Hardened structures are the buildings that resist attack because their walls are thick and heavy, and a settlement should harden its community buildings and refuge points, not every house. Mass is the cost: a 40 cm earth wall 2.5 m high weighs 1,500 to 2,000 kg per metre of length, which is 14.7 to 19.6 kN per metre, and on soft clay that needs a footing 0.31 to 0.82 m wide before any roof is added.
Part of Defensive Architecture
Hardened structures are buildings made resistant to attack by thick, heavy walls, strong doors, shuttered openings and safe rooms, used as the protective inner layer of a defence.
The Third Layer Is Made Of Mass
The defensive-architecture guide’s third layer is the buildings themselves: hardened walls on community buildings and refuge points, strong doors, interior shutters that can be closed and barred from inside because windows are the weakest points, and safe rooms for non-combatants. It adds a useful observation: earthbag, straw bale and rammed earth walls at their normal building thicknesses all give some protection, so no special fortification is needed, only the same methods used for ordinary buildings. The fortification guide puts the principle in one line: mass stops projectiles (Fortification Materials).
The next two articles deal with the walls (Ballistic Resistance by Material) and the buildings to harden (Reinforced Community Buildings); the refuge room has its own (Safe Rooms & Refuge Points).
Worked Example: What Mass Costs In Foundation
Inputs: wall 40 cm thick and 2.5 m high; earth density 1,500-2,000 kg/m³, the range the earth load guide uses (Earth Load Calculations); g = 9.81 m/s²; safe bearing 500-1,000 lb per square foot for soft clay and 2,000-4,000 for compact sand (Foundation Design); 1 lb per square foot = 0.0479 kPa. The wall’s own weight only; roof and floor loads are extra.
- Weight per metre of wall: 0.4 × 2.5 × 1,500 = 1,500 kg to 0.4 × 2.5 × 2,000 = 2,000 kg. In force: 1,500 × 9.81 = 14.7 kN to 2,000 × 9.81 = 19.6 kN per metre. (Kilonewtons are force; the kilograms are the mass.)
- Soft clay, 500-1,000 lb/ft² = 24-48 kPa. Footing width = load per metre ÷ bearing: 14.7 ÷ 48 = 0.31 m to 19.6 ÷ 24 = 0.82 m.
- Compact sand, 2,000-4,000 lb/ft² = 96-192 kPa: width 14.7 ÷ 192 = 0.08 m to 19.6 ÷ 96 = 0.20 m, less than the wall’s own width, so the wall itself is the footing.
On soft clay the footing is wider than the 40 cm wall, which means a gravel-filled trench wider than the wall, not just the first course. A hardened wall on bad ground without a proper footing settles and cracks, and the cracks let the wall’s protection and its fire resistance down together.
What You Need
- A soil test at the site (Soil Bearing)
- A rubble-trench foundation, below frost depth
- Wall material and tamping gear for earthbag, rammed earth or masonry
- Strong doors and interior shutters, with the exit rules below
- A decision on which buildings to harden
Method: Harden Selectively
Step 1: Choose the buildings: the gathering hall, the stores for food and water, the medical room and the refuge points.
Step 2: Test the ground and size the footing from the wall load.
Step 3: Build the walls at the thickness the ballistic table asks for the threat tier, with plaster for weather and fire.
Step 4: Fit shutters and doors, each with a way out.
Step 5: Keep the roof non-combustible, because fire is likelier than attack (Fire-Resistant Building Materials).
Common Mistakes
- Hardening every house. It costs the labour of a fortress and makes the settlement look like one.
- Ignoring the footing. The mass settles unevenly.
- Hardening the walls and leaving a thatch roof. Fire defeats it.
- Believing a wall of any material is bulletproof. See the ballistic article.
Safety
Exits and heavy walls
A drop bar suits a dwelling and is a crush mechanism on any room that can hold a crowd: such a room exits outward by pushing, with nothing to work first, and is secured from outside when empty. Heavy earth and masonry walls fall with great force; stay out of a leaning wall’s fall zone, which reaches as far as it is high, and never dig a footing trench deeper than 1.2 m without sloping or shoring the sides.
What’s Next
- Learn next: Ballistic Resistance by Material — what thickness does and does not buy
- Combine with: Reinforced Community Buildings — applying it