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# Soil Mechanics MCQs – Permeability – Validity of Darcy’s Law

1 - Question

1. The coefficient of permeability is equal to velocity of flow when _________ a) hydraulic gradient is equal to one b) hydraulic gradient is less than one c) hydraulic gradient is greater than one d) hydraulic gradient is equal to zero

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Answer: a
Explanation: According to Darcy’s law, we have,
V=ki
When i=1
V=k.
2 - Question

2. Darcy’s law of linear dependency is valid for only _______ flow conditions in soil. a) laminar and turbulent b) laminar c) turbulent d) intermediate

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Answer: a
Explanation: The linear dependency between velocity of flow and hydraulic gradient is valid only for laminar conditions in the soil.
3 - Question

3. For Darcy’s law to be valid, the Reynolds number should be ________ a) greater than one b) less than one c) equal to one d) greater than five

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Answer: b
Explanation: The Darcy’s law is valid if flow through soils is laminar. The flow will be laminar only if the Reynolds number is less than one.
4 - Question

4. For flow through soils, the characteristic length in Reynolds number is taken as ________ a) uniformity coefficient b) coefficient of curvature c) coefficient of percolation d) average diameter of the particles

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Answer: b
Explanation: The validity if the Darcy’s law is expressed by the Reynolds number in the form,
vDaρwηg≤1
Where, Da is the average diameter of the particles.
5 - Question

5. Darcy’s law in some cases is not valid in ______ a) clay b) silt c) fine sand d) coarse-gravel

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Answer: d
Explanation: The flow conditions in clay, sit and fine sand will be laminar, but whereas in the voids of the coarse gravel it will be a turbulent flow.
6 - Question

6. The critical Reynolds number varies from ________ for validity of Darcy’s law. a) 0.1-75 b) 75-100 c) 100-175 d) 175-200

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Answer: a
Explanation: The Reynolds number is the ratio of inertia to viscous forces. For the validity of Darcy’s law, the critical Reynolds number varies from 0.1-75. The critical Reynolds number is the condition when the flow changes from laminar to turbulent flow.
7 - Question

7. The linear dependency between velocity of flow and hydraulic gradient exists if effective size of soil _______ a) does not exceed 3mm b) exceeds 3mm c) equal to 3mm d) equal to 5mm

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Answer: a
Explanation: In 1892, Allen Hazen concluded that the linear dependency between velocity of flow and hydraulic gradient exists if effective size of soil did not exceed 3mm.
8 - Question

8. The drainage property of silty clay is ________ a) very good b) good c) fair d) very poor