The first two or three courses of an earthbag wall are filled with gravel only, with no clay or soil, so they act as a stem wall and moisture break that keeps water from wicking into the earth-filled courses above. Tamped courses are about 10-12 cm high, so two to three gravel courses lift the earth fill roughly 20-36 cm off the foundation.
Part of Earthbag Building
Gravel-filled bags at the base of an earthbag wall form a drained, non-wicking stem that separates the foundation from the earth fill.
Gravel Stops Water Climbing Into The Wall
Moisture climbs through fine soil by capillary action, in the same way it climbs a wick. The finer the pores, the higher it rises. Gravel has large voids between the stones, so it cannot lift water; it simply drains. A gravel-only base therefore breaks the connection between wet ground and the clay-bearing courses above. Earth-filled bags placed at ground level, with no gravel, wick moisture upward and soften.
What You Need
- Clean gravel, without clay or soil mixed in.
- Bags the same as for the rest of the wall.
- A tamper, a string line and a level.
- Barbed wire, laid between these courses as for every other course.
Method: Lay The Stem Courses
Step 1: Check the foundation top is level before the first bag goes down.
Step 2: Fill bags with gravel only. Fill about 80% of the bag volume so it can be closed and does not overstuff. Use the same funnel as for earth bags.
Step 3: Place and tamp the first course, fold side down, and tamp until the top is flat. Course height after tamping is roughly 10-12 cm.
Step 4: Lay two strands of barbed wire along the course before the next one.
Step 5: Repeat for the second and third course, stagger the joints half a bag length in running bond, then start the earth-filled courses; see Bag Filling and Placement.
Worked Example
- Inputs: tamped course height 10-12 cm, two to three gravel courses.
- Operation: height = number of courses x course height. Low: 2 x 10 cm = 20 cm. High: 3 x 12 cm = 36 cm.
- Result: the stem is about 20-36 cm tall. If heavy rain splashes up from the ground against the wall, or the ground slopes toward the building, add a course. This is an inference from the arithmetic, not a stated requirement.
Common Mistakes
- Mixing soil into the gravel. Fine particles give back the capillary path.
- Too few gravel courses for the splash zone. Rain striking bare ground bounces onto the wall base.
- Forgetting the wire. The gravel courses are part of the wall and need locking like the rest.
Safety
Heavy bags and loose gravel
A filled gravel bag is heavy; lift with a partner or fill the bag in place. Wear eye protection when tamping gravel, since stones chip.
What’s Next
- Learn next: Moisture Management — the rest of the water defences
- Combine with: Rubble Trench and Grade Beam — the base beneath