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Geotechnical Aspects
 

Geotechnical Aspects

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    Geotechnical Aspects Geotechnical Aspects Presentation Transcript

    • 1
      Geotechnical Aspects of Earthquake
      Engineering
      by
      Er. Kulbir Singh gill
      Department of Civil Engg
      GURU NANAK DEV ENGG COLLEGE
      Kulbirgillkulbir@yahoo.co.in
    • Geotechnical earthquake engineering is a young branch of earthquake engineering that developed in the last two decades or so. It is concerned with geotechnical aspects of earthquake engineering such as :
      Type of soil.
      Depth of foundation soil.
      Amplification of earthquake intensity by soil deposits.
      Liquefaction of soils.
    • The subsurface information required to evaluate the liquefaction includes :
      Location of water table.
      Mean grain size D50.
      Unit weight.
      Fines content of soil (percentage of weight passing I.S. sieve size 75µ).
      SPT blow count N or tip resistance/cone bearing of a standard CPT cone (qc).
    • 4
      Major Soil Groups
      0.002
      200
      63
      2.36
      0.075
      Granular soils or Cohesionless soils
      Cohesive soils
      Boulder
      Clay
      Silt
      Sand
      Gravel
      Cobble
      Grain size (mm)
      Fine grain soils
      Coarse grain soils
    • 5
      Grain Size Distribution
      Significance of GSD:
      To know the relative proportions of different grain sizes.
      An important factor influencing the geotechnical characteristics of a coarse grain soil.
      Not important in fine grain soils.
    • 6
      Grain Size Distribution
      sieve shaker
      soil/water suspension
      hydrometer
      stack of sieves
      Sieve Analysis
      Determination of GSD:
      In coarse grain soils …... By sieve analysis
      In fine grain soils …... By hydrometer analysis
      Hydrometer Analysis
    • Grain Size Distribution Curve
      can find % of gravels, sands, fines
      define D10, D30, D60.. as above.
    • 9
      f

      Mohr-Coulomb Failure Criterion


      failure envelope
      friction angle
      cohesion
      c

      f is the maximum shear stress the soil can take without failure, under normal stress of .
    • Shear strength
      In case of clayey soil C cannot be zero, there fore shear strength of soil cannot be zero but in case of cohesion less soil C= 0.Therefore
      S = σ tanΦ
      In saturated sandy deposits
      S = ( σ- u ) tanΦ
    • 11
      What is compaction?
      A simple ground improvement technique, where the soil is densified through external compactive effort.
      Compactive effort
      + water =
    • 12
      Compaction Curve
      air
      water
      soil
      Dry density (d)
      difficult to expel all air
      lowest void ratio and highest dry density at optimum w
      Water content
      What happens to the relative quantities of the three phases with addition of water?
    • 13
      Field Compaction
      Impact Roller
      • Provides deeper (2-3m) compaction. e.g., air field
    • 14
      Compaction Control
      • a systematic exercise where you check at regular intervals whether the compaction was done to specifications.
      e.g., 1 test per 1000 m3 of compacted soil
      • Maximum dry density
      • Range of water content
      Field measurements (of d) obtained using
      • sand cone
      • nuclear density meter
    • 15
      Compaction Control Test
      d
      d,field = ?wfield = ?
      w
      Compaction specifications
      Compare!
      compacted ground
    • solution cavities in limestone
      Pounder (Tamper)
      Cratercreated by the impact
      Dynamic Compaction
      - pounding the ground by a heavy weight
      Suitable for granular soils, land fills and karst terrain with sink holes.
      (to be backfilled)
    • Pounder (Tamper)Mass = 5-30 tonneDrop = 10-30 m
      Dynamic Compaction
    • Standard penetration test
    • CORRECTIONS TO N-VALUE
      Where
      N60 = SPT N-value corrected for field procedures
      Em= hammer efficiency
      CB= bore hole diameter correction
      Cs= sampler correction
      CR= rod length correction
      N= SPT N value recorded in the field
    • Hammer efficiency is given by the manufacturer and its value is different for different type of hammers. Bore hole, sampler and rod correction factors are given in the table below:
    • LIQUEFACTION ASSESSMENT
      Liquefaction research also has produced method of assessing the susceptibility of soil to liquefaction. Most of these methods use the cyclic stress approach. This method assesses the cyclic stress ratio anticipated at the site during the certain design earthquake and compares it to that required to produce liquefaction. Here we will confine our discussion to the simplified analysis based on standard penetration test data. The procedure for evaluating the liquefaction potential of a site essentially consists of two steps.
    • Step 1
      Evaluating stress induced using the following equation
      CSReq= 0.65(a max/g) x r d x (σ v/σ’v)
      Where
      r d is stress reduction factor, a max is peak ground acceleration and g is acceleration due to gravity.
      r d = 1 – 0.000765z for z ≤ 9.15 m and
      r d = 1 – 0.0267z for 9.15 < z ≤ 23
      where z is the depth below the ground surface in meters
      The maximum horizontal acceleration a max can be determined from the graph between epicentral distance and peak horizontal acceleration.
    • Step 2The cyclic strength of soil as CRR can be determined from the graph given below:
    • contd
      If the magnitude of earthquake is not 7.5, the value of CRR obtained is to be corrected using the relation:
      (CRR)m= ψ(CRR)7.5
      The value of ψdecreases with the increase in magnitude of earthquake. The value of ψfor different magnitudes is given by the various investigators. Once the values of CSR and CRR have been obtained the factor of safety against liquefaction is CRR / CSR. If the value of factor of safety is < 1 then the soil is liquefiable otherwise it is safe against liquefaction.
    • PREVENTION OF LIQUEFATION
      The following measures can be adopted to prevent liquefaction or to limit the damages caused by the liquefaction.
      Providing deep foundations
      Compaction of soil
      Replacing the liquefiable soil
      Grouting the soil
      Ground water pumping
      Drainage of soils
      Providing stone columns
      Application of surcharge.
    • Thanks