Lecture21 Dynamic Soil Properties Part1

8/12/2019 Lecture21 Dynamic Soil Properties Part1

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Dynamic Soil Properties

Part - I

Lecture-21

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There are several types of geotechnical engineering problems associated

with dynamic loading, examples include:

wave propagationmachine vibrationsseismic loadingliquefaction and cyclic transient loading

The mechanical properties associated with dynamic loading are termeddynamic soil properties and are listed below.

Density ( ) : mass per unit volumeshear wave velocity (V s): At ground surface, most energy arrives in the

form of vertically propagating shear waves. Velocity of these waves insoil is measured from field/laboratory studies.

Shear modulus (G): ratio of shear stress to shear strainDamping ratio (D): ratio of actual damping coefficient to the critical

damping coefficient.Poissons ratio (n): ratio of lateral strain to longitudinal strain

Important Dynamic Properties of Soils

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Measurement of Dynamic Properties of Soils

Dynamic properties of soils can be measured from in-situ as well aslaboratory tests.

The Field or in-situ tests have the advantage that the state of stress isinherently included in the procedure. However, laboratory tests need toconfine and consolidate the soil sample back to the state of stress to

replicate field conditions. Also getting undisturbed samples for loosesoils is almost impossible and the in-situ structure of the soil getsdisturbed in lab tests. Also there are sample size effects in lab tests.

Laboratory tests are particularly useful when the responses undercontrolled conditions are needed, creating which is not possible in field.

Also to study the influence of different parameters on the response, labtests are conducted.

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Dynamic Properties of Soils: v s

Shear wave velocity (V s) is the most commonly usedparameter used in soil characterization. It is used tocalculate several other parameters like density and shear

modulus in the elastic range of soil behavior.The importance in its utility is that the particle of motiontravels perpendicular to the direction of wave propagationand we are able to measure the shear properties of the soil

skeleton and not the fluids that cannot take shear.

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Dynamic Properties of Soils: GShear Modulus (G) is a calculated parameter based on the V s using thesimple elastic relationship

Gmax = Vs2

The mass density is often estimated or measured by a nearby subsurfacesampling or using correlations.

Advanced correlations to estimate the value of the dynamic shearmodulus are available based on the standard penetration test, AtterbergLimits (plasticity index) and grain size distributions. The shear modulus isused to perform more advanced soil modeling, and dynamic response ofthe soil-structure interactions.

Shear modulus at low strain levels as measured by geophysicaltechniques will provide the elastic parameter for machine foundationanalysis or earthquake engineering.

G can be used as a varying parameter with respect to strain, making thesoil response represent the real modulus degradation in soil behavior.

This parameter is used in defining the stiffness matrices for finiteelement analysis of earth structures and foundation soils. 5


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Dynamic Properties of Soils: D

Damping Ratio (D) is used in several dynamic analysis procedures to providea realistic motion attenuation. This ratio is based on the material dampingproperties.

The damping ratio vs. shear strain relationships for cohesionless andcohesive soils are provided by many researchers.

Since damping ratio is also shear strain dependent, it is required to haveseveral values with strain. Dynamic analysis results are also influenced bythe damping ratio. The effects of soil-structure interaction also influence thedamping of the system making it an area where recent research has focused.

The utility of this parameter is based on the ability of the system to absorbdynamic energy and how this will affect the duration and modes ofvibration.

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Dynamic Properties of Soils:

Poissons Ratio ( ) is a fundamental parameter that is difficult to measure and it isusually estimated in engineering calculations.

The ratio of horizontal to vertical strain is required to relate moduli and strains in asolid body. A suggested range of values for Poisson's ratio for soils is from 0.2 to 0.5,less common values may be as low as 0.1 for loess deposits.

This ratio can be calculated [ n = E/(2G-1)] based on laboratory tests at low strains if Gand E are obtained from torsional and longitudinal vibration respectively.

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Important Field tests for measuring dynamicproperties of soils

Low strain testsSeismic Reflection Test

Seismic Refraction Test

Suspension Logging Test

Steady-State-Vibration TestSpectral Analysis of Surface Waves (SASW) Test

Cross-Hole Test

Down-/Up-hole Test

High strain tests

Seismic Cone Penetration Test

Standard Penetration Test

Dilatometer Test

Pressuremeter Test8


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Important Laboratory tests for measuring dynamicproperties of soils

Element Tests

Resonant Column Test

Ultrasonic Pulse Test

Piezoelectric Bender Element TestCyclic Triaxial Test

Cyclic Simple Shear Test

Cyclic Torsion Test

Model Tests

Shake Table Test

Centrifuge Test

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Field Tests: Low Strain Test

(a) Vert ical Im pact (b) Shallo w Explo sive,

(c) Horizont al Im pact (d) Frequen cy-Con tro lled Surface Waves.

A source produces a pulse of waves, whose times of arrival aremeasured by receivers. The commonly used sources are hammer blowand explosive charges.

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Consideration of ground water table is very important for properinterpretation of results from these tests.

P-waves travel through ground water and soft saturated soils at almost samevelocity, hence making the detection of ground water very difficult

Failure to consider ground water effects results in overestimation of soilstiffness.

If the impulse wave generated is S wave, this problem is solved because, Swaves can not travel through fluids and the propagation of S waves belowGWT is only through soil skeleton, thus making proper estimation of soilproperties possible.

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Vertical hammer blows and explosive charges produce waves rich in p-wave content. Shear waves are produced by horizontal impact.

The resolution of S-waves could be improved by using reverse polarity.This can be achieved by striking a beam tightly pressed against theground surface in opposite directions and measuring the amplitudes.

Since the polarity of p-waves is not reversed, subtracting the reverserecord from the original will diminish the p-wave amplitude andenhance the S-wave amplitude.

Presence of ground water table may sometimes give inaccurateresults if p-wave velocity is considered. This can be avoided by using S-

waves which can not pass through water.

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Field Tests: Seismic Reflection Test

An impulse wave (rich in P-waves) is produced at the source SThe arrival time of P-wave is measured at receiver

The impulse produces stress waves that radiate away from the source in alldirections

Two waves are received at the receiver R1. Direct wave, which travels in direct path from S to R

2. The other wave travels downward and reflects back after striking theinterface of two layers

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2 i

S R

H

x

vp1

Wavefront

vp2

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2 i

S R

H

x

v p1

Wavefront

v p2

2 i

S R

H

x

v p1

Wavefront

v p2

Time taken for direct wave to reach the receiver = t d

Time taken for reflected wave to reach the receiver= t r

td = x/v p1

1

22

1

2

2

1

422

velocity

distance

2tan

p pr

v

x H

v

x H

t

H x

i

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Seismic Reflection

By measuring x and t r and knowing v p1 from direct wave calculations, thickness ofupper layer (H) can be determined.

If the layering is not horizontal, multiple measurements with receivers on either sideof the source are to be made to determine the layer thickness at one point and layerinclination.

22

1

2

1

22

2

1

4

xvt H

v x H t

pr

pr

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Field Tests: Seismic Refraction Test

The seismic refraction method is similar to the reflection method in that the sameinstruments and shock wave sources are used and the travel time of p or s waves ismeasured at receivers placed along the ground surface at different distances from thesource.

The advantage of refraction test over reflection test is that in refraction test, arrival time offirst wave is recorded, regardless of its path.

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Seismic Refraction

Vertical Geophones

Source(Plate)

Rock: V p2

Soil: V p1

oscilloscope

x1x2x3x4

t1t2

t3t4

zR

Determine depthto rock layer, z R

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Snells law:

The directions of all waves generated are related to the direction ofincident wave.

When the angle of incidence is i c, the refracted wave will be parallel tothe interface. This angle i c is called critical angle of incidence

ic = v1/v 2

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x

Sxc

v1

v2

Head waves

Direct waves

All receivers placed at distances greater than x c receive head waves before directwaves.

xc is the distance at which the angle of incidence of wavefront at the interface is i c.

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i c

S R

H

xn

v1

v2

i c

xn > xc

Determine H in terms of v 1, v2 and x c

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i c

S Rxn

v1

v2

i c

For a receiver placed at x n= xc,, thn = xc/v 1

12

12

2 vv

vv x H c

ci

H

cos

xn 2H tan i c

2

2

2

12

121

112

21

sin

costan2

cos

vv

H

v

xt

vv

i

iv H

vi H x

iv H

t

nhn

c

c

cn

chn

ci

H

cos

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Seismic Refraction: Travel time-distance diagram

0.000

0.005

0.010

0.015

0.020

T r a v e

l T i m

e ( s e c o n

d s

)

0 10 20 30 40 50Distance From Source (meters)

Horizontal Soil Layer over Rock

Vp1 = 1350m/s

1

Vp2 = 4880m/s

1z x

2 V V

V Vc

c p2 p1

p2 p1

Depth to Rock:

zc = 5.65 m

xc = 15.0 m

x values

t v a l u e s

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For multiple horizontal layers, travel time-distance diagram will have more than

one break in slope.The distances corresponding to break in slope and the slopes are used todetermine the thickness of layers and wave velocities in different layers.

For the case of multiple layers,

1

122

1

221

221

1

1

2

k

j k k

jk k jk k

j

j

k k

k k ck k

vv

vvvvvv

v

H

vvvv x

H

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Seismic Refraction Test: Limitations

The travel time distance curves are not exactly straight lines for realsituations of soil layers, where velocity of wave is not constant within alayer, as assumed.

Insufficient thickness of some layers may cause difficulties in detectingthem, causing blind zones

When velocity of top layer is high, refraction is not suitable.

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Inclined layers: Reverse profiling

When the boundaries between layers are not horizontal, refraction study isdone in two directions to get the thickness of layers and the inclinations.This is called Reverse Profiling

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Kramer, S.L. (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

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Lecture21 Dynamic Soil Properties Part1