Shallow Strip Foundation (MZLF) Calculator
Calculate materials and load bearing capacity for Shallow Frost-Protected Foundations (MZLF). Includes sand cushion volume and rebar schedules.
About
A Shallow Frost-Protected Foundation (often called MZLF in European engineering) is a cost-effective alternative to deep strip foundations. Instead of digging below the frost line, it relies on a non-heaving sand/gravel cushion and thermal insulation. However, the margin for error is smaller. The beam must be rigid enough to resist ground movement, and the cushion must be perfectly compacted.
This tool integrates a Load Bearing Check. It compares the weight of your structure against the bearing capacity of your soil. If the footing isn't wide enough, the house will settle unevenly. It also calculates the critical volume of the sand bedding - often 40% of the total material cost - and generates a reinforcement schedule specifically designed to counter tension forces from frost heave.
Formulas
Load Bearing Check: We estimate linear load (q) and compare pressure (P) to soil resistance (R):
Cushion Volume (Vsand): Calculated with a trapezoidal cross-section approximation (accounting for trench slope) and compaction factor (1.3):
Rebar Weight: Longitudinal bars (n) run the full perimeter; transverse stirrups are placed every 20 inches.
Reference Data
| Soil Type | Bearing Capacity (PSF) | Drainage | Frost Heave Risk | Rec. Cushion Depth |
|---|---|---|---|---|
| Gravel / Coarse Sand | 4000 - 6000 | Excellent | Low | 8 in |
| Fine Sand (Compact) | 3000 - 4000 | Good | Medium | 12 in |
| Sandy Loam | 2000 - 3000 | Moderate | High | 16 in |
| Clay (Dry/Hard) | 3000 - 4000 | Poor | Very High | 20 in + Drain |
| Clay (Soft/Wet) | 1000 - 2000 | Very Poor | Extreme | Not Recommended |
| Silt | 1000 - 2000 | Poor | Very High | 24 in + Drain |
| Rock (Bedrock) | 10000+ | Variable | None | 4 in (Leveling) |