Trails in Wet Areas (continued)

Turnpikes

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 geotextile 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.

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