UNIVERSIDAD NACIONAL MAYOR DE SAN MARCOS

FACULTAD DE INGENIERIA GEOLOGICA, MINERA,

METALURGIA Y GEOGRAFICA

ESCUELA DE POSGRADO

MECANICA DE SUELOS Y CIMENTACIONES

Mag. Ing. Lloyd Solrzano P.

Index Propierty of Soil

GO

Plasticity Index of Soils

A. Overview

soil texture degree of plasticity (Atterberg Limits)

Wentworth

Scale

Gravel 75-2 mm

Sand 2-0.075 mm

Silt and Clay 75mm)

Soil Texture

B. Overview of Mechanics

As water content increases,

the shear strength

decreases

Its all about shear strength

8

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)

(Holtz and Kovacs, 1981)

In percentage

C. Liquid Limit

Soil is practically a liquid Shows minimal shear strength Defined as the moisture content required

to close a distance of 0.5 inch along

the bottom of a groove after 25 blows

of the liquid limit device.

animation

C: Liquid Limit-LL

Casagrande Method

(ASTM D4318-95a)

Professor Casagrande standardized the test and developed the liquid limit device.

C: Liquid Limit-LL

D. Plastic Limit

Water content at which the soil is a plastic Less water content than liquid limit Wide range of shear strengths at plastic limit Defined as the moisture content % at which the soil begins to crumble when rolled into 1/8 diameter threads

animation

13

D: Plastic Limit-PL

The plastic limit PL is defined as the water content at which a soil thread

with 3.2 mm diameter just crumbles.

ASTM D4318-95a, BS1377: Part 2:1990:5.3

(Holtz and Kovacs, 1981)

E. Plasticity Index (PI)

Difference between Liquid Limit and Plastic Limit Important measure of plastic behavior

E. Plasticity Index (PI)

In general.

PI Degree of Plasticity

0 Nonplastic

1-5 Slightly plastic

5-10 Low plasticity

10-20 Medium plasticity

20-40 High plasticity

40+ Very high plasticity

(from Burmister, 1949)

Atterberg Limits Particle size is not that useful for fine grained soils

Figure: Moisture content versus volume relation during drying

SL - Shrinkage Limit

PL - Plastic Limit

LL - Liquid limit

Atterberg Limits

SL - Shrinkage Limit

PL - Plastic Limit

LL - Liquid limit

Plasticity Index = LL - PL = PI or Ip

Moisture contentmassof water

massof solids

Shrinkage Limit-SL

Definition of

shrinkage limit:

The water content at

which the soil volume

ceases to change is

defined as the

shrinkage limit.

(Das, 1998)

SL

Shrinkage Limit-SL (Cont.)

(Das, 1998)

Soil volume: Vi

Soil mass: M1

Soil volume: Vf

Soil mass: M2

)100)((M

VV)100(

M

MM

(%)w(%)wSL

w

2

fi

2

21

i

20

Plasticity Chart

(Holtz and Kovacs, 1981)

LL

PI

H L

The A-line generally

separates the more

claylike materials

from silty materials,

and the organics

from the inorganics.

The U-line indicates

the upper bound for

general soils.

21

Symbols

Soil symbols:

G: Gravel

S: Sand

M: Silt

C: Clay

O: Organic

Pt: Peat

Liquid limit symbols:

H: High LL (LL>50)

L: Low LL (LL

Procedure for AASHTO

Classification

(American Association of State

Highway and Transportation

Officials)

Developed in 1929 as the Public Road Administration Classification System

Modified by the Highway Research Board (1945)

Procedure for AASHTO

Classification

Determine the percentage of soil passing the #200 sieve

Determine the subgroups

For coarse-grained soils (gravel and sand), determine the percent passing the #10, 40,

and 200 sieves, AND

Determine the liquid limit and plasticity index

THEN, determine soil group or subgroup from Table 9.1

For coarse-grained soils (gravel and sand), determine the percent passing the #10, 40,

and 200 sieves. Cobble Gravel

Very Coarse to Med Sand

Fine/Very Fine Sand

Silt/Clay

3 #10

#40

#200

#10

#40

#200

Procedure for AASHTO

Classification

Determine the percentage of soil passing the #200 sieve

Determine the subgroups

For fine-grained soils (silt & clay), determine the liquid limit and plasticity index

Determine soil group or subgroup from Table 9.2

AASHTO Classification for Soils

Determine the group index

Determine the Group Index (usually reflects the relative strength of the material, where low values

have the greatest shear strength)

Determine the group index

fine

{e.g.: A-7-5(9)}

Example Problem

A-7-6(10)

Unified Soil Classification System

(USCS) Origin of USCS:

This system was first developed by Professor A. Casagrande (1948) for

the purpose of airfield construction during World War II. Afterwards, it

was modified by Professor Casagrande, the U.S. Bureau of Reclamation,

and the U.S. Army Corps of Engineers to enable the system to be

applicable to dams, foundations, and other construction (Holtz and Kovacs, 1981).

Four major divisions:

(1) Coarse-grained

(2) Fine-grained

(3) Organic soils

(4) Peat

Definition of Grain Size

Boulders Cobbles

Gravel Sand Silt and

Clay Coarse Fine Coarse Fine Medium

300 mm 75 mm

19 mm

No.4

4.75 mm

No.10

2.0 mm

No.40

0.425 mm

No.200

0.075

mm

No specific

grain size-use

Atterberg limits

General Guidance

Coarse-grained soils:

Gravel Sand

Fine-grained soils:

Silt Clay

NO.200

0.075 mm

Grain size distribution

Cu

Cc

PL, LL

Plasticity chart

50 %

NO. 4

4.75 mm

Required tests: Sieve analysis

Atterberg limit

LL>50

LL

Symbols

Soil symbols:

G: Gravel

S: Sand

M: Silt

C: Clay

O: Organic

Pt: Peat

Liquid limit symbols:

H: High LL (LL>50)

L: Low LL (LL

Unified Soil Classification To determine if W or P, calculate Cu and Cc

10

60

D

DCu

)( 1060

2

30

DD

DCc

x% of the soil has particles

smaller than Dx

Unified Soil Classification To determine W or P, calculate Cu and Cc

10

60

D

DCu

)( 1060

2

30

DD

DCc

0.0001 0.001 0.01 0.1 1 10 100

0

20

40

60

80

100

Particle size (mm)

% F

iner

x% of the soil has particles

smaller than Dx

Example

0.0001 0.001 0.01 0.1 1 10 100

0

20

40

60

80

100

Particle size (mm)

% F

iner

%fines (% finer than 75 mm) = 11% - Dual symbols required

Example

0.0001 0.001 0.01 0.1 1 10 100

0

20

40

60

80

100

Particle size (mm)

% F

iner

%fines (% finer than 75 mm) = 11% - Dual symbols required

D10 = 0.06 mm, D30 = 0.25 mm, D60 = 0.75 mm

Unified Soil Classification To determine W or P, calculate Cu and Cc

If prefix is G then suffix is W if Cu > 4 and Cc is between 1

and 3 otherwise use P

If prefix is S then suffix is W if Cu > 6 and Cc is between 1

and 3 otherwise use P

10

60

D

DCu

)( 1060

2

30

DD

DCc

Grading curves

0.0001 0.001 0.01 0.1 1 10 100

0

20

40

60

80

100

Particle size (mm)

% F

iner

W Well graded

Grading curves

0.0001 0.001 0.01 0.1 1 10 100

0

20

40

60

80

100

Particle size (mm)

% F

iner

W Well graded

U Uniform

Grading curves

0.0001 0.001 0.01 0.1 1 10 100

0

20

40

60

80

100

Particle size (mm)

% F

iner

W Well graded

U Uniform

P Poorly graded

Grading curves

0.0001 0.001 0.01 0.1 1 10 100

0

20

40

60

80

100

Particle size (mm)

% F

iner

W Well graded

U Uniform

P Poorly graded

C Well graded with some clay

Grading curves

0.0001 0.001 0.01 0.1 1 10 100

0

20

40

60

80

100

Particle size (mm)

% F

iner

W Well graded

U Uniform

P Poorly graded

C Well graded with some clay

F Well graded with an excess of fines

Plasticity Chart

(Holtz and Kovacs, 1981)

LL

PI

H L

The A-line generally

separates the more

claylike materials

from silty materials,

and the organics

from the inorganics.

The U-line indicates

the upper bound for

general soils.

Note: If the measured

limits of soils are on

the left of U-line,

they should be

rechecked.

Procedures for Classification

Coarse-grained

material

Grain size

distribution

Fine-grained

material

LL, PI

(Santamarina et al., 2001)

Highly

Example

Passing No.200 sieve 30 %

Passing No.4 sieve 70 %

LL= 33

PI= 12

53

Example

Passing No.200 sieve 30 %

Passing No.4 sieve 70 %

LL= 33

PI= 12

PI= 0.73(LL-20), A-line

PI=0.73(33-20)=9.49

SC

(15% gravel)

Clayey sand with gravel

(Santamarina et al., 2001)

Passing No.200 sieve 30 %

Passing No.4 sieve 70 %

LL= 33

PI= 12

Highly

54

Organic Soils Highly organic soils- Peat (Group symbol PT)

A sample composed primarily of vegetable tissue in various

stages of decomposition and has a fibrous to amorphous

texture, a dark-brown to black color, and an organic odor

should be designated as a highly organic soil and shall be

classified as peat, PT.

Organic clay or silt( group symbol OL or OH):

The soils liquid limit (LL) after oven drying is less than 75 % of its liquid limit before oven drying. If the above statement is true, then the first symbol is O.

The second symbol is obtained by locating the values of PI and

LL (not oven dried) in the plasticity chart.