Darcy's Law-
According to Darcy's Law, the velocity of flow is directly proportional to hydraulic gradient for laminar flow condition.
:. V & i or V = Ki, K is called coefficient of permeability. When i=1, V = K
Coefficient of permeability -
:. Coefficient of permeability is also defined as velocity of flow through soil under unit hydraulic gradient and has the same unit as the velocity expressed as mm/sec, m/hour,
m/dav.
But q=Av=A.ki
q= rate of flow or discharge per unit time.
A= total area of cross section of flow perpendiuplar to the direction of flow.
Soil type coefficient of.per- meability(cm/sec)
Clay. <10^-6
Silt. 10^-6 to 10^-3
Sand. 10^-3 to 1
Gravel 1 to 100
Average and seepage velocity of flow -
Sand ^^
- The flow which is taking place between the soil mass is obtained by Darcy's Law assuming that the flow in a place through the whole cross sectional area, A of Soil mass perpendicular to the direction of flow. The velocity of flow is known as theoretical or discharge or average velocity (V = q/A).
The total area of soil composed of area of voids (Au) and the area of solids (As). The actual velocity of flow, referred to as seepage velocity (VS) which is greater than the theoretical velocity obtained from Darcy's Law.
:. {Vs= q/Av}
::q = AV = Av .VS
Or Vs= V. A/Av=V.1/n[Au/A=Vv/V=n]
Or Vs=V/n.
n= Porosity of soil mass .
us = Seepage velocity.
V = Discharge velocity
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| Soil |
Factors affecting permeability of soil -
-(1) Particle size (2) Properties of pore fluid (3) Void ratio (4) Soil fabric and soil (5) Degree of saturation (6) Presence of foreign matter (7) Adsorbed water.
(1). Quick sand condiion occurs when hydraulic gradient is -
(a) maximum
(b) minimum
(c) critical
(d) none of the above
Answer: (c)
(2). In seepage analysis hydraulic gradient at any point on an equipotential line is -
(a) variable
(b) equal
(c) maximum
(d) minimum
Answer: (b)
(3). Which one of the following metbods is more appropriate for deciding the permeability of clayey soils?
(a) falling head permeameter
(b) constant head permeameter
(c) horizontal permeability test
(d) none of the above
Answer: (a)
(4). Increase in effective stress on a soil mass -
(a) increases the void ratio and decreases the permeability
(b) increases the void ratio and increases the permeability
(c) decreases the void ratio and increases the permeability
(d) decreases the void ratio and decreases the permeability
Answer: (d)
5. Due to a rise in tenperature, the viscosity and the unit wi. of a fluid percolating through a soil mass are reduced to 80% and 90%, respectively. If other facions remain . unaltered, the coefficient of permeability -
a) decreases in 25%
(b) increases by 12.5%
(c) decreases by 18%
d) increases by 28%
Answer:( a)
(6). The average coefficient of permeahility in nalural stratified deposits parallel to stratification is:
(a) always more than that perpendicular to stratification (b) always less than that perpendicular to stratification (c) always the same as in perpendicular direction
(d) Sometimes more and sometiines less than that in perpendicular direction, depending upon the soil type
Answer: (a)
7. Other factors remaining unaltered, an increase in temperature will
(a) decrease the permeability of a soli
(b) increase the permeability of a soil, Sometimes increases and sometimes decrease the permeability of a soil, depending upon a soil characteristics.
(d) not affect the permeability at all.
Answer :(b)
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