Effect of stress state on shear wave-velocity of Ganga sand using Bender elements Presented By : Debayan Bhattacharya Department of Civil Engineering IIT Gandhinagar 9/14/2014 IIT GANDHINAGAR 1 Project Advisor : Dr. Amit Prashant Department of Civil Engineering IIT Gandhinagar Main Author : SARASWATHI GUNDLAPALLI Department of Civil Engineering-IIT K (Sept.2007)
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Effect of stress state on shear wave-velocity of Ganga sand
using Bender elements
Presented By :
Debayan Bhattacharya
Department of Civil Engineering
IIT Gandhinagar
9/14/2014 IIT GANDHINAGAR 1
Project Advisor :
Dr. Amit Prashant
Department of Civil Engineering
IIT Gandhinagar
Main Author : SARASWATHI GUNDLAPALLI
Department of Civil Engineering-IIT K (Sept.2007)
Outline
Overview of the past work done
Important Conclusions of the study
Comparisons of Models/Justification of the model used
Isotropic Elasticity Model-Lade & Nelson
Future Work
9/14/2014 IIT GANDHINAGAR 2
Past work done by Gundlapalli.S (former Research fellow at IITK)
9/14/2014 3IIT GANDHINAGAR
A series of undrained triaxial compression tests on
i. 2% FC sand by isotropically consolidating to 100 kPa,200 kPa,300 kPa and 500
kPa.
ii. Natural composition of Ganga river sand(10% FC) by isotropically consolidating
to pi‟ = 200 kPa, 300 kPa and 500 kPa. The series was then repeated to the same
values of pi‟ but q/ pi‟ was kept at 0.85(anisotropic consolidation).
iii. Ganga sand – silt mix by isotropically consolidating to pi‟ =300 kPa with varying
fine content in the range of 2%,10%,31%,70% and 100%.
iv. Bender element readings were taken at every 2% of axial strain & it was
primarily used to measure the shear wave velocity(vs )-Cross-correlation has
been used to estimate the time-lag (phase-difference) .
Important Conclusions made from the study as conducted
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• Pore Pressure response at small strains was largely governed by pi’ –positive pore pressure
increased with pi’ ; at phase transformation (contractive to dilative response 1-2 % axial strains
); decrease in pore pressure was independent of pi’ .
• Bender-element tests data showed that vs increased with increase in pi‟ -found true for all the
tests carried out.
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200
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320
0 100 200 300 400 500 600 700
Effective mean stress(p')
Sh
ear
wav
e v
elo
cit
y(m
/sec)
pi΄=200kPa
pi΄=300kPa
pi΄=500kPa
(Shear wave velocity vs. Mean
effective stress of isotropic &
anisotropically consolidated
specimens of Ganga sand)
(*Source :Effect of Stress State & Silt-Content on
Shear Wave Velocity of Ganga Sand using Bender
Elements)
Important Conclusions made from the study as conducted
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• The failure point (often defined as either peak shear stress location or the location of
peak shear stress ratio is same as the angle of maximum obliquity (βmax).
• Peak shear stress occurred close to 24 % of axial strain (εa) & the peak of βmax
observed ≈ 10-15 % of axial strain.
• Shear wave velocity increased with the increase in initial mean effective stress and
deviatoric stress. Post peak shear stress ratio, the shear wave velocity decreased
although the mean effective stress and deviatoric stress both increased continuously
up to ultimate state.
• After anisotropic consolidation, the shear wave velocity showed sudden decrease in its
value when the specimen was sheared under undrained conditions
• Use of Isotropic Elasticity model using J2‟ effect (Lade Nelson‟s Model-Modelling the Elastic
Behavior of Granular Materials-1987)
Comparison/ Applicability of other Models to capture Elastic behaviour
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Models Limitations/Why the fail
Pa =atm. Pressure ;n = exponent =rate of variation of Ei with
σ3„ ; Ei = initial tangent modulus –power function of initially