Question

The effect of cohesion on a soil is to

• A. Reduce both active and passive earth pressure intensities
• B. Increase both active and passive earth pressure intensities
• C. Reduce active earth pressure intensity but increase passive earth pressure intensity
• D. Increase active earth pressure intensity but reduce passive earth pressure intensity

Hint:

Remember Rankine's equations: \[ \sigma_a=K_a\gamma z-2c\sqrt{K_a} \] \[ \sigma_p=K_p\gamma z+2c\sqrt{K_p} \] Cohesion always reduces active pressure and increases passive pressure.

Answer 1

The Correct Option is C

Concept: According to Rankine's earth pressure theory for cohesive soils, Active earth pressure is \[ \boxed{ \sigma_a=K_a\gamma z-2c\sqrt{K_a} } \] Passive earth pressure is \[ \boxed{ \sigma_p=K_p\gamma z+2c\sqrt{K_p} } \] where \[ c=\text{Cohesion}, \] \[ K_a=\text{Active earth pressure coefficient}, \] and \[ K_p=\text{Passive earth pressure coefficient}. \]

Step 1: Effect on active earth pressure. Since \[ 2c\sqrt{K_a} \] is subtracted, cohesion decreases the active earth pressure.

Step 2: Effect on passive earth pressure. Since \[ 2c\sqrt{K_p} \] is added, cohesion increases the passive earth pressure. Hence, \[ \boxed{\text{Cohesion reduces active earth pressure and increases passive earth pressure}.} \] Therefore, the correct option is \[ \boxed{(C)\;\text{Reduce active earth pressure intensity but increase passive earth pressure intensity}.} \]