Special Structures (continued)
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).
Construct wood crib by interlocking logs or beams, pinned or notched (if 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 62). 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, and are held in place by the fill. Outslope the tread to keep water from saturating the fill and excavation. Use guide structures to keep traffic off the edge.
Figure 62--The characteristics of a crib wall.
Treated logs are recommended.
Wood crib is also used to construct piers for bridges and to hold rock fill for abutments. Wood crib is easier to build than rock cribbing, but is less durable, especially in environments visited by rot or fire. Be sure to select rot-resistant logs if using native materials.
Rock or crib retaining walls are used when a sturdy wall is needed to contain compacted fill or to hold an excavation wall in place. Rock retaining walls are also called "dry masonry" because no mortar is used between stones. Rock, when available on site, is preferred over logs.
To build a rock wall, excavate a footing in soil or to solid rock. The footing should be insloped to match the designed batter angle and deep enough to support the foundation tier of stones (these are usually the largest stones in the wall) for the full width of the tread. Ideally, the footing is dug so that the foundation tier is embedded for the full thickness of the stones (Figure 63).
Figure 63--Rock placement and batter is
critical on the first courses.
Ideally, the stones should weigh at least 20 kg (45 lb). At least half of the stones should weigh more than 60 kg (130 lb). The ideal stone is rectangular with flat surfaces on all sides. The worst stone to use is rounded like river rock.
|In reality, you have to use the rock available. Small crib walls can be successfully constructed from smaller rocks. The key is the foundation platform and the batter. Remember to save some big rocks for the top course where you need them for capstones. A final point--most rock can be improved with a few good blows from a rock hammer. Placing the rock on dirt rather than another rock before striking will help ensure it breaks where you want it to.|
The batter should range from 2:1 to 4:1 (Figure 64). Factors determining this angle include the size and regularity of the rock, the depth of header stones, and the steepness and stability of the slope. At batter angles steeper than 4:1 or so, cement, or internal anchors (or both) may be needed for stability.
Figure 64--Terms used to describe crib walls.
On short walls, it may be possible to construct the entire structure starting upon a single keystone. The keystone is laid into the footing and successive tiers are laid. Each tier's face stones overlap the gaps between stones in the next lower tier. Each face tier includes tie or header stones that overlap the gaps between face stones and those deeper in the wall. The foundation tier (or the keystone) should be insloped slightly and rest on the excavated surface, not on fill. Each successive face tier should be staggered slightly into the hill to create the desired amount of batter. Header stones should also be used to tie deeper stones to those closer to the face. This is particularly important if the wall widens in cross section as it gains height.
Stones in each successive tier should be set so they have at least three points of good contact with the stones 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. Backfill and tamp as you build.
Other forms of retaining walls include gabion and variations of wet masonry. Gabion is a series of wire baskets filled with rock. The baskets are wired together in tiers and can be effective where no suitable source of crib stone is available. Gabion is more artificial looking than crib (in the eyes of traditionalists at any rate), and may have a shorter 'lifespan,' depending on the type of wire used and the climate.
Steps and stairways are used to gain a lot of elevation in a short distance. Steps are common on steep hiking trails in New England and elsewhere, less common (but not unheard of) on western trails used by horses and mules. Wooden steps of all configurations are common in coastal Alaska (Figure 65).
Figure 65--Step and run stairs in Alaska
(plank boardwalk in foreground).
Sometimes steps are used in an existing trail to fix a problem caused by poor trail location or design. The result often is out of character with the desired experience and esthetics of the trail. Before you construct steps, make sure they are consistent with the expectations of those the trail is designed to serve.
Your goal is to design the height (rise) and depth (run) of the steps to match the level of challenge desired. Steps are harder to negotiate as the rise increases. The difficulty also increases as the steps are closer together. Yet, as the trail becomes steeper, the step must either be higher or the distance between steps must be shorter. Steps can be built into a trail that traverses the slope. This allows the traveler to gain elevation rapidly, without the scary steepness of a stairway.
The components of a step are: the rise, the run, a landing on easier grades, and often retainer logs (Figure 66). The rise is the vertical distance gained at the face of each step. The run is the distance from the edge of one step to the base of the next step's face. The landing is the extension of the run above the step. In structures where the landing is composed of tamped fill material, retainer logs are used to retain the fill.
Figure 66--Common types of steps.
Hikers, especially backpackers, generally don't like steps and will walk alongside them if there is any opportunity. The steps need to be comfortable to climb or they won't be used. This means keeping the rise a reasonable 150 to 200 mm (6 to 8 in) and the run long enough to hold a hiker's entire foot rather than just their toe (Figure 67). It's helpful to armor the sides of steps with rocks to encourage users to stay on the steps.
Figure 67--A general rule of thumb for stairs:
Twice the riser plus the tread should
equal 635 to 686 mm (25 to 27 in).
In more primitive settings, you don't need a uniform flight of steps as long as the route is obvious and there is solid tread at each stepping point. In the Sierra, a cross between cobblestones and stairs, locally called riprap, is commonly used for this purpose. Elsewhere, riprap refers to rock forming a loose retaining wall.
The most important area of the step is usually in the run. This is where most traffic steps as it climbs. If the step is composed of something like a board on edge with fill behind it, then the traffic will step onto the landing. Almost all foot traffic descending the step will walk off the edge of the step. The top of the step (and landing) should be stable and provide secure footing. The edge of the step should be solid and durable. The face of each step should not contain a batter that creates a "face run" of over 50 mm (2 in) from top to bottom. This is particularly important as the rise of the step increases.
If the stairway climbs straight up the hill, each step should be slightly crowned to drain water to the edges or slightly sloped to one side. When the trail traverses a slope, each step and landing should be slightly outsloped. Water should not be allowed to descend long lengths of a set of steps or to collect on or behind a step on the landing. A drain dip where the trail approaches the top of the steps is a good idea.
Build stairways from the bottom up, at a break in the grade. The most common mistake is to start part way up a grade. If you do so, the trail will wash out below the stairs (Figure 68). The bottom stair should be constructed on a solid, excavated footing. If it is constructed on top of exposed rock, it should be well pinned to the footing. Each successive stair is placed atop the previous stair. Wood stairs are usually pinned to each other and into the footing. Dry masonry rock stairs usually rely on the contact with the stair below and with the footing to provide stability.
Figure 68--Begin laying steps at the bottom of
a grade rather than midway.
Steps with landings are a bit harder to secure in place because the stairs do not overlap. Each step can either be placed in an excavated footing and the material below the rise removed to form the landing of the next lower step. This is usually the most stable arrangement. Or the step can be secured on the surface and fill used to form a landing behind it. The material used to provide the rise does double duty as a retaining structure when the landing consists of tamped fill. These steps must be seated well to prevent them from being dislodged by traffic. For stock use, landings should be long enough to hold all four of the animal's feet.
Individual steps can be placed at any point in a trail. They are useful for retaining tread material in rocky pitches or to protect tree roots. Single steps installed midslope usually become high-step obstacles on stock trails or where there is heavy traffic or erosion.
In all steps, the key is to use the largest material possible and to seat it as deeply as possible. Rocks should be massive and rectangular. On steps that traverse a slope, it helps to seat the upper end of the step material in footings excavated into the slope.
Maintaining climbing turns and switchbacks requires working on the tread, maintaining 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 walls should be carefully checked for shifting, bulges, or loose structural material. Make sure that all the footings are protected from erosion. Guide structures should be secure. Check turn barricades for effectiveness and rebuild as necessary.
Remember, these special structures are relatively expensive and deserve careful attention to protect the investment.
Some special tactics include the use of:
- Internal anchors for increasing the stability of retaining walls.
- Precision blasting to obtain sufficient sized footings in rock.
- Power tamping equipment to strengthen fill.
- Cable systems for moving large rock or timbers to retaining wall locations.
The best way to learn how to build these structures is to seek someone who designs and builds well thought-out switchbacks, climbing turns, or walls. Have that expert conduct a seminar for your crew or actually participate in the construction.