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Dynamic properties of sands under cyclic torsional shear Uthayakumar, Muthukumarasamy
Abstract
Dynamic properties of soils have to be well understood in order to assure stability and acceptable performance of soil structures under seismic and wave loadings. It has been found the two important dynamic properties - shear modulus and damping factor are complex functions of many variables. In order to study the influence of various factors on shear modulus and damping factor, drained cyclic torsion shear tests were carried out in the hollow cylinder torsion device using medium Ottawa A S T M C-109 sand. Effects of shear strain amplitude, stress history, effective mean normal stress (σ'm = l/3(σ-'1 +σ'2 + σ'3)), principal effective stress ratio (R = σ1/σ3), intermediate principal stress parameter (b = σ2 —σ3)/(σ1 — σ-3)), void ratio, number of cycles of loading are some of the factors studied in this thesis. During the application of cyclic shear stress σ'm ,R and b were kept constant at pre-selected values for each test. This technique allows to study the effect cr'm, R and b independently. For example, the effect of R on dynamic properties can be isolated by a series of tests on specimens that have identical σ'm and b but different levels of R and all parameters σ'm, R and b are held constant during cyclic shear application. It is shown that shear modulus increases with number of cycles of a constant amplitude cyclic shear stress when the induced shear strain is higher than a certain threshold value. The damping, however, decreases with number of cycles even at strain amplitudes less than this threshold value. There is also a threshold value of shear strain below which zero volumetric strain occurs due to cyclic shear loading, and hence no pore pressure would develop if cyclic loading was undrained. Effects of stage testing and small strain history on dynamic properties is shown to be insignificant. With decrease of void ratio, shear modulus increases and damping factor decreases. It is shown that for a given b, the void ratio factor F(e) = (2.17 —e)² ( l + e), collapses the modulus degradation curves obtained at different void ratios in to a single curve. For a given initial stress state and shear strain amplitude, shear modulus obtained at different R levels do not show any significant difference when R < 3. Damping factors, however, seems to be unaffected by the change in R at all R levels. When R < 3, shear moduli in triaxial extension condition (b = 1) are found to be less than those in triaxial compression condition (b = 0) and damping factors for b = 1 are higher than those for b = 0. Both triaxial compression and extension state of loadings yielded same values shear modulus and damping factors at large amplitude of shear strain at R = 3. Test results indicate that when b < 1, the dynamic properties are independent of intermediate principal stress. Effects of stress history due to decrease in R from 3 to 2, is significant only in the small strain range, and as the strain level increase, the effects of stress history diminishes.
Item Metadata
Title |
Dynamic properties of sands under cyclic torsional shear
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Creator | |
Publisher |
University of British Columbia
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Date Issued |
1992
|
Description |
Dynamic properties of soils have to be well understood in order to assure stability
and acceptable performance of soil structures under seismic and wave loadings. It
has been found the two important dynamic properties - shear modulus and damping
factor are complex functions of many variables.
In order to study the influence of various factors on shear modulus and damping
factor, drained cyclic torsion shear tests were carried out in the hollow cylinder torsion
device using medium Ottawa A S T M C-109 sand. Effects of shear strain amplitude,
stress history, effective mean normal stress (σ'm = l/3(σ-'1
+σ'2 + σ'3)), principal effective
stress ratio (R = σ1/σ3), intermediate principal stress parameter (b = σ2 —σ3)/(σ1 —
σ-3)), void ratio, number of cycles of loading are some of the factors studied in this
thesis.
During the application of cyclic shear stress σ'm ,R and b were kept constant at
pre-selected values for each test. This technique allows to study the effect cr'm, R and
b independently. For example, the effect of R on dynamic properties can be isolated
by a series of tests on specimens that have identical σ'm and b but different levels of
R and all parameters σ'm, R and b are held constant during cyclic shear application.
It is shown that shear modulus increases with number of cycles of a constant
amplitude cyclic shear stress when the induced shear strain is higher than a certain
threshold value. The damping, however, decreases with number of cycles even at
strain amplitudes less than this threshold value. There is also a threshold value of
shear strain below which zero volumetric strain occurs due to cyclic shear loading, and
hence no pore pressure would develop if cyclic loading was undrained. Effects of stage
testing and small strain history on dynamic properties is shown to be insignificant.
With decrease of void ratio, shear modulus increases and damping factor decreases.
It is shown that for a given b, the void ratio factor F(e) = (2.17 —e)² ( l + e), collapses
the modulus degradation curves obtained at different void ratios in to a single curve.
For a given initial stress state and shear strain amplitude, shear modulus obtained
at different R levels do not show any significant difference when R < 3. Damping
factors, however, seems to be unaffected by the change in R at all R levels. When
R < 3, shear moduli in triaxial extension condition (b = 1) are found to be less than
those in triaxial compression condition (b = 0) and damping factors for b = 1 are
higher than those for b = 0. Both triaxial compression and extension state of loadings
yielded same values shear modulus and damping factors at large amplitude of shear
strain at R = 3. Test results indicate that when b < 1, the dynamic properties are
independent of intermediate principal stress.
Effects of stress history due to decrease in R from 3 to 2, is significant only in
the small strain range, and as the strain level increase, the effects of stress history
diminishes.
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Extent |
2394051 bytes
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Genre | |
Type | |
File Format |
application/pdf
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Language |
eng
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Date Available |
2008-12-18
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Provider |
Vancouver : University of British Columbia Library
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Rights |
For non-commercial purposes only, such as research, private study and education. Additional conditions apply, see Terms of Use https://open.library.ubc.ca/terms_of_use.
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DOI |
10.14288/1.0050466
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URI | |
Degree | |
Program | |
Affiliation | |
Degree Grantor |
University of British Columbia
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Graduation Date |
1992-11
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Campus | |
Scholarly Level |
Graduate
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Aggregated Source Repository |
DSpace
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Item Media
Item Citations and Data
Rights
For non-commercial purposes only, such as research, private study and education. Additional conditions apply, see Terms of Use https://open.library.ubc.ca/terms_of_use.