ContentsRef: - Supplementary Notes for TA C222

Soil Classification Grain Size Distribution Liquid Limit and Plastic Limit

Field Identification of Soils

Rock Cycle – Soil Formation

Soils The final products due to weathering are soils

IntroductionObject:To keep various types of soils into groups according to their propertiesSoil 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

Types of Soil Classification

1. Partcle 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 10% Gravel + 52% Sand + 38% Silt & Clay

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

Unified Soil Classification System (USCS)

BouldersSilt Clay

2.0 mm 0.425 mm

CobblesGravel Sand

Coarse Fine Coarse FineMedium

300 mm 80 mm

20 mm

4.75 mm 0.075 mm

0.002 mm

Coarse Grained SoilsFine

Grained Soils

Unified Soil Classification System

Coarse-grained soils:

Gravel Sand

Fine-grained soils:

Silt Clay

0.075 mm

•Grain size distribution

•Cu

•Cc

•PL, LL

•Plasticity chart

50 %

4.75 mm

Sieve analysis Atterberg limits

Unified Soil Classification System

Particle size

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

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 – 4.75 mm

0.075 – 0.425 mm

USCS

Particle size

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

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

0.425 – 2.0 mm

0.002 – 0.075 mm

USCS

Particle size

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

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

4.75 – 80 mm

< 0.002 mm

USCS

Grain Size Distribution

Coarse-grained soils:

Gravel Sand

Fine-grained soils:

Silt Clay

0.075 mm (USCS)

•Experiment

Sieve analysis Hydrometer analysis

Classification of Coarse Grained Soils

Procedure for grain size determination Sieving - used for particles > 75 m

Hydrometer test - used for smaller particles Analysis based on Stoke’s Law, velocity proportional to diameter

Schematic diagram of hydrometer test

At the beginning Towards the end of test

0.0001 0.001 0.01 0.1 1 10 1000

20

40

60

80

100

Particle size (mm)

% F

iner

CD

Du 60

10

CD

D Dc 302

60 10( )

x% of the soil has particles smaller than Dx

D60

Grain Size Distribution Curves

where CU is Coefficient of Uniformity and Cc is Coefficient of Curvature

Grain Size Distribution Curves

0.0001 0.001 0.01 0.1 1 10 1000

20

40

60

80

100

Particle size (mm)

% F

iner

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

A

C

B

D

E

To determine Well Graded (W) or Poorly Graded (P), calculate Cu and Cc.

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

CD

Du 60

10

CD

D Dc 302

60 10( )

Unified Soil Classification System (USCS)

where CU is Coefficient of Uniformity and Cc is Coefficient of Curvature

Describe the following Soil

2)9)(02.0(

)6.0(

)D)(D(

)D(C

curvatureoftCoefficien

45002.0

9

D

DC

uniformityoftCoefficien

2

6010

230

c

10

60u

mm9D

mm6.0D

)sizeeffective(mm02.0D

60

30

10

)(

631

)(

431

sandsfor

CandC

gravelsfor

CandC

soilgradedWell

uc

uc

Well Graded Soil

Criteria

USCS

QuestionWhat is the Cu for a soil with only one grain size?

D

Fin

er

1D

DC

uniformityoftCoefficien

10

60u

Grain size distribution

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

Question

-- Atterberg Limits (or Consistency Limits)

Shrinkage Limit, SL

Liquid Limit, LL

Liquid State

Plastic Limit, PL

Plastic State

Semisolid State

Solid StateDry Soil

Soil-water mixture

Incr

easi

ng w

ater

con

tent

Classification of Fine Grained SoilsUSCS

Volume Change with water content

Atterberg Limits (or Consistency Limits) - Cont.

Liquid and plastic limits

Question The figure shows how volume and water content are related in fine soils. Identify the consistency of zones A, B and C?

A. Semi-solid B. Plastic C. Liquid

Where

ws: shrinkage limit

wP: Plastic Limit

wL: Liquid Limit

Plastic Limit, Liquid Limit

Liquid and plastic limits

The lower and upper limits of the PLASTIC range are used to

classify the fine soils.

Plasticity Index = Liquid Limit – Plastic Limit

The difference between the liquid limit (wL) and plastic limit (wP) is called as

PLASTICITY INDEX (P.I.)

Plasticity Index

Liquid Limit - LL (or wL)

•Cone Penetrometer Method

This method is developed by the Transport and Road Research Laboratory, UK.

•Casagrande Method

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

LL - Casagrande Method

N=25 blows

Closing distance = 10 mm

•Device

The water content, in percentage, required to close a distance of 10 mm along the bottom of the groove after 25 blows is defined as the liquid limit

LL - Casagrande Method (Cont.)

12

21

/log,

NN

wwIindexFlow F

N

w

LL - Cone Penetrometer Method31o Cone of Stainless steel

Total sliding weight of 148 g

Cylindrical mould of 5 cm diameter and 5 cm height.

Water content w%Pen

etra

tion

of c

one

(mm

)

25 mm

LL

)15)(25(01.0 xxL wxww

Where x is the depth of penetration of cone in mmwx is water content corresponding to penetration x

For x = 20 to 30

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.

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 < 50)

Gradation symbols: W: Well-graded P: Poorly-graded

Ex: SW: Well-graded Sand

SC: Clayey Sand

SM: Silty Sand

)(

631

)(

431

sandsfor

CandC

gravelsfor

CandC

soilgradedWell

uc

uc

SYMBOLS Used for USCS Soil Classification

Above A-line use suffix C – ClayBelow A-line use suffix M – Silt

LL < 50% use Prefix LLL > 50% use Prefix H

Fine grained soils Silt (M) & Clay (C)To determine M or C use plasticity chart

0 10 20 30 40 50 60 70 80 90 10 0Liquid li m it

0

10

20

30

40

50

60

Plas

tici

tyin

dex

O

or

H

H

HO

or

C

CC

MM

M

LL

L

L

L

"" l n

Ai e

o paring soils at equal liq uid li mit

Toug h ness and dry stre ngth in crease

with increasin g plasticity in dex

Plasticity Chart

Classification of Fine Grained Soils

Use “O” below A – Line, if

Soil is Organic

A – Line

PI = 0.73(wL – 20)

USCS

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.

Classification of Fine Grained Soils - Cont.

Borderline Cases (Dual Symbols)

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.

USCS

Coarse-grained material

Grain size distribution

Fine-grained material

LL, PI

Highly

Unified Soil Classification System (USCS)

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

Passing No.200 (0.075 mm) sieve = 30 %

Passing No.4 (4.75 mm) sieve = 70 %

LL= 33%

PI= 12%

Highly

ExampleUSCS

Textural Classification1. IS 1498 – 1970 System

2. U S Bureau of Soils and Chemistry System

Grain size distribution of soil is required

Percentage of soil in each size group is determined

Ex:- Soil 10% Gravel + 52% Sand + 38% Silt & Clay

IS 1498 – 1970 System

U S Bureau of Soils and Chemistry System

First find the percentages of SAND, SILT and Clay. Based on the RELATIVE percentages of sand, silt and clay a

POINT is located on the TRIANGULAR CHART which indicates SOIL TYPE.

10 09080706050403020100

10 0

90

80

70

60

50

40

30

20

10

010 0

90

80

70

60

50

40

30

20

10

0

Silt Sizes ( %)

San d

Siz e

s(%

) Cla y

S izes( %

)

SandSilty San d Sandy Sil t

Clay -S an d Clay -S ilt

Sandy Clay Silty Clay

Clay

Public Roads Administration Classification System (AASHTO 1978 System)

Based on both particle size composition and plasticity characteristics

GROUP INDEX of the soil is determined in order to rate the soil as a SUBGRADE MATERIAL for PAVEMENT construction

HIGHER the value of GROUP INDEX, POORER is the QUALITY of the SOIL.

Field Identification of SoilsDistinguish Gravel from Sand

Sand from Silt

Silt from Clay

Grain Size

Dispersion Test

Dispersion Test Shaking (Dilatancy) Test Strength Test 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