4011 - GT Module 2 engineering 2021 revision

GEOTECHNICAL ENGINEERING
(4011)
1
SEMESTER : FOURTH
MODULE : 2
TOPIC:
PERMEABILITY OF
SOIL
Presented by,
Lecturer
Department Of Civil Engineering

PERMEABILITY OF SOIL
2
 The property of soil which permits flow of water through it, is called
permeability.
 The knowledge of permeability is essential in a number of soil
engineering problems such as settlement of buildings, yield of
wells etc.

DARCY’S LAW
3
• The flow of free water through soil is governed by Darcy’s law.
According to this law, in case of laminar flow in a homogenous soil
mass, the velocity of flow (v) is directly proportional to the hydraulic
gradient (i)
Vα i
• Introducing the coefficient of proportionality, k
velocity of flow V = ki
• The velocity is also known as discharge velocity and superficial
velocity. but, v = q/A , where q = discharge and A = cross-sectional
area
q = k iA
k= co efficient of permeability
i = hydraulic gradient = h/L ; Where L is the length of the specimen
When the i = 1,velocity of flow is equal to k (v=k)
ie, co efficient of permeability is defined as velocity of flow which will
occur at unit hydraulic gradient. (m/sec)

Discharge Velocity And Seepage Velocity
4
 Discharge velocity
The velocity of flow of water v, through soil mass is obtained
from Darcy's law assuming that the flow takes place through the
total cross section area (A) of soil mass. This velocity is referred
as discharge velocity or theoretical velocity
 Seepage velocity
The total area A is composed of the area of voids (Av),But flow
can only flow through area of voids. This velocity is known as
seepage velocity

Factors Affecting Permeability
5
 Particle size
 Structure of soil mass
 Shape of particle
 Void ratio
 Properties of water
 Degree of saturation
 Impurities inside the water
 Adsorbed water

 Particle size
In fine grained soil, particle size is less when compared to coarse
grained soil. And voids are higher in coarse grained soil,
when compared to fine grained.therfore permeability is higher
in coarse grained soil.
 Structure of soil mass
Structure of soil changes, voids are also changed. The size of
the flow passage depends upon the structural arrangement.
Consider the case of flocculated and dispersed soil structure.
6

The voids in flocculated is higher compared to dispersed soil
structures.so permeability is higher in flocculated structure
than dispersed structure
 Shape of particle
Consider round and angular shape particle, in round shape, voids
are greater .so permeability is higher in round shape than
angular shape
7

 Void ratio
When void ratio increases, permeability also increases.
 Properties of water
k= C* ( w/µ)*[e
ϒ 3 /(1+e)]D2
k is directly proportional to unit wt of water and inversely
proportional to viscosity. When temperature increases
viscosity decreases.so permeability of soil increases.
 Degree of saturation
Consider the case of fully saturated soil and partially saturated
soil, permeability is higher in fully saturated soil compared to
partially saturated soil. If the soil is not fully saturated, it
contains air pockets formed due to entrapped air or due to air
liberated from percolating water.
8

Whatever may be the cause of the presence of air in soils, the
permeability is reduced due to presence of air which causes
blockage of passage.
 Impurities inside water
Any foreign matter in water has a tendency to plug the flow
passage and reduce the effective voids and hence permeability
of soils. when water is impure ,then permeability is decreases
and when water is purred, permeability increases.
 Adsorbed water
Adsorbed water is the thin layer of water around each soil
particle.so permeability decreases in case of adsorbed
water. This is mainly seen in fine grained or clayey soil.
9

Determination Of Coefficient Of Permeability
10
 Laboratory methods
1) Constant Head Permeability Test
2) Variable Head Permeability Test
 Field Methods
1) Pumping Out Tests
2) Pumping In Tests

Constant Head Permeability Test
 The coefficient of permeability of a relatively more
permeable soil can be determined in a laboratory by constant
head permeability test.
 The test is conducted using an instrument known as constant
head permeameter.
 It consist of a metallic mould, 100mm internal dia., 127.3mm
of effective height and 1000 ml capacity.
 The mould is provided with a drainage base plate with a
recess
for porous stone.
 The mould is fitted with a drainage cap having an inlet
valve and an air release valve.
 The soil sample is placed inside the mould between
two porous disc.
 The porous disc is de-aerated before these are placed in
the mould.

11

 After the soil has been saturated, the constant head reservoir
is connected to the drainage cap.
 Water is allowed to flow out from the drainage base for
sometime
till a steady state is established.
 The water level in the constant head chamber bring to an
constant
 The water which enters the chamber after flowing through the
sample spills over the chamber and is collected in a graduated
jar for a convenient period.
q = kiA = k(h/l)A
K= (ql)/(Ah)
q= discharge, l= length of specimen, A =area of sample, h = 12

• The head causing flow(h) is equal to the difference in water
level between the constant head reservoir and constant
head chamber.
13

Variable Head Permeability Test
14
• For relatively collected less permeable soil the quantity of
water collected in the graduated jar of constant
head permeability test is very small and cannot be
measure accurately. For such soils the variable head
permeability test is used
• The permeameter mould is the same as that used in constant
permeability test (Internal dia 100 mm, effective height
127.3 mm, Capacity 1000 ml)
• A vertical graduated stand pipe of known diameter is fitted to
the top of permeameter
• The sample is placed between two porous disc
• The whole assembly is placed in a constant head chamber
filled with water to the brim at the start of test.

• The porous disk and water tubes should be de-aerated
before the sample is placed.
• Saturate the sample completely with soil
• The test started by allowing the water in the stand to flow
through the sample.
• During the test, the water level will continuously drop and
the height of water in stand pipe is recorded at several
time intervals during the test.
• Note the time required for water level to fall from a
known initial head (h1) to an known final head ( h2)
• h1 is noted and time taken to fall to h2 is recorded using
stopwatch.
• Coefficient of permeability (k) can be computed by using
the equation
k= ((2.30aL)/(At))Log10(h1/h2)
15

• a = area of stand pipe
• A = area of sample
• L = length of sample
• T= time required from h1 to h2
• h1 = intial head
• h2 = final head
16

GEOTECHNICAL ENGINEERING
(4011)
1
SEMESTER :FOURTH
MODULE : 2
TOPIC: SHEAR
STRENGTH
Presented by,
Lecturer
Depart
ment Of
Civil

Cohesive soils
2
Soils in which the adsorbed water and particle attraction act such
that it deforms plastically at varying water contents are known as
cohesive soils. The soil is classified as cohesive if the amount of
fines (silt and clay-sized material) exceeds 50% by weight.
Eg : Clays and plastic silt
Cohesionless soils
The soils composed of bulky grains are cohesionless regardless
of the fineness of the particles. Cohesionless soils are defined as
any free-running type of soil, such as sand or gravel, whose
strength depends on friction between particles
Eg : Non plastic silt, sand and gravel

• Cohesion of soil
It is the mutual attraction of soil particles due to molecular forces
and the presence of moisture films. It depends upon the fineness
of clay particles, type of clay mineral, amount of clay, and water
content of soil.
• Internal friction
The friction resistance between the individual soil particles
at their contact points is known as internal friction
• Angle of internal friction
The angle of internal friction is a physical property of earth
materials or the slope of a linear representation of the shear
strength of earth material. It represents frictional resistance
between soil particles.
3

Purely cohesive soil
These are the soils which exhibit cohesion but angle of
internal friction is zero.
Purely cohesionless soil
Cohesionless soils are defined as any free-running type of
soil, such as sand or gravel, whose strength depends on
friction between particles (measured by the friction angle,
Ø).
4

Shear strength of soil
5
•Shear strength of a soil is its maximum resistance to
shear stresses just before the failure.
•Shear failure of a soil mass occurs when the shears stresses
induced due to applied compressive loads exceed the shear
strength of the soil.
•Failure in the soil occurs by the relative movement of the
particles and not by the breaking of particles.
•It is a very important in estimating the bearing capacity
of foundations and in assessing the stability of retaining
walls, slopes, and embankments and the design and
construction of highway and airfield pavements

MOHR COULOMB THEORY
6
• Soil is a particulate material. The shear failure occurs in soils
by slippage of particles due to shear
• The failure is by shear, but the shear stresses at failure depends
on the normal stresses on the potential failure plane.
• Thus, according to Mohr-Columb theory, the failure is caused
by a critical combination of the normal and shear stresses.

• The soil fails when the shear stress on the failure plane
at
failure, (Ʈf) is a unique function of the normal stress, (σ) acting
on that plane.
Ʈf= f (σ)
• The shear stress on the failure plane at failure is defined as
the shear strength of soil, ‘S’
S = f (σ)
7

• A plot can be made between the shear stress and normal
stress at failure, and so obtained curve is termed as Mohr
envelope. Failure of the soil occurs when the Mohr circle of
the stresses touch this Mohr envelope.
8

• Any Mohr circle which does not touch the Mohr envelope and
lies below it represents a non-failure condition.
• The curved Mohr envelope was replaced by a straight line by
Coulomb, and shear strength of a soil at a point on a
particular plane was expressed as a linear function of
the normal stress as,
S= c + σ tan Ø
• The component, C of the shear strength is known as cohesion.
Cohesion holds the soil particles together as a soil mass
and is independent of the normal stress.
• The angle Ø is called the angle of internal friction. It
represents the frictional resistance between the soil
particles, which is directly proportional to the normal
stress, σ
9

DIRECT SHEAR TEST
12
• Shear box is made of brass or gunmetal which is either square
or circular in plan.
• A square box of size 60 x 60 x 50mm is commonly used.
• Box is divided horizontally such that the two halves of the box
are held together by locking pins.
• The box is provided with the grid plates which are toothed and
fitted inside it.
• Grid plates are plain for undrained tests and perforated
for drained tests
• Porous stones are placed at the top and the bottom of
the
specimen in drained tests.
• A pressure pad is fitted into the box to transmit the
normal load to the sample.
• Normal load from the loading yoke is applied on the top of the
specimen through a steel ball

• Lower half of the box is fixed to the base plate which is rigidly
held in position in a large container.
• Large container is supported on rollers
• Container can be pushed forward at a constant rate by a geared
jack. It can be operated manually or by an electric motor.
• A loading frame is used to support the large container. It has
the arrangement of a loading yoke and a lever system
for applying the normal load.
• Proving ring is fitted to the upper half of the box to measure
the shear force. As the box moves, the proving ring records
the shear force.
• Shear displacement is measured with a dial
• Another dial gauge is fitted to the top of the pressure pad to
measure the change in the thickness of the specimen.
13

TEST
• A soil specimen of size 60 × 60 × 25 mm is taken. It may be either
an undisturbed sample or made from compacted and remoulded soil.
• A porous stone is placed in the box. For undrained tests, a plain
grid is kept on the porous stone.
• For drained tests, perforated grids are used instead of plain grids.
• upper grid, porous stone and the pressure pad are placed on the
specimen.
• The loading yoke is mounted on the steel ball placed on the pressure
pad.
• The dial gauge is fitted to the container to give the shear
displacement. The other dial gauge is mounted on the loading yoke
to record the vertical movement.
• Locking pins are removed and the upper half box is slightly raised
• Normal load is applied to give a normal stress of 25 kM/m2.
• The test is repeated under the normal stress of 50, 100, 200 and
400
kN/m2
• A plot made between shear stress and shear strain 14

VANE SHEAR TEST
16
• In-situ vane-shear test is conducted to determine the shear strength
of a cohesive soil in its natural condition
• The apparatus consists of a vertical steel rod having four thin
stainless steel blades, 100mm long (vanes) fixed at its bottom end.
• the height H of the vane should be equal to twice the overall
diameter D.
• The diameter and the length of the rod are recommended as 2.5
mm and 60 mm respectively.
• For conducting the test in the laboratory, a specimen of the size 38
mm diameter and 75 mm height is taken in a container which is
fixed securely to the base.
• The vane is gradually lowered into the specimen till the top of
the vane is at a depth of 10 to 20 mm below the top of the24
mm specimen.

• The readings of the strain indicator and torque indicator are taken.
• Torque is applied gradually to the upper end of the rod at the rate of
about 6° per minute (i.e. 0.1°2.5per second).
• The torque required to shear the cylinder of the soil is measured by
means of a spring balance. The torque continued till the soil fails in
shear.
• The vane-shear test is extremely useful for determining the in-situ
shear strength of very soft and sensitive clays, for which it is
difficult to obtain undisturbed samples.
• The test can also be used even for determining the shear strength of
stiff, fissured clays.
• However, the method cannot be used for sandy soils.
17

4011 - GT Module 2 engineering 2021 revision

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