An earthbag building is heavy, with a single-story 5 x 5 m building and 40 cm walls weighing 20-30 tonnes, so it needs a rubble trench foundation dug below frost depth and topped with two or three courses of gravel-filled bags. The trench spreads the weight and drains water, and the gravel courses stop moisture rising into the earth-filled bags above.
Part of Earthbag Building
The foundation system of an earthbag building is a drained rubble trench with gravel-filled courses forming a stem wall.
Weight And Water Are The Two Foundation Problems
Earth walls are heavy and earth absorbs water: a single-story 5 x 5 m building with 40 cm walls weighs 20-30 tonnes. The foundation has to do two jobs: spread that weight over enough ground that the soil does not sink, and keep ground water away from the earth-filled bags. A rubble trench with a drain pipe handles both, and the first two or three courses, filled with gravel instead of soil, act as the stem wall.
What You Need
- A trench below frost depth, 50-60 cm wide and wider than the wall bags.
- A perforated drain pipe laid along the bottom and sloped to an outlet.
- Compacted gravel or rubble, placed in 15 cm lifts.
- Gravel-filled bags for the first two to three courses.
Method: From Trench To Stem Wall
Step 1: Dig the trench. Take it below frost depth, 50-60 cm wide. See Rubble Trench and Grade Beam.
Step 2: Lay the drain pipe, perforated, at the bottom, sloped to an outlet.
Step 3: Fill with gravel or rubble in lifts, compacting each 15 cm lift.
Step 4: Lay the first two to three courses of gravel-filled bags. These form the stem wall; see Gravel-Filled Stem Courses.
Step 5: Manage moisture around the base by sloping the ground away and keeping roof water off the wall; see Moisture Management.
For the general principles behind sizing a foundation to the ground, see Foundation Design and Soil Bearing Capacity.
Common Mistakes
- Putting soil-filled bags directly at ground level. Moisture wicks into them.
- A trench with no outlet. A drain that cannot discharge only stores water.
- Skipping compaction between lifts. A loose foundation settles unevenly.
Safety
Trenches below frost depth can collapse
Frost depth may be well over 1.2 m, and an unsupported trench wall deeper than about 1.2 m can bury a person. Slope the sides back or shore them, keep spoil away from the edge, and never work in a deep trench alone. See the full precautions in Rubble Trench and Grade Beam.
What’s Next
- Learn next: Wall Construction — building the walls on the base
- Combine with: Foundation Types — alternatives to a rubble trench