Friction Angle of Organic Soil and Peat: Unit Weight, Cohesion, Permeability
The friction angle of organic soil and peat typically runs from 15 to 25 degrees, with a bulk unit weight of 10 to 13 kN/m3 and effective cohesion of 5 to 20 kPa. Permeability is on the order of 1e-6 to 1e-8 m/s. The table collects the screening values and the note explains what moves them.
| Property | Typical range |
|---|---|
| Friction angle | 15 to 25 degrees |
| Bulk unit weight | 10 to 13 kN/m3 |
| Effective cohesion | 5 to 20 kPa |
| Permeability | 1e-6 to 1e-8 m/s |
What moves the numbers
The lightest and most compressible material on this list: settlement of half its thickness is possible, which is why the usual advice is removal. These are screening ranges for preliminary work and study: design values come from site specific testing, and the tools below show how each property is used once you have it.
Use these values
- Free Online Slope Stability Risk Calculator to turn these ranges into a probability of failure.
- Lateral Earth Pressure Calculator for the pressures this friction angle implies.
- Hazen Permeability Calculator for estimating permeability from grain size.
- The Property Library for every soil, rock, and mineral on this site.
Related Calculators and Reference
- Terzaghi Bearing Capacity Calculator for Shallow Foundations, for the bearing capacity that follows from this friction angle.
- Retaining Wall Pressure Calculator (Rankine and Coulomb), since retaining wall pressures come straight off this angle.
- Void Ratio, Porosity and Unit Weight Converter, for converting between void ratio, porosity and unit weight.
- Slope Stability Factor of Safety Calculator, once the question becomes stability rather than bearing.
- Friction Angle of Stiff Clay, for the material most often confused with it in the field.
- ASTM D2487, the standard practice behind the classification these values are grouped by.