3_engineering geology and soil mechanics_chapter 4_soil classification

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SOIL MECHANICS AND GEOLOGY Dr. Paul Ho

Sept/2009

CHAPTER 3: GRAIN SIZES AND GRAIN SIZE DISTRIBUTION Page 1 of 20

3 PARTICLE SIZE DISTRIUTION, SOIL CONSISTENCY,

SOIL CLASSIFICATION AND DESCRIPTION

3.1 Introduction

The classification of soil is a good guide to a soils functional character as amaterial for engineering use.

Different soil types have different shear strength and settlement

characteristics, meaning they differ in their response to loads induced on

them by structures.

Different soil types have different seepage characteristics, meaning they

differ in the ease with which water or other liquids flow through them.

3.2 Basic Soil Groups

There are three main groups of soil:

Coarsegrained soils examples: cobbles, gravels, sands

Individual grains can be seen with unaided eyes.

There is no cohesion between grains (i.e., cohesionless).

Particles flow freely when dry.

Water can flow through them freely.

Retain little or no water when drained.

Finegrained soils examples: silts, clays

Individual grains cannot be seen with unaided eyes.

There is some form of cohesion between grains (i.e., cohesive).

Form slumps when dry.

Water does not drain out - retain water.

Organic - example: peat soils or muskeg (fibrous):

Decayed plant remains mixed with silt and clay.

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ENGINEERING GEOLOGY AND SOIL MECHANICS

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3.3 Typical features of Engineering Soils:

Major classes and features of engineering soil are summarised in Table 3.1.

Table 3.1 Major Classes and Feature of Engineering Soils

Coaresed Grain Fine Grained Organic

Soil types Cobble, Gravel, Sand Silt, Clay Peat

Particle shape Round to angular Flaky Fibrous

Particle or grain size Coarse Fine ---

Porosity or void ratio Low High High

Permeability High Low to Variable

impermeable

Inter-particle cohesion None to very low High Low

Inter-particle friction High Low None tolow

Plasticity Very low Low to high Low to

moderate

Compressibility Very low Moderate to Usually

very high very high

Rate of compression Immediate Moderate to slow Moderate

to rapid

Effect of particle size Important Relatively -

distribution on Important (silt)Engineering Relatively

Behaviour unimportant (clay)

Effect of water on Relatively Important -

engineering unimportant

behaviour exception:

(very fine sand)

3.4 Particle Sizes

The range of particle sizes encountered in soils is very wide, from boulder size

larger than 200 mm down to the colloidal size of some clays of less than 0.001

mm. Although natural soils are mixtures of various sized particles, it is

common to find a predominant grading with a relatively narrow band of sizes.

Table 3.2 shows the British Standard (BS)of particle size limits for use in soil

engineering.

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NGINEERING GEOLOGY AND SOIL MECHANICS

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Table 3.2 British Standard of Particle Size Limits of Engineering Soils

Type Range of particle size, mm

Boulder > 200

Cobble 200 - 60

GravelCoarse gravel 60 - 20

Medium gravel 20 - 6

Fine gravel 6 - 2

Sand

Coarse sand 2 - 0.6

Medium sand 0.6 - 0.2

Fine sand 0.2 - 0.06

Silt

Coarse silt 0.06 - 0.02Medium silt 0.02 - 0.006

Fine silt 0.006 - 0.002

Clay Less than 0.002

3.5 Particle Size Distribution

Both the size of particles and the distribution of particles sizes are

important. Sievingtests (for coarse grain soils) and hydrometertests (for fine grained

soils) are used to define the distribution of grain sizes.

Classification of soils according to particle sizes varies slightly between

different classification system. In Hong Kong a system based on the British

Soil Classification System (BSCS)is commonly used.

In discribing the size of a soil particle, either a dimension or name as shown

in Table 3.2 is used.

The particle size refers to an equivalent particle diameter as found from

sieve analysis.

The British Standard Sieve Sizes as shown in Table 3.3 are commonly used

in Hong Kong.

The range of particle sizes varies from 200 mm > D (grain size diameter)>

0.002 mm, hence the particle size distribution is examined on a logaritmic

scale as shown in Figure 3.1

Coarse

Fine

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hydrometer

sieve test

smallest sieve size

at 0.063mm

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Table .3.3 British Standard Test Sieve Sizes

75 mm, 63 mm, 50 mm, 37.5 mm, 28 mm, 20 mm, 14 mm, 10 mm, 6.3 mm, 5

mm, 3.36 mm, 2 mm, 1.18 mm, 600 m, 425 m, 300 m, 212 m, 150 m, 63

m

(1 m = 0.001 mm)

Figure 3.1 Particle Size Distribution Chart (BS range of particle sizes)

Determination of Particle-size Distribution

Basically, in terms of grain size, soil is described as either coarse-grainedor

fine-grained.

Coarse-grained soil: one in which more than 35% of the grains, by weight, are

greater than 0.06 mm in diameter (BSCS).

Fine-grained soil: one in which more than 65% of the grains, by weight, are

smaller than 0.06 mm in diameter (BSCS).

In British Standard, the size 0.06 mm is the dividing line between silt and sand

(see Table 3.2) and represents the smallest particle that can be distinguished as

a discrete grain by the naked eye.

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65%

35%


at discrete sieve sizes

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Sieving and Sedimentation

Two methods are used to determine the particle-size distribution of soils. One

is for coarse-grained material which uses sieves. The other is for fine-grained

material which uses the technique of sedimentation; one example is the

hydrometermethod.

Most natural soil is a mixture of coarse-grained material (sand and gravel) and

fine-grained material (silt and clay). Separation of the coarse and fine

materials is necessary for proper testing. This is done by an initial wet sieving

in which the soil is completely washed through a 63m (.063 mm) sieve by a

stream of water. The soil retained on the sieve is greater than 0.063 mm in

grain size. The particle-size distribution of the retained fraction can then be

done using other larger size sieves. Alternatively, the sieving of the coarse

fraction can also be done on the dried sample. This is known as dry sieving.

Sieving

Sieve Analysis is used to determine the distribution of the larger grain sizes.

The soil is passed through a series of sieves with the mesh size reducing

progressively (Figure 3.2), and the proportions by weight of the soil retained on

each sieve are measured. There are a range of sieve sizes that can be used, and

the finest is usually a 63 m sieve. Sieving can be performed either wet or dry.

Because of the tendency for fine particles to clump together, wet sieving isoften required with fine-grained soils.

Figure 3.2Sieves and Shaker

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;


currently used in most of

laboratories.

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Sedimentation (Hydrometer)

To determine the grain size distribution of material passing the 63 m sieve the

Hydrometer method is commonly used (Figure 3.3). The soil is mixed with

water and a dispersing agent, stirred vigorously, and allowed to settle to the

bottom of a measuring cylinder. As the soil particles settle out of suspensionthe specific gravity of the mixture reduces. An hydrometer is used to record the

variation of specific gravity with time. By making use of Stokes Law, which

relates the velocity of a free falling sphere to its diameter, the test data is

reduced to provide particle diameters and the % by weight of the sample finer

than a particular particle size.

Figure 3.3 Hydrometer

Particle Size Distribution Curve

Most soils are composed of particles of various sizes. Some soils have a more

homogeneous (same) combination of particle sizes while other soils have a

mixture of grain sizes. The sieving analysis (or together with hydrometer) of

soil particle sizes is usually recorded on a Particle Size Distribution (PSD)

Chart and the curve so ploted is referred as the Particle Size Distribution

(PSD)Curve (or Grading Curve) as shown in Figure. 3.4.

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Particle Size Distribution Chart

The PSD Chart is a semi-logarithmic chart.

The horizontal scale is a logarithmic scale (log10) of the particle size diameter

over a range of 0.0001 mm to > 100 mm.

The vertical scale is the percentage by weight of the soil grains that are finer

than a given size. For example, point A in Figure. 3.4 represents 60% by

weight of that soil is finer than 2.0 mm. The percentage is always designed as

percent passing or finer (a certain sieve size) or as a summation percentage.

Figure 3.4 Typical Particle Size Distribution (PSD) curves

Some typical grading (PSD) curves are shown on the figure. The following

descriptions are applied to these curves

W Well graded material

U Uniform material

P Poorly graded material

C Well graded with some clay

F Well graded with an excess of fines

Another quantity analysis of grading curves may be carried out using certain

geometric values known as grading characteristics. For example, in Fig. 3.5,

D10= diameter of grain (mm) for which 10 % is finer -- effective size

D30= diameter of grain (mm) for which 30 % is finer

D60= diameter of grain (mm) for which 60 % is finer

0.0001 0.001 0.01 0.1 1 10 100

0

20

40

60

80

100

Particle size (mm)

%F

iner

A

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2mm


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Coefficient of uniformity, Cu = D60/D10 - measures spread of particle size

distribution

Coefficient of curvature, Cc= (D30)2/(D60x D10) - measures slope of the grading

curve

GRADING of coarse-grained soils (gravel and sand):

Well-graded soil (gravel or sand):

Cu > 4 and 1 < Cc< 3 (well-graded gravel)

Cu > 6 and 1 < Cc< 3 (well-graded sand)

Poorly-graded soil: (gravel or sand)

If Cu is small, soil is poorly-graded (uniform)If Cc

> 3 or 1< , soil is poorly graded (gapped graded)

Figure 3.5 Grading Characteristics

Curves can also be used to obtained percentages of gravel, sand and fines (silt

and clay).

For example, for the curve in Figure 3.5:

gravel = (100-46) = 54%

sand = (46-18) = 28%Fine (silt + clay) = (18-0) = 18%

0

20

40

60

80

100

0.001 0.01 0.1 1 10 100

Grain size (mm)

D30

sievehydrometer

D10 = 0.013 mm

D30 = 0.47 mm

D60 = 7.4 mm

sands gravelsfines

%P

as

sing

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10

30


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CHAPTER 4 Further Worked Examples

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Further Worked ExamplesCHAPTER 4

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3.6 Consistency of Fine-grained Soils

Atterberg Limits

If we take a very soft (high moisture content) clay specimen and allow it to drywe would obtain a relation similar to that shown in Figure 3.6.

As the soil dries its strength and stiffness will increase. Three limits are

indicated, the definitions of which are given below. The liquid and plastic

limits appear to be fairly arbitrary, but recent research has suggested they are

related to the strength of the soil.

Figure 3.6. Volume - Moisture Content relationship for fine-grained soils

(SL) The Shrinkage Limit - This is the moisture content the soil would have

had if it were fully saturated at the point at which no further shrinkage occurs

on drying.

(PL) The Plastic Limit - This is the minimum water content at which the soilwill deform plastically (i.e., the soil can be molded)

(LL) The Liquid Limit - This is the minimum water content at which the soil

will flow under a small disturbing force

(PI or Ip) The Plasticity Index. This is derived simply from the LL and PL

IP = LL - PL (3)

It measures the range of water within which the soil is plastic.

PI

Decreasing Strength

Semi-solid/

Semi- lastic

Solid Plastic Liquid

LLMoisture Content(%)SL PL

Volum

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Volume


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The shrinkage limit (SL) is the water content where further loss of moisture will not result in any more volume

reduction.[2]The test to determine the shrinkage limit isASTM InternationalD4943. The shrinkage limit is

much less commonly used than the liquid and plastic limits.


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(LI) The Liquidity Index - This is defined as

PI

PLw

PLLL

PLwLI

=

= (4)

where w= the natural moisture content

It tells which state (semi-solid, plastic, or lquid) the soil is at its natural

condition.

The Atterberg Limits and relationships derived from them are simple measures

of the water absorbing ability of soils containing clay minerals. For example, if

a clay has a very high LI and LL it is capable of absorbing large amounts of

water, and for instance would be unsuitable for the base of a pavement. The LL

and PL are also related to the soil strength.

Remember that only the fraction finer than 425 m is tested in the Atterberg

Limits tests Liquid Limit and Plastic Limit). If this fraction is only small (that is,

the soil contains significant amounts of sand or gravel) it might be expected

that the soil would have better properties. While this is true to some extent and

it is important to realise that the soil behaviour is controlled by the finest 10 -

25 % of the particles.

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OR 0.425mm


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Liquid Limit Tests

These tests are only used for the fine-grained fraction (silt and clay) of a soil.

Determination of Liquid Limit (Cone Penetrometer Method)

Figure 3.7 Conepenetrometer

Figure 3.8 Typical results of Cone Penetration Test

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Determination of Liquid Limit (Casagrande Method)

British Soil Classification System (BSCS)

The standard system used worldwide for most major construction projects is known as

the Unified Soil Classification System (USCS). This is based on an original systemdevised by Cassagrande. Soils are identified by symbols determined from sieve

analysis and Atterberg Limit tests.

Coarse Grained Materials

Figure 3.9 Casagrande Method

Figure 3.10 Typical results of Casagrande Mehtod

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3.7 British Soil Classification System

The standard system discussed here is the British Soil Classification System (BSCS)

which is used in Hong kong. Soils are identified by symbols (Table 3.4) determined

from sieve analysis and Atterberg Limit tests.

Figure Table 3.4 Symbols used for BSCS

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Coarse-grained Soils

If more than 65% of the material is coarser than the 60 m, the soil is classified as

coarse. The following steps are then followed to determine the appropriate symbols

(Primaryprefix and Secondarysuffix).

Steps:

1.Determine the prefix

If more than half of the coarse fraction is sand then use prefix S

If more than half of the coarse fraction is gravel then use prefix G

2.Determine the suffix

This depends on the uniformity coefficient Cu and the coefficient of curvature Cc

obtained from the grading curve, and also on the percentage of fines, and the type of

fines.

First determine the percentage of fines, that is the % of material smaller than the 60

m.

Then if % fines is

< 5% use W or P (Pu or Pg) as suffix (u = uniform, g = gap graded)

between 5% and 15% add M or C as suffix in addition to W or P(Pu or Pg)

between 15% and 35% use M or C together with degree of plasticity (L, I, H, V, E)

as suffix and no W or P(Pu or Pg) is required

If W or P are required for the suffix then Cuand Ccmust be evaluated

C D

Du =

60

10

C D

D Dc =

30

2

60 10( )

If prefix is Gthen suffix is Wif Cu> 4 and Ccis between 1 and 3, otherwise use (Pu

or Pg)

If prefix is S then suffix is Wif Cu> 6 and Ccis between 1 and 3, otherwise use (Pu

or Pg)

If Mor Care required they have to be determined from the procedure used for fine-

grained materials discussed below. Note that Mstands for Silt and Cfor Clay. This isdetermined from whether the soil lies above or below the A-line in the plasticity chart

shown in Figure 3.11.

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a.

b.

If Cc > 3 or 1< , soil is poorly graded (gapped graded)


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Fine-grained Soils

These are classified solely according to the results from the Atterberg Limit tests.

Values of the Plasticity Index and Liquid Limit are used to determine a point on the

plasticity chart shown in Figure 3.12. The classification symbol is determined from theregion of the chart in which the point lies.

Examples CH High plasticity clay

CL Low plasticity clay

MH High plasticity silt

ML Low plasticity silt

Figure 3.11 Plasticity chart for laboratory classification of fine grained soils

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Fine-grained Soils

If more than 35% of the material is finer than the 60 m, the soil is classified as fine-

grained soil. The following steps are then followed to determine the appropriate

symbols.

Steps:

1. Determine the prefix

If the point (PI, LL) is plotted aboce the A-line, the soil is Clay and use symbol C

If the point (PI, LL) is plotted below the A-line , the soil is Silt and use symbol M

2. Determine the suffix

This depends on the amount of fine materials and the types of coase materials presentin the soil:

Then If % fines is:

from 65% - 100% use degree of plasticity (L, I, H, V, E) as suffix (no need to

worry about the coarse materials)

from 35% - 65%, in addition to plasticity (L, I, H, V, E), add G to the suffix if the

coarse material is Gravel or S if the coarse material is Sand

The complete procedure for BSCS is summarised in Table 3.5

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above


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Table 3.5 British Soil Classfication System (BSCS)

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The final stage of the classification is to give a description of the soil to go with the

symbol class. For a coarse grained soil this should include:

the percentages of sand and gravel

maximum particle size

angularity surface condition

hardness of the coarse grains

local or geological name

any other relevant information

If the soil is undisturbed mention is also required of

stratification

degree of compactness

cementation

moisture conditions

drainage characteristics

All information required can be found in the list of reference (GEO Guide 3: Guide to

Soil and Rock Description).

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Example - Classification using USCS

Classification tests have been performed on a soil sample and the following

grading curve and Atterberg limits obtained. Determine the BSCS classification.

Given Atterberg limits: Liquid limit LL = 32, Plastic Limit, PL =26

Step 1: Determine the % fines from the grading curve

%fines (% finer than 60 m) = 10% ( 4 and Cc is between 1 and 3)

sand/gravel

silt/sand

well-graded silty SAND

>50%


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3.1


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Refer to Chapter 1 page 15


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Figure 3.11.

Table 3.5.

(b)

(b)


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(a)

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(c)

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(f)

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(b)

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(Marine Mud).

Stiff, moist, ,

dark brownish grey,slightly sandy SILT/CLAY.

(Marine Sand).

Loose, moist,

light brown,

slightly gravelly fine to coarse SAND.

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Loose, moist,

light brown,

slightly silty/clayey, gravelly SAND

with interbedded soft, moist,greyish brown,

sli htl sand SILT/CLAY Alluvium .

Layer 1. Dense, dry,

yellowish brown (large cobbles and boulders are light grey),

bouldery COBBLES

with much finer material (slightly gravelly, sandy

silt/clay). (Colluvium).

Layer 2. Very stiff, dry,

yellowish brown,

sli htl ravell , sand SILT/CLAY Colluvium .

Layer 3. Very stiff, moist,

dark brown (boulders are light grey),

slightly sandy gravelly SILT/CLAY (Colluvium)

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Layer 1.

Soft, dry,

light yellowish brown,

sandy SILT/CLAY (Fill)

Layer 2.

Soft, moist,

brownish red,

slightly sandy SILT/CLAY (Fill)

Layer 3.

Soft to firm, wet,

dark greyish brown,

slightly gravelly sandy SILT/CLAY (Fill).

Layer 4.

Firm, wet,

brown,

slightly sandy SILT/ CLAY (Fill)

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Chapter 4 Soil Classification and Description Engineering Geology & Soil Mechanics

Soil Mechanics_Chapter 4_Class Practice_2014

Chapter 4Particle Size Distribution, Soil Consistency, Soil Classification and Description

Class Practice

Q.1 State the differences between coarse-grained and fine-grained soils in terms of the

following soil properties:

(i) particle shape

(ii) porosity or void ratio

(iii) permeability

(iv) inter-particle cohesion

(v) inter-particle friction

(vi) plasticity

(vii) compressibility

(viii) rate of compression

Q.2 A soil sample was carried out a sieve analysis in laboratory. The mass of soil

retained on each sieve was measured and shown below.

Sieve size

(mm)

Mass of soil retained on sieve

(g)

63 0

37.5 52

20 45

10 386.3 32

5 27

2 25

1.18 23

0.6 19

0.425 17

0.3 16

0.212 140.15 13

0.063 10

Pan 5

(i) Plot the particle size distribution curve using the provided PSD chart and

determine the followings for the soil

(ii) Find D10, D30and D60

(iii)Calculate coefficient of uniformity, Cu and coefficient of curvature, Cc

(iv)Classification for the soil


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Q.3 The following results were obtained from a liquid limit test using a cone

penetrometer.

Cone penetration (mm) 7.5 13.5 15 26 32.5

Water content (%) 7 12.5 14 24 30

(i) Find the liquid limit of the soil sample.

(ii) Calculate the plasticity index and liquidity index if the plastic limit was 15%

and the natural moisture content 20%

(iii) What is the classification of this soil

Q.4 A cone penetrometer test was carried out on a sample of clay with the following

results:

Cone Penetration

(mm)

16.1 17.6 19.3 21.3 22.6

Moisture Content

(%)

50.0 52.1 54.1 57 58.2

The results from the plastic limit test were:Test

No.

Mass of

container (g)

Mass of wet soil +

container (g)

Mass of dry soil +

container (g)

1 8.1 20.7 18.7

2 8.4 19.6 17.8

(i) Plot the cone penetration against moisture content and determine the

liquid limit.

(ii)

Determine the plastic limit and the plasticity index of the soil.


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Q.5 The results of particle size distribution analysis on a soil are summarized below:

Sieve size (mm) Mass retained (g)

10 0

5 4.9

2.0 24.7

1.18 22.8

0.6 21.5

0.3 12.2

0.15 9.9

0.063 7.1

pan 6.5

(i) Tabulate the particle size distribution results and plot the particle size

distribution curve for the soil.

(ii) Determine the D10, D30, D60, Cuand Cc.

(iii) If LL and PL of the fines portion (


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Q.6 Classify soils

You have carried out a sieving test on the soil sample. The particle size distribution

curve of the soil sample (marked 'A') is shown below. The fine portion (particle size

3_engineering geology and soil mechanics_chapter 4_soil classification

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