It is the property of the soil which allows passage of
fluid through it
Permeable soil: When a soil has
continuous voids.
Example: Gravel ,
sand
- A soil is said to be highly permeable when K> 10-1 cm/sec.
- Soil is said to be impermeable when K< 10-7 cm/sec.
- Example: stiff clay
Darcy’s law: For
flow through soils
V=ki or Q=k.i.A.
Where
- V = discharge velocity of flow = Q/A = Discharge /Area
- i = hydraulic gradient = AhIL = loss of head per seepage length
- A = perpendicular cross sectional area
- “V” is called discharge velocity or superficial or fictitious velocity
- k = Coefficient of permeability--cm/sec or mm/sec
- When i= 1 ,V=k
- :. k is velocity of flow under unit hydraulic gradient
Validity of Darcy’s
law:
- For laminar flow conditions only
- Generally, laminar flow will prevail in clays, silts and fine sands.
- Flow will be laminar as long as Reynolds number <1, when Re is calculated considering characteristic length as the average particle diameter.
Seepage velocity(V)
or actual or true velocity:
- Vs = Discharge/Area of voids
- Vs = V/n = Discharge velocity/porosity
- Vs is always greater than V.
Coefficient of
percolation(kp):
- Vs ∝ i ∴ Vs =k.i
- kp= coefficient of percolation = k/n
- ∴ kp >k
Where
- Ds = effective grain size = D10
- yw= unit weight of percolating liquid
- e = void ratio
- C = shape constant depends upon type of soil, shape of soil particle and packing.
- u= viscosity of percolating fluid “
Factors affecting k:
- Size of particle : k ∝ D102.
- Shape of particle : For same void ratio, the soils with angular particle are less permeable than those with rounded particles, k is inversely proportional to specific surface area
- Void ratio: k ∝ e3/1+ e >>Paradoxically, the soils with largest void ratio (clays) have less permeability.
- Properties of percolating fluid: k = yw/uViscosity changes drastically withtemperaturek is reported at standard temperature of27°C as per Indian Standards.
- kt = permeability at temperature of t°C when viscosity is ut
- k27 = permeability at temperature 27°C when viscosity is u27 permeability increases with an increase in temperature due to reduction of the viscosity.
Degree of
saturation : Permeability of partially saturated
soil is considerably smaller than that of
a fully saturated soil dueto presence of air.
>>Adsorbed water:
Adsorbed water or diffuse double layer
decreases permeability
Determination of
permeability : by using permeameter.
Constant head
test:
- Suitable for coarse grained soils like Gravel, sand with k> 10-1 cm/sec
- head is kept constant and the discharge is measured, knowing the head causing flow, the hydraulic gradient ìs calculated. The “k” is determined using the formula,
Q=k.i.A.
Variable head test:
- For fine sands and silts with k = 10-2 to 10-4 cm/sec.
- Discharge is not required to be measured.
- k is calculated
using the formula
Where, a = area of
stand pipe
L = length of soil
specimen
a = cross sectional
area of soil specimen
t2-t1 = time interval in
which head drops from h1 to h2.
Consolidation test
data: Suitable for clays with k < 10-6 cm/sec
Capillary-permeability
test:
- Suitable to find k of partially saturated soil.
- Capillarity and permeability both can be found.
Pumping out test:
- Most accurate among all tests, since large area surrounding the well is influenced.
- Used for large engineering projects.
- It is expensive
Pumping ¡n test:
- Commonly used for testing rocks and individual stratum.
- Example: Open-end test, Packer test(for rocks)
Permeability of
stratified soil deposits:
(a) Flow parallel to
planes of stratification
- Loss of head or hydraulic gradient is same for all layers.
- Total discharge, Q
= q1 + q2 + q3
(b) Average permeability
for perpendicular to flow, kv.
kh is always greater than kv
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