The simplest adequate foundation for a straw bale wall is a rubble trench 40-50 cm wide, dug below frost depth, which varies by region from 30 to 150 cm, filled with compacted gravel or rubble around a drain pipe that slopes to daylight if possible. The stem wall is built on top, and the trench both carries the load and drains water away from the wall base.
Part of Straw Bale Construction
A rubble trench is a gravel-filled drained footing under a stem wall.
A Trench Both Carries And Drains
Frost heave lifts shallow footings, so the trench goes below the depth to which the ground freezes. The gravel fill spreads the wall load and gives water an easy path to the drain pipe. Straw bale walls are light compared with earth walls, but the same logic about spreading load and draining water applies; see Foundation Design for sizing a footing against the ground and Soil Bearing Capacity for the ground itself.
What You Need
- A trench 40-50 cm wide.
- Compacted gravel or rubble.
- Perforated drain pipe with a route to daylight, or a dry well lower than the trench.
- Shoring if the trench is deeper than about 1.2 m.
- A level or water level; see Levels and Plumb Bobs.
Method: Dig, Drain, Fill, Build
Step 1: Find your frost depth from local knowledge; the range is 30-150 cm depending on region.
Step 2: Mark the trench 40-50 cm wide along the wall line.
Step 3: Dig with a level floor or a steady fall toward the outlet, whichever the drain needs.
Step 4: Lay the drain pipe at the bottom, sloping to daylight if possible.
Step 5: Fill with compacted gravel or rubble, in lifts, and compact each.
Step 6: Build the stem wall on the compacted top; see Stem Wall Height and Materials.
Common Mistakes
- Stopping above frost depth. Heave cracks the wall.
- A drain with no outlet.
- Uncompacted fill. It settles unevenly.
Safety
Deep trenches collapse and bury people
Soil weighs about 1,500-2,000 kg per cubic metre, and a trench at the deep end of the frost-depth range (150 cm) is well past the point of about 1.2 m where walls need support. Slope the sides back to about 1.5 m horizontal for each 1 m of depth in loose or wet soil, or shore them with planks and cross-braces. Keep spoil, tools and vehicles at least 0.6 m from the edge. Never work alone in a trench, keep a ladder in it, and climb out at the first crack, slump or seep, especially after rain. For lined shafts see Well Digging.
What’s Next
- Learn next: Stem Wall Height and Materials
- Combine with: Foundations and Stem Walls