Special Structures (continued)

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.

Crib Walls and Other Retaining Structures

Retaining structures are designed to keep dirt and rock in place. The crib wall keeps fill from following the call of gravity and taking the tread with it. Retaining structures are useful for keeping scree slopes from sliding down and obliterating the tread, for keeping streams from eroding abutments, and for blocking traffic from going places it shouldn't.

The most common retaining structure is the crib wall. "Crib" is used primarily to keep compacted fill in place. Well-built crib is the most stable kind of uncemented retaining structure (except perhaps wire gabion).

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