CE 5320 Soil Dynamics (2014)CE 5320 Soil Dynamics (2014)

D i S il P iDynamic Soil Properties-Lab TestsLab Tests

Prof. A. BoominathanDepartment of Civil Engineering

IIT M dIIT Madrasboomi@iitm.ac.in

Prof. Boominathan, IIT Madras

Laboratory measurement of dynamic soil propertiescan be used to supplement or confirm the results ofcan be used to supplement or confirm the results offield measurements.

Lab tests are necessary Lab tests are necessary to establish values of damping and modulus of

strains at larger than these that can be attained ingthe field

to measure the properties of materials that do notp pexist in the field, such as soils to be compacted.

Two types of loading: Dynamic loads. Cyclic but slow enough that inertial effects do noty g

occur.

Laboratory TestsLaboratory TestsLaboratory TestsLaboratory Tests

Low strain element testsLow strain element tests Low strain element testsLow strain element tests Resonant column testResonant column test Bender element testBender element test Bender element testBender element test

High strain element testsHigh strain element testsC li i i l C li i i l Cyclic triaxial testCyclic triaxial test

Cyclic direct simple shear testCyclic direct simple shear test Cyclic torsion shear testCyclic torsion shear test

Prof. Boominathan, IIT Madras

Resonant Column Test

This testing technique applies cyclic

force to a soil specimen at various

frequencies.

A cylindrical test specimen (solid or

hallow) is excited harmonically in

torsional or axial loading .

Fig. shows the soil specimen is fixed

at the bottom and and is subjected to

torsition at the toptorsition at the top.

The resonant (natural) frequency of

the soil element can be obtainedthe soil element can be obtained

For example, when there is no top mass, the natural period of soil

column, Tn, in torsional shear is given by

T 4H/VTn = 4H/VsWhen the resonance or the maximum response occurs at the

l di i d f ( h f f ) i h /1loading period of (or the frequency of ) in the test

this experimental value and the theoretical value are equated.

nn/1

Hence,

2)/4(,/4/,/4 THGandTHGVVH )/4(,/4/,/4 nnssn THGandTHGVVH

By equating the theoretical and experimental resonant frequencies, the shear modulus G is obtainedshear modulus, G, is obtained.

Interpretation of resonant column test The dynamic response of a specimen to this force is measured in terms of velocity and /or acceleration.

While a precise measurement of small displacement (deformation) is difficult, velocity and acceleration at a high gfrequency are large enough to be measured.

By varying the loading frequency, the variation of amplification in amplitude ofamplification in amplitude of response is plotted against the frequency. Damping from Free or

Forced vibration test data.

Lf2 )FLf(4G r2

Prof. Boominathan, IIT Madras

Typical results of R.C tests on Clay

Prof. Boominathan, IIT Madras

Limitations of Resonant Column Test

The number of loading cycles that a soil specimen experiences during tests is p p gsignificantly greater than that in real earthquakes.qThe obtained modulus and damping corresponds to those of many cycles.corresponds to those of many cycles.

R.C. tests on water saturated specimens under large strain amplitudes cause under large strain amplitudes cause liquefaction (high frequency shaking makes free drainage of pore water impossible) free drainage of pore water impossible)

BENDER ELEMENT TEST

Bender elements are constructed by bonding two piezoelectrical materialspiezoelectrical materials together in such a way that a voltage applied to their faces causes to expand while the other contracts causing the entire element to bend .

Si il l l t l di t b Similarly , a lateral disturbance of the bender element will produce a voltage , so that the bender elements can be used be de e e e ts ca be usedas both s-wave transmitters and receivers

Positive voltage causes element to bend one way way. Negative voltage causes it to bend the other.

Shearwavepropagationthroughalaboratorysoilsamplep p g g y p

Measurement of wave propagation velocity by bender elements

By measuring the time

required for the wave to travelrequired for the wave to travel

from the source to the receiver,

and knowing the distanceand knowing the distance

between each, the shear wave

velocity of the specimen can bey p

measured nondestructively.

the maximum shear modulus

Gmax (with < 10-6)

Gmax = 2sVmax sV

Prof. Boominathan, IIT Madras

Measurement of Vs in the lab Bender Element Test

LocationDepth

(m)Soil

description (soilclassificati

Gs NMC (%)

Atterberg limits (%)

LL PL PI(on)

LL PL PI

Tondiarpet 10.00 Soft clay 2.7 60 77 28 49

Siruseri 2 25 Stiff clay 2 7 34 85 26 59

Size of specimen: 50 x 100 mm2

Siruseri 2.25 Stiff clay 2.7 34 85 26 59

Waveform: sinusoidal Frequency: 5000 Hz Input voltage amplitude: 20 Vpp S/N ratio: 4 db Estimation of travel time: peak

to peak

14

Comparison of Field and Lab Test Results (Tondiarpet)

Vs = 120 m/s

V fili f b d l Wave trace

Vs profiling from bender element test

15

Measured VMeasured Vss is about 20 % and 14 % lower than the field MASW for soft is about 20 % and 14 % lower than the field MASW for soft

and stiff clay respectively.and stiff clay respectively.

Gmax for Kalpakkam sand from Bender element test Bender element test

(Jaya et al., 2008)

140

120

130

140

Dr = 20 %D = 50 %

90

100

110Dr = 50 %

Dr = 65 % Dr = 85 %

Pa)

70

80

90

Gm

ax (M

P

40

50

60

0 20 40 60 80 100 120 140 160 180 200 22030

Effective Confining Stress (kPa)Prof. Boominathan, IIT Madras

Th li t i i l t t d i ll d ib d i The cyclic triaxial test procedures is well described in ASTM D 3999:

In the triaxial test, a cylindrical specimen is placed between In the triaxial test, a cylindrical specimen is placed between top and bottom loading platens and surrounded by a thin rubber membrane. Th i i bj d di l ll The specimen is subjected to a radial stress usually applied pneumatically and an axial stress.

The difference between the axial stress is called the The difference between the axial stress is called the deviator stress.

The deviator stress is applied cyclically either under stress controlled conditions or under strain controlled conditions.

Frequency range of modern cyclic triaxial apparatus - 0.1 to 10 Hzto 10 Hz.

Simulation of earthquake loading in cyclic triaxial apparatusapparatus

Shear deformations are resulting from propagatedresulting from propagated waves

Ishihara (1996)Seed and Lee (1966)Prof. Boominathan, IIT Madras

CYCLIC TRIAXIAL TEST SETUP

Test ProcedureTest Procedure Sample Preparation Size: Diameter 50/70/100 mm Air pluviation, water pluviation and moist tamping

techniquestechniques Saturation Degree of saturation (B > 0.96)g ( )Consolidation Saturated sand specimens are consolidated to the required

ff ti i t i teffective isotropic stresses.Cyclic deformation Sine wave with a frequency of 0 1-10 Hz Sine wave with a frequency of 0.1-10 Hz. During cyclic loading the cell pressure is kept constant. The cell pressure, axial load, axial deformation and pore p , , p

water pressure are monitored using a built-in data acquisition system. Prof. Boominathan, IIT Madras

A t i l h t i l i th t t i l t bt i d f th li A typical hysteresis loop in the stressstrain plot obtained from the cyclictriaxial test.

Modulus is determined from the slope of the line joining the end points ofth lthe loop.

Damping characteristics can be calculated from the area of the hysterisiscurve which represents the energy dissipated during the strain cycle. Thedamping ratio is proportional to the ratio of the loop to the area of thedamping ratio, is proportional to the ratio of the loop to the area of the

triangle OAB

1 4

area of hysteresis looparea of triangle OAB

Typical Stress- Strain Loops from Cyclic Triaxial Tests

1 cycle

d1 d2 1 2E ( ) /( )

W

d110

d

s

W4 W

G = E/2(1+)1

2

G = E/2(1+)d2

At 1.5% (200 kPa)Deviator Stress Axial Strain

( )

Shear modulus and Damping f l kk dcurves for Kalpakkam sand

60

40

50

(MPa

)

20

30

'0'0

200kPa

100kPa

mod

ulus

(

10

20

Shea

r m

0.01 0.1 1 100

Shear strain (%)

Jaya et.al, (2008)Prof. Boominathan, IIT Madras

Anumberoffieldtestsmeasurelowstrainproperties,

particularly wave propagation velocities Among theseparticularlywavepropagationvelocities.Amongthese

testsSeismiccrosshole,MASW,andSeismicrefractionare

widelyused.

OtherfieldtestssuchasSPT,BVT,andCPLTmeasurethe

propertiesatrelativelyhigherstrainlevels.

Shear modulus G is commonly obtained from laboratory ShearmodulusGiscommonlyobtainedfromlaboratory

testssuchasresonantcolumn,benderelementandcyclic

i i ltriaxialtests.

Prof. Boominathan, IIT Madras

Prof. Boominathan, IIT Madras