Civil Engineering MCQAE Level Civil Engineering
The critical hydraulic gradient i_c for upward seepage in soil (causing quicksand condition) is:
Soil MechanicsCritical Hydraulic GradientMEDIUM
Choose the correct answer
A
i_c = n (porosity)
B
i_c = G_s - 1
C
i_c = (G_s - 1)/(1 + e)
D
i_c = γ_sat/γ_w
Correct Answer
C. i_c = (G_s - 1)/(1 + e)
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Quick condition: seepage force = buoyant weight. i_c × γ_w = (G_s - 1)γ_w/(1+e) → i_c = (G_s-1)/(1+e). For typical soil with G_s=2.67, e=0.67: i_c ≈ 1.0. Effective stress reaches zero at this gradient.
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