MSE, BARIPADA

DEPARTMENT OF CIVIL ENGINEERING

SUBJECT – GEO-TECH. ENGG.

TEACHER – ER. ANKIT JOSHI

PRESENTATION

ON

SOIL CLASSIFICATION

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INTRODUCTION

• Soil classification is the arrangement of different soils with similar properties into groups and subgroups based on their application.

• Soils may be classified in a general way as:

• Cohesive vs. cohesionless

• Fine- grained vs. coarse grained

• Residual vs. Transported

• However these terms are too general and cover too wide range of physical and engineering properties.

• A more refined classification is necessary to determine the suitability of a soil for specific engineering purposes.

• Therefore, these terms are collected into SOIL CLASSIFICATION SYSTEMS, usually with some specific engineering purpose in mind.

• A number of classification systems have been proposed since the 1^st quarter of the 20^th century.

NEED FOR SOIL CLASSIFICATION SYSTEMS

• A soil classification system represents, in effect, a language of communication between engineers.

• It enables one to use the engineering experience of others.

• The engineering properties have been found to correlate quite well with the index and classification properties of a given soil deposit.

• Therefore, by knowing the soil classification, the engineer already has a fairly good general idea of the way the soil will behave.

• Most of the soil classification systems that have been developed for engineering purposes are based on simple index properties such as particle-size distribution and plasticity.

Classification System

“Language”

Engineering Properties

Permeability, shear strength, compressibility,

swell-shrinkage etc.

Engineering Purpose

(Foundation, Dams, Highways, Airfields, etc.)

Role of Soil Classification in Geotechnical Engineering

Classification and index properties

(w, e, γ , s, GSD, LL, PI, etc.)

Why more than one Classification System are in use?

• Classification systems are used to group soils in accordance with their general behavior under given physical conditions.

• Soils that are grouped in order of performance for ONE SET of Physical CONDITIONS will not necessarily have the same order for performance under other set of physical conditions.

• This led to classifying soil by use, and each agency (Like FAA, AASHTO, USBR) has in mind specific use for the soils.

SOIL CLASSIFICATION SYSTEMS

1. Grain-size classification

Limitations:

• Gives only border between groups (i.e. gravel, sand, silt, clay) but does not give us a name for a given bulk of soil.
• In most cases, natural soils are mixtures of particles from several size groups.

2. TEXTURALCLASSIFICATION

• The texture of a soil is its appearance or ”feel” and it depends on:

• Generally the term texture is refereed wholly to the size characteristics of the soil particles.

• Large soil particles with some small particles will give COARSE-APPEARNCE or COARSE-TEXTURED soil.

• Smaller particles give a MEDIUM TEXTURED soil.

• Fine-grained particles give FINE-TEXTURED soil.

• In the textural classification system, the soils are named after their principal components, such as sandy clay, silty clay, and so forth.

• The relative sizes of the particles
• Range or distribution of these sizes
• Shapes of the particles

• A number of textural classification systems were developed in the past by different organizations.

• The following figure shows the textural classification system developed by the USDA. It is based on the particle size limit as shown in Table 2.3 in the textbook.

Loam is soil composed of sand, silt, and clay in relatively even concentration (about 40-40-20% concentration, respectively).

• If there were no gravel in the soil, the line representing the three sizes included in the chart would intersect at a common point.

• The chart is based on only the fraction of soils that passes through No. 10 sieve (opening = 2 mm).

• In concrete technology called Feret Triangle.

• Percentage of each class can be determined from GSD (without GSD, percentage can be determined based on sieve size opening).

• If a certain percentage of the soil particles are > 2 mm in diameter, then a correction will be necessary.

REMARKS

Example 1

Let it be desired to determine the textural classification of a soil whose grading is as follows:

The two lines intersect within the area labeled LOAM, which is the textural class of this soil.

Notes

We could have used the point of intersection of:

• Silt-size+ Clay-size
• Sand-size + Clay-size

Example 2

The textural classification is CLAY.

Find the modified textural composition:

Let it be desired to determine the textural classification of a soil whose grading is as follows:

Because of the large percentage of gravel, it may be called GRAVELLY CLAY.

• They are based entirely on particle-size distribution and does not consider PLASTICITY which to great extent influences the physical properties of soils.

Limitations of Textural Classification Systems

• The two elaborate systems used at present are AASHTO and USCS. Both systems take into account the particle-size distribution and plasticity.

• Because textural classification systems do not take plasticity into account and are not totally indicative of many important soil properties, they are inadequate for most engineering purposes.

• The AASHTO classification system is used mostly by highway departments. Geotechnical engineers generally prefer the Unified system.

3. CLASSIFICATION BY USE

A. AASHTO CLASSIFICATION SYSTEM

• The AASHTO soil classification system was originally developed in the late 1920’s (1929) by the U.S. Bureau of Public Roads (BPR) for the classification of soils for highway subgrade use.

• It was developed as a result of the work of Hogentogler in the 1920’s.

• Originally, the system classified soil as being either a group A or a Group B.

• AASHTO : Acronym of American Association of State Highway and Transportation Officials.

• Adopted by Bureau of Public Roads in 1931.

• It went through various revisions since 1929, and the classification system received its last revision in 1974.

• ASTM D-3282; AASHTO method M145.

• A Group A soil was able to maintain uniform pavement support at all location whereas the Group B soils were not.

• The B designation was subsequently deleted, leaving only A soils in the classification system.

• Consequently, the “A” still remains in an AASHTO classification of a soil type, but it no longer has any real significance.

• The A soils were subdivided into eight subgrade soil groups. A-1 through A-8.

Tests Required:

• Grain-size analysis
• Liquid Limit
• Plastic Limit

Criteria:

Remarks

• In addition to Sieve no. 10 & 200 also sieve no. 40 is involved in AASHTO classification which separates between medium and fine sands.
• In the classification, differentiation between silt and clay soils is based on plasticity.

18

4.75 mm

2.0 mm

0.425 mm

0.075 mm

(AASHTO)

Gravel

(USCS)

Sand

Sieve No. involved in AASHTO Soil Classification System

3 Groups

6 Subgroups

4 Groups

2 Subgroups

For classification starts apply the test data from left to right, top to bottom. By process of elimination, the first group from the left into which the test data fit is the correct classification.

No. 10

No. 40

No. 200

• If the soil is A-1 or A-3 we cannot use this chart (mainly non plastic soils).

• The plot below is for the range of the liquid limit and the plasticity index for soils that fall into groups A-2, A-4, A-5, A-6, and A-7.

Note:

Differentiation between A-2’s and other group is based on %passing Sieve No. 200

• According to this system, soil is classified into eight major groups, A-1 through A-8.

• Soil group A-8 is peat (very organic) or muck (thin very watery, and with considerable organic material).

• A soil is classified according to the table by proceeding from left to right, top to bottom, column by column on the table to find the first group in which the soil test data will fit.

• The first group from the left into which the test data will fit is the correct classification.

REMARKS

• The classification process stops at this point regardless if another column farther to the right can also qualify.

GROUP INDEX

• Soils containing fine-grained material are further identified by a number called GROUP INDEX (GI). This was to establish the relative RANKING of a soil within a subgroup or a group. This help in evaluating the quality of a soil as a highway subgrade material.

• It is dependent on:

1. Percentage of the soil passing the No. 200 (0.075 mm) sieve.

2. Liquid limit, LL

3. Plasticity Index, PI

• The index is given by the following empirical formula:

F₂₀₀= % passing No. 200 sieve.

Rules for Determining Group Index

1. If GI is negative value take it as zero.
2. GI is rounded off to the nearest whole number.
3. There is no upper limit for GI.
4. The group index belonging to groups A-1-a, A-1-b, A-2-4, A-2-5 and A-3 will always be zero. Why?
5. When calculating the group index for soils belonging to groups A-2-6 and A-2-7, the partial group index for PI should be used, or

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• The group index value is written in parenthesis next to the AASHTO symbol. Ex. A-2-6(3), A- 4(5) etc.
• In general the higher the GI, the less desirable is the soil for use as a subgrade. A GI of 0 indicates a “good’ subgrade , and a GI> or equal 20 indicates a “very poor” subgrade material.

Example 1

The sieve analysis and plasticity data for two soils are given in the table below. Classify soils according to the AASHTO soil classification system.

Soil 2:

A-1-a(0)

110

Soil 1:

A-7-5

Because %Passing #200 > 35%

The soil is classified as A-7-5 and not A-2-7.

Example 2

#200 86%

LL 70%

PI 32%

Because %Passing #200 > 35%

The soil is classified as A-7-5 and not A-2-7.

Example 3

The grain size distribution curve, natural water content, liquid limit, and liquidity index are give below. Classify the soil according to the AASHTO Classification System.

PL =21.25%

PI = 18.75%

40

Given

Solution

From the AASHTO Table the soil is classified as: A-2-6 (0)

Because %Passing #200 < 35%

The soil is classified as A-2-6 and not A-6.

Note: If the location comes along the border line, we need to consult the table and the grain size will tell the classification of the given soil.

Remarks About AASHTO System

• Boulders (retained on 75 mm (3 inch) sieve] should be excluded from the portion of the sample to which the classification is applied, but the percentage of such material, if any, in the sample should be recorded.

• Differentiation between A-7-5, A-7-6 is according to the following criterion:

PI <= LL- 30 ------🡪A-7-5

PI > LL- 30--------🡪 A-7-6

• Group A-3 is placed before group A-2 in the table since:

• It is better as a subgrade
• It is based only on grain size

So it was easer to put A-3 before A-2 in order to facilitate left to right and top to bottom procedures applied in this system.

• For A-1, A-3, A-2-4, A-2-5…>> F<= 35% , PI < 15, therefore GI –ve (i.e. GI always zero).

Or instead we use the chart

B. UNIFIED SOIL CLASSIFICATION SYSTEM (USCS)

• This system was developed by Arthur Casagrande in 1942 for use in the air field construction works undertaken by the Army Corps of Engineers during WW II.

• To make it applicable to DAMS and other constructions besides airfields, it was revised in 1952 in cooperation with the USBR.

• The system was last revised in 1984 by the ASTM by the addition of a GROUP NAME to the group symbol. This modification has not been adopted by some agencies which use USCS to classify soils.

• ASTM Test Designation D-2487.

1. Coarse-grained soils < =50% passes sieve No. 200
2. Fine-grained soils > 50% passes sieve No. 200
3. Organic

• This system classifies soils under three broad categories:

• Criteria for USCS:

1. Grain size
2. C_u, C_c
3. Plasticity (Plasticity chart)

• Tests required

• Grain-size analysis
• Liquid Limit
• Plastic Limit

• This system is the most popular soil classification system among geotechnical engineers.

• All soils are classified into 15 groups each group is designated by two letters called a GROUP SYMBOL.

• The first letter of the group symbol is termed the PREFIX and the second letter is termed the SUFFIX. These letters are abbreviations of certain soil characteristics as follows:

Use Plasticity Chart

Gravels

Sands

High

Plasticity

Low

Plasticity

Sands

Gravels

AASHTO

USCS

To determine if well graded (W) or poorly graded (P), calculate C_u and C_c

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Coefficient of uniformity

Coefficient of gradation

A-Line

U-Line

OH

or

OH

or

OH

or

OH

or

OL or OH based on the value of a

Organic Soils

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

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• The second symbol is obtained by locating the values of PI and LL (not oven dried) in the plasticity chart.

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• 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.

Coarse-grained soils (Gravel (G) or Sand (S))

Fine-grained soils (Silt (M) or Clay (C))

% Passing sieve No. 200 (0.075 mm)

> 50%

< = 50%

% Coarse soil (Co) = 100 - % Passing # 200

% Gravel (G) = 100 - % Passing # 4

G > 1/2 Co

G < 1/2 Co

Gravel (G)

Sand (S)

A-Line

OH

or

OH

or

OL

or

OL

or

OL or OH based on the value of α

Example 1

The sieve analysis and plasticity data for two soils are given in the table below. Classify the soils according to the USCS soil classification system.

Soil 1:

Soil 2:

SC

​

MH

Example 2

Classify the following soils Using USCS

47

SC

​

CH

​

SM

​

SC

CH

SM

Example 2 (Cont.)

Example 2 (Cont.)

49

50

Example 3

51

Soil A is then classified as SP-SM

(Poorly graded sand with silt and gravel)

Example 4

#4 = 95%

#200 = 11%

Coarse fraction =100-11 = 89%,

Gravel = 100 -95 = 5% (5/89 = 5.6% < 50%) -🡪 Sand

From Atterberg Tests

LL = 32, PL = 26

Example 4 (cont.)

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

C_u = 0.75/0.06 = 12.5 C_c = 0.25X0.25/(0.75X 0.06) = 1.39

∴ Well graded

From Atterberg Tests

LL = 32, PL = 26

PI = 32-26 = 6

The soil is classified as SW-SM (Well-graded sand with silt)

The fine part is silt

Remarks

• If for coarse-grained soils, the percent passing through the No. 200 sieve is between 5 and 12% dual symbols are needed (i.e. GW-GC, GP-GC, GW-GM, GP-GM, SW-SC, SP-SC, SW-SM, SP-SM).

• A soil that plots in the area to the left of the A-line in the plasticity chart bounded by 4<= PI <=7, the A-line, and the left of the boundary chart has a dual-classification of CL-ML. This is the only fine-grained soil that has such a dual classification.

• Peat is visually identified

• Whether the fine-grained soil is organic or not, that is determined as follows:

Comparison of the USCS and AASHTO Classification Systems

• In AASHTO if 35% passes No. 200 🡺 fine-grained

In USCS if 50% passes No. 200 🡺 fined-grained

• In AASHTO Sieve No. 10 is used to separate gravel from sand, in USCS it is Sieve No.4.

• In USCS, the gravely and sandy soils are clearly separated, in the AASHTO system they are not.

• The symbols GW, SM, CH and others that are used in the USCS are more descriptive of the soil properties than the A symbols used in the AASHTO system.

• The classification of organic soils such as OL, OH, and Pt has been provided in the USCS. In AASHTO system, there is no place for organic soils. (A-8 has been taken out).

• In AASHTO PI is used to distinguish between silt and clay (LL appears only in distinguishing A-7-5 and A-7-6). In USCS both PI and LL (plasticity chart) are used.

• USCS distinguishes high and low plastic fine-grained soils.

• Both AASHTO and USCS are better than most other available systems when applied to engineering or construction applications.

• Both AASHTO and USCS systems have the advantage of having been used for many years and having gained acceptance in the engineering and construction fields.