Soil Mechanics

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Transcript of Soil Mechanics

  • 1

  • Soil Mechanics


    Engr Saadat Ali Khan



    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 , shrinkageof soil , plasticity index , liquidity index , flow index of soil , toughness index , volumetric

    Outlines of the PresentationOutlines of the Presentation


    shrinkage and shrinkage ratio.2. Soil classification.

    A) American Association of StateHighway 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 .


    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



    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



    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



    plasticity index.Mathematically,

    P.I = L.L P.L

    In general.

    PI Degree of Plasticity0 Nonplastic1-5 Slightly plastic5-10 Low plasticity10-20 Medium plasticity20-40 High plasticity40+ Very high plasticity(from Burmister, 1949)

  • Liquidity index:The ratio between the difference

    of water content and plastic limit to the plasticity



    index is called liquidity index . The formula used to find the liquidity index is given by

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



    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.



    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.



    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,



    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.


    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 SystemAASHTO 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 distribution2. Liquid Limit

    3. Plasticity Index


  • AASHTO SYSTEMKey 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 sieve2. 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.


  • 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-grained2. Fine-grained3. Organic soils4. Peat


  • 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 (

  • Unified Soil Classification system, USCS