lec 8 soil shear strength

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					              Lecture-8
      Shear Strength of Soils




Dr. Attaullah Shah
                                1
       Strength of different materials



  Steel             Concrete                 Soil



Tensile           Compressive              Shear
strength          strength                 strength


                               Presence of pore water
              Complex
              behavior
                                                    2
       What is Shear Strength?
•   Shear strength in soils is the resistance
    to movement between particles due to
    physical bonds from:
    a. Particle interlocking
    b. Atoms sharing electrons at surface contact
       points
    c. Chemical bonds (cementation) such as
       crystallized calcium carbonate

                                                    3
     Influencing Factors on Shear
               Strength
• The shearing strength, is affected by:
  – soil composition: mineralogy, grain size and grain size
    distribution, shape of particles, pore fluid type and
    content, ions on grain and in pore fluid.

  – Initial state: State can be describe by terms such as:
    loose, dense, over-consolidated, normally
    consolidated, stiff, soft, etc.

  – Structure: Refers to the arrangement of particles
    within the soil mass; the manner in which the particles
    are packed or distributed. Features such as layers,
    voids, pockets, cementation, etc, are part of the
                                                           4
    structure.
       Shear Strength of Soil
• In reality, a complete shear strength
  formulation would account for all
  previously stated factors.

• Soil behavior is quite complex due to the
  possible variables stated.

• Laboratory tests commonly used:
  – Direct Shear Test                         5
Soil Failure and shear strength.
• Soil failure usually occurs in the form of
  “shearing” along internal surface within the
  soil.
• Thus, structural strength is primarily a
  function of shear strength.
• Shear strength is a soils’ ability to resist
  sliding along internal surfaces within the
  soil mass.
                                             6
Slope Stability: Failure is an
Example of Shearing Along
      Internal Surface




                                 7
Mass Wasting: Shear Failure




                              8
Shear Failure: Earth Dam




                           9
Shear Failure Under Foundation
             Load




                                 10
                  Shear failure
 Soils generally fail in shear

                             embankment

       strip footing



                                   mobilized shear
                                   resistance

failure surface

At failure, shear stress along the failure surface
reaches the shear strength.
                                                     11
Shear failure



        failure surface
        The soil grains slide over
        each other along the
        failure surface.


         No crushing of
         individual grains.

                                     12
    Shear failure mechanism




At failure, shear stress along the failure surface ()
reaches the shear strength (f).
                                                    13
   Shear failure of soils
     Soils generally fail in shear


Retaining
wall




                                     14
         Shear failure of soils
           Soils generally fail in shear


                                     Mobilized
      Retaining
                                     shear
      wall
                                     resistance

                                Failure
                                surface




At failure, shear stress along the failure surface
(mobilized shear resistance) reaches the shear strength.
                                                           15
    Mohr-Coulomb Failure Criterion

               

                    f  c   tan 
                                                

                               friction angle
    cohesion
                       f
               c
                   
                                                    
f is the maximum shear stress the soil can take
without failure, under normal stress of .              16
      Mohr-Coulomb Failure Criterion
            (in terms of total stresses)
               

                    f  c   tan 
                                              

                             Friction angle
    Cohesion
                       f
               c
                   
                                                  

f is the maximum shear stress the soil can take without
failure, under normal stress of .                    17
      Mohr-Coulomb Failure Criterion
           (in terms of effective stresses)
            

                  f  c' ' tan  '
                                                       '  u
                                            ’
                                                      u = pore water
Effective
                                                          pressure
cohesion                      Effective
                      f   friction angle
            c’
                 ’                              ’

f is the maximum shear stress the soil can take without
failure, under normal effective stress of ’.         18
     Mohr-Coulomb Failure Criterion
      Shear strength consists of two
      components: cohesive and frictional.




                              f  c' ' f tan  '
f
                ’f tan ’                   frictional
       ’                                    component

c’              c’
              ’f             '


                                                          19
     Mohr-Coulomb Failure Criterion
      Shear strength consists of two
      components: cohesive and frictional.





f
                                f  c   f tan 
                    f tan                       frictional
                                                 component
 c                  c
               f                 
           c and  are measures of shear strength.
                                                               20
       Higher the values, higher the shear strength.
21
22
23
Determination of shear strength parameters of
soils (c,  or c’, ’)

Laboratory      tests   on                        Field tests
specimens     taken   from
representative undisturbed
samples

Most common laboratory tests             1.   Vane shear test
to determine the shear strength          2.   Torvane
parameters are,                          3.   Pocket penetrometer
                                         4.   Fall cone
1.Direct shear test                      5.   Pressuremeter
2.Triaxial shear test                    6.   Static cone penetrometer
                                         7.   Standard penetration test
Other laboratory tests include,
Direct simple shear test, torsional
ring shear test, plane strain triaxial
test, laboratory vane shear test,                                  24
laboratory fall cone test
Laboratory tests
Field conditions



 A representative
 soil sample
                            z                      z
                vc                    vc + D


       hc            hc        hc        hc


                vc                    vc + D


    Before construction         After and during
                                                       25
                                construction
                                                   vc + D
Laboratory tests
                                           hc           hc
 Simulating field conditions
 in the laboratory
      0               vc                          vc + D

                                                   vc
0          0     hc         hc
                                                              

     0                 vc                 

Representative     Step 1                          vc
soil    sample                           Step 2
taken from the     Set the specimen in
site               the apparatus and     Apply             the
                   apply the initial     corresponding field
                   stress condition      stress conditions 26
Direct shear test
 Schematic diagram of the direct shear apparatus




                                                   27
  Direct shear test
Direct shear test is most suitable for consolidated drained tests
specially on granular soils (e.g.: sand) or stiff clays

   Preparation of a sand specimen


Porous
plates




Components of the shear box         Preparation of a sand specimen
                                                                28
Direct shear test
Preparation of a sand specimen             Pressure plate




Leveling the top surface         Specimen preparation
of specimen                      completed


                                                        29
 Direct shear test
                              P               Steel ball
 Test procedure
                                              Pressure plate
Porous
plates

                                                                  S



                                                           Proving ring
                                                           to measure
                                                           shear force



Step 1: Apply a vertical load to the specimen and wait for consolidation

                                                                           30
  Direct shear test
                               P               Steel ball
Test procedure
                                               Pressure plate
 Porous
 plates

                                                                   S



                                                            Proving ring
                                                            to measure
                                                            shear force



 Step 1: Apply a vertical load to the specimen and wait for consolidation

                                                                           rate
 Step 2: Lower box is subjected to a horizontal displacement at a constant31
Direct shear test
                         Dial gauge to
                         measure vertical
    Shear box            displacement




                                   Proving ring
                                   to measure
                                   shear force

 Loading frame to      Dial   gauge    to
 apply vertical load   measure horizontal
                       displacement

                                            32
     Direct shear test
                           Analysis of test results



                              Normal force (P)
   Normal stress 
                     Area of cross section of the sample

                   Shear resistance developed at the sliding surface (S)
  Shear stress 
                           Area of cross section of the sample

  Note: Cross-sectional area of the sample changes with the horizontal
        displacement




                                                                   33
Direct shear tests on sands
Stress-strain relationship




                          Shear stress, 
                                                                  Dense sand/
                                                                  OC clay
                                             f
                                                                   Loose sand/
                                                          f       NC clay



                                                  Shear displacement
                               Expansion
       Change in height
       of the sample




                                                                   Dense sand/OC Clay

                                                   Shear displacement
                               Compression




                                                                   Loose sand/NC Clay
                                                                                 34
Direct shear tests on sands
How to determine strength parameters c and 
       Shear stress, 




                                                     Normal stress = 3
                                                          Normal stress = 2
                                                           Normal stress = 1
                                     f2
                                           f1
                               f3
                                            Shear displacement
 Shear stress at failure, f




                                                          Mohr – Coulomb failure envelope
                                           


                                                                                            35
                                                  Normal stress, 
Direct shear tests on sands
Some important facts on strength parameters c and  of sand


                          Direct shear tests are
Sand is cohesionless      drained and pore water
hence c = 0               pressures            are
                          dissipated, hence u = 0

                                               Therefore,
                                               ’ =  and c’ = c = 0




                                                                   36
Direct shear tests on clays
In case of clay, horizontal displacement should be applied at a very
slow rate to allow dissipation of pore water pressure (therefore, one
test would take several days to finish)

Failure envelopes for clay from drained direct shear tests
  Shear stress at failure, f




                                            Overconsolidated clay (c’ ≠ 0)


                                            Normally consolidated clay (c’ = 0)
                                ’



                                     Normal force, 

                                                                             37
Interface tests on direct shear apparatus
In many foundation design problems and retaining wall problems, it
is required to determine the angle of internal friction between soil
and the structural material (concrete, steel or wood)
                               P




                            Soil
                                                             S

                     Foundation material




  f  ca   ' tan 
                                           Where,
                                           ca = adhesion,
                                                                            38
                                            = angle of internal friction
Triaxial Shear Test
                                   Piston (to apply deviatoric stress)



         Failure plane
                                              O-ring


                                             impervious
                                             membrane
Soil sample                    Soil
at failure                     sample
                                                Porous
                     Perspex                    stone
                     cell
                                              Water


          Cell pressure
         Back pressure                             Pore pressure or
                               pedestal            volume change

                                                                         39
Triaxial Shear Test
Specimen preparation (undisturbed sample)




               Sampling tubes


                                            Sample extruder
                                                          40
Triaxial Shear Test
Specimen preparation (undisturbed sample)




     Edges of the sample                    Setting up the sample
     are carefully trimmed                  in the triaxial cell
                                                                41
Triaxial Shear Test
Specimen preparation (undisturbed sample)




    Sample is covered
    with   a      rubber                    Cell is completely
    membrane and sealed                     filled with water
                                                                 42
Triaxial Shear Test
Specimen preparation (undisturbed sample)
                                  Proving ring to
                                  measure     the
                                  deviator load


                                  Dial gauge to
                                  measure vertical
                                  displacement




                                                     43
Unconfined Compression Test (UC Test)

    1 = VC + D



          3 = 0




 Confining pressure is zero in the UC test
                                             44
Unconfined Compression Test (UC Test)

   1 = VC + Df




                           Shear stress, 
         3 = 0

                           qu


                                             Normal stress, 




            τf = σ1/2 = qu/2 = cu
                                                           45
46
The End


          47

				
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