Section 4: Soil and Water Conservation Engineering
Soil Mechanics
Subtopic: Engineering properties of soils
Question Statement:
The statement which is not correct for the porous medium is
Options:
(A) Seepage velocity is always greater than the Darcy's velocity
(B) Darcy's velocity is not exclusively controlled by soil porosity
(C) Seepage velocity increases with increasing surface ponding of water
(D) Darcy's velocity in unsaturated soil is always greater than that in saturated soil
Official Answer Key:
D
Step-by-Step Solution & Explanation
Darcy (superficial) velocity v_D relates to seepage (actual pore) velocity v_s by v_s = v_D/n (n = porosity), and since porosity n<1, v_s is always greater than v_D — so (A) is a TRUE statement. Darcy's velocity v_D = K×i (hydraulic conductivity × gradient); K itself depends on pore-size distribution/permeability, not porosity alone, so (B) is TRUE. Increased ponding raises the hydraulic gradient, increasing both Darcy and seepage velocity — (C) is TRUE. Statement (D) is FALSE: unsaturated soils have LOWER hydraulic conductivity (and hence Darcy's velocity) than saturated soils of the same type, because air-filled pores reduce the effective flow area and increase tortuosity — not “always greater” as claimed. Answer: (D) is the incorrect (NOT correct) statement, matching what the question asks for.
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