A rubble trench foundation for an earthbag wall is dug below frost depth, 50-60 cm wide, with a sloped perforated drain pipe at the bottom and compacted gravel or rubble placed in 15 cm lifts. For a 5 x 5 m building with 40 cm walls, the 20-30 tonne wall load works out at roughly 18-32 kPa on the ground, a figure that overlaps what soft clay can carry, so soft ground needs a wider trench.
Part of Earthbag Building
A rubble trench is a drained, gravel-filled footing; a grade beam is a stiff continuous beam on top of it that ties the wall base together.
A Drained Trench Spreads The Load And Carries Away Water
The trench does two jobs. The compacted rubble spreads the wall load over a wider strip of ground, and the perforated pipe at the bottom takes water away so the ground under the wall is not soaked. The trench goes below frost depth so freezing soil cannot heave the footing.
A grade beam is a continuous, stiff beam built on top of the rubble to tie the whole base into one unit, so a soft spot under one section does not let one length of wall sink on its own. In a rebuilding setting it can be a reinforced lime-concrete beam or a continuous timber or stone course. No tested sizes exist for an earthbag grade beam, so size it from the principles in Foundation Design and test it on the actual ground.
What You Need
- A trench below frost depth, 50-60 cm wide.
- Perforated drain pipe and an outlet lower than the trench bottom.
- Compacted gravel or rubble in 15 cm lifts.
- A tamper, string line and level.
- Shoring materials if the trench is deeper than about 1.2 m.
Method: Dig, Drain, Fill, Compact
Step 1: Mark the trench a little wider than the planned wall base.
Step 2: Dig to below frost depth, sloping the sides back or shoring them.
Step 3: Lay the perforated pipe along the bottom, sloped toward the outlet so water flows out and does not sit.
Step 4: Fill with gravel or rubble in 15 cm lifts, compacting each before adding the next.
Step 5: Build the grade beam if the ground is uneven or soft, then start the gravel-filled bags; see Gravel-Filled Stem Courses.
Worked Example
Check the ground pressure for a 5 x 5 m building with 40 cm walls, using the 20-30 tonne wall weight stated for that building.
- Wall centreline, assuming 5 m is the outside dimension: each side is 5 m minus one wall thickness (0.4 m) = 4.6 m, so the perimeter is 4 x 4.6 m = 18.4 m.
- Footing area at 0.5 m wide: 18.4 m x 0.5 m = 9.2 m². At 0.6 m wide: 18.4 m x 0.6 m = 11.04 m².
- Force: mass x 9.81 m/s2. 20 t = 20,000 kg x 9.81 = 196,200 N = 196 kN. 30 t = 30,000 kg x 9.81 = 294,300 N = 294 kN.
- Pressure = force / area. Lowest: 196 kN / 11.04 m² = 17.8 kPa, about 18 kPa. Highest: 294 kN / 9.2 m² = 32.0 kPa.
The Foundation Design page gives 500-1,000 lb per sq ft for soft clay. At 47.88 Pa per lb/ft2, that is 24-48 kPa. The top of the earthbag range (32 kPa) sits inside the soft-clay range. This sum leaves out the roof and floor loads, so soft ground should get a wider trench than 50-60 cm.
Common Mistakes
- Trench pipe without an outlet. The pipe fills and the foundation sits in water.
- Uncompacted lifts. A loose footing settles, and the wall cracks.
- Judging the soil by eye. A site that looks firm can be soft clay; see Soil Bearing Capacity.
Safety
Trench collapse buries people
Any trench or pit deeper than about 1.2 m needs its walls sloped back or shored. That figure is the strictest of the real ones: British Columbia’s occupational health regulation requires protection before a worker enters any excavation over 1.2 m, and Ontario’s guidance says never to enter a trench deeper than 1.2 m unless the walls are sound, while US OSHA and Alberta both set 1.5 m and the UK sets no depth at all, requiring support wherever collapse is possible. This guide uses 1.2 m throughout. Soil weighs about 1,500-2,000 kg per cubic metre, so even a small slab falling in is lethal. In loose or wet soil, slope the sides back to about 1.5 m horizontal for every 1 m of depth (the slope US excavation rules assume for the weakest soil class), or shore them with planks and cross-braces. Keep spoil, tools and vehicles at least 0.6 m back from the edge, and farther for deeper trenches, because the pile loads the wall. Never work alone in a trench, keep a ladder in it, and get out at the first sign of cracking, slumping or water seeping in, especially after rain. For hand-dug shafts, see the lining method in Well Digging.
What’s Next
- Learn next: Gravel-Filled Stem Courses — the first courses on the trench
- Combine with: Moisture Management — keeping the trench draining