Due to a rise in the water table, the effective stress in soil:
Try it first: pick an option, then read the explanation.
Asked in SSC JE 2005
Detailed Explanation
Effective stress is σ′ = σ − u. Below the water table, soil is buoyant: its effective unit weight is the submerged unit weight γ′ ≈ γsat − γw, which is about half its moist unit weight.
Why “decreases” Is Correct
When the water table rises through the soil, soil that was above it (counted with its full unit weight) becomes submerged (counted with γ′). Pore pressure increases more than total stress, so the effective stress decreases. This is the official answer.
One exception worth knowing: if the water table is already at or above the ground surface (e.g. a pond), raising it further increases σ and u equally, so σ′ does not change. The book notes this, but the commission accepted (b).
Why the Other Options Are Wrong
- A. increases: Effective stress never increases due to a rising water table.
- C. does not change: It does not change only in the special case where water is already above the ground surface; generally it decreases.
- D. may increase or decrease depending on the position of the water table: It never increases, so ‘may increase or decrease’ is wrong.
Key Point
Water table rises in the soil → σ′ falls → strength and bearing capacity fall.
Key Concepts for Students
- Lowering the water table (dewatering) increases σ′ and causes settlement.
- Bearing capacity is roughly halved when the water table rises to the footing base.
- σ′ at depth z below a water table at the surface = γ′z.
Practice more: this is question 49 in Shear Strength of Soils MCQs – Part 4. Try the whole set and check every answer instantly.