A river 5 m deep consists of a sand bed with saturated unit weight of 20 kN/m³ . v_w = 9.81 kN/m³. The effective vertical stress at 5 m from the top of sand bed is

ANS:B - 51 kN/m²

Total stress= 20 x 5 = 100.
Pore water pressure = 9.8 x 5 = 49.
Effective stress = 100 - 49 = 51.

Basically, when the water tables fluctuate above the ground surface it will not increase or decrease the effective stress.

Effective stress will increase or decrease when the water table decreases or increases below the ground surface.