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Geotechnical Aspects of Earthquake Engineering

Department of Civil EnggGURU NANAK DEV ENGG COLLEGE

Kulbirgillkulbir@yahoo.co.in

by

Er. Kulbir Singh gill

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).

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Major Soil Groups

0.002 200632.360.075

Grain size (mm)

BoulderClay Silt Sand Gravel Cobble

Fine grain soils

Coarse grain soils

Granular soils or Cohesionless soils

Cohesive soils

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Grain Size Distribution

• 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.

Significance of GSD:

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Grain Size Distribution

• In coarse grain soils …... By sieve analysis

Determination of GSD:

In fine grain soils …... By hydrometer analysis

Sieve Analysis Hydrometer Analysis

soil/water suspension

hydrometer

stack of sieves

sieve shaker

Grain Size Distribution Curve

• can find % of gravels, sands, fines

define D10, D30, D60.. as above.

0

20

40

60

80

100

0.001 0.01 0.1 1 10 100

Grain size (mm)

D30

sievehydrometer

D10 = 0.013 mm

D30 = 0.47 mm

D60 = 7.4 mm

sands gravelsfines

% P

as

sin

g

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Mohr-Coulomb Failure Criterion

tancf

c

failure envelope

cohesion

friction angle

f is the maximum shear stress the soil can take without failure, under normal stress of .

f

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

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What is compaction?

A simple ground improvement technique, where the soil is densified through external compactive effort.

+ water =

Compactive effort

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Compaction CurveWhat happens to the relative quantities of the three phases with addition of water?

Water content

Dry

den

sity

( d

)

soil

water

air

difficult to expel all air

lowest void ratio and highest dry density at optimum w

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Field Compaction

Provides deeper (2-3m) compaction. e.g., air field

Impact Roller

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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

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Compaction Control Test

compacted ground

d,field = ?wfield = ?

Compaction specifications

Compare!

w

d

Dynamic Compaction

- pounding the ground by a heavy weight

Suitable for granular soils, land fills and karst terrain with sink holes.

Crater created by the impact

Pounder (Tamper)

solution cavities in limestone

(to be backfilled)

Dynamic Compaction

Pounder (Tamper)Mass = 5-30 tonneDrop = 10-30 m

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:

Factor Equipment variables ValueBorehole diameter 65 – 115 mm 1.00factor 150 mm 1.05 200 mm 1.15Sampling method factor Standard sampler 1.00 Sampler without liner 1.20Rod length factor 3 – 4 m 0.75 4 – 6 m 0.85 6 – 10 m 0.95 >10 m 1.00

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 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 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 ψ decreases with the increase in magnitude of earthquake. The value of ψ for 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