An earth-sheltered building must carry three loads: the weight of soil on the roof, the sideways pressure of soil on the walls, and the added weight and pressure when that soil is saturated with water. Soil weighs about 1,500-2,000 kg per cubic metre, so even a thin earth cover loads a roof with tonnes, and a failed earth roof gives no warning, which is why the parent article sizes the roof structure at least three times the expected dead load.
Structure in an earth-sheltered building means resisting vertical earth load on the roof, lateral earth pressure on the walls and the effects of water in the soil.
Three Loads, And Water Makes Each Worse
- Vertical load on the roof: the weight of the earth cover, plus water in it, plus any live load such as people or snow. See Roof Structures for Earth Loading.
- Lateral load on the walls: soil pushes sideways, and saturated soil with groundwater pushes far harder. See Retaining Wall Design.
- The numbers behind both: Earth Load Calculations.
The general vocabulary is in Dead Loads and Live Loads. Dead load is the structure and earth itself; live load is everything else. Earth is a dead load, the biggest one in this kind of building. Soil weighs about 1,500-2,000 kg per cubic metre, so even a thin cover loads a roof with tonnes.
What You Need
- Knowledge of the soil: its weight, its type, and whether it swells when wet. Expansive clay can crush walls.
- A drainage plan, because water is the multiplier.
- A structural shell that is strong in compression: arches, vaults, thick masonry, or heavy timber with short spans.
Method: Think In This Order
Step 1: Fix the earth cover depth. Less cover means less load; more cover means more insulation and stability.
Step 2: Compute the roof load. Use the weight of dry soil and then the saturated figure.
Step 3: Pick a roof form that suits your materials; see the roof article.
Step 4: Check the walls against soil and water pressure.
Step 5: Add the margin. Size the roof at least three times the expected dead load, because there is no warning before collapse.
Common Mistakes
- Sizing for dry soil. After rain the cover is heavier.
- Forgetting the sideways load. Walls designed for a roof’s weight can fail under soil pressure.
- Ignoring drainage. Water raises the wall load several times over.
Safety
A collapsed earth roof kills the people under it
Earth-loaded roofs fail without warning. Overbuilding the roof is not wasteful. Do not stand or work under a roof structure that has not been checked, and never add load, such as a pile of spoil or a vehicle, to an earth roof.
What’s Next
- Learn next: Earth Load Calculations — the arithmetic
- Combine with: Soil Bearing Capacity — what the ground under the building can carry