Transcript Slide 1

IDENTIFICATION AND CLASSIFICATION OF
PROBLEMATIC SOILS
Presented by
Kulbir Singh Gill
Associate Professor, Deptt. of Civil Engineering,
GNDEC,Ludhiana
([email protected])
FORMATION OF SOIL
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Soil is formed either by physical weathering or by chemical
weathering.
Because of different processes of weathering ,soils exhibit
different characteristics.
Physical weathered rock to some extent represents the parent
rock mass ,for instance sand and gravel.
Where as chemically weathered rocks results in the
formation totally different material such as clay.
Needless to say that sands and gravels are considered to be
the best material from civil engineering point of view.
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Except the situation where permeability is to be controlled.
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Clays shows huge volume change when exposed to moisture.
CONTD….
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Soils are heterogenious in nature.
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Soils are also anisotropic.
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If the wind is the weathering agent ,it results in aeoline deposits which
are cohesion less in nature such as sands.
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If water is the agent for movement of weathered rock products, it results
in the formation of alluvial deposit and their suitability as construction
material is varying from poor to fair.
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Other deposit are glacial, marine , beach, etc. Residual deposits are the
one which is not transported to farther distances.
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Suitability of any soil can be assessed based on its properties.
PROPERTIES OF SOILS
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The soil properties include index and engineering properties.
The index properties are specific gravity, void ratio , liquid limit,
plastic limit, shrinkage limit, relative density, dry density,
porosity, initial water content, grains size distribution etc.
The engineering properties are shear strength, compressibility
and permeability.
Unlike other material, soil behavior is influenced by many factors
such as mineralogy, water content, void ratio, soil structure, pore
fluid characteristics (Ion concentration , valancy of ion , dielectric
constant) , temperature, drainage, condition, strain rate, aging
etc
IDENTIFICATION AND CLASSIFICATION OF
SOILS
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Beside the complexity of understanding soil, geotechnical
engineers made their best efforts to group the soil based on its
specific response to different environmental conditions
Soil can be classified as highly compressible and soil of low
compressibility, expansive & non expansive, sensitive &
insensitive, high plastic & low plastic, very soft to stiff clay, loose
and dense sand etc.
In this note, the identification and classification of different soils
are presented in order to classify the good and poor soil,
otherwise called as Problematic soil.
LIQUID LIMIT
Liquid limit values of soils may be described as low,
intermediate, high very high or extra high plasticity as given
below in Table .
Plasticity Classified as
Liquid limit (%)
Low
20 to 35
Intermediate
35 to 50
High
50 to 70
Very high
70 to 90
Extra high
Over 90
PLASTICITY INDEX
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No uniform standard is adopted in classifying degree of plasticity
of soils. However, the classification given below is approximately
the one which is often used and hence is recommended
Soil classified as
Plasticity Index (%)
Non – Plastic
0-5
Moderately Plastic
5-16
Plastic
16-35
Highly plastic
Over 35
SHRINKAGE LIMIT
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Shrinkage limit of soil is an indication of not only the shrinkage
potential of clays but also an indicative of swelling nature.
Swelling and Shrinkage Classification based on Shrinkage Limit
Classified as Shrinkage
table /
Swelling detail
Shrinkage limit
(%)
Very Low
<1
Low
8 to 10
Intermediate
11 to 15
High
16 to 20
Very high
Over 20
INDIAN STANDARD CLASSIFICATION
SYSTEM
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The fine-grained soils in ISC system are subdivided into three
categories of low, medium and high compressibility .
Coarse-grained soils, when 50% or more of the total material by
weight is retained on 75 micron IS sieve.
Fine-grained soils, when more than 50% of the total material
passes 75 micron IS sieve.
If the soil is highly organic and contains a large percentage of
organic matter and particles of decomposed vegetation, it is kept
in a separate category marked as peat (Pt).
MAJOR SOIL GROUPS
Cohesive
soils
Clay
Granular soils or
Cohesionless soils
Silt
0.002
Sand
0.075
Gravel
2.36
Cobble
63
Boulder
200
Grain size
(mm)
Fine
grain
soils
Coarse
grain soils
10
100
80
% Passing
hydrometer
sieve
60
fines
sands
gravels
40
20
D
30
D10 = 0.013 mm
D30 = 0.47 mm
D60 = 7.4 mm
0
0.001
0.01
0.1
1
10
Grain size (mm)
GRAIN SIZE DISTRIBUTION CURVE
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Can find % of gravels, sands, fines
 Define
D10, D30, D60.. as above.
100
CLASSIFYING FINES
Purely based on LL and PI
Intermediate plasticity
Liquid Limit
60
Low
plasticity
High
plasticity
40
Clays
Silts
20
0
0
20
35
50
Liquid Limit
100
12
ATTERBERG LIMITS
Border line water contents, separating
the different states of a fine grained soil
0
Shrinkage
limit
brittle-solid
Plastic
limit
semi-solid
Liquid
limit
plastic
water
content
liquid
13
FIELD IDENTIFICATION OF SOILS
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The soils can be identified in the field by conducting the following
simple tests.
The sample is first spread on a flat surface.
If more than 50% of the particles are visible to the naked eye
(unaided eye), the soil is coarse-grained; otherwise, it is fine
grained
CONTD.
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To differentiate fine sand from silt, dispersion test is adopted.
When a spoonful of soil is poured in a jar full of water, fine sand
settles in a minute or so, whereas silt takes 15 minutes or more.
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Dilatancy (reaction to shaking) test.
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Toughness test.
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Dry strength test.
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These tests helps in classifying the different types of soil.
CLASSIFICATION BASED ON FIELD TESTS
Test
ML
CL
OL
MI
CI
OI
MH
CH
OH
Dilatan
cy
Quick None to
very slow
Slow
Quick to
slow
None
Slow
Slow to
none
None
None to
very
slow
Toughne
ss
None Medium
Low
None
Medium
Low
Low to
medium
High
Low to
mediu
m
Low
Low
Medium
to high
Low to
medium
Low to
medium
High to
very
high
Medium
to high
Dry
Streng
t
h
None of
low
Medium
GENERAL BEHAVIOR OF
Soil Group
Permeability
GRAVELS
Compressibility
Shear Strength
Workability
\
GW
GP
GM
GC
Pervious
Negligible
Excellent
Negligible
Good
Good
Good
Good
Very pervious
Semi-pervious to
impervious
Negligible
Impervious
Very low
Good to fair
Excellent
Good
GENERAL BEHAVIOR OF SAND
Soil Group
SW
Permeability
Pervious
Compressibility
Negligible
Shear Strength
Workability
Excellent
Excellent
Good
Fair
SP
Pervious
SM
Semi-pervious to
impervious
Low
Good
Fair
SC
Impervious
Low
Good to fair
Good
Very low
GENERAL BEHAVIOR OF SILT AND CLAY OF MEDIUM
PLASTICITY
Soil Group
ML, MI
Permeability
Semi-pervious to impervious
Compressibility
Medium
Medium
CL, CI
Impervious
Shear Strength
Fair
Fair
Fair
Medium
Semi-pervious to impervious
OL, OI
Workability
Good to fair
Fair
Fair
GENERAL BEHAVIOR OF SILTS AND CLAYS OF HIGH
PLASTICITY
Soil Group
Permeability
Compressibility
Shear Strength
Workability
MH
Semi-pervious to
impervious
Fair to poor
High
Poor
CH
OH
Impervious
Impervious
High
Poor
Poor
High
Poor
Poor
CLASSIFICATION OF EXPANSIVE SOILS
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Damages to structures, property and life resulting from swellshrink characteristics of expansive soils have been reported from
many parts of the world including India.
India, Africa, Australia, Israel, South America and United States
of America possess vast tracts covered with such soils.
Our Black Cotton soil is an expansive soil.
It extends nearly one-fifth of our country, chiefly in the states of
Tamil Nadu. Maharashtra, Gujarat, Madhya Pradesh, Southern
Uttar Pradesh, Eastern Rajasthan, Karnataka and parts of
Andhra Pradesh
CONTD.
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Most of the expansive soils found in India are black in
color and are good for growing cotton.
Some of these soils are reddish brown and yellowish grey
in color.
These soils are generally found near the surface, with
layer thickness varying from 0.5 m to 10.m, and sometime
more than 10 meters.
CONTD.
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Once an expansive soil is encountered at the project site, the
following items need to be given specific attention.
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Swell – shrink characteristics of soil encountered.
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Thickness and depth of various underlying soil layers
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Depths of significant moisture variation.
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Local climate and hydrology.
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Floor-foundation system; ability to accommodate and tolerate the
soil behavior.
Methods of improving the soil behavior
CHECKLIST FOR RECOGNITION OF EXPANSIVE SOIL
S.No
Item to be checked
1
Are the soils near by the project area
known to be expansive?
2
Are the evidences of cracks in walls, curb,
sidewalks and pavements etc. in nearby
construction?
3
Are there shrinkage cracks in the soils in
dry season?
4
Does the soil behaves very sticky and
sticking to the shoes?
5
If you take a lump of dry soil and try to
break it between the fingers, do you find
the soil hard and difficult to break.
Answers
Yes
No
IDENTIFICATION OF EXPANSIVE SOILS
S.No
Degree of
expansion
Liquid limit
(%)
Shrinkage
limit
(%)
Plasticity
index
(%)
1
2
3
4
Very high
High
Medium
60-70
40-60
30-40
>30
20-30
10-20
>35
20-35
10-20
Low
20-30
<10
<10
IDENTIFICATION OF EXPANSIVE SOIL ON THE BASIS
OF GSD
S.No
Degree of
expansion
Clay fraction
(%)
Colloidal content
(%)
1
Very high
>28
>28
2
High
20-28
20-28
3
Medium
12-20
15-20
4
Low
0-12
<15
SENSITIVITY OF CLAYS
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Some clays have a curious property called sensitivity, which
means their strength in a remolded or highly disturbed condition
is less than that in an undisturbed condition at the same
moisture content.
These highly sensitive clays are called quick clays, are found in
certain areas of Eastern Canada, parts of Scandinavia, and else
where.
This behavior occurs because these clays have a very delicate
structure that is disturbed when they are remolded.
The degree of sensitivity is defined by the parameter S1, the ratio
of undisturbed shear strength to the remolded shear strength
CLASSIFICATION OF SENSITIVE SOILS
Classification
Sensitivity, S1
Low sensitivity
2-4
Medium sensitivity
4-8
High sensitivity
8-16
Quick
>16
CLASSIFICATION OF SOILS ON THE BASIS OF IN SITU
TESTS
Correlation between N and Denseness of Sand
N
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Denseness
0-4
Very Loose
25° - 32°
4-10
Loose
27° - 35°
10-30
Medium
30° - 40°
30-50
Dense
35° = 45°
> 50
Very Dense
> 45°
CONTD.
Correlation between N and qu
Consistency
qu (kN / m2)
0-2
Very Soft
< 25
2-4
Soft
25 – 50
4-8
Medium
50 – 100
8 - 15
Stiff
100 – 200
15 – 30
Very Stiff
200 – 400
> 30
Hard
> 400
N
IDENTIFICATION OF DISPERSIVE SOILS
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Dispersion occurs in soils when the repulsive forces between
clay particles exceed the attractive forces thus bringing about
deflocculating so that in the presence of relatively pure water
the particles repel each other to form colloidal suspensions.
Dispersive soils have a moderate to high clay material
content but there are no significant differences in the clay
fractions of dispersive and non-dispersive soils, except that
soils with less than 10% clay particles may not have enough
colloids to support dispersive piping.
Dispersive soils contain a higher content of dissolved sodium
(up to 12%) in their pore water than ordinary soils.
CONTD.
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The sodium adsorption ratio (SAR) is used to quantify the
role of sodium where free salts are present in the pore
water and is defined as:
SAR= Na/ 0.5(Ca+Mg ) with units expressed in meq/litre of
the saturated extract.
. Gerber and Harmse (1987) considered an SAR value
greater than 10 indicative of dispersive soils, between 6
and 10 as intermediate, and less than 6 as non-dispersive.
CONTD.
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The presence of exchangeable sodium is the main chemical factor
contributing towards dispersive behavior in soil.
This is expressed in terms of the exchangeable sodium
percentage (ESP):
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ESP= Exchangeable sodium x 100/cation exchange capacity.
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Where the units are given in meq/100 g of-dry clay.
CONTD.
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Soils with ESP values above 15% are highly dispersive (Bell and
Maud, 1994). Those with low cation exchange values (15 meq/100
g of clay) have been found to be completely non- dispersive at
ESP values of 6% or below.
Unfortunately, dispersive soils cannot be differentiated from nondispersive soils by routine soil mechanics testing.
Although a number of special tests have been used to recognize
dispersive soils, no single test can be relied on completely to
identify them (Bell and Maud, 1994). These can be divided into
physical and chemical tests. The former include the crumb test,
the dispersion or double hydrometer test, the modified
hydrometer or turbidity ratio test and the pinhole test.
CONTD.
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Serious piping damage to embankments and failures of earth
dams have occurred when dispersive soils have been used in
their construction (Bell and Maud, 1 994).
Severe erosion damage also can form deep gullies on earth
embankments after rainfall.
In many areas where dispersive soils are found there is no
economical alternative other than to use these soils for the
construction of earth dams. However, experience indicates that if
an earth dam is built with careful construction control and
incorporates filters, then it should be safe enough even if it is
constructed with dispersive soils.
COLLAPSIBLE SOIL
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Collapsible soils, which are sometimes referred to as metastable
soils, are unsaturated soils that undergo a large volume change
upon saturation.
This volume change may or may not be the result of the
application of additional load.
Foundations that are constructed on such soils may undergo
large and sudden settlement if and when the soil under them
becomes saturated with an unanticipated supply of moisture.
This moisture may come from several sources, such as (a) broken
water pipelines, (b) leaky sewers, (c) drainage from reservoirs
and swimming pools, (d) slow increase of groundwater, and so on.
This type of settlement generally causes considerable structural
damage. Hence identification of collapsing soils during field
exploration is crucial.
CONTD.
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The majority of naturally occurring collapsing soils are aeolin
that is, wind- deposited sand and/or silts, such as loess, aeolic
beaches, and volcanic dust deposits.
These deposits have high void ratios and low unit weights and
are cohesionless or only slightly cohesive.
Loess deposits have silt-sized particles. The cohesion in loess may
be the result of the presence of clay coatings around the silt-size
particles, which holds them in a rather stable condition in an
unsaturated state.
In the United States, large parts of the Midwest and arid West
have such types of deposit. Loess deposits are also found over 1
5%-20% of Europe and over large parts of China
RELATION OF COLLAPSE POTENTIAL TO THE
SEVERITY OF FOUNDATION PROBLEMS
Cp(%)
Severity of problem
0-1
No Trouble
1-5
Moderate Trouble
5-10
Trouble
10-20
Severe Trouble
>20
Very Severe Trouble
SUMMARY
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Stability of any civil engineering structures lies primarily with
the response of soil under the influence of external loading.
It is a must for any civil engineer to understand the type of soil
and their engineering characteristics prior to the use of same for
any applications.
If there is no proper importance given to the soil before start of
construction activities in the beginning itself, then the
rectification of damage to the structure, because of soil
movement, if any would be much higher than the cost of the
project itself.
THANKS