Trail Foundation
Here's how you can make sure your trail has a strong, long-lasting foundation.
Constructing contour trails into the sideslope requires excavating the side of the hill to provide a solid, stable trail tread. Stay away from flat areas because water has nowhere to go. Keep grades sustainable by using the half rule and add plenty of grade reversals. Slightly outsloping the tread (about 5 percent) is a must to help move water across the trail.
Trail professionals almost always prefer full-bench construction. A full bench is constructed by cutting the full width of the tread into the hillside and casting the excavated soil as far from the trail as possible (figure 26). Full-bench construction requires more excavation and leaves a larger backslope than partial-bench construction, but the trailbed will be more durable and require less maintenance. You should use full-bench construction whenever possible.
Figure 26—A full-bench trail is constructed by cutting the full width of the
tread into the hillside. The tread needs to be outsloped at least 5 percent.
Partial-bench construction is another method to cut in a trail, but it takes a good deal of trail-building experience to get this method right. The trail tread will be part hillside and part fill material (figure 27).
Figure 27—With partial-bench construction, the trail tread is part hillside and
part fill material. The tread needs to be outsloped at least 5 percent.
The fillslope needs to be composed from good, solid material like rock or decay-resistant wood. And it has to get compacted evenly—this is the puzzle to solve. Solving Sudoku puzzles doesn't guarantee you'll get this one!
Backslope—The backslope is the excavated, exposed area above the tread surface. The backslope should match the angle of repose of the parent material (the sideslope). You may come across trail specifications calling for 1:1 backslope. This means 1 meter vertical rise to 1 meter horizontal run.
Most soils are stable with a 1:1 backslope. Solid rock can have a steeper 2:1 backslope, while less cohesive soils may need a 1:2 backslope (figure 28).
Figure 28—Backslopes are noted as a ratio of vertical rise
to horizontal distance,
or "rise" to "run."
Bottom line, angle the backslope until loose material quits falling down onto the trail tread. Stabilize the entire backslope by compacting it with the back of a McLeod.
Stable Backslopes
Look at the surrounding landscape and soil to see areas that are stable. Create a somewhat gentler backslope than you think necessary. Although you will initially expose more raw soil, the chances of your trail remaining stable and revegetating are greater than if you leave a backslope so steep that it keeps sloughing.
One option to reduce backslope excavation is to construct a retaining wall. This can be less obtrusive than huge backslope excavations and more stable if the wall is well constructed.
Fillslope—The fillslope is that area below the tread surface on the downhill side. A full-bench tread will not have any fill on this side of the trail. Fillslopes are critical. Fillslopes often need to be reinforced with retaining or crib walls to keep them from failing. Fillslope failures are common and will wipe out the trail. That's why most trailbuilders prefer full-bench trails.
Looking at construction plans is one thing, but going out and building a rolling contour trail is quite another. Here is a proven method that works even for the complete novice. This procedure is for the actual dirt moving once vegetation has been cleared.
- Place pin flags to keep the diggers on course.
- Straddle a centerline flag and face uphill. Swing your Pulaski
or other tool to mark the area to be cleared. Where the
tool strikes the hillside will be approximately the top of the
backslope. The steeper the slope, the higher the backslope. Do this at each centerline flag, then scratch a line between
the tool strikes. This defines the area to be dug to mineral
soil. Clear about the same distance below the flag. Keep the
duff handy by placing it uphill. It will be used later. Don't
clear more trail than can be dug in a day unless you know it
isn't going to rain before you can complete the segment.
- Stand on the trail and work the tread parallel to the direction
of travel. Level out the tread and get the right outslope.
Don't continue facing uphill when you're shaping the tread,
despite the tendency to do so.
- Make sure that the width of the rough tread is about the
length of a Pulaski handle. The finished tread will be about
right for a good hiking trail.
- Make sure grade reversals and other drainage structures are
flagged and constructed as you go.
- Shape the backslope about as steep as the original slope.
Backslope ratios are hard to understand. Instead, look at the
natural slope and try to match it.
- Round off the top of the backslope, where the backslope
meets the trail tread, and the downhill edge of the trail.
Keeping these areas smooth and rounded will help water
sheet across the trail.
- Walk the trail to check the tread's outslope. If you can feel
your ankles rolling downhill, there is too much outslope
(figure 29). The outslope should be barely detectable to the
eye. A partially filled water bottle makes a good level or you
can stand a McLeod on the trail tread—the handle should
lean slightly downhill.
- Compact the entire tread, including the backslope, with the
back of a McLeod. Don't leave compaction up to trail users.
They will only compact the center, creating a rut that funnels
water down the middle of the trail.
- Place the duff saved earlier onto the scattered dirt that was
tossed downhill. The duff helps naturalize the outside edge
and makes the new trail look like it has been there for years.
- Be careful not to create a berm with the duff.
Figure 29—If your ankles start to roll, the tread has too
much outslope.
