lecture23 dynamic soil properties part3

8/12/2019 Lecture23 Dynamic Soil Properties Part3

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Dynamic Soil Properties

Part - III

Lecture-23

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Large Strain Tests

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Seismic Cone Penetration Test

4Source: google images


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Push the seismic CPT probe to the desired test

depth (shear wave velocities will be measuredat multiple depths in the same push.)

Generate a seismic pulse that moves throughthe soil. Data acquisition begins as soon as thehammer hits the shear plate.

The shear wave travels from the shear plate,through the soil, and excites the accelerometeron the seismic CPT probe as it moves past theprobe's position.

The accelerometer data is displayed and thenstored on a laptop computer. Multiple seismictraces are generated at each depth.

Push the probe to the next measurementdepth.

SCPT

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Source: http://www.conepenetration.com


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Seismic Cone Penetration TestThe shear wave velocity, V S, is calculated by dividing the difference in travel pathbetween two depths by the time difference between the two signals recorded.

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Piezocone

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Pressure Meter TestIt is the only field test capable of stress-strain as well as strength behaviour

9Source: http://www.geotechdata.info/geotest/pressuremeter-test


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Pressure Meter Test

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DilatoMeter Test (DMT)

Front and side view of dilatometer

The flat dilatometer is a stainlesssteel blade having a flat, circularsteel membrane mounted flush onone side as shown in figure. The

blade is connected to a controlunit on the ground surface by a pneumatic-electrical tube runningthrough the insertion rods. A gastank, connected to the control unit

by a pneumatic cable, suppliesthe gas pressure required toexpand the membrane.

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Testing Procedure

The test starts by inserting the dilatometer into the ground. Soonafter penetration, by use of the control unit, the operator inflates themembrane and takes, in about 1 minute, two readings:

1) The pressure, required to just begin to move the membrane

against the soil ("lift-off")

p0 2) the pressure, required to move the center of the membrane 1.1mm against the soil- p 1.

The blade is then advanced into the ground of one depth increment(typically 20 cm) and the procedure for taking p0 , p1 readingsrepeated at each depth.

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Dilatometer Parameters The horizontal stress index

where u0 is the pre-insertion in-situ pore pressure.

Dilatometer Modulus E D = (p 1 po)

Value of Depends upon dimensions of blade and membrane.For standard equipment, its value is 34.7.

The Material index

Dilatometer parameters are related to low strain G and alsoliquefaction resistance of soil.

'vo

oo D

u p K

oo

o D u p

p p I

1

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Standard Penetration Test

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Because of their variability, sensitivity to test procedure, anduncertainty, SPT N-values have the potential to providemisleading assessments of liquefaction hazard, if the tests arenot performed carefully.

The engineer who wants to utilize the results of SPT N-valuesto estimate liquefaction potential should become familiar withthe details of SPT sampling as given in ASTM D 1586 (ASTM,1998) in order to avoid some of the major sources of error.


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It has been suggested that the corrections should be applied accordingto the following formula:

(N1)60 = Nm Cn Ce Cb Cr Cs

Where

Nm

= SPT raw data, measured standard penetration resistance fromfield

Cn = depth correction factor

Ce = hammer energy ratio (ER) correction factor

Cb = borehole diameter correction factor

Cr = rod length correction factor

Cs = correction factor for samplers with or without liners

Standard Penetration Test: Corrections

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The procedures that relate SPT N-values to liquefaction

resistance use an SPT blow count that is normalized to aneffective overburden pressure of 100 KPa (or 1.044 tons persquare foot). This normalized SPT blow count is denoted asN1, which is obtained by multiplying the uncorrected SPT

blow count by a depth correction factor, C n.A correction factor may be needed to correct the blow countfor an energy ratio of 60%, which has been adopted as theaverage SPT energy.

Additional correction factors may need to be applied toobtain the corrected normalized SPT N-value, (N 1)60 .


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N value normalized to an overburden pressure of 1 t/ft 2

(N1) = N CN Where N is the SPT value measured

CN = over burden correction factor

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N C

Where is the effectiveoverburden stress in ton/ft 2 which is based on watertable during SPT

'0

Fig: SPT over burden correctionfactor after Liao & Whitman (1986)

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Standard Penetration Test: (N 1)60Standard penetration test results are used in evaluatingmany important dynamic properties of soils

The (N 1)60 value is especially used for estimating theliquefaction potential of sands

(N1)60 value is the N value normalized to an overburden

pressure of 1 t/ft2

and corrected to an energy ratio of 60%Energy ratio = E m/Eeff

Where E m = Measured Hammer energy

Eeff = Theoretical free-fall hammer energy(N1)60 = N CN (Em/0.60 E eff )

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Standard Penetration Test: (N 1)60

'0'

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21om

K

Fig. Relation between the cyclic stress ratio causingliquefaction and (N 1)60 (after Seed et al., 1975)

Shear modulus of the soil can beobtained from the SPT N value usingempirical correlations

Where K 0 is the coefficient of lateralearth pressure and o is theeffective vertical overburdenpressure

2/1'3/1601max 200010 mN G

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Kramer (1996) Geotechnical Earthquake Engineering, Prentice Hall.

Braja M. Das, Ramana G.V. (2010) Principles of soil dynamics, C L Engineering.

Prakash, S. (1981) Soil Dynamics, McGraw-Hill.

Kearey P., Brooks, M. Hill I. (2002) An Introduction to Geophysical Exploration,

Wiley-Blackwell.

Burger H.R, Sheehan A.F., Jones, C.H. (2006)Introduction to Applied Geophysics:

Exploring the Shallow Subsurface, W. W. Norton & Company.

http://civil.iisc.ernet.in/~madhavi/ce202/lecture3.pdf

References
http://civil.iisc.ernet.in/~madhavi/ce202/lecture3.pdfhttp://civil.iisc.ernet.in/~madhavi/ce202/lecture3.pdf

lecture23 dynamic soil properties part3

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