Standards and Procedures:
Rock Retaining Wall
Retaining walls are constructed to hold the tread in place on sections of trail where the treadway is unstable and erodes down the side slope. These are often areas with very steep side slopes, or spots where the trail was built along a river embankment. A well-built rock retaining wall is among the longest lasting and most beautiful of trail structures. Build these walls with the mindset that they should last 100 years or more.
Rock retaining walls require a considerable amount of material and time to build well. If possible, it is generally preferable to redig the stretch of trail to full bench specifications, pulling the tread back from the unstable edge of the slope. If there’s no way to establish a stable tread without shoring up the edge of the trail, then some form of retaining structure is needed and rock is the most sustainable material.
In the Pisgah Ranger District, the ability to build with rock is limited. We are constrained by NEPA rules, which limit the harvesting of rock to that which is already within the trail corridor. Harvesting rock outside the corridor requires additional NEPA surveys and approvals, a process which can take years and so must be reserved for major projects.
Due to the limitations in rock, it may often be necessary to use black locust instead to construct a log crib, which serves the same function as a rock retaining wall but is quite different in form.
This document will outline the most important guidelines for building sustainable rock retaining walls that stand the test of time.
Below is a recommended list of essential tools for building a rock retaining wall.
- Rock bars (at least 2)
- Pick mattock
- Double-jack sledge hammer
- Single-jack sledge hammer (several)
- Prospectors pick/mini-pick/rapid-digger - for digging in tight spaces
- Cargo/rock net
- Drag slings
- Buckets or canvas rock bags for transporting gravel/crush
- Carbide-tipped stone-shaping hammers and chisels
- Rotary hammer drill with carbide-tipped auger or chipper bits (gas powered, or electric with portable generator)
- Feathers/pins-and-wedges - sized to match your drill bit.
- Griphoist rigging setup for major rock projects
- Large stones are preferable (at least 2-3 cubic feet), although a variety of sizes can be used and some smaller stones will be necessary to piece together a tight wall.
- The best stones are large and rectangular. Angular edges make for easier installation.
- If possible, flat surfaces should be faced to the outside of the wall, creating an aesthetically-pleasing wall.
- The base layer of rocks should be set in an excavated footing of mineral soil that is angled back into the hillside (batter).
- The base layer of rocks should ideally be buried in mineral soil so that they are flush with the excavated footing.
- The base layer should be composed from among the largest rocks available.
- Rocks should have good contact with the rocks on either side.
- If possible, tie either end of the retaining wall into existing bedrock or boulders to provide immovable anchors for the wall.
- On multiple tier walls, each rock in a tier should have at least 3 points of good contact with the rock layer below it.
- On multiple tier walls, take care to stagger joints. This means the above rock should be contacting at least two rocks below it, and the joints should never line up.
- Preserve the angle of the wall (batter - i.e. tilted back toward the trail) all the way to the top of the wall.
- The higher the wall, the more batter is needed for stability (i.e. a higher wall is less steep).
- Fill behind with crushed rock as you build each tier, tamping with hammers to pack fill into all crevices and prevent shifting and settling later on.
- Use smaller stones as necessary to fill gaps and piece together a solid wall. Take care not to shim from the outside with smaller stones, however, where they could pop out over time.
- Use occasional tie rocks, also called “dead men” or headers, to extend beyond the back of the building stones and tie the wall more securely into the hillside. Reserve longer rocks for this purpose.
- Reserve some of your largest stones (besides the base layer) for the capstones. The weight and size of the capstones holds the lower stone tiers in place and protects them from traffic.
- Be sure to maintain a 2-5% tread outslope that allows sheet flow of water over the finished retaining wall. Retaining walls should not trap water on the trail.
These procedures are intended to provide a helpful reference guide. If you have never built one of these structures before, please learn from an experienced trail builder.
A. Survey the Site
1. Determine the length of the wall.
2. Determine how far out you must start to incorporate the width of the wall and the batter (inward angle).
3. Are there existing rocks you could tie in with?
B. Excavate Footing
1. Dig out a stable footing into mineral soil, with proper batter tilt.
C. Quarry Rock
1. Use rock within the trail corridor. Dig off-trail in NEPA approved sites only.
2. Search for large, rectangular rocks and rocks with angular surfaces
3. Collect various smaller rocks for filling gaps and crevices.
4. Dig/transport to site with rock bars, nets, slings, and/or rigging
D. Install Base Layer
1. Bury the bottom layer of rocks flush with the mineral soil in the footing, tilted in to the hill.
E. Build Intermediate Tiers
1. Stack layers one by one, stagger joints.
2. Check for good contact, fill gaps.
3. Fill behind with crush/gravel and tamp thoroughly to fill spaces.
4. Repeat until only enough space remains for capstones.
F. Install Capstones
1. Use large rectangular rocks, large enough to withstand traffic. Ensure there is no wobbling.
G. Finish Work
1. Add/Tamp Crush where needed.
2. Top exposed crush with mineral soil.
3. Re-grade, reestablish tread outslope.
4. Revegetate work area, including visible craters from quarrying.