The effect of cohesion on a soil is to:
Try it first: pick an option, then read the explanation.
Correct answer: C. reduce active earth pressure intensity but increase passive earth pressure intensity
Asked in SSC JE (previous year paper)
Detailed Explanation
Cohesion is extra shear strength. In the active case strength helps the soil hold itself up, so the wall carries less. In the passive case strength must be overcome to push the soil, so the resistance is greater.
\[ p_a = K_a\gamma z - 2c\sqrt{K_a} \qquad p_p = K_p\gamma z + 2c\sqrt{K_p} \]
Why “reduce active earth pressure intensity but increase passive earth pressure intensity” Is Correct
The cohesion term is subtracted in the active equation and added in the passive equation. So cohesion reduces active pressure and increases passive pressure.
Why the Other Options Are Wrong
- A. reduce both active and passive earth pressure intensities: Cohesion increases passive pressure.
- B. increase both active and passive earth pressure intensities: Cohesion reduces active pressure.
- D. increase active earth pressure intensity but reduce passive earth pressure intensity: This is the reverse of the actual effect.
Key Point
Cohesion: active ↓ (−2c√Ka), passive ↑ (+2c√Kp).
Key Concepts for Students
- Near the top, active pressure in clay becomes negative (tension zone, depth 2c/(γ√Ka)).
- Designers often ignore the tension zone and assume cracks fill with water.
- Bell's equations extend Rankine's theory to c–φ soils.
Practice more: this is question 21 in Earth Pressure and Retaining Wall MCQs – Part 2. Try the whole set and check every answer instantly.