07 - nptel.ac.innptel.ac.in/courses/105101001/downloads/L7.pdf · range over which the soil remains...

Prof. B V S Viswanadham, Department of Civil Engineering, IIT Bombay

07


Soil Classification


Engineering use of Atterberg Limits-Often used directly in specifications for controlling soil for use in fill.

-Used for predicting Activity of clay, Frost susceptibility.

-The PI, indicating the magnitude of water content range over which the soil remains plastic, and the liquidity index, indicating the nearness of a natural soil to the LL, and are particularly useful characteristics of soil.

-Used for classifying fine-grained soil.


Engineering use of Atterberg Limits-Greater the Liquid Limit, greater the compressibility of a soil.

-Liquidity and Consistency Indices are good indicators of the consistency of the soil.

Limitation-The Atterberg limits give no indication of particlefabric or residual bonds between particles which mayhave been developed in the natural soil but aredestroyed in preparing the specimen for thedetermination of limits.


Activity

The plasticity of a given clay is depend on:

(i) The nature of clay mineral

(ii) The amount of clay mineral present

Based on laboratory tests for several soils,Skempton (1953) made the observation that, for agiven soil, the plasticity index is directlyproportional to the percent clay-size fraction (i.e.percent by weight finer than 0.002 mm in size).


Activity

CPIAc =

Where PI = Plasticity Index

C = Percent Clay – Size Fraction, by Weight

Activity Ac = f( type of clay mineral present in it)

Activity is used as an index property to determine the swelling potential of expansive clays.


Activity Values of Minerals

Mineral Activity ValueNa - Montmorillonite 4 - 7

Ca - Montmorillonite 1.5Illite 0.5 – 1.3

Kaolinite 0.3 – 0.5

Halloysite (hydrated) 0.1

Quartz 0

After Skempton (1953)


ActivityClay minerals with kaolinite, a stable clay mineral, will have low activity, whereas those soils with montmorillonite, known to be a type subject to large volume changes depending on available water, will have a high activity value.

Soil classification based on activity value:Activity Classification< 0.75 Inactive Clays0.75 – 1.25 Normal Clays> 1. 25 Active Clays


Classification of the soil according to activity


After Seed et al. 1964

Plastic property of several artificial blends

(C’ = 9)


Soil Classification SystemsClassification systems generally group together broad categories of the soils that have similar features or properties, which are considered to be of importance.

As a result, a classification system is not necessarily an identification system in which all pertinent engineering properties of a material are determined.

Because of this, soil classification system should not be used as sole basis for design or construction planning.


Requirements for a satisfactory engineering classification systems include:

(i) Limited number of groupings – so that the system is easy to remember and use.

⇒ Groupings should be on the basis of only of a few similar properties and generally similar behavioural characteristics.

(ii) Properties and behavioural characteristics should have meaning for the engineering use and construction profession (i.e. relate to soils handling characteristics, shear strength, volume change characteristics and permeability).


Requirements for a satisfactory engineering classification systems include:(iii) Descriptions used for each grouping should be in

terms that are easily understood and are in common use for indicating the soil type and its properties.

(iv) Classification into any grouping should be possible on the basis of visual identification (limited to GSD and Atterberg Limits) without special tests or equipment.


Soil Classification SystemsFundamental idea:Collect soil samples from the field

Perform easy and inexpensive tests on the soil samples (typically GSD tests and Atterberg limit tests)

Based on results of these tests, classify the soil (s) in question.

Based on the classifications of the soil (s), whether or not might be appropriate for the intended usage.

If Yes, perform more extensive lab tests on the soil (s) as needed.


Soil Classification SystemsHistorically, the most widely used method of classifying

soils has been through:

(i) Visual identification

(ii) Size of soil grains

(iii) Plasticity of the soil

being used as the basis for indicating the soil type.

For example, how to distinguish between Silt and Clay?


Distinction between Silt and Clay

Reaction to shaking

Resistance to breaking

f(Plasticity, Colloidal Fraction content of the soil


Classification of soil on the basis of grain size

Clay Silt Sand Gravel Cobble

F CM F C

80204.7520.075 0.4250.002

According to BIS: 1498 (particle sizes in mm)

CobbleClay Silt Sand Gravel

F CM F C

76194.7620.074 0.4200.002

According to Unified Soil-Classification System


Classification of soil on the basis of grain sizeAccording to American Society for Testing and Materials

Clay Silt Sand Gravel Cobble

F CM F C

60620.660µ 0.22µ

F M C

6µ 20µ

M

20 200

According to British Soil Classification System

Clay Silt Sand Gravel

F CM

4.7520.075 0.4255µ1µ

colloids


Soil Classification Soil classification is the arrangement of soils into various groups or subgroups to provide a common language to express briefly the general usage characteristics without detailed descriptions.

Two widely used soil classification systems:

-AASHTO system and

-Unified Soil Classification system

Both systems use simple index properties such as GSD, LL, and PI of the soil.


AASHTO Classification SystemConsiders both texture (GSD) and Atterberg Limits

Originally proposed in 1919; the system was last modified in 1945.

Widely used by highway and transportation engineers

Performed on that part of a soil sample that falls in the Gravel ----Clay Size range.

Once Group classification has been found, a so-called Group Index (GI) can be computed to further classify soils within a group.


AASHTO Classification SystemDifferent AASHTO Groups: A-1-a, A-1-b, A-2-4, A-2-5, A-2-6, A-2-7, A-3, A-4, A-5, A-6, A-7-5, A-7-6, A-8

GI = f( GSD, Shape, and Surface Area)

0.075 mm sieve

And above


AASHTO Classification SystemGI is a means of rating the value of soil as a subgrade material within its own group.

Higher the value of GI, poorer is the quality of the material.

-Always begin on the LHS with A-1-a group and check each of the criteria.

-If any criterion is not met, step to the right and repeat the process.

-Do not begin at the middle of the chart.

Useful Hints:


AASHTO Classification SystemExample 1:

Group classification is A-4 and

from the given data F = 58 ⇒GI = 3.45 (3)

Hence Group classification is: A-4 (3)

[Fair to poor Subgrade rating]

i.e. 2 mm sieve i.e. 0.425 mm


AASHTO Classification SystemExample 3:

Group classification is A-7-6 and

from the given data F = 95 ⇒GI = 42

Hence Group classification is: A-7-5 (42)

[Poor Subgrade rating]


AASHTO Classification System

Limitation: The criteria for the groupings are logical, but

shortcomings include requirement for laboratory testing, in order to determine a classification, and the difficulty in using code designation and remembering the requirement for each of the designations.


Unified Soil Classification System (USCS)First devised in 1942 and last modified in 1991.

Like the AASHTO system it uses both GSD and Atterberg Limits.

0.425 mm sieve


Unified Soil Classification System (USCS)Classification Procedure


Unified Soil Classification System

-Originally proposed by A. Casagrande in 1942

-Revised by the Corps. of Engineers and US Bureau of Reclamation in 1952.

-Widely used by various organizations, geotechnical engineers in private consulting business, and building codes.


Unified Soil Classification SystemSoil

Coarse-Grained Soil

Fine-Grained Soil

Peat

Gravel SandSilt

Inorganic clay

Organic Silt/clay

Casagrande’s extended classification system



Two major divisions:-A soil is coarse-grained (gravelly and sandy) if more than 50 % is retained on a No. 200 sieve and

-as a Fine-grained soil (Silty and clayey) if more than 50 % is passing through a No. 200 sieve.

Soil is further classified by a number of sub-divisions with primary and secondary characteristics.



The following symbols are used in USCS:

G: GravelS: SandW: Well-gradedP: Poorly (gap) gradedC: Well-graded

with some clayF: Well-graded with

excess fines

M: Silt (In Swedish Mo = Silt)C: ClayO: OrganicPt: Peat (highly organic)L: Low PlasticityI: Intermediate PlasticityH: High Plasticity



Criteria of classification for coarse-grained soils

into four groups.

W: Well-graded (Cu > 4 for gravels Cu > 6 for sands and

Cc: 1 – 3) with fines (finer than 75µ) less than 5 %

P: Poorly-graded (Cu < 4 for gravels Cu < 6 for sands and Cc not between 1 – 3) with fines less than 5 %

C: Plastic clayey fines (PI > 7) with fines more than 12 %

M: Non-plastic silty fines (PI < 4) with fines more than 12 %



Criteria of classification of Fine-grained soils

- Casagrandes Plasticity Chart

Basis of Plasticity chart:Experimental results from soils tested from different parts of the world were plotted on a graph of plasticity index (ordinate) vs. liquid limit (abscissa). It was found that clays, silts, and organic soils lie in distinct regions of the graph.


U-line

Ip = 0.9(wl-8)

Casagrandes Plasticity Chart



Features:

1) Chart is divided into six regions, three above A – line [delineates the boundaries between clays (above the line) and silts and organic soils (below the line)] and three below.

)20(73.0 −= lp wIEquation for A – line:



-U-line, defines the upper limit of the correlation between PI and LL.

If the results of soils tests fall above the U-line, repeat Atterberg tests.

Equation for U – line: )8(9.0 −= lp wI


Casagrandes Plasticity Chart2) All points representing inorganic clays lie above A-line, and all points for inorganic silts lie below it.

Points representing organic clays are usually located within the same region as those representing inorganic silts of high compressibility and organic silts in the region assigned to inorganic silts of medium compressibility.

In doubtful cases, LL should be determined for an oven-dry specimen as well as a fresh one. If drying decreases LL by 30 % or more, the soil may be classified as organic.



3) As the LL increases, the plasticity and compressibility of soils also increase.

4) The dry strength of inorganic soils represented by points on lines located above A-line increases from medium to samples with a LL < 30 to very high for samples with a LL > 100.


Major division Typical names Criteria of classificationCourse-grained soils (percent passing No. 200 sieve < 50)Gravels (percent of coarse fraction passing No.4 (4.75 mm) sieve < 50%)Gravels with little or no fines

Gravels with fines

-Well-graded gravels, gravel-sand mixtures (little or no fines) GW-Poorly graded gravels, gravel-sand mixtures (little or no fines) GP-Silty gravels, gravel-sand-silt mixtures GM-Clayey gravels, gravel-sand-clay mixtures GC

between 1 and 3

Not meeting the two criteria for GW

Atterberg limits below A-line or PI < 4Atterberg limits above A -line with PI > 7

260 30

10 10 60

( )4;uD DC CcD D D

⟩ =×


Major division Typical names Criteria of classification*Sands (percent of coarse fraction passing no.4 (4.75 mm) sieve > 50)

Clean sands (little or no fines)

Sands with fines (appreciable amount of fines)

Well-graded sands, gravelly sands (little or no fines) SW

Poorly graded sand, gravelly sands (little or no fines) SP

Silty sands, sand-silt mixtures SM

Clayey sands, sand-clay mixtures SC

Between 1 and 3

Not meeting the two criteria for SW

Atterberg limits below A- line or PI < 4

Atterberg limits above A- line with PI > 7

260 30

10 10 60

( )6;uD DC CcD D D

⟩ =×


Major division Typical names Criteria of classificationFine grained soils (percent passing No. 200 (0.0075 mm) sieve > 50 %)

Silts and clay (LL < 50)

Inorganic silts, very fine sands, rock flour, silty or clayey fine sands ML

Inorganic clays (low to medium plasticity), gravelly clays, sandy clays, silty clays, lean clays CL

Organic silts, organic siltyclays (low plasticity) OL

Plasticity Chart


Silts and clay (LL > 50)

Highly organic soils

Inorganic silts, micaceousor diatomaceous fine sandy or silty soils, elastic silts MH

Inorganic clays (high plasticity), fat clays CH

Organic clays (medium to high plasticity), organic silts OH

Peat, mulch, and other highly organic soils Pt

Plasticity Chart

Major division Typical names Criteria of classification


Percent passing No. 200 Classification

< 5

12

Between 5 to 12

GW, GP, SW, SP

GM, GC, SM, SC

Borderline-dual symbols requiredSuch as GW-GM, GW-GC, GP-GM, GP-SC, SW-SM, SW-SC, SP-SM, SP-SC

Classification based on percentage of fines

Atterberg limits above A - line and plasticity index between 4 and 7 are borderline cases. It needs dual symbols.


On or Above A-line – Inorganic claysVery high oven dried strength Very low k; Hard at wp

Below A-line – Silts & Organic soils

Very friable as w wp; Lower oven dried strength



Different samples from the same soil stratum


Location of common clay minerals in Casagrande’s Plasticity Chart

After Holtz and Kovacs, 1981)


Modified after Liu, 1970 and Al-Hussaini, 1977)

Atterberg limits rages for subgrad subgroups A-4, A-5, A-6 and A-7

To obtain ranges of LL and PI for groups A-4 to A-7Highly organic soils (Peats and mucks placed in group A-8)


For a soil specimen, given:Passing 2 mm sieve = 100 %; Passing 0.425 mm sieve = 85 %;

Passing No. 200 sieve = 38 %

LL = 20 % and PI = 12 %

Classify the soil by the Unified soil classification system

Example 2


Solution for Example 2

- Soil is a coarse grained soil (Percent passing No. 200 sieve < 50).

- Sands (percent of coarse fraction passing No. 4 sieve > 50)

- Since more than 12 % passes No. 200 sieve, it must be SM or SC

- PI = 20 – 12 = 8 > 7 [above A-line]

Hence the soil classification is SC


Example 3

For a soil specimen, given:Passing No. 4 sieve = 92 %; Passing No. 40 sieve = 78 %;

Passing No. 10 sieve = 81 %; Passing No. 200 sieve = 65 %

LL = 48 % and PI = 32 %

Classify the soil by the unified classification system

No. 4 sieve = 4.75 mm

No. 10 sieve = 2 mm;

No. 40 sieve = 0.425 mm sieve



Since more than 50 % is passing through a No. 200 sieve, it is a fine-grained soil, i.e., it could be ML, CL, OL, MH, OH, CH or OH.

Now, if we plot LL =48 and PI = 32 on the plasticity chart, it falls in the zone CL.

So the soil is classified as CL


Example 4

Limit tests performed on a clay indicate a liquid limit of 67 and a plastic limit of 32. From a hydrometer analysis to determine particle sizes, it is found that 80 % of the sample consists of particles smaller than 0.002 mm. From this information, indicate the activity classification and the probable type of clay mineral.



PI = LL – PL

= 67 – 32

= 35 CPIAc =

Ac = 35/80 = 0.44

The clay mineral is Kaolinite as Ac: 0.3 – 0.5


Example 5:

Use the grain-size distribution curve shown below toclassify soils A and B using the USCS. Soil B’s Atterberglimits are LL = 49% and PL = 45%.


For soil A, G = 2%, S = 98%;

M = 0% & C = 0%.

CU = 1.4/0.5 = 2.8; Cc = 0.952/(1.4)*(0.5) = 1.29

Soil A is a poorly graded sand (SP)

For soil B, G = 0%, S = 61%;

M = 35% & C = 4%.

CU = 0.45/0.005 = 90

Soil A is a very well graded silty sand (SM)

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