- Climbing Turns
- Crib Walls and Other Retaining Structures
- Maintaining Special Structures
This section covers some of the more complex trail structures. Switchbacks, crib walls, and similar structures are common in trail construction. They are often relatively expensive and difficult to design and construct correctly. Inadequate maintenance greatly shortens their useful lives. However, a well-designed, well-built trail structure can last for decades and be quite unobtrusive.
Special structures are more frequently used on low-challenge trails where easy, wide, and smooth grades are the management goal. The cost of building relatively easy trail into progressively more difficult terrain quickly becomes prohibitive (remember how many fortunes and lives were poured into constructing railroads). Keep in mind the Minimum Tool philosophy and build only as many structures as you absolutely need to reach your goal.
Switchbacks and climbing turns are used to reverse the direction of travel on hillsides and to gain elevation in a limited distance. What is the difference between the two? A climbing turn is a reversal in direction that maintains the existing grade going through the turn without a constructed landing. A switchback is also a reversal in direction, but has a relatively level constructed landing (Figure 57). Switchbacks usually involve special treatment of the approaches, barriers, and drainages. They are used on steeper terrain, usually steeper than 15 to 20 percent. Both of these turns take skill to locate and are relatively expensive to construct and maintain. Choosing when to use each one is not always easy.
Figure 57--Climbing turns can be built on gentler slopes,
usually not more than 15 to 20 percent. Switchbacks are
needed on steeper sites.
Trail designers should make every effort to minimize the use of these turns. Planning carefully to avoid impassable or very difficult terrain reduces the need for switchbacks and climbing turns.
User psychology (human or animal) is more important to the success of these structures than any other trail structure. The turns must be easier, more obvious, and more convenient than the alternatives. They work best when terrain or vegetation screens the view of travelers coming down the upper approach toward the turn. Avoid building sets of these turns on open hillsides unless the terrain is very steep. It's usually best not to build turns, or the connecting legs of a series of turns, on or across a ridge. The local critters have traveled directly up and down these ridges since the last ice age. They are not going to understand why you are building low hurdles in their path...and they will not be forced onto your trail and turns.
Try to avoid "stacking" a set of switchback turns on a patch of hillside. Long legs between turns help reduce the temptation to shortcut. Staggering the turns so that all legs are not the same length reduces the sense of artificiality. Keep the grade between turns as steep as the design challenge level allows. Remember, travelers will cut switchbacks when they feel it's more convenient to cut the turn rather than stay on the tread. The designer's goal is to make travel on the trail more attractive than the shortcut (Figure 58).
Figure 58--Long sections of trail between switchbacks
are usually better than short--fewer switchbacks
will be needed, with fewer turns to shortcut.
Next to waterbars, climbing turns are the trail structure most often constructed inappropriately. The usual problem is that a climbing turn is built (or attempted) on steep terrain where a switchback is needed. A climbing turn is built on the slope surface, and where it turns, it climbs at the same rate as the slope itself. If the slope is 40 percent, the turn forces travelers to climb at 40 percent. It is almost impossible to keep a climbing turn from eroding and becoming increasingly difficult to travel if the slope is steeper than 20 percent.
The advantages of climbing turns in appropriate terrain is that a larger radius turn (4 to 6 m, 13 to 20 ft) is relatively easy to construct. Trails that serve off-highway-vehicle traffic often use insloped, or banked, climbing turns so that riders can keep up enough speed for control (Figure 59). Climbing turns are also easier than switchbacks for packstock to negotiate. Climbing turns are usually less expensive than switchbacks because much less excavation is required, and fill is not used.
Figure 59--Climbing turns continue the climb through
the turn. They can be insloped or outsloped.
The tread at each end of the turn will be full bench construction, matching that of the approaches. As the turn reaches the fall line, the amount of material excavated will decrease. In the turn, the tread will not require excavation other than that needed to reach mineral soil.
Guide structures should be placed along the inside edge of the turn. Temptation-reducing barricades can be added if necessary. The psychologically perfect place to build climbing turns is through dense brush or dog-hair thickets of trees. Be sure to design grade dips into the approaches.
Switchback turns are harder to build correctly, but retain stable tread on steeper terrain. Most switchbacks are constructed to a much lower standard than is needed. The key to successful switchback construction is making an adequate excavation, using appropriate structures to hold the fill in place, and building psychologically sound approaches.
Look for "natural" platforms when you are scouting for possible switchback locations. Use these for control points when locating the connecting tread. These will save you a lot of time later by reducing the amount of excavation and fill needed.
A switchback consists of two approaches, a landing or turn platform, a drain for the upper approach and platform, and guide structures. The upper approach and the upper half of the turn platform are excavated from the slope. Part of the lower approach and the lower half of the turn are constructed on fill (Figure 60).
Figure 60--Switchback with a radial turning platform.
The approaches are the place where most of the trouble with switchback turns start. The approaches should be designed for the primary user group. In general, the last 20 m (65 ft) to the turn should be as steep as the desired challenge level will allow. This grade should be smoothly eased to match that of the turn in the last 2 to 3 m (6.5 to 10 ft).
Do not "flatten" the grade 20 m (65 ft) before the turn. If anything, steepen the approach grades to foster the sense that the switchback is the most convenient way of gaining or losing altitude. There is absolutely nothing as infuriating as walking a nearly flat grade to a distant switchback turn while looking several meters over the edge at the nearly flat grade headed the other direction. You can build a Maginot Line of barricades and still not prevent people, packstock, and wildlife from cutting your switchback. The only exception is a trail designed primarily for wheeled vehicles. The flatter approach makes control easier.
As the upper approach nears the turn, a grade dip should be installed. The tread below this point should be insloped until the halfway point in the turn. Both sides of this trench should be backsloped to an angle appropriate for the local soil. As the turn is reached, the tread should be widened 0.5 m to 1 m (1.6 to 3.2 ft) wider than the approach tread. This is particularly important on small radius turns and for wheeled vehicles. It's less necessary for hikers and packstock.
The turn can be a smooth radius ranging from 1.5 to 3 m (5 to 10 ft) or a simple Y-shaped platform. A smooth radius turn is important if the trail's use includes wheeled traffic or packstrings. The Y platform works for hikers (Figure 61). The turn platform is nearly flat, reaching no more than a 5-percent grade. The upper side is excavated from the side slope and the borrow used to construct the fill on the lower side. Switchbacks on steep sideslopes can require very large excavations to reach a stable backslope angle and provide clearance for packstock loads. The greater the turn radius, the wider the platform, or the flatter the turn, the greater the excavation required. A point may be reached where a crib wall is needed to keep the backslope to a reasonable size.
Figure 61--Switchback with a "Y" turning platform.
The amount of tamped fill required on the lower side of the turn will usually be at least as much as excavated from the upper side unless a crib wall is used to support the fill. A crib wall is absolutely necessary where the terrain is steeper than the angle of repose for the fill material.
The tread in the upper portion should be insloped, leading to a drain along the toe of the backslope. This drain should extend along the entire backslope in the trench and be daylighted (have an outlet) where the excavation ends. Construct a spillway to protect the adjacent fill from erosion. You may need guide structures--rock walls or logs are common--on the inside of the turn to keep traffic on the trail.
Construct the approach on the lower side of the turn on tamped fill. The crib wall should extend for most of this length. The tread on the lower portion of the turn should be outsloped. The fill section transitions into the full bench part of the approach; the approach changes grade to match the general tread grade.