Standards and Procedures:
Locust Log Cribbing
Crib walls, also known as retaining walls, are constructed to hold or repair 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.
Log cribs are one of the more time consuming and challenging trail structures to build. They also require a large amount of black locust logs - a precious resource within the district. For these reasons, think hard about whether a crib wall is really needed. 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. Also consider whether building with rock might be an option. A well-constructed rock wall can last virtually indefinitely, while a log crib will eventually rot out. NEPA regulations and poor rock availability in many areas mean that sometimes, log cribbing might be the only option. When this is the case, it’s critical to build log cribbing of the highest quality, making the best use of limited black locust resources within the forest and creating a structure that lasts as long as possible.
Log Cribbing - Recommended Tools and Equipment
Below is a list of the essential items for building log cribbing.
- Chainsaw with a sharp chain
- Extra sharpened chain recommended
- Draw knives for peeling bark
- Power drill capable of drilling into locust wood (portable battery powered or gas powered are both fine, just make sure they can handle locust wood and bring extra batteries/fuel)
- 3/8 ” auger drill bit (or to match rebar diameter)
- 3/8” landscape spikes 8-10” long, or rebar for larger logs
- Single-jack Sledge hammer
- Tape measure
- Buckets or canvas rock bags to transport gravel.
- Assorted digging tools (picks, pulaskis, etc.)
Log Cribbing: General Standards
- Relatively thicker logs are preferable for cribbing, as they have more weight and stability.
- Use the heaviest logs at the bottom of the crib wall.
- Bury the bottom log tier in the ground for maximum foundational strength.
- Use headers, also known as “dead men” or tie logs, in between each tier. These logs are notched into the face logs and extend into the hillside as far as the inside edge of the trail. Headers give the wall much more stability and hold it in place.
- Header/deadmen logs should be spaced no more than 8’ apart.
- Angle the wall slightly in toward the hillside. This angle is known as “batter” and helps the wall withstand the pressure from the combined weight of fill, tread, and trail users over time.
- All logs should be pinned to the logs directly below, using spikes or rebar.
- Use miter joints to join logs if necessary to span the length of the crib.
- Stagger any joints as you add tiers to the wall. Joints should not stack one over another.
- Use notching to attach header/tie logs, creating a “lincoln logs” style structure with maximum stability.
- Fill behind the crib with rock and gravel, using larger rocks to plug gaps between logs in the wall face and smaller rock/gravel to tightly fill empty spaces.
- Pack all fill rock with hammers as you go so that fill is compacted and doesn’t shift and settle later on.
- Top the crib with mineral soil and grade to create a stable, smooth treadway with proper 2-5% outslope. Cribbing should not trap water on the trail.
Log Cribbing Construction Procedures
These procedures are intended to provide a helpful reference guide to building log ladder staircases, ensuring that you cover the essentials. If you are new to building this structure, it is best to learn hands-on 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).
B. Excavate Footing
1. Dig out a stable footing into mineral soil, with proper batter tilt.
C. Prep Locust Logs
1. Peel bark
2. Determine best logs and best orientations
3. Cut logs to length
D. Install Base Layer
1. Bury the bottom log flush with the mineral soil in the footing.
E. Build Intermediate Tiers
1. Notch “dead men”/tie logs perpendicular to base log at 5-8’ intervals, extending into treadway
2. Drill/Spike in place
3. Fill/tamp with crush - use larger rock to block gaps between logs
4. Notch the following face log so it sits on top of the dead men logs
5. Drill/Spike in place
6. Fill/tamp with crush - use larger rock to block gaps between logs
7. Repeat until wall is level with tread
F. 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