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Soil Mechanics

Instructor:

Engr Saadat Ali Khan

Lecture#3

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Engr Saadat Ali Khan

Department of Civil Engineering

University of Engineering and Technology, Peshawar

1. Some basic Definitions. (Atterburg limits)

Liquid limit of soil , plastic limit of soil , shrinkage

of soil , plasticity index , liquidity index ,

flow index of soil , toughness index , volumetric

Outlines of the PresentationOutlines of the Presentation

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shrinkage and shrinkage ratio.

2. Soil classification.A) American Association of State

Highway and TransportationOfficials System (AASHTO)

B) The Unified Soil ClassificationSystem (USCS)

Liquid limit of soil :

It is the minimum water content

at which twenty five (25) blows will close a groove

of the standard dimensions for a length of half inch,

Definitions .Definitions .

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of the standard dimensions for a length of half inch,

When the groove is made in soil , placed in the liquid

limit device.

It is represented by L.L

Plastic limit of soil :

It is the minimum water

content at which the soil mass crumbles (small

fragments or parts) when rolled into a thread of

Definitions.Definitions.

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fragments or parts) when rolled into a thread of

1|8 inch diameter.

Mathematically ,

Wp = P.L

Shrinkage of soil:

The water content at which

further loss of water from the soil mass will not

Definitions.Definitions.

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cause any reduction in the volume of the soil.

Mathematically,

Ws = S.L

Plasticity index:

The difference between the

liquid limit and the plastic limit is called

Definitions.Definitions.

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plasticity index.

Mathematically,

P.I = L.L – P.L

In general….

PI Degree of Plasticity

0 Nonplastic

1-5 Slightly plastic

5-10 Low plasticity

10-20 Medium plasticity

20-40 High plasticity

40+ Very high plasticity

(from Burmister, 1949)

Liquidity index:

The ratio between the difference

of water content and plastic limit to the plasticity

Definitions.Definitions.

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index is called liquidity index . The formula used

to find the liquidity index is given by

L.I = W-P.L |P.I

The range of the liquidity index is from 0 to 1.

Flow index of soil:

The slope of the flow curve is

called the flow index of the soil.

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Flow curve consists of two parameters , one is the

water content and the other is the number of

blows.

Flow index is represented by F.I

Toughness index:

The ratio of the plasticity index

to the flow index is called toughness index.

DefinitionsDefinitions

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Mathematically,

T.I = P.I | F.I

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• Volumetric shrinkage :

The decrease in volume

expressed as percentage of the dry volume , when

the water content is reduced to shrinkage limit.

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the water content is reduced to shrinkage limit.

V.S = (V-Vd)*100|Vd

Shrinkage ratio:

The ratio of the volumetric

shrinkage to the decrease in the water content.

Mathematically,

Definitions.Definitions.

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Mathematically,

S.R = V.S |W -Ws

Classification Systems

•Soils in nature rarely exist separately as gravel, sand, silt, clayor organicmatter, but are usually found as mixtures with

varying proportions of these components.•Classifying soils into groups with similar behavior, in terms of simple indices ,can provide geotechnical engineers general

guidance about engineering properties of the soils through the accumulated experience.

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Two commonly used systems for Classifying soils based on particle distribution and Atterberg limits:

1. AASHTO System: American Association of State Highway and Transportation Officials.

2. 2. USCS: Unified Soil Classification System.

AASHTO System

•AASHTO system of soil classification was developed by

Hogentogler and Terzaghi in 1929

The system is based on the following three

soil properties:soil properties:

1.Particle-size distribution

2. Liquid Limit

3. Plasticity Index

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AASHTO SYSTEM

Key Elements:

1. Grain Size:

• Gravel: Fraction passing 75mm sieve and retained on #10 (2mm) US sieve

• Sand: Fraction passing #10 sieve and retained #200 sieve

• Silt and Clay: Fraction passing #200 sieve

2. Plasticity:

• Term silty is applied when fine fractions have a PI < 10• Term silty is applied when fine fractions have a PI < 10

• Term clayey is applied when fine fractions have PI > 11

3. Groups: (see Tables)

• Soils are classified into eight groups, A-1 through A-8.

• The major groups A-1, A-2, and A-3 represent the coarse grained soils.

• The A-4, A-5, A-6, and A-7 represent fine grained soils.

• The A-8 are identified by visual inspection.

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AASHTO SYSTEMThe ranges of the LL and PI for groups A-2 ,A-4,

A-5 , A-6 and A-7

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Unified Soil Classification system, USCS

• Origin: The Unified Soil Classification system was first

developed by Professor A. Casagrande in 1942.

• The USCS is based on the recognition of the type and

predominance of the constituents considering grain-size,

gradation, plasticity and compressibility.

• It classifies soils into Four major categories:• It classifies soils into Four major categories:

1. Coarse-grained

2. Fine-grained

3. Organic soils

4. Peat

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Unified Soil Classification system, USCS

• Procedures for Classification:

• From sieve analysis and the grain-size distribution curve determine

the percent passing as the following:

• First, Find % passing # 200.• First, Find % passing # 200.

• If 5% or more of the soil passes the # 200 sieve, then conduct

Atterberg Limits (LL & PL).

• If the soil is fine-grained (≥50% passes #200) follow the guidelines for

fine- grained soils.

• If the soil is coarse-grained (<50% passes #200) follow the guidelines

for coarse-grained soils: Find % Gravel & Sand, calculate C u & C c ,

LL, PL &PI

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Unified Soil Classification system, USCS

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