Trail Foundation
The Trailbed
On hillside trails, the trailbed is excavated into the side of the hill
to provide a slightly out–sloped travel path. Depending on the slope of the hill, the
amount of excavation and the use of the excavated material varies
(Figure 10).
Figure 10—Typical trailway cross sections. Full–bench
construction will give you the fewest problems—especially on steep slopes.
On steep slopes, full–bench construction is usually needed. Soil
excavated from the hill is cast aside as far as possible from the trail and not used at
all in the fillslope. Especially on steep slopes, relying on fill for part of the trailbed
is a bad idea. This soft material is likely to erode away quickly, creating dangerous soft
spots on the downhill edge of the trail. If fill is used, it often needs to be reinforced
with expensive crib or retaining walls. As the slope of the hillside decreases, it becomes
more feasible to use fill material as part of the trailbed. However, even though it requires
more hillside excavation, full–bench trailbeds will generally be more durable and
require less maintenance than partial bench construction. There is a tradeoff, though.
Full–bench construction is often more costly because more excavation is needed, and it
also results in a larger backslope. Most trail professionals will usually prefer
full–bench construction.
Constructing Sidehill Trails
Looking at construction plans is one thing, but going out and building a
sidehill trail is quite another. Here is a proven method that works even for the complete
novice. This is for the actual digging part once vegetation has been cleared.
- Mark the centerline of the trail with wire flags no more than 3 m
(10 ft) apart. These wire flags are the key to explaining how to dig the tread, and they
keep the diggers on course.
- Remove leaf litter, duff, and humus down to mineral soil. To mark the area to be cleared,
straddle the flag facing the uphill slope. Swing your Pulaski or other tool. Where the tool
strikes the ground is approximately the upper edge of the cut bank. The steeper the slope, the
higher the cut bank. Do this at each centerline flag, then scratch a line between them. This
defines the area to be raked to mineral soil. Clear about the same distance below the flag.
Keep the duff handy, as 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.
- For a balanced bench trail, the point where the wire flag enters the ground is the
finished grade. Scratch a line between flags to keep yourself on course. Facing the uphill
slope, begin digging about 150 mm (6 in) from the flag cutting back into the slope. Imagine
a level line drawn from the base of the flag into the bank. Dig into the bank down to this
line, but not below (Figure 11). Pull the excavated material to the
outer edge. Tamp this fill material as you go. On a full–bench trail, the wire flag
essentially ends up at the outside edge of the trail. For less than a full–bench trail,
the flag ends up somewhere between the centerline and outside edge. Keep this in mind when
you place the wire flags.
Figure 11—Basic sidehill trail building.
- There is a tendency to want to stay facing uphill. To properly shape the tread, you
need to stand on the trail and work the tread parallel to the trail direction to level out
the toe of the cutslope and to get the right outslope.
- There is a tendency to make the trail too narrow. If the width of rough tread equals the
length of a Pulaski handle, the narrower finished tread will be about right for a good hiking
trail.
- Make sure grade dips and other drainage structures are flagged and constructed as you go.
- If you try to slope the cut bank close to the original surface, you will usually get
somewhere close to what is needed. Slope ratios are hard to understand. Instead, look at
the natural slope and try to match it.
- Round off the top of the cutslope. The easiest way to do this is to rake parallel to the
cut edge with a fire rake.
- The best way to check the outslope is to walk the tread. If you can feel your ankles
rolling downhill, there is too much outslope (Figure 12). The outslope
should be barely detectable to the eye. If you can see a lot of outslope, it's probably too
much. A partially filled water bottle makes a good level.
Figure 12—If your ankles start to roll,
there is too much outslope.
- Once the bench construction is finished, stand on the tread and pull the reserved duff up
onto the fillslope with a fire rake. This helps stabilize the fill (especially important in high
rainfall areas), and makes the new trail look like it has been there for years. Be careful not
to create a berm with the duff. On full–bench trails there will be no need for the duff,
as the outside edge of the trail has not been disturbed. Sometimes contract specifications call
for scattering rather than reserving the duff.
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