Dr. Ravi Kant mittal
Assistant Professor, BITS Pilani
E- Mail:
ravi.mittal@rediffmai.com
ravimittal@bits-pilani.ac.in
Mobile: 9887692025
CE C361 Soil Mechanics and Foundation Engg.1
Contents
• Soil Formation
• Clay Minerals
• Field Identification of Soils
• SOIL CLASSIFICATION
2
Definition
Soil may be defined as an assemblage of discrete solid
particles of organic or inorganic composition with air
and/or water occupying the void space amongst the
particles.
Soil can thus have all three phases present in it
- solid, water and air.
If there is no AIR present Æ SATURATED SOIL
If there is no WATER present Æ DRY SOIL
3
Rock Cycle – Soil Formation
Soils
The final products
due to weathering are
soils
Soil Formation
Soil is formed by the process of weathering of rocks,
i.e., disintegration and decomposition of rocks and
minerals at or near the earth’s surface through the
actions of natural or mechanical and chemical agents
into smaller and smaller grains.
The factor of weathering such as changes in
temperature
and
pressure;
erosion
and
transportation by wind, water and glaciers; chemical
action such as crystal growth, oxidation, hydration,
carbonation and leaching by water.
Soil Formation
Soils formed by mechanical weathering bear a similarity in certain
properties to the minerals in the parent rock.
Chemical changes destroys the identity. In chemical weathering
some minerals disappear partially or fully, and new compounds
are formed.
The intensity of chemical weathering depends on the presence of
water and temperature and the dissolved materials in the water.
Carbonic acid and oxygen are the most effective dissolved
materials found in water which cause the weathering of rocks.
95% of the earth crust consists of Igneous rock and remaining 5%
consists of sedimentary and metamorphic rocks. However,
sedimentary rocks are present on 80% of the earth’s surface area.
Residual and Transported soils
Soils which are formed by weathering of rocks may remain in
position at the place of region are called as RESIDUAL SOILS.
Soils may get transported from place of origin by various
agencies such as wind, water, ice, gravity etc. these soils are
called as TRANSPORTED SOILS.
Residual soils differ very much from transported soils in their
characteristics and engineering behavior.
Transported soils may also be referred to as SEDIMENTARY
soils since the sediments formed by weathering of rocks will be
transported by agencies such as wind and water.
A high degree of alteration of particle shape, size and texture
occurs during transportation and deposition.
Transported soils
Transported soils can be classified as below based on the
transporting agency and the place of deposition.
ALLUVIAL SOILS: Soils transported by rivers and streams:
sedimentary clays
AEOLINE SOILS: soils transported by wind: Loess
GLACIAL SOILS: soils transported by glaciers: Glacial till.
LACUSTRINE SOILS: soils deposited in lake beds: Lacustrine silts
and lacustrine clays.
MARINE SOILS: soils deposited in sea beds: Marine silts and marine
clays.
Major Soil deposits in India
10
SOIL FORMATION & COMPOSITION
Major Soil deposits in India:
(i)Marine deposits:
¾ Very soft to soft clay (thickness from 5 to 20 meters).
¾ Medium sensitive & inorganic.
¾ Need pretreatment before load application.
¾ Controlled loading to prevent failure.
11
SOIL FORMATION & COMPOSITION
Major Soil deposits in India:
( (ii)Black cotton Soils:
¾ Expansive due to presence of Illite & Montmorillonite
clays.
¾ Thickness upto max. 20m.
¾ Crack depth & pattern varies.
¾ Surface is hard in summer & slushy in rainy season.
¾ Seasonal w/c change causes volume change upto
max. 1.5m depth.
¾ Due to swelling & shrinkage characteristics, soil
should be pretreated.
12
1. SOIL FORMATION & COMPOSITION
(iii)Laterites & lateritic soils:
¾ Thickness more than 30m.
¾ Laterisation is the process of rock removal, silica
removal, base removal, aluminum & iron
accumulation at the top of soil profile.
¾ If approximately 90% coarse grained: laterite.
¾ Mostly fine grained: lateritic.
¾ Has high strength when cut & dried in heat (due to
iron oxide dehydration & halloysite presence).
¾ Strength of hardened soil not affected due to water
presence.
(
13
1. SOIL FORMATION & COMPOSITION
( (iv)Alluvial Soils:
¾ Exhibits alternate layers of sand, silt & clay.
¾ In some locations organic layers are also found.
¾ Depth upto 100m.
¾ Alluvial sand: Used as fine aggregate.
¾ Alluvial clay: For brick manufacturing.
14
1. SOIL FORMATION & COMPOSITION
(VI)Boulder Deposits:
¾ Boulder deposits due to rivers flowing in hilly
terrains.
¾ Their properties depend on relative proportion of
boulder and soil matrix.
¾ Boulder-to-boulder contact results in large friction
and angle of shearing resistance.
¾ Due to large size, laboratory sample is not
representative of natural deposit, hence field
investigations are carried out to find properties
needed in design.
15
1. SOIL FORMATION & COMPOSITION
(V)Desert Soils:
¾ Wind blown deposits in the form of sand dunes.
¾ Formed under arid conditions.
¾ Mostly fine or silty sand.
¾ Water scarcity is a serious problem.
(
16
17
SAND:
„
Sand particles are made up of rock minerals. They have the same
composition as that of big boulder from a rock mass. They are only
smaller in size.
„
The processes of weathering reduces boulders to cobbles, cobbles
to gravel, gravel to sand, sand to silt and even silt to rock dust which
have particles of clay size.
„
Particles of rock minerals are ELECTRICALLY NEUTRAL. They are
acted upon only by the gravitational force.
CLAYS:
„
Clay particles have a net electrical charge on them. Usually a
negative charge on their faces and a positive charge on their ends.
„
They are made up of clay minerals.
„
Three important clay minerals are Kaolinite, Illite, Montmorillonite
„
Clay mineral particles have a net electrical charge on them on
account of a phenomenon that occurs during their formation.
Clay Formation
„
Clay particles < 2 μm
„
Compared to Sands and
Silts, clay size particles
have undergone a lot
more “chemical
weathering”!
19
Clay vs. Sand/Silt
„
Clay particles are generally more platy in
shape (sand more equi-dimensional)
„
Clay particles carry surface charge
„
Amount of surface charge depends on
type of clay minerals
„
Surface charges that exist on clay
particles have major influence on their
behavior (for e.g. plasticity)
20
Clay Minerals
„
Kaolinite family
Kaolinite
(ceramic industry, paper, paint,
pharmaceutical)
„
Smectite family
Montmorillonite
(weathered volcanic ash,
Wyoming Bentonite, highly expansive,
used in drilling mud)
„
Illite family
21
Clay Morphology
Scanning Electron
Microscope
(SEM)
„ Allows us to study
morphology of
clay minerals
„ Used in mineral
identification
„
22
Origin of Clay Minerals
“The contact of rocks and water produces clays, either at or near the surface of
the earth”
„
„
Rock +Water → Clay
For example,
„The CO2 gas can dissolve in water and form carbonic acid, which will become
hydrogen ions H+ and bicarbonate ions, and make water slightly acidic.
„
„
CO2+H2O → H2CO3 →H+ +HCO3-
The acidic water will react with the rock surfaces and tend to dissolve the K ion
and silica from the feldspar. Finally, the feldspar is transformed into kaolinite.
„
„
„
„
Feldspar + hydrogen ions+water → clay (kaolinite) + cations, dissolved silica
2KAlSi3O8+2H+ +H2O → Al2Si2O5(OH)4 + 2K+ +4SiO2
Note that the hydrogen ion displaces the cations.
1.2 Characteristics
(Holtz and Kovacs, 1981)
Soil Classification
Object:
To keep various types of soils into groups
according to their properties
ÆSoil consisting of similar characteristics Can be
placed in the SAME Group
Need:
To find the suitability of the soil for construction of
dams, highways and foundations
Field Identification of Soils
Distinguish Gravel from Sand Æ Grain Size
Sand from Silt
Silt from Clay
Æ Dispersion Test
Æ Dispersion Test
Æ Shaking (Dilatancy) Test
Æ Strength Test
Æ Rolling (Toughness) Test
Organic Content and Color
Organic soils usually have a distinctive odour of decomposed organic
matter, which can be detected by heating.
Acid Test – use dilute HCL to check the presence of Calcium Carbonate
Shine Test – Highly Plastic soil is more Shine than Low Plastic soil
Types of Soil Classification
1. Particle Size Classification
2. Unified Soil Classification System (USCS)
3. Textural Classification
4. Public Roads Administration Classification
(AASHTO, 1978)
Particle Size Classification
™ Soils are according to Grain Size
™ Various grain size classifications are in use
™ Grain size distribution of soil is required
™ Percentage of soil in each size group is
determined
Example: Soil Æ10% Gravel + 52% Sand + 38% Silt & Clay
Unified Soil Classification System (USCS)
Origin of USCS:
This system was first developed by Professor Casagrande
(1948) for the purpose of airfield construction during World
War II. Afterwards, it was modified by Professor Casagrande
to enable the system to be applicable to dams, foundations, and
other construction
Four major divisions:
(1) Coarse-grained
(2) Fine-grained
(3) Organic soils
(4) Peat
IS 1478 is adopted USCS after rounded up
Fine
Grained
Soils
Coarse Grained Soils
Gravel
Silt
Sand
Clay
Boulders Cobbles
Coarse
300 mm
Fine
Coarse
Medium
Fine
0.075
mm
4.75 mm
80 mm
20 mm
2.0 mm
0.425 mm
IS 1478 Soil Classification System
0.002
mm
IS 1478 Soil Classification System
50 %
Coarse-grained soils:
Fine-grained soils:
Gravel
Silt
Sand
Clay
4.75 mm
0.075 mm
•Grain size distribution
•PL, LL
•Cu
•Plasticity chart
•Cc
Sieve analysis
Atterberg limits
Particle size
IS 1478
Question
For the purpose of engineering descriptions, soils are divided into classes of similar grain size. The
NOUNS used to describe a size class refer to a specific range of sizes.
What is the range of sizes of SAND?
0.075 mm to 0.425 mm
0.425 mm to 4.75 mm
0.075 mm to 4.75 mm
2.0 mm to 4.75 mm
0.075 – 4.75 mm
What is the range of sizes of FINE SAND particles?
0.075 mm to 0.425 mm
0.425 mm to 4.75 mm
0.075 mm to 4.75 mm
2.0 mm to 4.75 mm
0.075 – 0.425 mm
Particle size
IS 1478
Question
For the purpose of engineering descriptions, soils are divided into classes of similar grain size. The
NOUNS used to describe a size class refer to a specific range of sizes.
What is the range of sizes of MEDIUM SAND?
0.075 mm to 0.425 mm
0.425 mm to 2.0 mm
0.075 mm to 4.75 mm
2.0 mm to 4.75 mm
0.425 – 2.0 mm
What is the range of sizes of SILT particles?
0.002 mm to 0.425 mm
0.002 mm to 4.75 mm
0.002 mm to 0.075 mm
<0.002 mm
0.002 – 0.075 mm
Particle size
IS 1478
Question
For the purpose of engineering descriptions, soils are divided into classes of similar grain size. The
NOUNS used to describe a size class refer to a specific range of sizes.
What is the range of sizes of Gravel?
2 mm to 4.75 mm
4.75 mm to 20 mm
4.75 mm to 80 mm
20 mm to 80 mm
4.75 – 80 mm
What is the range of sizes of CLAY particles?
0.002 mm to 0.425 mm
0.002 mm to 4.75 mm
0.002 mm to 0.075 mm
<0.002 mm
< 0.002 mm
Classification of Soils
Grain Size Distribution
•Experiment
Coarse-grained soils:
Gravel
Sand
Fine-grained soils:
Silt
Clay
0.075 mm (USCS)
Sieve analysis
Hydrometer analysis
Procedure for grain size determination
„
Sieving - used for particles > 75 μm
„
Hydrometer test - used for smaller particles (<
‹
75 μm)
Analysis based on Stoke’s Law, velocity proportional to diameter
At the beginning
Towards the end of test
Schematic diagram of hydrometer test
Hydrometer Analysis – Stoke’s Law
Assumption
( γ s − γ w )D 2
v=
18η
Reality
Sphere particle
Platy particle (clay particle) as D
≤ 0.005mm
Single particle
Many particles in the suspension
(No interference
between particles)
Known specific
gravity of
particles
Terminal velocity
Average results of all the
minerals in the particles,
including the adsorbed water
films.
Note: the adsorbed water films
also can increase the resistance
during particle settling.
Brownian motion as D ≤ 0.0002
mm
Grain Size Distribution Curves
100
% Finer
80
60
40
20
D60
0
0 .0 0 0 1
0 .0 0 1
0 .0 1
0 .1
1
10
100
P a r ti c l e s i z e ( m m )
Cu =
Cc =
D 60
D10
2
30
D
( D60 × D10 )
x% of the soil has particles
smaller than Dx
where CU is Coefficient of Uniformity and Cc is Coefficient of Curvature
Grain Size Distribution Curves
100
B
E
A
% Finer
80
60
D
40
C
20
0
0 .0 0 0 1
0 .0 0 1
0 .0 1
0 .1
1
P a r ti c l e s i ze ( m m )
A
Well graded Soil
B
Uniform Soil (or Poorly Graded Soil)
C
Gap Graded Soil (or Poorly graded soil)
D
Well graded with some fines
E
Well graded with an excess of fines
10
100
IS 1478 Soil Classification System
To determine Well Graded (W) or Poorly Graded (P), calculate Cu and Cc.
Cu
D 60
=
D10
D302
Cc =
( D60 × D10 )
where CU is Coefficient of Uniformity and Cc is Coefficient of Curvature
If prefix is G (Gravel) then suffix is W if Cu > 4 and Cc is between 1 and 3
otherwise use P
If prefix is S (Sand) then suffix is W if Cu > 6 and Cc is between 1 and 3
otherwise use P
„
IS 1478
Describe the following Soil
D10 = 0.02 mm (effective size)
D 30 = 0.6 mm
D 60 = 9 mm
Criteria
Well − graded soil
Coefficient of uniformity
D
9
C u = 60 =
= 450
D10 0.02
1 < Cc ≤ 3 and Cu > 4
Coefficient of curvature
1 ≤ Cc ≤ 3 and Cu > 6
Cc =
2
2
( for gravels)
(D 30 )
(0.6)
=
=2
(D10 )(D 60 ) (0.02)(9)
Æ Well Graded Soil
( for sands )
Question
„What is the Cu for a soil with only one grain
size?
„
Finer
Coefficient of uniformity
D
Cu = 60 = 1
D10
D
Grain size distribution
Question
The grading curve for a soil gives the size
characteristics:
d10 = 0.16 mm and d60 = 0.47 mm
What is the Uniformity coefficient
(Cu) and gradation of the soil?
0.34, well graded
2.94, well graded
2.94, uniformly graded
0.34, uniformly graded
2.94, Uniformly (or Poorly) Graded Sand
Classification of Fine Grained Soils
IS 1478
-- Atterberg Limits (or Consistency Limits)
Moisture content =
Increasing water content
Soil-water
mixture
massof water
massof solids
Liquid State
Liquid Limit, LL
Plastic State
Plastic Limit, PL
Semisolid State
Shrinkage Limit, SL
Solid State
Dry Soil
Atterberg Limits (or Consistency Limits) - Cont.
Volume Change with water content
Classification of Fine Grained
Soils
„
„
„
„
The classification system uses the term “fines” to describe
everything that passes through a # 200 sieve (<0.075mm)
No attempt to distinguish between silts and clays in terms
of particles sizes since the biggest difference between silt
and clay is not their particle sizes, but their physical and
chemical structures
The soil consistency is used as a practical and an
inexpensive way to distinguish between silts and clays
Plasticity property is important because it describes the
response of a soil to change in moisture content
Why Plasticity?
„
Water Content Significantly affects properties of Silty and
Clayey soils (unlike sand and gravel)
‹
‹
‹
‹
‹
Strength decreases as water content increases
Soils swell-up when water content increases
Fine-grained soils at very high water content possess
properties similar to liquids
As the water content is reduced, the volume of the soil
decreases and the soils become plastic
If the water content is further reduced, the soil becomes
semi-solid when the volume does not change
Liquid and plastic limits
The lower and upper limits of
the PLASTIC range are used to
classify the fine soils.
Plastic Limit, Liquid Limit
Plasticity Index
The difference between the liquid limit
(wL) and plastic limit (wP) is called as
PLASTICITY INDEX (P.I.)
Plasticity Index = Liquid Limit – Plastic Limit
Atterberg Limits
• Atterberg limits are important to describe the consistency
of fine-grained soils
• The knowledge of the soil consistency is important in
defining or classifying a soil type or predicting soil
performance when used a construction material
• A fine-grained soil usually exists with its particles
surrounded by water.
• The amount of water in the soil determines its state or
consistency
• Four states are used to describe the soil consistency;
solid, semi-solid, plastic and liquid
50
Atterberg Limits (cont.)
Wetting
Semi
Solid
Volume, v or e
Solid
Solid
State
Plastic
Liquid
vi
S = 100 %
vf
SL
PL
LL
PI
Drying
w%
51
Atterberg Limits
• Liquid Limit (LL) is defined as the moisture content at which
soil begins to behave as a liquid material and begins to flow
(Liquid limit of a fine-grained soil gives the moisture content
at which the shear strength of the soil is approximately 1.7 to
2kN/m2 = 17-20 gm/cm2)
• Plastic Limit (PL) is defined as the moisture content at
which soil begins to behave as a plastic material
• Shrinkage Limit (SL) is defined as the moisture content at
which no further volume change occurs with further reduction
in moisture content.
(SL represents the amount of water required to fully saturate
the soil (100% saturation))
52
53
Liquid Limit (LL)
• In the lab, the LL is defined as the
moisture content (%) required to close a 2mm wide groove in a soil pat a distance of
0.5 inch along the bottom of the groove
after 25 blows.
• ASTM D 4318 (IS2720)
• Soil sample size 150g passing 425 micron
• Equipment: Casagrande liquid limit device
54
Liquid Limit - LL (or wL)
•
Casagrande Method
Professor Casagrande standardized the test and
developed the liquid limit device.
•
Cone Penetrometer Method
This method is developed by the Transport and Road
Research Laboratory, UK.
LL - Casagrande Method
•Device
N=25 blows
Closing distance =
12 mm
The water content, in percentage, required to close a distance of 12 mm
along the bottom of the groove after 25 blows is defined as the liquid limit
Source: http://www.wku.edu/~matthew.dettman/matt/prof/ce410/ll.htm
Liquid Limit (Procedure)
„
„
„
„
„
„
„
„
„
150g air dry soil passing 425 micron (# 40 sieve)
Add 20% of water - mix thoroughly
Place a small sample of soil in LL device (deepest part about 810mm)
Cut a groove (2mm at the base)
Run the device, count the number of blows, N
Stop when the groove in the soil close through a distance of 0.5in
Take a sample and find the moisture content
Run the test three times [N~(10-20), N~(20-30) and N~(35-45)] and
Plot number of blows vs moisture content and determine the liquid
limit (LL) (moisture content at 25 blows)
Determining LL
Log Scale
LL - Casagrande Method (Cont.)
w
N
w1 − w2
Flow index, I F =
log( N 2 / N1 )
Determining LL by single test
LL - Cone Penetrometer Method
30o Cone of Stainless steel
Total sliding weight of 148 g
Penetration of cone (mm)
Cylindrical mould of 5 cm diameter and 5 cm height.
20 mm
LL
Water content w%
For D = 14 to 28mm
Where x is the depth of penetration of cone in mm
wx is water content corresponding to penetration D, here it is based on
assumption that shear strength of soil at LL is about 15 to 17.6 g/cm2
LL - Cone Penetrometer Method
31o Cone of Stainless steel
Total sliding weight of 148 g
Penetration of cone (mm)
Cylindrical mould of 5 cm diameter and 5 cm height.
20 mm
LL
Water content w%
wL = wx + 0.01(25 − x)( wx + 15) For x = 14 to 28mm
Where x is the depth of penetration of cone in mm
wx is water content corresponding to penetration x, here it is based on
assumption that shear strength of soil at LL is about 15 to 17.6 g/cm2
4.2.3 Comparison
A
good
correlation
between the
two
methods can
be observed
upto LL is
less
than
100.
Littleton and Farmilo, 1977 (from Head, 1992)
Plastic Limit (PL)
„
„
„
The moisture content (%) at which the soil when
rolled into threads of 3.2mm (1/8 in) in diameter,
will crumble.
Plastic limit is the lower limit of the plastic stage
of soil
Plasticity Index (PI) is the difference between
the liquid limit and plastic limit of a soil
Plastic Limit – PL (or wP)
The plastic limit is defined as the water content at which a
soil thread with 3 mm diameter just crumbles.
Plastic Limit (cont.)
Plastic Limit (Procedure)
„
„
„
„
„
„
Take 20g of soil passing #40 sieve into a dish
Add water and mix thoroughly
Prepare several ellipsoidal-shaped soil masses
by quizzing the soil with your hand
Put the soil in rolling device, and roll the soil until
the thread reaches 1/8 in
Continue rolling until the thread crumbles into
several pieces
Determine the moisture content of about 6g of
the crumbled soil.
Plasticity Index, PI
„
„
Plasticity Index is the difference between the
liquid limit and plastic limit of a soil
PI = LL – PL
After finding LL and PI use plasticity chart to
classify the soil
SYMBOLS Used for USCS Soil Classification
„
„
„
„
„
„
„
Soil symbols:
G: Gravel
S: Sand
M: Silt
C: Clay
O: Organic
Pt: Peat
„
„
„
„
„
„
„
Liquid limit symbols:
H: High LL (LL > 50)
I: Intermediate
(35<LL<50)
L: Low LL (LL < 35)
Gradation symbols:
W: Well-graded
P: Poorly-graded
Well − graded soil
Ex:
SW: Well-graded Sand
SC: Clayey Sand
SM: Silty Sand
1 p Cc ≤ 3 and Cu > 4
( for gravels)
1 ≤ Cc ≤ 3 and Cu > 6
( for sands)
Classification of Fine Grained Soils
IS1478
Fine grained soils Æ Silt (M) & Clay (C)
To determine M or C use plasticity chart
Plasticity
Chart
A – Line
PI = 0.73(wL – 20)
L iq u id li m it
Above A-line use suffix C – Clay
Below A-line use suffix M – Silt
LL < 35% use Prefix L
35% < LL < 50% use Prefix I
LL > 50% use Prefix H
Use “O” below
A – Line, if
Soil is Organic
Classification of Fine Grained Soils -
USCS
Cont.
Borderline Cases (Dual Symbols)
Fine-grained soils with limits within the shaded zone (i.e.,
PI between 4 and 7 and LL between 12 and 25).
It is hard to distinguish between the silt and claylike
materials.
Use: CL-ML: Silty clay &
SC-SM: Silty, clayed sand.
Coarse-grained soils with 5% - 12% fines.
The first symbol indicates whether the coarse fraction is
well or poorly graded.
The second symbol describe the contained fines (M/C).
Example: SP-SM: Poorly graded Sand with Silt.
Classification of Coarse Grained Soils
Classification of Fine Grained Soils
Structure of Soils
The structure of soil is defined as the MANNER OF
ARRANGEMENT and state of aggregation of soil grains.
Single grained structure:
-- is a characteristic of coarse
grained soils. Gravitational forces
predominate the surface forces and
hence grain to grain contact results.
Honey – Comb Structure
-- occur in fine grained soils (in
particular silts and rock flour). Due to
smaller size of grains, besides
gravitational forces, inter-particle
surface forces also play a major role.
Structure of Soils
Flocculent Structure:
-- is characteristic of fine grained soils such as clays. Interparticle forces play a predominant role in the deposition.
-- very fine particles of colloidal size (<0.001 mm) may be in a
flocculated or dispersed state. The flaky particles are oriented
edge - to - edge or edge – to – face or face – to – face.
Flaky particles of clay minerals tend to form card house /dispersed structure.
Flocculated structure
Card-house structure
Dispersed structure
Textural Classification
1. IS 1498 – 1970 System
IS 1498 – 1970 System
™ Grain size distribution of soil is required
™ Percentage of soil in each size group is determined
Ex:- Soil Æ10% Gravel + 52% Sand + 38% Silt & Clay
Plasticity Characteristics
Plasticity Index
Plasticity
0
1–5
5 – 10
10 – 20
20 – 40
> 40
Non-Plastic
Slight
Low
Medium
High
Very High
Range of Plasticity Index
Indices
Shrinkage Index =
Plastic Limit – Shrinkage Limit
Flow Index = (w2-w1)/(log10(N1/N2))
(from Liquid limit test)
Toughness Index =
Plasticity Index/Flow Index
Indices – Liquidity Index/ Consistency Index
„
Liquidity index LI
„
Consistency index CI
„
For scaling the natural water
content of a soil sample to the
Limits.
„
For scaling the natural water
content of a soil sample to the
Limits.
w − wp
w − wp
wl − w p
wl − w wl − w
=
CI =
PI
wl − w p
w is the water content
w is the water content
LI =
PI
=
LI > 1 Æ soil is in the Liquid state
LI = 1 Æ soil is at the liquid limit
LI = 0 Æ soil is at the Plastic limit
CI < 1 Æ soil is in the Plastic state
CI = 1 Æ soil is at the Plastic limit
CI = 0 Æ soil is at the Liquid limit
LI + CI = 1
4.1 Atterberg Limits
„
The presence of water in fine-grained soils can significantly affect
associated engineering behavior, so we need a reference index to clarify the
effects. (The reason will be discussed later in the topic of clay minerals)
In percentage
(Holtz and Kovacs, 1981)
Consistency Classification
Consistency
Index
Liquidity
Index
Consistency
1.00 – 0.75
0.00 – 0.25
Stiff
0.75 – 0.50
0.25 – 0.50
Medium soft
0.50 – 0.25
0.50 – 0.75
Soft
0.25 – 0.00
0.75 – 1.00
Very soft
Indices- ACTIVITY
„
Activity, A
PI
% clay size material present
clay fraction : < 0.002 mm
A=
•Purpose
- Indicates of the type of clay
present in soil
Normal clays: 0.75<A<1.25
„Inactive clays: A<0.75
„Active clays: A> 1.25
„
High activity:
„large volume change when wetted
„Large shrinkage when dried
„Very reactive (chemically)
„
Examples:
Kaolinite – Inactive
Illite – Normal
Montmorillonite – Active
Indices - Sensitivity
„
Sensitivity St (for clays)
Strength(undisturbed )
St =
Strength(disturbed )
Unconfined compressive strength
Clay
particle
w > LL
Water