PERMEABILITY
PERMEABILITY
• Ease with which water flows through a soilPermeability
• Study of flow of water in soil - Darcy
Darcy’s law and its validity
• Velocity of flow proportional to hydraulic
gradient
V=ki
K- coefficient of permeability (cm/sec)
- If discharge q flows per unit of time through a
soil of cross sectional area A and length L
V= q/A,
i=h/L
h= Head causing the flow
Validity
1. Flow is laminar
• Velocity- hydraulic gradient relationship is
linear
• Flow in soil is laminar if Reynolds number is in
range 1 to 10
2. Flow is through saturated soil mass
- If degree of saturation less than 100%:pockets
of trapped air causes resistance to flow
Hydraulic conductivity of different types of soils
Soil type
Clean gravel
Clean sand
Sand mixture
Fine sand
Silty sand
Silt
clay
K values
1 and greater
1- 1x 10-2
1x 10-2 -.5x 10-2
.5x 10-2 - 1x 10-3
.2x 10-3 - 1x 10-4
.5x 10-4 - 1x 10-5
1x 10-6 and smaller
Factors affecting Permeability
1. Void ratio : Permeability increases with increase of
void ratio
2. Degree of saturation : Permeability increases with
increase of degree of saturation
3. Composition of soil particles:
• For sands and silts this is not important
• For soils with clay minerals this is one of the most
important factors.
• Permeability in clay minerals, depends on the thickness
of water held to the soil particles
• coefficient of permeability decreases with increasing
thickness of the diffuse double layer
4. Soil structure:
• Fine-grained soils with a flocculated structure
have a higher coefficient of permeability than
those with a dispersed structure
5. Density of packing of the soil particles
• represented by void ratio (e) or porosity (n)
• In sands, permeability can be empirically
related to the square of some representative
grain size from its grain-size distribution.
• For filter sands,
k =100 (D10)2 cm/s
D10= effective grain size in cm.
For clays
Ck- permeability change index
ek -reference void ratio.
6. Effect of properties of pore fluid
• Permeability directly proportional to unit
weight of water and inversely proportional to
viscosity
• Unit weight do not change with change in
temperature, viscosity changes
7. Effect of adsorbed water
• Adsorbed water surrounding the fine soil
particle is not free to move- so reduces
effective pore space available for passage of
water- permeability decreases
8. Effect of foreign matter
- Organic matter gets entrapped in voidsleading to decrease in permeability
- Refer photostat for some other equations and
graphs
EFFECT OF ENVIRONMENT ON
GEOTECHNICAL PROPERTIES OF SOIL
• Important Engineering properties :
Permeability, Compressibility and shear
strength
• Changes in these properties with environment
DIELECTRIC CONSTANT
• Ratio of permittivity of a substance to
permittivity of vaccum
• Permitivitty: Measure of ability of a material
to be polarized by an electric field
• Dielectric constant is influenced by ionic
concentration and type of ions in soil
• As dielectric constant decreases, swelling
increases
ZETA POTENTIAL
• Due to movement of colloidal particles- electric
potential developes at solid liquid interface
• Region containing the double layer is sheared at
some distance from the solid surface creating a
thin film associated with the solid.
• The electrical potential at the shearing plane is
the zeta potential
• Thickness of double layer affect zeta potential
• Influenced by ion exchange capacity & size of ion
radius
Activity of clay
• Ratio of plasticity index to percent by weight
of soil particles of diameter smaller than 2
microns present in the soil
• Relates plasticity to clay sized particle
• Activity – obtain from lab tests : wet analysis,
liquid limit and plastic limit
• Activity of given soil depend on type of clay
minerals present in it
• Typical activity values are:
Koalinite
0.4- 0.5
Illite
0.5-1
Montmorillonite
1- 7
Based on Activity Number: soil
Classification
< .75
0.75-1.40
>1.40
INACTIVE
NORMAL
ACTIVE
•High activity signifies large volume change when
wetted and large shrinkage when dried.
•Soils with high activity are very reactive chemically.
Sensitivity
• Degree to which a clay soil will undergo a
reduction in undrained shear strength as it
undergoes any disturbance that causes
remoulding / volume change
• Ratio of unconfined compression strength in
natural or undisturbed state to that in the
remoulded state without change in the water
content
• Higher the sensitivity, the greater the loss of
strength
SENSITIVITY
CLASS
<1
INSENSITIVE
1-2
SLIGHTLY SENSITIVE
2-4
MEDIUM SENSITIVE
4-8
VERY /QUICK SENSITIVE
8-16
SLIGHTLY QUICK
16-32
MEDIUM QUICK
32-64
VERY QUICK
>64
EXTRA QUICK
Causes of sensitivity
1.
2.
3.
4.
5.
6.
Metastable fabric
Cementation
Weathering
4. Thixotropic hardening
Formation of dispersing agent
Leaching, ion exchange, change in
monovalent and divalent cation ratio
1. Metastable Fabric
• For fine grained soil, particles tend to
flocculate
• After sedimentation, they have edge to edge
and edge to face association
• During consolidation, high effective stress- if
arranged in parallel position
• If remoulded, fabric is disrupted- effective
stress reduced : Becoz vol decrease- strength
is decreased
2. Cementation
• Soil contain free carbonate, iron oxide
alumina, organic matter – cause interparticle
contact ( cementing agent)
• When remoulded ( any disturbance)- bonds
are destroyed – loss of strength
3. Weathering
• After any disturbance – weathering can alter
flocculation and defloculation tendencies of
soil
• Weathering cause changes in relative
proportion of ions and type of ions in solution
• Strength and sensitivity of soil – depend on
nature of changes in ionic distribution
• Affect mainly salt content and proportions of
Na, K, Ca, Mg etc…
4. Thixotropic hardening
• Sedimentation, remoulding and compaction of
soil produce a structure compactible with the
conditions at that time
• When external force applied, structure losses
its equillibrium- may reorganise to a state of
low energy structure
• Due to this sensitivity may go upto 8
5. Leaching, iron exchange and change
in monovalent/divalent ion
• Undisturbed strength reduces 64%
• Leads to formation of Quick clays
• If sea water is leached from clay, without
much change in concentration of ions such as
Na, Mg etc- High sensitivity clay is not formed
• Fresh water leaching: decreases salt content ,
increases pHand increases % of monovalent
cation- Quick clays
6. Dispersing Agent
• At the time of sediment deposition- organic
substances are introduced
• In pores of clay- variety of organic compounds
are present
• These compounds act as dispersing agents
• Lead to increased double layer repulsion –
sensitiviy is increased
Effect of organic matter on properties
of soil
• Improves soil structure and porosity, increases
infiltration rate, increases available water
capacity.
• Humus possesses a cation exchange capacity
leading to retention of cations (Ca, Mg, K)and
buffering against acid rain.
• High shrinkage, high plasticity, low strength
and low permeability
• Interaction b/w soil and organic matter
possible b/w 5 groups
- Carbohydrate
- Proteins
- Fats
- Hyrdrocarbons
- Carbon
• 1-2% organic content – increases Atterbergs
limit by 10-20%
• Increase in organic matter
– Decreases max. dry density
- Decrease Unconfined compressive strength
- Increases Optimum moisture content
Refer graphs from photostat
Effect of pH on properties of soil
• When the soil pH is high (i.e., more basic, low
concentration of H+), more base cations will be on the
particle exchange sites and thus be less susceptible to
leaching.
• However, when the soil pH is lower (i.e., less basic,
higher concentration of H+), more H+ions are available
to “exchange” base cations, thereby removing them
from exchange sites and releasing them to the soil
solution (soil water)
• As a result, exchanged nutrients are either taken up by
the plant or lost through leaching or erosion.
Influence of Exchangable cations of
soil
• As activity of clay increases- influence of
cations will also increase
• Amount of swelling properties exhibited by
expansive clay minerals depends on type of
cation
Eg:
1. Sodium and lithium montmorillonite –
unrestricted swelling-@ low electrolytic
concentration & at low confining pressure
2. Divalent and trivalent form of montmorilloniteexpand upto 17 A0
• In case of non- expansive clays- type of
adsorbed clay is important
Eg: Sodium and lithium cation- cause
defloculation
Divalent & Trivalent cation -floculation
Clay Mineral Structure Crystal lattice
• Atoms in crystal are arranged in definite
manner- form 3D network called Lattice
• Positions within lattice where atoms or group
of atoms are located- Lattice point
• Smallest subdivision of crystal that posses
characteristics of crystal- Unit cell
Importance of Mineralogical
composition in soil expansion
• Expansive soil – contain montmorillonite &
vermiculate
• Mineral structure & presence of interlayer
materials- significant in swelling property of
soils
1. Crystal lattice configuration effect
• Lattice charge – influence the swelling
property
• For charge deficiency of 1/unit cell- great
expansion
• As b-dimension of montmorillonite crystal
lattice increases- swell decreases
Mineral
Negative charge/unit
cell
Tendency to
expand
Illite
>12
Only with drastic
chemical change
Vermiculate
1.4-0.9
Expanding
Montmorillonite
1- 0.6
Readily expanding
2. Hydroxy interlayer
• Conditions for interlayer formation:
1. Sufficient supply of aluminium ions
2. Moderately acid pH
3. Low oxygen condition
4. Frequent wetting and drying
• Formation and properties of hydroxy
interlayer- effect physical properties of
expansive soil
• Randomly distributed ions of interlayer
material bind adjacent layers together
• Eg:Fe-OH, Al-OH,Mg-OH
Eg: In acid soils- interlayer is Hydroxy aluminium
In alkaline soil- interlayer Mg (OH)2