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
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.