Geotechnical Aspects
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1. Geotechnical earthquake engineering is concerned with assessing how soil properties influence earthquake shaking and effects like liquefaction. Key soil properties include grain size, density, and water content. 2. Evaluating liquefaction risk involves determining the soil's standard penetration test value, grain size, water content, and comparing the expected earthquake shear stresses to the soil's shear resistance. 3. Methods to prevent liquefaction include deep foundations, compacting or replacing liquefiable soils, installing drains or stone columns, dewatering, and applying surcharges.
Geotechnical Aspects
- 1. 1Geotechnical Aspects of EarthquakeEngineering byEr. Kulbir Singh gillDepartment of Civil EnggGURU NANAK DEV ENGG COLLEGEKulbirgillkulbir@yahoo.co.in
- 2. 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.
- 3. 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. 4Major Soil Groups0.002200632.360.075Granular soils or Cohesionless soilsCohesive soilsBoulderClaySiltSandGravelCobbleGrain size (mm)Fine grain soilsCoarse grain soils
- 5. 5Grain Size DistributionSignificance 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. 6Grain Size Distributionsieve shakersoil/water suspensionhydrometerstack of sievesSieve AnalysisDetermination of GSD:In coarse grain soils …... By sieve analysisIn fine grain soils …... By hydrometer analysisHydrometer Analysis
- 7. Grain Size Distribution Curvecan find % of gravels, sands, finesdefine D10, D30, D60.. as above.
- 9. 9fMohr-Coulomb Failure Criterionfailure envelopefriction anglecohesioncf is the maximum shear stress the soil can take without failure, under normal stress of .
- 10. 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. 11What is compaction?A simple ground improvement technique, where the soil is densified through external compactive effort.Compactive effort+ water =
- 12. 12Compaction CurveairwatersoilDry density (d)difficult to expel all airlowest void ratio and highest dry density at optimum wWater contentWhat happens to the relative quantities of the three phases with addition of water?
- 13. 13Field CompactionImpact RollerProvides deeper (2-3m) compaction. e.g., air field14Compaction Controla systematic exercise where you check at regular intervals whether the compaction was done to specifications.e.g., 1 test per 1000 m3 of compacted soilMaximum dry density
- 14. Range of water contentField measurements (of d) obtained using sand cone
- 15. nuclear density meter15Compaction Control Testdd,field = ?wfield = ?wCompaction specificationsCompare!compacted ground
- 16. solution cavities in limestonePounder (Tamper)Cratercreated by the impactDynamic Compaction- pounding the ground by a heavy weightSuitable for granular soils, land fills and karst terrain with sink holes.(to be backfilled)
- 17. Pounder (Tamper)Mass = 5-30 tonneDrop = 10-30 mDynamic Compaction
- 20. CORRECTIONS TO N-VALUEWhere N60 = SPT N-value corrected for field proceduresEm= hammer efficiency CB= bore hole diameter correction Cs= sampler correction CR= rod length correction N= SPT N value recorded in the field
- 21. 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:
- 22. LIQUEFACTION ASSESSMENTLiquefaction 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.
- 23. Step 1Evaluating 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 andr d = 1 – 0.0267z for 9.15 < z ≤ 23 where z is the depth below the ground surface in metersThe maximum horizontal acceleration a max can be determined from the graph between epicentral distance and peak horizontal acceleration.
- 24. Step 2The cyclic strength of soil as CRR can be determined from the graph given below:
- 25. contdIf the magnitude of earthquake is not 7.5, the value of CRR obtained is to be corrected using the relation:(CRR)m= ψ(CRR)7.5The 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.
- 26. PREVENTION OF LIQUEFATIONThe following measures can be adopted to prevent liquefaction or to limit the damages caused by the liquefaction.Providing deep foundations Compaction of soilReplacing the liquefiable soilGrouting the soilGround water pumpingDrainage of soilsProviding stone columnsApplication of surcharge.
- 27. Thanks