What is the difference between saturated and unsaturated soil and why it matters for stability?

• A. Saturated soil has no pore water pressure.
• B. Unsaturated soils are always stronger.
• C. Saturated soil has pore water pressure affecting effective stress and strength.
• D. Both are equally stable.

Answer: (C)

The main idea here is how pore water pressure controls the effective stress that resists shear in soil. Stability depends on the stress that actually acts to hold the grains together, not the total stress that includes the pores.

In saturated soil, the spaces between grains are filled with water, and external loads increase the pore water pressure. Since effective stress equals total stress minus pore pressure, this rise in pore pressure lowers the effective stress that can mobilize shear. That reduction weakens the soil and makes it more susceptible to failure, especially during rainfall infiltration or rapid loading.

In unsaturated soil, the pores are not fully water-filled; the presence of suction (negative pore pressure) adds to the bonding between grains, effectively increasing the soil’s shear strength. But as the soil wets and suction diminishes, strength can decrease. This contrast explains why drainage conditions and moisture changes critically affect stability.

So, the statement that saturated soil has pore water pressure affecting effective stress and strength correctly captures the mechanism driving stability in many soil conditions. The other ideas—no pore pressure in saturated soil, unsaturated soils always being stronger, or both being equally stable—don’t reflect how pore pressure and suction govern strength.