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Soil Classification

ADNAN REHMAT

Sieve Analysis, Liquid Limit and Plastic Limit

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If I give you a bag of 1-Kg soil taken from an under construction site and ask you the

following questions. • What is the most basic classification of soil?

• What are the methods of soil gradation or grain size distribution?

• How do you define the soil types? Clay, Silt, Sand, Gravel or cobble and boulder

• Calculate D10, D30 and D60 of this soil using the sieve analysis?

• Calculate both the Cu and CC of this soil?

• Is this soil poorly, gap or well graded, Liquid limit and Plastic limit? How do you define theses terms?

SOIL CONSISTENCE Soil consistence is a physical property to describe the resistance of a soil to

mechanical stresses or manipulations at various moisture contentsOR

Soil consistence provides a means of describing the degree and kind of cohesion and adhesion between the soil particles as related to the resistance of the soil to deform or rupture.

Since the consistency varies with moisture content and clay minerals, the consistence can be described as dry consistency, moist consistency, and wet consistency.

Consistency that is evaluated includes rupture resistance and stickiness.

The rupture resistance is a field measure of the ability of the soil to withstand an applied stress or pressure as applied using the thumb and forefinger.

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SOIL STRENGTH

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Sieve Sizes in Descending Order

• 75 mm (3 in.) • 63 mm (2-1/2 in.) • 50 mm (2 in.) • 37.5 mm (1-1/2 in.) • 25.0 mm (1 in.) • 19.0 mm (3/4 in.) • 12.5 mm (1/2 in.) • 9.5 mm (3/8 in.)

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• 4.75 mm (No. 4) • 2.36 mm (No. 8) • 1.18 mm (No. 16) • 600 μ m (No. 30) • 300 μ m (No. 50) • 150 μ m (No. 100) • 75 μ m (No. 200)

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Purpose: ( Grain size Analysis)

• This test is performed to determine the percentage of different grain sizes contained within a soil.

• The mechanical or sieve analysis is performed to determine the distribution of the coarser, larger-sized particles, and the hydrometer method is used to determine the distribution of the finer particles.

Significance:

• The distribution of different grain sizes affects the engineering properties of soil.

• Grain size analysis provides the grain size distribution, and it is required in classifying the soil.

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Major Soil Groups

0.002 4.750.075

Grain size (mm)

BoulderClay Silt Sand Gravel Cobble

Fine grain soils

Coarse grain soils

Granular soils or Cohesionless soils

Cohesive soils

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Grain Size Distribution

To know the relative proportions of different grain sizes.

An important factor influencing the geotechnical characteristics of a coarse grain soil.

Not important in fine grain soils.

Significance of GSD:

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Grain Size Distribution

In coarse grain soils …... By sieve analysisDetermination of GSD:

In fine grain soils …... By hydrometer analysis

Sieve Analysis Hydrometer Analysis

soil/water suspension

hydrometer

stack of sieves

sieve shaker

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Sieve Analyses

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Sieve Analysis

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Sieve Designation - Large

Sieves larger than the #4 sieve are designated by the size of the openings in the sieve

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Sieve Designation - Smaller

10 openings per inch

# 10 sieve

1-inch

Smaller sieves are numbered according to the number of openings per inch

Sieve Analysis Test Procedure

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Prepare the sample Fine Aggregate

Select a representative sample of approximately 500 g from material that has been thoroughly dried at a temperature of 110 ± 5 ° C (230 ± 9 °F).

Weigh the sample and record its mass to the nearest 0.1 g.

Coarse Aggregate Dry the sample in an oven to a constant mass, and then allow cooling to room temperature.

Record the total dry mass of the sample to the nearest gram.

Minimum mass of samples required in each sieve sizes from 9.5mm to 75mm are as shown below

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Sieving procedure(1) Write down the weight of each sieve as well as the bottom pan to be used in the analysis.(2) Record the weight of the given dry soil sample.(3) Make sure that all the sieves are clean, and assemble them in the ascending order of sieve numbers (#4 sieve at top and #200 sieve at bottom). Place the pan below #200 sieve. Carefully pour the soil sample into the top sieve and place the cap over it.(4) Place the sieve stack in the mechanical shaker and shake for 10 minutes.(5) Remove the stack from the shaker and carefully weigh and record the weight of each sieve with its retained soil. In addition, remember to weigh and record the weight of the bottom pan with its retained fine soil.

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Data Analysis:(1) Obtain the mass of soil retained on each sieve by subtracting the weight of the empty sieve from the mass of the sieve + retained soil, and record this mass as the weight retained on the data sheet. The sum of these retained masses should be approximately equals the initial mass of the soil sample. A loss of more than two percent is unsatisfactory.

(2) Calculate the percent retained on each sieve by dividing the weight retained on each sieve by the original sample mass.

(3) Calculate the percent passing (or percent finer) by starting with 100 percent and subtracting the percent retained on each sieve as a cumulative procedure.

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For example: Total mass = 500 g,

Mass retained on No. 4 sieve = 9.7 g

For the No.4 sieve:

Quantity passing = Total mass - Mass retained

= 500 - 9.7 = 490.3 g

The percent retained is calculated as;

% retained = Mass retained/Total mass

= (9.7/500) X 100 = 1.9 %

From this, the % passing = 100 - 1.9 = 98.1 %

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Grain size distribution

0.0001 0.001 0.01 0.1 1 10 1000

20

40

60

80

100

Particle size (mm)

% F

iner

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Unified Soil ClassificationEach soil is given a 2 letter classification (e.g.

SW). The following procedure is used.

Coarse grained (>50% larger than 75 mm)

Prefix S if > 50% of coarse is SandPrefix G if > 50% of coarse is Gravel

Suffix depends on %fines

if %fines < 5% suffix is either W or Pif %fines > 12% suffix is either M or Cif 5% < %fines < 12% Dual symbols are used

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Unified Soil ClassificationTo determine W or P, calculate Cu and Cc

CDDu 60

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CD

D Dc 302

60 10( )

0.0001 0.001 0.01 0.1 1 10 1000

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40

60

80

100

Particle size (mm)

% F

iner

x% of the soil has particles smaller than Dx

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Grading curves

0.0001 0.001 0.01 0.1 1 10 1000

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40

60

80

100

Particle size (mm)

% F

iner

W Well graded

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Grading curves

0.0001 0.001 0.01 0.1 1 10 1000

20

40

60

80

100

Particle size (mm)

% F

iner

W Well graded

U Uniform

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Grading curves

0.0001 0.001 0.01 0.1 1 10 1000

20

40

60

80

100

Particle size (mm)

% F

iner

W Well graded

U Uniform

P Poorly graded

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Grading curves

0.0001 0.001 0.01 0.1 1 10 1000

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40

60

80

100

Particle size (mm)

% F

iner

W Well graded

U Uniform

P Poorly graded

C Well graded with some clay

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Grading curves

0.0001 0.001 0.01 0.1 1 10 1000

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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

Grain Size Distribution Curve

can find % of gravels, sands, fines

define D10, D30, D60.. as above.

0

20

40

60

80

100

0.001 0.01 0.1 1 10 100

Grain size (mm)

D30

sievehydrometer

D10 = 0.013 mmD30 = 0.47 mmD60 = 7.4 mm

sands gravelsfines

% P

assi

ng

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To determine W or P, calculate Cu and Cc

CDDu 60

10

CD

D Dc 302

60 10( )

0.0001 0.001 0.01 0.1 1 10 1000

20

40

60

80

100

Particle size (mm)

% F

iner

D90 = 3 mm

x% of the soil has particles smaller than Dx

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Well or Poorly Graded Soils

Well Graded Soils Poorly Graded Soils

Wide range of grain sizes present

Gravels: Cc = 1-3 & Cu >4Sands: Cc = 1-3 & Cu >6

Others, including two special cases:(a) Uniform soils – grains of same

size(b) Gap graded soils – no grains in a specific size range

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Atterberg Limits

Border line water contents, separating the different states of a fine grained soil

Liquidlimit

Shrinkagelimit

Plasticlimit

0 water content

liquidsemi-solid

brittle-solid

plastic

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Purpose:This lab is performed to determine the plastic and liquid limits of a fine grained soil. The Atterberg limits are based on the moisture content of the soil. The plastic limit: is the moisture content that defines where the soil changes from a semi-solid to a plastic (flexible) state. The liquid limit: is the moisture content that defines where the soil changes from a plastic to a viscous fluid state.The plasticity index of a soil is the numerical difference between its liquid and plastic limits

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Liquid Limit Definition

The water content at which a soil changes from a plastic consistency to a liquid consistency

Defined by Laboratory Test concept developed by Atterberg in 1911.

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The liquid limit (LL) is arbitrarily defined as the water content, in percent, at which a pat of soil in a standard cup and cut by a groove of standard dimensions will flow together at the base of the groove for a distance of 12 mm under the impact of 25 blows in the devise. The cup being dropped 10 mm in a standard liquid limit apparatus operated at a rate of two shocks per second.

Defined by Laboratory Test concept developed by Atterberg in 1911.

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Atterberg Limits

Liquid Limit (wL or LL):Clay flows like liquid when w > LL

Plastic Limit (wP or PL):Lowest water content where the clay is still

plasticShrinkage Limit (wS or SL):

At w<SL, no volume reduction on drying

Procedure of Liquid Limit Test

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Adjust the liquid limit device by means of the adjustment plate on the device and the gauge on the handle of the grooving tool. Adjust so the center of the wear point on bottom of cup is lifted exactly 1 cm above the base.

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LL Test Procedure Place approximately 100

grams of prepared soil in the evaporating dish and add 15 to 20 ml of water and mix water thoroughly with the soil .

Place mixture in the cup over the spot where the cup rests on the base and spread into place with as few strokes as possible to a depth of 10 mm.

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LL Test ProcedureCut groove in

soil paste with standard grooving tool

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LL Test ProcedureTurn the crank to

raise and drop the cup twice per second until the two sides of the grooved sample come in contact at the bottom of the groove for a distance of 12.5 mm.(1/2 inch)

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Procedure of Liquid Limit Test Record the number of blows. If the number of blows is between 15 and 40, take a representative sample

of the soil in the cup for moisture content. If the number of blows is outside the range of 15 to 40, transfer the soil

from the cup to the evaporating dish. Adjust the moisture content by mixing, with or without the addition of water and repeat the test until the blows fall within the range of 15 -40 blows.

Record the numerical difference between the wet and dry weight as weight of moisture.

The "weight of moisture" divided by the "dry weight of sample" and

multiplied by one hundred is the percent moisture.

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LL Test Procedure

Obtain water content for each testPlot water content versus number of blows on semi-log paper

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LL Test Results

Log N

water content, %LL= w%

Interpolate LL water content at 25 blows

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Calculation

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The moisture content and corresponding number of blows for the two liquid limit determinations is used to calculate the liquid limit (wL) at 25 blows. Adjust the moisture content of the sample by dividing by a denominator selected from the following chart:

Calculation

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Calculate the liquid limit for each test and average the results as shown in the following example.

Test 1 2

Blows 19 31

Moisture 33.79 31.50 Test 1: WL(25 blows)= = 32.6%Test 2: WL (25 blows)= = 32.4%

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LL Values < 16 % not realistic

16 Liquid Limit, %

PI, %

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LL Values > 50 - HIGH

Liquid Limit, %

PI, %

50

H

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LL Values < 50 - LOW

Liquid Limit, %

PI, %

50

L

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Plastic LimitThe minimum water content at which a soil

will just begin to crumble when it is rolled into a thread of approximately 3 mm in diameter.

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Plastic Limit w% procedureUsing paste from LL test, begin dryingMay add dry soil or spread on plate and

air-dry

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Plastic Limit w% procedureWhen point is reached where thread is

cracking and cannot be re-rolled to 3 mm diameter, collect at least 6 grams and measure water content. Defined plastic limit

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1. Calculate the water content of each of the plastic limit moisture cans after they have been in the oven for at least 16 hours.

2. Compute the average of the water contents to determine the plastic limit, PL.

Calculation of Plastic LimitRecord the difference between the wet and dry

weights as the weight of moisture. Calculate the plastic limit Wp by dividing the

"weight of moisture" by the "dry weight of sample" and multiply by 100.

Plastic Limit (Wp) = x 100

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Definition of Plasticity IndexPlasticity Index is the numerical difference

between the Liquid Limit w% and the Plastic Limit w%

w% LLPL

PI = LL - PL

Plasticity Index = Liquid Limit – Plastic Limit

plastic (remoldable)

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Low plasticity wL = < 35%Intermediate plasticity wL = 35 - 50%High plasticity wL = 50 - 70%Very high plasticity wL = 70 - 90%Extremely high plasticity wL = > 90%

Plasticity Chart

Compaction TestStandard and Modified Proctor

test

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Soil is air dried, pulverized & passed thru #4 sieve. Separated into 4 to 6 samples. Adjust the water content of each sample by adding water. Using the proctor mould (1/30th cubic foot) place & compact soil in 3

layers. Each layer should receive 25 drops of the compaction hammer. After the last layer, use a straight edge to trim the excess soil leveling to

the top of the mould. Determine the weight of the mould with the compacted moist soil. Extrude from mould and collect a sample for water content determination. Repeat for each sample over a range of moisture contents. After collecting all pertinent weights, calculate dry density and plot vs.

water content

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CBR (California Bearing Ratio) Test

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California bearing ratio test is the ratio of force per unit area required to penetrate a soil mass with standard circular piston at the rate of 1.25 mm/min, to that required for the corresponding penetration of standard material or (Crushed Stone).

CBR = * 100

Sample PreparationPrepare the remolded specimen at Proctors maximum dry density or any other density at which CBR is required.Maintain the specimen at optimum moisture content or field moisture as required.The material used should pass 20mm Sieve but should be retained on 4.75mmPrepare the specimen either by static or dynamic compaction.

Dynamic CompactionTake 4.5 to 5.5Kg of soil and mix thoroughly with the required water

(OMC)Fix the extension Collar and the base plate to the mould, Insert the spacer disc over the base, and place the filter paper over the

spacer disc.Compact the mix soil in the mould either using light compaction or

heavy compaction.Light Compaction: - Compact the soil in 3 equal layers, each layer being

given 55 blows by the 2.6Kg rammerHeavy Compaction: - Compact the soil in 5 equal layers, each layer being

given 55 blows by the 4.89Kg rammerRemove the collar and trim off the soilTurn the mould upside down and remove the base plate and the

spacer discWeigh the mould with compacted soil and determine the bulk

density and dry density

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Observation & Recording• Optimum water content (%)• Weight of mould + Compacted specimen (gm)• Weight of empty Mould (gm)• Weight of Compacted Specimen (gm)• Volume of Specimen (cm2)• Bulk density g/cc• Dry density g/cc

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Light Compaction Calculate the weight of the wet soil at the required water content to

give desired density when occupying standard specimen volume in the mould from the expressionW = Desired dry density * ( 1+w ) V

W = Weight of wet soilw = Desired Water ContentV = Volume of Specimen in the mould = 2250cm3

Take the weight W (calculated as above) of the mix soil and place it in the mould

Place a filter paper and the displacer disc on the top of soil. Keep the mould assembly in static loading frame and compact by

pressing the displacer disc till the level of disc reaches the top of the mould.

Keep the load for some time and then release the load, remove the displacer

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The test may be conducted for soaked as well as unsoaked conditions, if the sample is soaked, (for both light and heavy compactions), put a filter paper on the top of the soil and place the adjustable stem and perforated plate on the top of filter paper.

Put annular weights to produce a surcharge equal to weight of base material and pavement expected in actual construction, each 2.5kg weight is equivalent to 7cm construction.

A minimum of 2 weights should be used Immerse the mould assembly and weights in a tank of water and soak it

for 96hours and remove the mould for compaction test.Observations and recordings for light compaction

Dry density gm/cc Moulding water Content Wet weight of the compacted soil, gm Period of soaking 96hrs (4days)

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Procedure for Penetration testSet the stress and strain dial gauge to read zero,Place the mould assembly with the surcharge weight on the penetration test machine, but in no case in excess of 4kg so that full contact of the piston on the sample is established.Apply the load on the Piston so that the penetration rate is about 1.25mm/minRecord the load readings at penetration of 0.5, 1.0, 1.5, 2.0, 2.5, 3.0, 4.0, 5.0, 7.5, 10, 12.5mm

Note the maximum load and corresponding penetration if it occurs for a penetration less than 12.5mm Detach the mould from the loading equipment.Take about 20 to 50 gm. of soil from the top 3cm layer and determine the moisture content.

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CBR

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Interpretation and RecordingIf the initial portion of the curve (load vs Penetration) is concave upwards, apply correction by drawing a tangent to the curve at the point of greatest slope and shift the origin.Find and record the correct load reading corresponding to each penetration.

CBR = * 100

PT = Corrected test load corresponding to the chosen penetration from the load penetration curve

PS = Standard load for the same penetration

Interpretation and RecordingPenetration of crushed stone and the corresponding load that causes the penetration

The CBR values are usually calculated for penetration of 2.5mm and 5mm.Generally the CBR value of 2.5mm will be grater than that at 5mm and in such a case the former (2.5mm CBR value) should be taken as a design CBR.If CBR for 5mm exceeds that for 2.5mm, the test shall be repeated.If Identical results follow, the CBR corresponding to 5mm penetration should be taken for design.

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