ce222 sm 06 soil plasticity
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Soil Mechanics I CE - 222
Lecture 6
Soil Plasticity Atterberg Limits
Asif Riaz
SCEE
NUST, Sector H-12, Islamabad
Off : +92-51-90854008
Email: asifriaz@scee.nust.edu.pk
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Special Instructions
No mercy on low attendance
No Cross Talking
among students during lecture
No Mobile Phone during Class
2
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Lecture No
Sections Covered/Reading Assignment
1 Introduction
2 Soil Formation and Nature of Soil constituents Assignment-1
3 Mechanical Analysis of Soil (sieve analysis & hydrometer analysis) Quiz -1
4 Clay Soil Formation
5 Phase Relationships (Weight-Volume relationship) Assignment-2
6 Soil Plasticity Quiz -2
7 Soil Plasticity (Specific Gravity)
OHT - 1
8 Soil Classification systems
9 Soil Compaction: Introduction, theory, and lab tests Quiz -3
10 Soil Compaction: Field Density Determination Assignment-3
11 Soil Permeability Quiz -4
12 Seepage and Flow Nets
OHT - 2
13 Seepage Examples
14 In-Situ Stresses Assignment-4
15 Stresses in Soil
End Semester Exam
Course outline
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Classification of fine-grained soils
The Unified Soil Classification System uses the term fines to describe everything that passes through a No. 200 sieve (
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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
5
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Attreberg 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 as 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.
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Attreberg Limits 7
SL PL LL
Solid
State
Vo
lum
e, v
or
e
Water content, w %
Solid Semi
Solid Plastic Liquid
S = 100 %
Drying
Wetting
vf
PI
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Attreberg Limits 8
Liquid Limit, LL
Liquid State
Plastic Limit, PL
Plastic State
Shrinkage Limit, SL
Semisolid State
Solid State Dry Soil
Fluid soil-water
mixture
Incr
easi
ng w
ater
con
tent
Consistency Limits of Soil
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Atterberg Limits
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, i.e., S = 100%).
Plastic Limit (PL) is defined as the moisture content at which soil begins to behave as a plastic material.
Liquid Limit (LL) is defined as the moisture content at which soil begins to behave as a liquid material and begins to flow (LL of a fine-grained soil gives the moisture content at which the shear strength of the soil is approximately 2.5kN/m2).
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Liquid Limit (LL)
In the lab, the LL is defined as the moisture content (%) required to close a 2-mm wide groove in a soil pat a distance of 0.5 in along the bottom of the groove after 25 blows.
Standard: ASTM D 4318.
Soil sample size: 150g passing No. 40 sieve.
Equipment: Casagrande liquid limit device.
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Casagrande Apparatus 11
Before Test After Test
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12
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Liquid Limit (Procedure)
150g air dry soil passing No. 40 sieve.
Add 20% of water - mix thoroughly.
Place a small sample of soil in LL device (deepest part about 8-10 mm).
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 0.5 inch.
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)
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Liquid Limit 14
Flow Curve
25
46.39
y = -7.55Ln(x) + 70.69
44
46
48
50
1 10 100
Number of drops
Wate
r conte
nt
(%)
- LL values
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LL values 50 (high) 16
Liquid Limit, %
PI,
%
50
H
- LL values
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Plastic Limit
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. It is the dividing line between the plastic and semisolid states.
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Liquid Limit, LL
Liquid State
Plastic Limit, PL
Plastic State
Shrinkage Limit, SL
Semisolid State
Solid State Dry Soil
Fluid soil-water
mixture
Incr
easi
ng
wat
er c
on
ten
t
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Plastic Limit 19
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Plastic Limit (Procedure)
Take 20g of soil passing No. 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 inch.
Continue rolling until the thread crumbles into several pieces.
Determine the moisture content of about 6g of the crumbled soil.
20
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Plasticity Index & Consistency Index
Plasticity Index is the difference between the liquid limit and plastic limit of a soil
PI = LL PL
Consistency Index (CI)
CI = LL - w
LL PI
Conditions for Maximum and Minimum CI ????
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Activity 22
Soil type Activity, A
Inactive clays A < 0.75
Normal clays 0.75 < A < 1.25
Active clays A >1.25
Properties of Active Clays When Wet = Large Volume Change When Dry = Large Shrinkage Very Reactive (Chemically)
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Activity Values of different Soils 23
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Plasticity Chart 24
Liquid Limit
Pla
stic
ity
Ind
ex
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Plastic soils 25
Plastic soils
plot above the
A-Line on a
Plasticity Chart
A-Line
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Non-Plastic Soils 26
Non-plastic or
slightly plastic
soils plot below
the A-Line on a
Plasticity Chart
A-Line
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U-Line significance 27
Correct tests
never plot
above U-line
and LL values
are never < 16
16
U-Line
A-Line
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Plasticity Chart 29
Low plasticity/ compressibility
Medium plasticity/ compressibility
High plasticity/compressibility
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Plasticity Chart 30
Cohesionless soil
Organic clays
Silt
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Shrinkage Limit Calculations 31
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