  Privacy
| Legal |
Table of Contents Back | Next | Home | Cover Page |
 Technology & Development Center |
Trail Construction and Maintenance Notebook
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).
![[diagram] Switchback](images/fig060.jpg)
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.
![[diagram] Switchback with retaining wall](images/fig061.jpg)
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.
 |
E-mail: