Switchback Theory and Principles:

Traverses.

If you have not read the definition of 'switchback' , please do so now, as that material is a prerequisite for what follows.

So a switchback (zig-zag) is basically a traverse with a kink. Before focusing in on the kink let us examine some basics of traverses.

A traverse is the crossing of a slope from one side to the other. Of course trails can be built on level terrrain, and there are lot of interesting challenges in that context. But the context of switchbacks is on slopes, and our focus here is entirely on traverses crossing slope. (And the occasional kink.)

Geometry.

The geometry of a traverse is very simple. Imagine an inclined plane, the slope. It need not be flat, though for convenience we will assume so. Now imagine the another inclined plane, the trail tread. The intersection of the this plane with that of the slope determines the line of the grade. Alternately, given the slope and the line we can determine the plane of the tread. Indeed, that is the core task of trail design: finding a line on the slope that determines a suitable plane for the tread.

At any point on this line where the slope and the tread intersect imagine a vertical plane at right angles to the line. On this vertical plane can be drawn a cross-section of the trail, showing the angle of the slope and the width of the bench on which the trail is built. (See the Half-bench diagram below for an illustration.) The cross-section of the tread (nominally horizontal, ignoring any in- or out-sloping¹ forms the base of an oblique triangle, which also includes the cut-slope (or back-slope) and the original surface of the slope which was cut away. The relationships between the parts of this triangle (such as the three angles, lengths of the three sides, and height of the triangle) are described by the Law of Sines (consult any trigonometry book). For here it is sufficient – and important! – to note that only three variables (one of which must be a length) are needed to fully describe the triangle. If you are given slope angle and tread width, then you can select a cut-slope angle, or cut-slope height, but not both, as specifying either one of those then "solves" the triangle in all other aspects.

The other geometrical point to note is that the angle of the line of the grade with any contour line (any linne of constant elevation) reflects the grade of the trail. If the line is level – neither rising nor falling – it is parallel with the contour, in both map view and side view. As the grade increases, the angle with the contour lines increases (map view and side view), the grade reaching its maximum value (equal to the slope) when perpendicular to the contour lines (i.e., straight up-and-down the slope). This is important enough that you should take a moment to let it sink in: lines that are level (constant elevation) are parallel to the the contour, lines that ascend or descend (i.e., change elevation) cross the contours at some angle, and are steepest when perpendicular to the contour. This should recall the previous discussion of the fall line.

Other considerations.

The purpose of switchback theory is to establish the physical and geometrical foundations for building "good" trail, where "good" has various aesthetic aspects but includes sustainability (loosely defined as resistance to failure). The focus here is primarily on avoiding loss of tread due to erosion because that is a principal mode of failure. But there are other modes of failure, and getting the grade right will hardly matter if something else fails. So I would like to touch on some fundamental aspects of building traverses that, strictly, are not part of switchback theory, but are necesssary for its success.

Half-bench versus full-bench.

Let us consider half-bench construction. The trail builders from my era seem to have mostly learned better, so I am surprised (and annoyed) to see half-bench construction still being used.

Above: Actual cross-section of tread that lost its outer edge. The shiny black area near the center is the outside edge of the hardened soil. The downward migration of loose material originally on top has been checked by the layer of roots at the lower left. The new tread was cut inward of, and even slightly below, the zone of roots, which became the outside margin.

"Half-bench" – the United States Forest Service (USFS) calls it "balanced bench" – is the idea of cutting the bench (or shelf) for a traverse into the slope by half the width of the bench, and then using the cut out soil to fill on the outside to support the other half of the bench. (See the Half-bench Construction diagram, right) Twenty or so years ago this was still recommended, but practice has shown it to be less than worthless. There is some appeal to it, but only in a scissors-and-paper kind of geometry.

The first problem is that the soil dug out is loose, and wants to slide down hill. The maximum slope at which a loose material will hold – its "angle of repose" – is, for most dry materials, around 40 degrees. So if your slope is around 40 degrees, you are pretty much trying to make a new slope nearly parallel to the existing slope, but standing out from it. A simple geometrical consideration is that you may have to fill all the way down to the bottom of the slope. Even on slopes of much lower angle I have noticed that, lacking any kind of retaining structure, loose soil will spread out so much that anything less than three-quarters bench is not really viable.²

A second problem is that the filled portion of the bench is never as solid or hard as as the cut portion. (We used to do a lot of "compacting". Well, we thought it was a lot, but it was nothing compared to a couple thousand feet of ice sitting on top of the ground for a couple thousand years. Which is typical of much of the glacial till in the northern United States.) The filled portion may shift downhill, or settle. It also erodes faster, being softer material. All of these factors lead to a characteristic typical of half-bench construction: split trail. There is a little ridge down the middle, and a quite excessive outslope on the outer portion of the tread. The outer portion quickly erodes away, forcing users onto the outer edge of the half-trail that remains, which quickly succumbs, leading to trail creep, or even a blowout. (The red lines in the diagram show the typical evolution of half-bench built tread.)

As the USFS Trail Construction and Maintenance Notebook says (all editions, p. 24): "... relying on fill for part of the trailbed is a bad idea." It bears repeating: a BAD IDEA! The Forest Service states that full-bench "is often more costly", but that is only the initial cost. Factor in the cost of having to come back in a year or two to rebuild it, and it is cheaper to do it right the first time – with full-bench construction.

The bench (or shelf) on which the tread is built is cut full-width on the solid, undisturbed soil (brown). The fill (yellow) is used to raise the tread up to grade.

Even on a low-angle slope: do not use half-bench. If a rock or other obstacle prevents making a full width cut into the slope at the elevation desired, try this: cut down the entire width of the trail to full-bench, but slightly below grade, then redistribute the soil across the entire width. You still have loose, soft fill, but now it is supported on top of a full width subgrade bench. Any settling or compaction will be uniform across the entire width, not concentrated on one side, or (worse) on a wedge that is being forced down slope. Fill should always be uniform in cross-section and depth, lest it develop an irregular shape that contributes to failure.

The basic appeal of half-bench construction is that in respect of cut and fill it is "balanced", with no excess to dispose of. As an advocate of full-bench construction I am sometimes asked, what do you do with all that excavated material? As a practical matter I have always found places to use it. It may have to be moved a little ways, but that's what wheelbarrows are for.³ Slopes are always imperfect, with lots of dips and little gullys to fill, and roots to ramp over; fill can always be used.

There will be occasions where it may be useful to build out – that is, to fill. To avoid the problems of half-bench construction two things must be done. First, protect the outside edge! Even if you decide a wall is not necessary, be sure to protect the outside edge, either by armoring it with a curb log or such, or providing a wide margin. Second, give the tread uniform density by cutting down and filling, as described above.

Edges and blowouts.

Some initial missteps on the edge, then a damaged edge starts moving in. The rock on the right is probably not helping.

The outside edge of the tread is the most vulnerable part of a trail. If someone stomps hard on the inside edge not much is going to happen: the soil is supported on all sides, and is not going anywhere. But in typical trail construction the outside edge – even if it is full-bench construction – has a side. If someone over-steps the edge they likely will carve some material off that side and push it down slope. This steepens the side at that spot, and may even undercut the edge. If they come down on top of the edge, the force transmitted downward is unopposed on the side, which is likely to result in soil being moved outwards, and then lost down slope.

If this happens only rarely, on a seldom visited trail, the result is barely consequential, and an annual flying visit to fix any divots and such may be an adequate remedy.

But on a frequently used trail, one with even a few users per day, the results can be insiduously accumulative. One misstep is soon joined by another; the edge is worn down, and creeps closer to the center of the trail. This exposes it to even more traffic, until that spot becomes the familiar half-bowl shape where every step grinds on a surface that has become too steep. Both the process and the result are called a blowout. What started out as a minor edge problem can become so extensive that repair requires a substantial wall.

Right: Narrow spot on a busy trail: a big problem is developing.

The prevention of blowouts is simple: keep traffic off of the edge of the trail. On a busy trails this starts by making the trail wide enough to accommodate the peak volume of traffic, which means wide enough for concurrent uphill and downhill traffic.⁴ On both well used and lesser used trail the physical edge needs to be protected from errant footsteps by a buffer. This is done by cutting the bench wide enough to leave a margin on either side of the tread. Ideally the outer margin will be on a thick layer of roots and vegetation that not only can resist and recover from the occasional errant footstep, but will be distinctly uninviting to walk on. (If not, consider augmenting it with a lot of three-inch diameter or larger rock. See picture.) Note that the inside margin also protects the outside edge, as it reduces the tendency of vegetation to push people off the trail. (I don't know about the rest of the country, but in the Pacific Northwest some of the ferns are pretty intimidating.) So if your specification calls for a two-foot wide tread, cut the bench at least three feet wide, leaving a six-inch margin on either side. (And this is the bare minimum for a narrow trail.)

As a trail gets busier it is important to ensure that there is adequate width for users to pass without having to step off the trail. Too many hikers are in too much of a hurry to wait for on-coming hikers to clear a narrow spot; they will be off the trail and going across rough and steep slopes with nary a slowdown.

Crib logs and walls.

There will be places where there will be no room for margins, or the edge will be raw fill (perhaps crossing a small gully). In such places we used to place so-called "curb logs". Not much in the way of logs (typically only two or three inches thick, always small enough to move by hand, and pinned in place by one-inch pegs), they were hardly effective curbs, and seemed more like a woodsman's version of a fog line at the edge of a highway. Over the years I realized that in such cases it was much better to bury a substantial curb (or "crib") log at the outside edge. This is less likely to get kicked aside, and much more able to protect the edge.

Log on rock bolsters.

The proper use of curb logs is a minor art form in itself, which we will not take up. But I strongly recommend that they be:

• a minimum of six inches in diameter,
• settled on well-founded rock bolsters (see picture, right),
• continuous across the gap that is being filled, ⁵
• set into the fill, not set above it, and
• only one log high. ⁶

Nice crib wall with rock filler. ⁸

A retention "structure" only one log high – even if that log is three feet in diameter – is fairly straightforward. But try stacking a second log on top of that, and matters can get very tricky. Any kind of structure more than one log high is a wall⁷, and walls can be very tricky to do right. You might decide that you do not need a wall. But if you do need a wall, then you must build it properly. This can be done with logs or dimensional timber, but consider that those will eventually have to be replaced. Rock is best, but this is a very large topic which will not be addressed here. It can also be a very serious effort. If that is more than you can handle, consider if another route is better. Or if another group can help. But don't take shortcuts here. If you can't do it right, don't do it, as doing something wrong is often worse than doing nothing at all.

Summary

The basis and fundamental requirement for building good switchbacks is the ability to build good traverses. The material that has been presented here is not a complete course in the building of traverses, but covers some crucial but often overlooked aspects, such as full-bench construction, using margins, and protecting the outside edge of the trail.

Back to Switchback Theory and Principles.

Notes.

1. The proper amount of in- or out-sloping is between 2 and 4%, perhaps as much as 6%. Enough for water to drain, not so much as to turn an ankle. This is too subtle for untrained volunteers. Even experienced trail builders, going by eye and feel alone, often overdo it. If you don't have (or use) a level I would recommend ignoring outslope. Get the grade right, and the cross-section as nearly horizontal as you can; come back and tweak the outslope if there are any problems. But it would be better to measure it!
2. Without a retaining structure of some kind, pushing fill over the edge is such an inefficient way of building tread out that it is really wasteful. My experience suggests that moving cut material up or down the grade to fill dips and hollows is much more efficient, and more useful.
3. Use wheelbarrows with the smaller four cubic-foot tray. Even if only half of your volunteers are strong enough and skilled enough to use them, the rest can shovel and rake. Your productivity will be double or more than with buckets, and less likely to drive volunteers away. See tip.
4. Harvey Manning – founder of the Issaquah Alps Trails Club – used to decry "freeways in the woods". Like many of us, he preferred a narrow, woodsy trail with ferns tickling our ankles. But such a sylvan trail could hardly survive the stresses of any popular trail. Once the traffic reaches a certain density a wide, hardened tread is necessary to avoid loss of the trail.
5. I.e., supported on solid ground (via the rock bolsters) on either end. Short pieces not well-founded are easily kicked aside, and amount to little more than decoration.
6. Strictly speaking, cribbing, or cribwork, would be a framework of timbers for holding back fill, usually extending into the fill. This can be very dicey for trail work, as in 15 to 25 years the cribbing rots out, and everything has to be dug out and replaced. (Or that piece of trail abandoned.) For a single log at the edge of the trail this is not too great of a problem.
7. A key difference between curb (or crib) logs, as I envision them, and walls, is that curb logs (properly done!) are supported at the ends, while walls stand on a base. For walls, and especially rock walls, it is essential to build a proper base that will support it, and tie it into the slope. There are other possibilities, but I think I am safe in saying that in all cases involving fill there must be either a proper wall, or a curb log (or something like it) to protect the outside edge.
8. Picture of a very nice crib wall on the Mount Si (WA) trail. Though somewhat problematical for the distinction I make in the note above. It should be noted that by "walls" (of the retaining kind), and especially rock walls, is usually meant what is more accurately called a gravity wall, meaning a kind of retaining wall that relies on massiveness – and the pull of gravity, and a good base – to resist the lateral force of the fill. A curb log (properly emplaced) also relies on gravity and a certain amount of massiveness to stay in place, but the force resisted is transmitted not to a base, but to the ends. (And hopefully to proper bolsters that can resist that force.)
      "Cribbing" – the use of a framework of small timbers and such in a retaining structure – is somewhat bastardized in that it depends on both methods. (Note both the longitudinal pieces, and the ballast of small rock.) Because cribbing itself is relatively flimsy, it also depends on tying the structure into the fill (note the ends of the transverse pieces sticking out of the wall). Which means that the entire fill must rest on a suitable base, lest it go sliding down hill in the manner of half-bench construction (bad!).
      The difficulties (and potential liabilities) of designing retaining walls is illustrated by the cautiousness of engineering authorities (e.g., for walls six to eight feet high the City of Seattle requires "four-man" rock – around 3,000 pounds each), and in the comment of one structural engineer that the design of retaining walls is "best left to the competition". Trail builders seldom have that option; we really need a definitive, comprehensive guide to the construction of walls suitable for trails.