A log crib wall is a hollow box of horizontally laid, notched logs filled with earth, gravel or rubble: the logs hold the shape and tie the two faces together, and the fill provides nearly all the mass. It lasts only as long as the logs do, because rot lets the fill spill, and it must be tied through its thickness or the earth pushes the faces apart.
A log crib is a cribwork of stacked, notched logs forming cells that are packed with earth or stone to make a thick wall or bank.
The Fill Does The Work, The Logs Hold It In
The parent article describes a rectangular frame of logs notched at the corners like a log cabin, with the interior filled with earth, gravel or rubble, and calls it a timber-faced rammed earth wall. That is the right picture: the mass is in the fill and the logs are the formwork that stays in place. A crib therefore combines the speed of timber with some of the weight of earth.
The parent article also says a crib 1 m thick filled with earth “stops anything short of explosive projectiles”. No threat, cartridge or range is named and this corpus has no test data, so that sentence is untested and should not be relied on. What can be said is arithmetic. Fill 1 m thick at an assumed 1,600 kg/m³ is 1 x 1,600 = 1,600 kg/m² of wall face, about twice the 810 kg/m² of a rammed earth wall 0.45 m thick at 1,800 kg/m³. The face logs add little; fill in a crib is also usually less well compacted than rammed earth, and it settles. Treat the figure as a way to compare walls, not a rating.
Why The Faces Need Ties
Earth in a box presses outward. Using the standard active-pressure formula for granular fill (the same one used in Retaining Wall Design), for fill with an internal friction angle of about 30 degrees (recalled typical value), the coefficient is Ka = (1 − sin 30°) / (1 + sin 30°) = 0.5 / 1.5 = 0.33. At the base of fill 2 m high at 1,800 kg/m³ (17.7 kN/m³), the sideways pressure is 0.33 x 17.7 x 2 = 11.8 kPa, over a tonne pushing on each square metre of the lowest face logs. The logs alone, notched at the corners, rely on the corner joints and on header logs that run through the thickness to hold the two faces back.
What You Need
- Logs — straight, 15 to 25 cm across, long enough for the notched cell size; see Log Construction for choosing and notching.
- Fill — earth, gravel, rubble, or broken masonry.
- Footing material — gravel or flat stones for a dry base.
- Axes, saws, shovels, rammers.
Method: Build The Cribwork In Lifts
Step 1: Prepare a dry footing. Lay sill logs on a bed of gravel or flat stones above ground level, so the lowest logs stay out of standing water. These are the logs that rot first.
Step 2: Lay the outer and inner face logs. Two parallel lines of horizontal logs, notched where they cross the header logs. Keep each course level.
Step 3: Set header logs through the thickness. Logs running across the width, from the outer face to the inner face, at regular intervals in every course or every few courses, notched into the face logs. They are the ties; without them the faces separate.
Step 4: Fill and tamp each lift. Fill the cells one or two courses at a time and tamp, rather than filling to the top at the end. A tall column of loose fill compacts under its own weight and settles, leaving a gap beneath the cap. Fill against the face logs and tamp well at the edges.
Step 5: Cap the top. Finish with a layer of earth or logs sloped to shed rain. Water entering from above is what rots a crib from the inside.
Durability And Fire
The parent article gives a crib 10 to 30 years. The part that goes first is the face logs at and just above the ground; when one fails, fill escapes through the gap and the wall lowers. Keep an eye on the lowest courses and replace logs as you go.
Fire: the log faces burn and the earth fill does not, so a fire damages a crib but the mass behind it survives. The fill protects the inside of the wall, not the face logs. Keep brush and fuel back from the face, and see Timber Fortifications for the wider fire problem.
Common Mistakes
- No header logs. The faces bow outward and the crib opens up.
- Filling after building to full height. The fill never compacts and settles.
- Logs lying directly on earth. Rot starts at once. Put them on gravel or stone.
- Wet fill sealed in. Use drained gravel and rubble in the lower courses.
A crib collapses by the face
A tall, filled crib releases tonnes of earth when a face lets go. Do not stand on a crib wall with a bowing face, do not undercut the footing, and keep everyone out of the line of the fill when you remove logs for repair.
What’s Next
- Learn next: Timber Framing for Defense — a skeleton instead of a solid wall
- Combine with: Rammed Earth Walls — compacting the fill properly