The installations of sand drains in clayey soil causes the soil adjacent to the sand drains to undergo which one of the following?
- AIncrease in porosity
- BIncrease in compressibility
- CDecrease in horizontal permeabilityCorrect
- DDecrease in shear strength
Solution
Concept
Smear effect of sand drains
Step-by-step solution
1. Installing sand drains (driving or boring) remoulds the clay around the drain, forming a smear zone. 2. The smear zone has lower horizontal permeability than the undisturbed clay.
Explanation
Disturbance during installation of drains smears the clay along the drain boundary and reduces its horizontal permeability, which slows consolidation. This is called the smear effect.
Shortcut
Sand drain → smear zone → lower horizontal permeability.
Common mistake
Choosing a decrease in shear strength.
Correct answer: C) Decrease in horizontal permeability