Additional Trail Elements Cont.
The amount of tamped fill required on the lower side of the turn will usually be at least as much as was excavated from the upper side unless a retaining wall is used to support the fill. A retaining 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 and be daylighted (have an outlet) where the excavation ends. Construct a spillway for the drain 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 retaining 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.
Try to avoid "stacking" a set of switchback turns on a hillside. Long legs between turns help reduce the temptation to shortcut. Staggering the turns so that legs are not the same length reduces the sense of artificiality (figure 72). Keep the grade between turns as steep as the challenge allows. Remember, travelers will cut switchbacks when they feel it's more convenient to do so than to stay on the tread. The designer's goal is to make travel on the trail more attractive than the shortcut.
Maintaining climbing turns and switchbacks requires working on the tread, improving drainage, and doing any necessary work on retaining walls, guide structures, and barricades. The tread should be insloped or outsloped as necessary, slough should be removed to return the tread to design width, and tread obstacles should be removed.
Retaining structures keep dirt and rock in place. The retaining wall keeps fill from following the call of gravity and taking the tread with it. Retaining walls are useful for keeping scree slopes from sliding down and obliterating the tread, for keeping streams from eroding abutments, and for holding trail tread in place on steep sideslopes.
Two common retaining structures are the rock retaining wall and the log crib wall. Of course, rock is more durable and lasts longer than wood.
Rock retaining walls are used when a sturdy wall is needed to contain compacted fill (figure 73) or to hold a steep excavated backslope in place (figure 74). Rock retaining walls are also called dry masonry because no mortar is used between the rocks.
Ideally, the bigger the rock, the better. Big rocks are less likely to shift or become dislodged. At least half of the rocks should weigh more than 60 kilograms (130 pounds). The best rock is rectangular with flat surfaces on all sides. Round river rock is the worst.
To build a rock retaining wall, excavate a footing to firm, stable dirt or to solid rock. Tilt the footing slightly into the hillside (batter) so the rock wall will lean into the hill and dig it deep enough to support the foundation tier of rocks (these are usually the largest rocks in the wall). Ideally, the footing is dug so that the foundation tier is embedded for the full thickness of the rocks.
The batter should range from 2:1 to 4:1 (figure 75). Factors determining this angle include the size and regularity of the rock, the depth of header rocks, and the steepness and stability of the slope. At batter angles steeper than 4:1 or so, cement, internal anchors, or both, may be needed for stability.
Drawing of a rock retaining wall on a sloped hillside. The drawing includes the text:
The keystone is laid into the footing and successive tiers are laid. For each tier, overlap the gaps between rocks in the next lower tier, called breaking the joints. Each tier should be staggered slightly into the hill to create the desired amount of batter. Header rocks are long rocks turned and placed so that they extend deep into the hillside. Using header rocks is particularly important if the wall's cross section widens as the wall gets higher.
Rocks in each successive tier should be set so they have at least three points of good contact with the rocks below. Good contact is defined as no wobble or shifting under a load without relying on shims (or chinking) to eliminate rocking. Shims are prone to shifting and should not be used to establish contact, especially on the face of the wall, where they can fall out. Add backfill and tamp crushed rocks into the cracks as you build.
The Right Rock
In reality, you have to use the rock that is available. Small walls can be constructed successfully from small rocks. The key is the foundation and batter. Remember to save some large rocks for the capstones. A final point--most rock can be shaped with a few good blows with a rock hammer and carbide-tipped rock chisel. Placing rock on dirt rather than another rock before striking it will help ensure that the rock breaks where you want it to.
Log walls are designed to keep compacted fill in place (figure 76). Construct wood walls by interlocking logs or beams, pinned or notched (for logs) at the joints. Lay sill logs at right angles to the direction of travel and alternate tiers of face logs and header logs (figure 77).
Each successive tier is set to provide enough batter to resist creep pressure from the slope and to reduce pressure on the face logs from the fill. The ends of the header logs are seated against the backslope of the excavation for stability. As fill is tamped in place, filler logs are placed inside the structure to plug the spaces between the face logs. Filler logs are held in place by the fill.
Drawings of crib walls. One looking into hillside and the other a side view. The drawing includes the text:
Outslope the tread to keep water from saturating the fill and excavation. Use guide structures to keep traffic off the edge of the tread.
All retaining structures should be checked carefully for shifting, bulging, or loose structural material. Make sure that all the footings are protected from erosion. Anchor guides should be secure.