The shear strength of soils, including sand, clay, and organic soils, depends on whether the soil is considered drained or undrained. Here’s a breakdown of typical values:
1. Sand (Granular Soils)
  • Drained Shear Strength: Governed by effective stress and friction angle (ϕ′\phi’ϕ′). Sand exhibits high drained shear strength due to frictional resistance.
  • Undrained Shear Strength: Not typically applicable, as sands tend to drain quickly under loading due to high permeability. In some cases (e.g., very loose sands under rapid loading), temporary undrained conditions might occur.
Soil Type Friction Angle (ϕ′\phi’ϕ′) Cohesion (c’) (kPa) Undrained Shear Strength (kPa)
Loose Sand 28° – 34° 0 –
Medium Sand 30° – 36° 0 –
Dense Sand 35° – 40° 0 –
2. Clay (Cohesive Soils)
  • Drained Shear Strength: Controlled by friction angle (ϕ′\phi’ϕ′) and cohesion (c′c’c′). Drained conditions occur over long periods under sustained loading.
  • Undrained Shear Strength: Determined by Su (Undrained Cohesion), used for short-term stability analysis (e.g., during excavation).
Soil Type Friction Angle (ϕ′\phi’ϕ′) Cohesion (c’) (kPa) Undrained Shear Strength (Su) (kPa)
Soft Clay 15° – 20° 10 – 25 12 – 50
Firm Clay 20° – 25° 25 – 50 50 – 100
Stiff Clay 25° – 30° 50 – 100 100 – 200
Very Stiff Clay 30° – 35° 100 – 150 200 – 300
Hard Clay 35° – 40° 150+ 300+
3. Organic Soils (e.g., Peat, Muskeg)
  • Organic soils have very low shear strength due to high compressibility and water content. Their properties are highly variable and sensitive to loading conditions.
Soil Type Friction Angle (ϕ′\phi’ϕ′) Cohesion (c’) (kPa) Undrained Shear Strength (Su) (kPa)
Organic Clay (Soft) 15° – 25° 0 – 20 10 – 40
Peat (Highly Organic) 5° – 15° 0 – 10 5 – 20
Muskeg (Variable) 5° – 20° 0 – 15 5 – 30
Summary
  • Sands: High drained shear strength, negligible undrained strength.
  • Clays: Significant difference between drained and undrained strengths due to low permeability.
  • Organic Soils: Very low shear strength, highly compressible, challenging for construction projects.