Cutting and Filling Tread

A common problem commonly faced by the average Appalachian Trail section overseer involves trail tread that is cupped.

Cupped treadCupped tread

This results from a combination of factors, including:

  • Erosion
  • Trail design & layout
  • Erosion
  • Years of foot travel
  • Erosion
  • Compacted soil
  • Erosion

Is there a common theme that commonly causes this common problem?

You bet. What is it?

 Erosion

As you see in this photo of the A.T. leading north out of Brown Gap, the tread has cupped into itself between the outer and inner sides; sides that have become like the banks of a stream. Why? Because rain or storm water turns the trail into a stream.

Original Treadway

Nearly 70 years ago, the Civilian Conservation Corps (CCC) relocated the A.T. here to remove it from the route of the new Skyline Drive. In many cases, the CCC crews built cribbing of stone and brought in mineral soil (clay) to top off their cribbing. After millions of footfalls and foot-pounds and gallons of rain water later, the Trail in many areas showed moderate to worse damage.

Original trail diagram

Trail construction techniques the CCC used dictated a level surface from side-to-side of the trail. The sad fact is, that this technique often results in erosion, as the treadway surface forms a path straight-way downhill. Rain water, like so many other inanimate objects (students, politicians, zealots of any ilk, and so forth) automatically chooses to follow the Path of Least Resistance. Therefore, the bulk of the tread material—the clay soil—was washed away over the years. That this erosion wasn’t so bad is due mainly to the good grade on which the Trail was built.

Cupped?

The diagram above shows how the trail was constructed. The grade was not excessive, but it went up and over the contours instead of working with the topography. Also, the tread itself was not sloped outward, or “out-slopped,” so run-off tended to run down the trail, causing it to become a stream bed. Especially after 70 years of hikers had trod the section.

Cupped trail diagram

Solution: Start Your McLeods!

The only real “solution” to this problem is to cut-and-fill the cupped treadway section. This requires you to cut dirt from the uphill section and pull it into the cupped treadway. You will stabilize the tread for a period of time, but it won’t fix the problem permanently. To do that, a crew would need to pull enough dirt out into the trail to form a “lip” over the capstones of the crib wall and grade the tread to at least a 1% grade out to the edge of the capstones. There may not be enough dirt available, in some cases, to do that.

It’s time to introduce our favorite tool for situations like this: the Venerable McLeod. The McLeod lets you cut duff and mineral soil using the tines, pull the mineral soil (clay) using the straight edge, and grade and tamp it using the flat side of the blade.

Take Care of the Crib Wall

An interesting thing happened over the 70 years since the CCC built this section of the Appalachian Trail—erosion. The fact is, that the only evidence you see of the original crib wall is the line of capstones along the side of the trail. The rest of the crib wall lies buried under the soil that washed down off of the ridge and off of the original treadway. This next drawing illustrates the situation.

Erosion effect diagram

Theoretically, you could dig soil from the crib wall and replace it into the treadway. Unfortunately, Mother Nature had other plans for that material: it is no longer mineral soil (that is, clay) but has been transformed into, essentially, topsoil. Trees and shrubs and weeds and Poison Ivy and vine of all descriptions grow in it. It contains a lot of duff, and duff is not good for tread filler. (It compacts and causes more run-off problems too quickly.) Still, I have, with a crew, removed a significant amount of clay from underneath the duff and replaced it in the tread. Most of the mineral soil used, of course, came from cutting the uphill bank and pulling it into the treadway.

Handle Capstones Carefully

Shenandoah National Park is committed to preserving the structures built by the CCC, including capstones.Do not remove them!

You may find that the original capstones are bowed-out and no longer sat atop the now-buried CCC crib wall. As this touched-up photo shows, some force pushed a section of CCC capstones off of the crib wall and over the side. This probably resulted both from water rushing over the stones and by hikers who started to hike upon the stones as the treadway narrowed from the uphill side.

Cut-n-fill diagram

You have to remove all dirt from around the original capstones to expose the crib wall underneath. Then, you need to replace and re-set the stones and bring dirt back into the tread to stabilize both the stones and the Trail. This is not quite so easy to do as it is to describe.

The Prime Directive

Yes, there is a Prime Directive applied to stone masonry: The Rock Can’t Rock!

It’s not enough to set the capstones back where they appear to belong. That part is easy. They must, however, stay where they’re placed.

After all, some hiker will step on them—if they move, the hiker may lose his or her balance, trip, and fall. Besides, the hiker will upset and kick out the capstone in doing this, and that is not good.

So, we who work with stone have to find smaller stone as shims to level and hold the larger stones in place, largely through friction-induced inertia. When that’s done, and done for each stone in the line, you take mineral soil and pack it into the spaces between the capstones and the smaller rocks you’ve set in place.

Clay (mineral soil) is great for this purpose; it acts much like concrete mix would (if we could use it). When all is thought to be done, it remains to test your work to ensure you have satisfied the Prime Directive.

Finally…

Just when you think you’re finished, you still have something to help protect all the work you’ve done from the evils of—gasp—Splash Erosion!

Splash erosion is the effect of raindrops hitting loosely compacted clay. Each drop falls far to Earth from somewhere Up There. The impact of a single drop is insignificant; however, the cumulative effect of many drops can cause serious problems before the newly re-established tread is fully set.

How to handle this issue? Simple: scatter leaves over the new tread! I believe that, in eons past, some disgruntled trail crew worker snorted in disgust and claimed that his (or her) Boss wanted him (or her) to litter the trail with leaves. Hence, we know this material as “Leaf Litter.”

Scatter it with glee, for:

  • You are about done
  • Leaves don’t weigh as much as a tool
  • The kid in you just loves to throw leaves