Upload
david-miller
View
212
Download
0
Tags:
Embed Size (px)
Citation preview
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 [email protected]
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