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GE

Unit - 4

Shear Strength of Soil

Q1) Explain Shear strength as an Engineering property?

A1)

  • In most of the problems in soil mechanics such as those concerning the foundations of structures, earthwork engineering etc., the soil mass has to withstand shearing stresses, which are unlike in nature than the compressive stresses.
  • Shearing stresses tend to displace part of the soil mass relative to rest of the soil mass, Shear strength of a soil is the capacity of the soil to resist shearing stress.
  • It can be defined as the maximum value of shear stress that can be mobilized within a soil mass. If this value is equalled by the shear stress on any plane or surface at a point, failure will occur in the soil because of movement of a portion of the soil mass along that plane or surface.
  • The soil is then said to have failed in shear.
  • The shear strength depends upon:
  • Type of soil
  • Cohesion
  • Compaction.
  • Water content
  • Internal friction
  •  

    Q2) What are the principal stresses in Mohr’s circle?

    A2)

  • There are two principal stresses like major and minor principal stresses are to be considered and third principal stress in the third direction is not much relevance.
  • Major principal stress (
    1): The normal stress without shear stress acting on the major principal plane is called as major principal stress (
    ).
  • Minor principal stress (
    ): the normal stress without shear stress acting on the minor principal plane is called as minor principal stress (
    .
  • The intermediate principal stress (
    ) is not much relevant, hence not considered in the many problems in geotechnical engineering, only major principal stress (
    ) and minor principal stress (
    ) are important.
  • Following equation shows the equation of circle,
  • +2   =2                         

    Where, = Normal stress

    = Shear stress

     1= Major principal stress

    = Minor principal stress

    The co-ordinates of centre of circle are =0 and

    = and its radius is

     

    Q3) Explain Effective Stress principle?

    A3) The principle of effective stress can be stated as

    a)     Effective stress governs volume changes in soils.

    b)    Effective stress ( l) Total stress ()-pore pressure(uw)

    c)     Effective stress controls shearing strength of soils.

  • Many properties like volume change, permeability, shearing strength, compressibility etc. depends upon the effective stresses.
  • The voids can totally be filled with water or partly with water and air. Shear stresses are to be carried only be the skeleton of solid particles. But in general, the total normal stress on any planes are the sum of two components.
  • :. Total stress = component of stress carried by solid particles + pressure in the fluid in the void space.

     

    Q4) What is Peak shear strength?

    A4)

  • Resistance to a particulars maximum shear stress caused only by strain hardening is termed as peak shear strength or only shear strength of the soil.
  • The maximum shearing resistance decreases after yield point in some soils. Hence when the maximum shearing resistance decreases after the yield level, then soil is said to be strain softening.
  • Yield level is considered as unstable level and at the maximum peak strength, the soil is said to fail.
  •  

    Q5) What is Residual shear strength?

    A5) When the shearing resistance comes at a constant level after a continued large strain then corresponding shear resistance at a constant level is termed residual shear strength.

     

    Q6) What are the factors which affects shear strength?

    A6) For Cohesionless soil:

  • Gradation: Exhibit greater strength in case of well graded sand.
  • Shape: Max. angular and sharp edge particle more will be the strength
  • Pressure: With increase in confining pressure shear strength increases.
  • Denseness: More is denseness more is shear strength
  • Moisture: IFS and is saturated apparent cohesion is destroyed.
  • For Cohesive Soil:

  • Plasticity index: Value of decreases with increase in plasticity index
  • Clay content: As clay content increases angle of shearing resistance decreases.
  • Drainage condition: Less strength if drainage is not proper
  • Pressure: Shear strength of clay increases with increase in confining strength
  •  

    Q7) Explain stress-strain behavior of sand?

    A7)

  • The stress-strain behaviour of saturated sand specimen can be well studied by performing drained tests in direct shear tests or triaxial tests.
  • Two specimens of saturated sand are taken and one specimen is kept at a very high initial void ratio i.e., loose sand and other is kept at a very low initial void ratio i.e., dense sand. These two specimens can be used in consolidated drained (CD) tests. The test gives the results in the form of deviator stress and axial strain.
  • Fig. shows the graph of deviator stress against axial strain taken from the test results.
  • H:\unit4\IMG_20210519_202243.jpg

    Fig.: Typical stress-strain behavior of sands from CD test

    In both the specimen, the cell pressure is kept constant and axial stress is increased the sample specimen fails in shear.

  • The stress-strain curve for the dense sand shows a definite peak or ultimate point of stress relatively low strain.
  • In Fig, when strain goes on increasing, the peak stress decreases and then stress becomes more or less constant. For further continuation of shearing of soil, the stage of deviator stress is termed as ultimate stress.
  • For shearing of loose sand, the stress increases with strain more gradually as compared to the dense sand and maximum deviator stress comes very close to ultimate stress value of dense sand.
  • The peak stress is used to define the shear strength. In case of loose sands, the maximum risers or stress value at an arbitrarily selected value of strain is used to define the shear strength.
  •  

    Q8) Explain stress-strain behavior of clay?

    A8)

  • Fig. shows typical stress-strain behaviour of clays obtained from unconsolidated undrained conditions test (UU tests). Curve 'A' shows a sharp peak for undisturbed clays of high sensitivity at a low strain.
  • H:\unit4\IMG_20210519_202306.jpg

    Fig.: Typical stress-strain behavior of clays from UU test

    H:\unit4\IMG_20210519_202328.jpg

    Fig.: Brittle and plastic failure resp.

  • The sample specimen shears very clearly along a well-defined shear plane as shown in Fig. This shear failure is termed as brittle failure.
  • Curve 'D' shows plastic failure for remoulded sensitive clays i.e., soft clays due to continuous yield at a more or less constant stress resulting in the bulging effect on the cross-sectional area of specimen as shown in Fig.
  • In Fig., the curve 'B' shows an increase of stress with strain up to a certain stage and then there is slightly stress drop.
  • Fig. shows a typical stress-strain curves for normally consolidated and over consoildated clay obtained from consolidated drained (CD) test.
  • H:\unit4\IMG_20210519_202351.jpg

    Fig.: Stress-strain behavior of clay obtained from CD test

  • In Fig., the curve 'A' shows a greater strength for over consolidated clays than normally consolidated clays as shown by curve B. In curve A, peak occurs quite early and the stress falls off as the strain increases which is a phenomenon called as work-softening or strain softening.
  •  

    Q9) Write advantages and disadvantages of direct shear strength?

    A9) Advantages:

  • Test is simple and fast.
  • Drainage is quick due to less thickness of sample.
  • CD and CU tests takes relatively small period, because of quick drainage and rapid dissipation of pore water pressure.
  • Test is ideally suited for drained tests on cohesionless soils.
  • The direct shear test apparatus is relatively cheap.
  • Disadvantages:

  • Failure of soil specimen is always along a horizontal plane, which may not be very realistic.
  • If any large soil particles or stones etc. are present at failure plane, it will give wrong results.
  • Actual field condition is not simulated in the set up.
  • Measurement of pore pressure is not possible.
  •  

    Q10) Write advantages and disadvantages of triaxial test?

    A10) Following are the various advantage of triaxial test:

  • A Casagrade developed the triaxial test in his research and removed the disadvantages of direct shear test.
  • There is a complete control during conducting the shear test under all the three drainage conditions.
  • Porewater pressure and volume change are precisely measured during the test.
  • There is uniform stress distribution on the failure plane.
  • State of stress within the soil sample during any stage of the test and at the failure of soil sample is completely determinate.
  • Disadvantages of Triaxial Test:

  • The apparatus in triaxial test is bulky, costly and elaborate.
  • Triaxial test takes longer period in case of drained test as compared with that in a direct shear test.
  •  

    Q11) Write advantages and disadvantages of unconfined compression test?

    A11) Advantages of Unconfined Compression Test:

  • It is quick and convenient for meaning shear strength.
  • It is useful for homogeneous cohesive clayey soil.
  • It is suitable for measuring unconsolidated, undrained shear strength of saturated class
  • It is used to measure inside strength and very useful for field test.
  • Disadvantages of Unconfined Compression Test:

  • This test cannot be carried out on coarse grained soils like sands and gravels and also not carried out on fissured clays.
  • The test result may be misleading for the soils for which
    0.
  •  

    Q12) What are the drainage condition for shear test?

    A12) There are three types of drainage conditions for shear tests as explained follows:

    1. Consolidated-undrained conditions (CU condition)

  • Before shear, the specimen or soil sample is allowed to consolidate and the drainage is allowed until the consolidation is completed in first stage.
  • During shear, no drainage is allowed or drainage is stopped in second stage this test is also termed as CU test or 'R' test.
  • 2. Consolidated drained conditions (CD condition)

  • Before shear, the specimen is allowed to consolidate during drainage conditions in first stage.
  • During shear; drainage is allowed in second stage. Note that rate of shearing is kept very low to ensure that a fully drained condition exists such that excess pore water is zero.
  • This test is also termed as CD test.
  • 3. Unconsolidated-undrained condition (UU condition)

  • Before and after shear, no drainage and consolidation is allowed; that means before application of axial stress or shear in first stage, drainage and consolidation is not allowed and after shear or application of axial stress, no drainage and consolidation of soil sample is permitted in second stage of test.
  • Due to unconsolidated and undrained condition, the test can be carried out quickly in a less time and therefore this test is also called as 'quick test' or 'UU test.
  • Q13) What is Sensitivity?

    A13)

  • Definition: The ratio of unconfined compressive strength of an undisturbed sample of soil to the unconfined compressive strength of the remoulded sample of same soil at the same water content as in undisturbed soil is called as sensitivity (St).
  • St =

    Where, qu= unconfined compressive strength of an undisturbed sample of soil or clay

    qu’=unconfined compressive strength of remoulded sample of soil or clay at the same water content as in the undisturbed soil

  • Sensitivity is a measure of the loss in strength of soils and remoulding on the consistency of a cohesive soil.
  • Following table shows natural clay deposits classified into four different categories based on the value of sensitivity:
  • Sr.no.

    Sensitivity

    Classification

    1

    1-14

    Normal

    2

    4-8

    Sensitive

    3

    8-16

    Very sensitive

    4

    16-32

    Slightly quick

    5

    32-64

    Medium quick

    6

    Greater than 64

    Quick

    7

    Less than 1

    Stiff clay

     

    Q14) What is thixotropy?

    A14)

  • Thixotropy is a Greek word in which thix means touch and tropo means change, overall meaning of thixotropy is any change which occurs by touch.
  • Thixotropy is the property of certain clays or soil by virtue of it gradually regains its lost strength with time if remoulded clay is allowed to rest without change in water content.
  • In short, if a remoulded soil sample having the sensitivity more than one is allowed to stand without change in water content and disturbance, then it may regain some parts of its original strength and stiffness. This increase in strength of soil sample is termed as thixotropy.
  • Thixotropy has more practical importance in geotechnical engineering. In pile-driving operations, thixotropy plays important role. For example. There is a strength loss of soil, when a pile is driven into the ground. In such case, one can easily know regaining of shear strength after the pile has been driven and left in place for some time with the help of thixotropy.
  •