Chapter 4--Designing Trail Elements--Continued
On hillsides, excavate the trailbed into the hill to provide a slightly outsloped travel path. Figure 4-13 shows cross sections of a trail with a relatively flat trailbed, full-bench construction, ¾-bench construction, and a balanced section. Full-bench construction is preferred because it produces a more durable trail that requires less maintenance. During full-bench construction, excavated soil from the hill is cast as far as possible from the trail since it is not needed for fill (figure 4-14). Partial-bench construction incorporates part of the cut material in a process known as sliver fill. Because it is difficult to compact the fill evenly, the trail may be prone to failure, especially on the downhill side. If a slope needs to be filled, reinforce it with retaining walls or use step cuts and fills (see figure 4-13) to key the fill material into the slope.
Proper drainage is vital trails because it reduces erosion from runoff and boggy conditions from water pooling in flat areas. Poor drainage increases tread damage by all trail users. Figure 4-15 shows an advanced case of poor trail drainage on a popular shared-use trail. For further information on trail drainage, refer to Appendix B--Trail Libraries, Trail Organizations, and Funding Resources.
One way to avoid water damage on relatively flat or level ground is to crown the tread--keep it higher in the center than on the edges. Usually, treads are crowned 2 to 5 percent. Soil composition, texture, type, and the trail's use determine how often crowned tread needs to be maintained. Tread quickly becomes trenched on trails that are not maintained or that have significant traffic. Turnpikes are structures with a crowned tread that are sometimes used when trails cross boggy areas. Don't crown short sections of trail paved with asphalt or cement.
Although waterbars are common on trails, they often work poorly and require substantial maintenance. In theory, water running down the trail is deflected by the waterbar and runs off the trail's lower edge. In reality, waterbars fill in with soil, wash out, dislodge, or deteriorate over time. In the process, the anchors holding waterbars in place may become exposed, creating a significant tripping hazard. Wildlife often go around waterbars, which also is the natural inclination for horses and mules. These unwelcome detours widen treads. When waterbars on horse trails are unavoidable, construct them of rock or wood.
Rock--or armored--waterbars are occasionally used where the trail grade is less than 5 percent (figure 4-16). On steeper grades--15 to 20 percent--waterbars are likely to clog if the waterbar is set at an angle of less than 45 degrees to the trail. When grades are steeper than 20 percent, waterbars are ineffective. At such steep grades, there is a fine line between clogging the waterbar and eroding it away.
Grade reversals are used on new outsloped trails to shed water from the tread. In a grade reversal, the vertical tread alignment levels out and then drops subtly for 10 to 50 linear feet (3 to 15.2 meters) before rising again. Water flows down the drop, running off at the low spot before the water gains significant momentum or volume. Contour trails with grade reversals are often referred to as rolling contour trails. Retrofitted trails generally incorporate knicks or rolling grade dips. A knick is appropriate for draining puddles on relatively flat ground. A knick (figure 4-17) consists of a subtle, semicircular depression in the trail, about 5 to 10 feet (1.5 to 3 meters) long. The depression is angled about 15 percent so water runs off the edge of the trail. A rolling grade dip (figure 4-18) is similar to a knick. A rolling grade dip has an outsloped depression with a ramp built from the removed soil. The ramp is outsloped like normal tread, up to 5 percent. Rolling grade dips are 15 to 30 feet (4.6 to 9.1 meters) long and are more suitable than knicks for relatively steep trails. Stock tolerate grade reversals, knicks, and rolling grade dips well. Grade reversals, knicks, and rolling grade dips are preferred over waterbars in nearly all situations.
Where running water crosses the trail, culverts may be needed. Construct culverts of rock (figure 4-19), treated timbers, plastic, concrete, or metal, and surface them with at least 6 inches (152 millimeters) of suitable tread material. Bare culverts are slippery and have other undesirable features. The hollow sound of horseshoes hitting a bare culvert and the metal's bright reflections or odd contrast can spook stock. Consider using tapered end sections (figure 4-20), painting the culvert ends, or screening the edges with rock or timber for safety and esthetics. The tread surface over culverts has a tendency to erode and needs to be replaced regularly.
Figure 4-20--This trail culvert has a flared
end piece that is partially concealed
with rock. It is attractive, durable, conforms
to the slope, and improves waterflow.
Culverts with flared or covered ends are more
horse-friendly than exposed culverts because
they look more natural and there is no exposed
metal to make noise when a
horse steps on it.
--Courtesy of Kandee Haertel.
Any grates should be strong enough to support the weight of stock safely. Grate patterns should not catch horseshoes. Small grates placed to the side of the tread are better than grates that encroach on the center of the trail. Long, narrow grates are more likely to be accepted by stock than large square ones. Horses and mules often avoid grates because their surface does not appear solid and they make noise when stock step on them.
The large size of stock and their loads requires plenty of maneuvering space. While curves and switchbacks designed to accommodate riders are usable by many recreationists, the design parameters are slightly different than those for other users, such as bicyclists. Refer to Chapter 1--Understanding Horses and Mules for the design dimensions of horses.
On trail curves and turns, the minimum comfortable radius is 5 feet (1.5 meters). When turns are any tighter, stock may stumble over their own legs. Turns with a radius of 6 to 8 feet (1.8 to 2.4 meters) are more comfortable for both animal and rider.
Table 4-5 shows the minimum suggested turning radius on horse trails with different levels of development. Wider turns are preferred. In addition to handling increased traffic volume and being more comfortable, wider turns may better suit tread width, site conditions, and trail users' experience levels. Allow additional clearance for packstock equipped with side panniers or for stock that are pulling carts.
|5 to 6||6 to 8||8 to 10|
When trails are in steep terrain, other trail users can find it challenging to move aside for stock. Incorporate passing areas on narrow trails, particularly those on steep hillsides. A space 5 feet (1.5 meters) wide by 10 feet (3 meters) long will allow a single trail animal to pull off the tread. Locate passing areas in natural openings if possible. Larger passing areas, where large groups or packstrings may move off the trail while another group goes by, are sometimes needed. Plan these areas to handle the expected traffic volume and group sizes.
Switchbacks reduce the grade on a trail by incorporating sharp turns on one or more trail segments. Several switchbacks may be needed to traverse a steep area effectively. Switchbacks consist of an upper and lower approach, guide structures, a landing--or turn platform--and a drain for the upper approach and landing. Figure 4-21 illustrates suggested guidelines for trail switchbacks on horse trails.
Figure 4-21--A switchback with a retaining wall.
Drawings of a switchback with a retaining wall. In the drawings the text reads, Backslope, Inslopped trail tread with 5% slope, fillslope, 5 ft min., Outsloped trail tread, Retaining wall, Original ground plane, Drainage swale, Upper (descending) leg shoulder with 10% max. slope, Begin inslope 20 ft from landing, 5 ft min. radius landing, Crowned landing slope 5% in all directions, Lower (climbing) leg shoulder with 15% max. slope, Outslope with 5% max. slope, and Downslope.
Where appropriate, climbing turns are an alternative to switchbacks and are easier for packstock to negotiate. A climbing turn (figure 4-23) follows the natural slope. When the tread turns, it climbs at the same rate as the slope. The advantage of climbing turns is that a larger radius turn is easier to construct. Construction is much less expensive because less excavation is required and fill is not needed. The minimum suggested radius for a climbing turn is 20 feet (6.1 meters). Climbing turns work best when built on slopes of 15 percent or less. In steeper areas, switchbacks are a better choice.