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

Special Instructions

No mercy on low attendance

No Cross Talking

among students during lecture

No Mobile Phone during Class

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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

Classification of fine-grained soils

The Unified Soil Classification System uses the term fines to describe everything that passes through a No. 200 sieve (

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

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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

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

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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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

LL values 50 (high) 16

Liquid Limit, %

PI,

%

50

H

LL values

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

Plastic Limit 19

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.

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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)

Activity Values of different Soils 23

Plasticity Chart 24

Liquid Limit

Pla

stic

ity

Ind

ex

Plastic soils 25

Plastic soils

plot above the

A-Line on a

Plasticity Chart

A-Line

Non-Plastic Soils 26

Non-plastic or

slightly plastic

soils plot below

the A-Line on a

Plasticity Chart

A-Line

U-Line significance 27

Correct tests

never plot

above U-line

and LL values

are never < 16

16

U-Line

A-Line

Plasticity Chart 29

Low plasticity/ compressibility

Medium plasticity/ compressibility

High plasticity/compressibility

Plasticity Chart 30

Cohesionless soil

Organic clays

Silt

Shrinkage Limit Calculations 31