is used to provide a stable trail base in areas of high water table and fair to well drained soils. Turnpikes are practical up to 10 percent trail grade (Figure 40).
Figure 40--Trail turnpike.
A turnpike should be used primarily in flat areas with 0 to 20 percent sideslope where there is wet or boggy ground. The most important consideration is to lower the water level below the trail base and carry the water under and away from the trail at frequent intervals. Turnpikes requires some degree of drainage. When the ground is so wet that grading work cannot be accomplished and drainage is not possible, use puncheon surfacing instead. However, a turnpike is easier and cheaper to build and may last longer than puncheon. A causeway is another alternative where ground water saturation is not a problem but a hardened tread is needed.
Begin your turnpike by clearing the site wide enough for the trail tread plus a ditch and retainer log or rocks on either side of the trail tread. Rocks, stumps, and stobs that would protrude above the turnpike tread or cause large rips in 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 gotextile or other geosynthetic materials and retainer rocks or logs. Geotextile and geogrid should go under any retainer rocks or logs (Figure 41). Lay the geotextile over the top with no excavation, then fill over with high quality fill. An alternative method, one that not only provides for separation between good fill and clay but also keeps a layer of soil drier than the muck beneath, is called encapsulation, or the sausage technique. Excavate 250 to 300 mm (10 to 12 in) of muck from the middle of the turnpike. Lay down a roll of geotextile the length of the turnpike, wide enough to fold back over the top with a 300-mm (1-ft) overlap (Figure 42). Place 150 mm (6 in) of good fill, or even rocks, on top of the single layer of geotextile, then fold the geotextile back over the top and continue to fill with tread material.
Figure 41--Place the geotextile under
the retainer logs or rocks before staking it.
Figure 42--Sausage or encapsulation method.
Rocks or logs can be used for retainers. Rocks last longer. If you use logs, they should be at least 150 mm (6 in) in diameter and peeled. Lay retainer logs in one continuous row along each edge of the trail tread. The logs can be notched to join them, if desired. However, in some species notching may cause the logs to rot faster (Figure 43).
Figure 43--Notched retainer log.
Anchor the logs with stakes or, better yet, large rocks along the outside. Inside, the fill and surfacing hold the retainer logs (Figure 44).
Figure 44--Try this old Alaska trick if your stakes
tend to work up out of boggy ground.
Firm mineral soil; coarse-grained soils or granular material; or small, well-graded angular rock are needed for fill. Often it is necessary to haul in gravel or other well-drained material 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 50 mm (2 in) or a minimum of 2-percent grade above the retainers. It doesn't hurt to overfill to begin with, as the fill will settle.
Construct a dip, waterbar, or a drainage structure at each end of the turnpike where necessary to keep water from flowing onto the structure. Keep the approaches as straight as possible coming onto a turnpike, to minimize the chance that stock or motorbike users will cut the corners and end up in the ditches. Turnpike maintenance, especially recrowning, is particularly important the year after construction; most of the soil settling occurs during the first year.
A more environmentally friendly relative of the turnpike is the causeway, essentially a turnpike without side ditches (Figure 45). Causeways filled with crushed rock have been successfully used throughout the Sierra Nevada and elsewhere to create an elevated, hardened tread across seasonally wet alpine meadows. Often multiple parallel paths are restored and replaced with a single causeway. Causeways create less environmental impact than turnpikes because ditches are not used and the water table is not lowered. The risk is that in highly saturated soils the causeway could sink into the ground, a problem that geotextile can help prevent.
Figure 45--Causeways create an
elevated, hardened tread across seasonally wet areas.
Puncheon is a wooden walkway used to cross bogs or deep muskeg, to bridge boulder fields, or to cross small streams. It can be used where uneven terrain or lack of tread material makes turnpike construction impractical (Figure 46). Puncheon is also preferred over turnpike where firm, mineral soil cannot be easily reached; puncheon can be supported on muddy surfaces better than turnpike, which requires effective drainage.
Puncheon resembles a short version of the familiar log stringer trail bridge. It consists of a deck or flooring made of sawn, treated timber or native logs placed on stringers to elevate the trail across wet areas that are not easy to drain. Puncheon that is slightly elevated is termed surface puncheon. Puncheon placed flush with the wetland surface is known as subsurface puncheon.
Sooner or later, you'll probably hear the term, corduroy. Corduroy is basically a primitive type of puncheon. It consists of laying three or more native logs on the ground as stringers with cross logs laid side by side across the stringers and bound together with wire or nails (Figure 47). Corduroy should always be buried, with only the tread exposed. Corduroy is notorious for not lasting very long and consuming large amounts of material. It should only be used as a temporary measure.
Figure 47--Cordurory should be considered a temporary
fix. until a more permanent structure an be installed.
Here's how to build puncheon. First of all, the entire structure must extend to solid mineral soil so soft spots do not develop at either end. Approaches should be straight for at least 3 m (10 ft) coming up to a puncheon. Any curves either approaching or while on the puncheon add to the risk of slipping, especially to stock and to mountain bike and motorcycle users.
To begin construction, install mud sills. These support the stringers. Mud sills can be made of native logs, treated posts, short treated planks, or precast concrete parking lot curb blocks. The mud sills are laid in trenches at both ends of the area to be bridged at intervals of 1.8 to 3 m (6 to 10 ft) (Figure 48). They are approximately two-thirds buried in firm ground. If firm footing is not available, use rock and fill to solidify the bottom of the trench, increase the length of the sill log to give it better flotation, or use more sills for the needed floatation. Enclosing rock and fill in geotextile minimizes the amount of rock and fill required. For stability, especially in boggy terrain, the mud sills should be as long as practical up to 2.5 m (8 ft).
Figure 48--Mud sill and stringer layout.
Stringers made from 200-mm (8-in) peeled logs or treated timbers are set on top of the mud sills. They should be at least 3m (10 ft) long and matched by length and diameter. Stringers also need to be level with each other so the surface of the puncheon will be level when the decking is added. Two stringers are sufficient for hiking trails, but for heavier uses, such as stock use, three are recommended.
Notch the mud sills, if necessary, to stabilize the stringers and to even out the top surfaces (Figure 49). To hold the stringers in place, toenail spikes through the stringers to the mud sills or drive Number 4 rebar (1/2 in) through holes in the stringers.
Figure 49--When using logs, notch the mudsill--not
the stringer. Do not notch them more than one third
of their diameter.
Next comes the decking. The thickness needs to be strong enough for the loads the structure will need to support. Lengths can be as narrow as 460 mm (18 in) for a limited-duty puncheon for hikers. The decking should be 1.2 to 1.5 m (4 to 5 ft) long for puncheon suitable for stock use.
Do not spike decking to the center stringer, if you have one, because center spikes may work themselves up with time and become obstacles. Leave at least a 20-mm (3/4-in) gap between decking pieces to allow water to run off (Figure 50). Decking should be placed with tree growth rings curving down. This encourages water to run off rather than soak in and helps to prevent cupping.
Figure 50--Place decking planks on stringers to
provide bearing for the full width of the plank.
Running planks are often added down the center for stock to walk on. Often the running planks are untreated because horseshoes cut out the plank before wood has a chance to rot. Do not leave gaps between running planks because they can trap mountain bike or motorcycle wheels.
Curb logs, also called bull rails, should be placed along each side of the puncheon for the full length of the structure to keep traffic in the center. To provide for drainage, nail spacers between the curb logs and the decking.
Finally, a bulkhead or backing plate needs to be put at each end of the structure to keep the stringers from contacting the soil (Figure 51). If the plate stays in place, do not spike it to the ends of the stringers. Spiking causes the stringers to rot faster.
Figure 51--Place a bulkhead or backing plate at each end
of the puncheon. Approaches should have a rising grade
so water will not run onto the structure.
In the rare case where puncheon is constructed on grades steeper than 5 percent, treat the surface to reduce slipping. Use cleats, commercial fish netting, mineral roofing, or other surfacing.
Subsurface puncheon involves construction with the mud sill, stringers, and decking under the surface. This design depends on continual water saturation for preservation (Figure 52). Moisture, air, and wood are needed before wood can rot. Remove any one of these and rot won't occur. A good rule for reducing rot is to keep the structure continually dry or continually wet. Totally saturated wood will not rot because no air is present. Cover the surface between the curb logs with a layer of gravel, wood chips, or soil to help keep everything wet.
Figure 52--Subsurface puncheon with covered tread surfacing.