Cohesion given Shear Strength of Cohesive Soil Calculator

Cohesion Formula:

\[ c = \tau_f - (\sigma_n \times \tan(\phi)) \]

Shear Strength (τ_f):

kN/m³

Normal Stress (σ_n):

kN/m²

Friction Angle (φ):

Uncertainty Factor:

Cohesion (c): kN/m²

1. What is Cohesion in Soil Mechanics?

Definition: Cohesion is the component of shear strength in soils that is independent of interparticle friction and normal stress. It represents the "stickiness" of soil particles.

Purpose: This calculator determines the cohesive strength of soil based on measured shear strength, normal stress, and internal friction angle.

2. How Does the Calculator Work?

The calculator uses the Mohr-Coulomb failure criterion formula:

\[ c = \tau_f - (\sigma_n \times \tan(\phi)) \]

Where:

\( c \) — Cohesion (kN/m²)
\( \tau_f \) — Shear strength (kN/m³)
\( \sigma_n \) — Normal stress (kN/m²)
\( \phi \) — Angle of internal friction (degrees)

Explanation: The shear strength is reduced by the frictional component to isolate the cohesive component of soil strength.

3. Importance of Cohesion Calculation

Details: Accurate cohesion values are critical for slope stability analysis, foundation design, and earth retaining structure calculations.

4. Using the Calculator

Tips: Enter the measured shear strength, normal stress, and internal friction angle. The uncertainty factor (default ±5%) accounts for measurement variability.

5. Frequently Asked Questions (FAQ)

Q1: What's a typical cohesion value for clay?
A: Soft clay: 5-20 kN/m², medium clay: 20-40 kN/m², stiff clay: 40-75 kN/m², hard clay: 75-150 kN/m².

Q2: How is shear strength measured?
A: Typically through triaxial tests or direct shear tests in a geotechnical laboratory.

Q3: Why include an uncertainty factor?
A: Soil properties are naturally variable, and test results have measurement errors.

Q4: Can cohesion be negative?
A: No, negative values suggest an error in input parameters as cohesion cannot be negative.

Q5: How does water content affect cohesion?
A: For cohesive soils, increased water content generally decreases cohesion.