Friction Angle of Stiff Clay: Unit Weight, Cohesion, Permeability
The friction angle of stiff clay typically runs from 20 to 28 degrees, with a bulk unit weight of 18 to 21 kN/m3 and effective cohesion of 10 to 30 kPa. Permeability is on the order of 1e-9 to 1e-11 m/s. The table collects the screening values and the note explains what moves them.
| Property | Typical range |
|---|---|
| Friction angle | 20 to 28 degrees |
| Bulk unit weight | 18 to 21 kN/m3 |
| Effective cohesion | 10 to 30 kPa |
| Permeability | 1e-9 to 1e-11 m/s |
What moves the numbers
Good bearing today, but fissures, swelling minerals, and softening with time keep the long term case in charge of design. 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, when the next question is what the ground will carry.
- Retaining Wall Pressure Calculator (Rankine and Coulomb), for the earth pressures this angle produces behind a wall.
- Void Ratio, Porosity and Unit Weight Converter, when your source quotes one density term and you need another.
- Slope Stability Factor of Safety Calculator, if the same soil is holding up a cut or an embankment.
- Friction Angle of Soft Clay, for the next entry along when you are bracketing a description.
- ASTM D2487, the standard practice behind the classification these values are grouped by.