Correct Answer - Option 2 : Decreases
Explanation:
Effective stress is the ratio of force at the contact of particles of soil to the total area. It cannot be obtained practically but we can calculate the effective stress by measuring total stress and pore water pressure as:
Total stress (σ) = effective stress (σ’) + pore water pressure (u)
Effective stress (σ') = Total stress (σ) - pore water pressure (u)
If we increase the ground water table then value of pore water pressure increases and effective stress decreases.
If we lowering the groundwater table below ground then the value of pore water pressure decreases and effective stress increases.
