Rock Cribbing

Sometimes, we need to build retaining walls, or crib walls to build trail to cross deep gullies or to go around rock ledges that we find along the survey line of a new section of hiking trail.

CCC Crib Wall at BlackrockCCC Crib Wall at Blackrock

Note: This article describes how we do that, conforming to accepted standards such as those published by the USDA Forest Service, Trail Construction and Maintenance Notebook.

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 side slopes.

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 or to hold a steep excavated backslope in place. 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 130 pounds. The best rock is rectangular with flat surfaces on all sides. Round river rock is the worst.

Construction

Crib Wall ConstructionCrib Wall Construction

Footing

  • Excavate a footing to firm, stable dirt or to solid rock
  • Tilt the footing slightly into the hillside (batter) so the rock walls into the hill
  • Dig it deep enough to support the foundation tier of rocks (these are usually the largest rocks in the wall)
  • Ideally, dig the footing so that the foundation tier is embedded for the full thickness of the rocks.

Batter

  • The batter should range from 2:1 to 4:1
  • Factors determining this angle include the
    • Size and regularity of the rock
    • Depth of header rocks
    • 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.

Keystone

  • On short walls, it may be possible to construct the entire structure starting upon a single keystone
  • Lay the keystone into the footing before laying successive tiers

Tiers

  • For each tier, overlap the gaps between rocks in the next lower tier, called breaking the joints
  • Stagger each tier 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 cross section of the wall widens with height
  • Set rocks in each successive tier 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 use the rock that is available.
  • Small walls can be constructed successfully from small rocks. The keys are the foundation and batter.
  • 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.
Carefully unspool the wire
Rig the line
Set the shackling
Rig the rock
Carefully haul in the line
Check the rigged rock
Pry the rock into the cribwall
Check the cribwall alignment
Modify as needed
Backfill the tread

Terminology

Batter The amount that the wall leans into the hillside.
Outslope Out-sloping lets water sheet across the trail naturally. The tread should be out-sloped at least 2-5%.
Tread The actual travel surface of the trail.
Backfill Mineral soil and/or small rock.
Capstone Rock with sufficient mass and shape to provide a stable top course.
Header rock Rock that extends or “ties” the wall to the backfill.
Foundation course The bottom layer of rock that provides a stable and in-sloped base—usually the largest rocks. These must be keyed into solid ground, not fill.