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Lecture-7
SOIL MECHANICS
AND FOUNDATION ENGINEERING-I (CE-210)
PERMEABILITY
Definition: Permeability is defined as the property of porous
material which permits the passage or seepage of water (or other
fluids) through its interconnecting voids.
Factors Affecting Permeability
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Grain Size
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Property of the pore fluid
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Void ratio of the soil
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Structural arrangement of the soil particles
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Entrapped air and foreign matter
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Adsorbed water in clayey soils
PERMEABILITY DETERMINATION IN THE LAB
• CONSTANT HEAD PERMEABILITY TEST
• 𝑘=
𝑉𝐿
𝐴 𝑡 Δ𝐻
•
k = Coefficient of permeability
•
V = Quantity of water collected
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t = time in which water is collected
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L = length of sample
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A = area of cross section
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DH = head of water
PERMEABILITY DETERMINATION IN THE LAB
• VARIABLE HEAD PERMEABILITY TEST
• 𝑘=
𝑎𝐿
𝐻𝑜
2.3 𝑙𝑜𝑔10
𝐴𝑡
𝐻1
•
k = Coefficient of permeability
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a = area of cross section of tube
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L = length of sample
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A = area of cross section
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Ho = Initial head
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H1 = Final head
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t = time elapsed to come water head from H 0 to H1
PROBLEMS
1. Calculate the coefficient of permeability of a soil sample, 6cm in
height and 50 cm2 in cross sectional area, if a quantity of water
equal to 430ml passed down in 10 minutes, under an effective
constant head of 40 cm.
2. In a falling head permeability test, the initial head is 40 cm. The
head drops by 5cm in 10 minutes. Calculate the time required to
run the test for the final head to be 20cm. If the sample is 6cm in
height and 50 cm 2 in cross sectional area, calculate the
coefficient of permeability, taking area of stand pipe as 0.5cm 2.