Definition of failure in cyclic DSS tests on NC clay and ... Simple Shear (DSS) Tests Direct simple...
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Definition of failure in cyclic DSS tests on NC clay and presentation of shear strain contour diagrams D. Zografou, N. Boukpeti, S.M. Gourvenec & C. D. O’Loughlin 5 th International Conference on Geotechnical and Geophysical site investigation Gold Coast, Queensland 5-9 September 2016 Centre for Offshore Foundation Systems e: [email protected]
Transcript of Definition of failure in cyclic DSS tests on NC clay and ... Simple Shear (DSS) Tests Direct simple...
Definition of failure in cyclic DSS tests on NC clay and presentation
of shear strain contour diagrams
D. Zografou, N. Boukpeti, S.M. Gourvenec & C. D. O’Loughlin
5th International Conference on Geotechnical and
Geophysical site investigationGold Coast, Queensland5-9 September 2016
Centre for Offshore Foundation Systems
Outline of Presentation
• Direct Simple Shear (DSS) Tests background
• Testing objectives
• Discussion of results
• Conclusions
Slide 1
Direct Simple Shear (DSS) Tests
Direct simple
shear
from NGI website
Example of
application
Non-symmetrical DSS
τave
Symmetrical DSS
0
Slide 2
Cyclic DSS
Monotonic
Cyclicτσv’
γ=x/h
x
h
DSS test description
DSS Apparatus: Geocomp ShearTrac II
Sample Box free to move
on the horizontal
axis
Top of
sample
fixedDSS tests =constant
volume:
• no radial deformation
• specimen height
constant by adjusting
vertical stress (Δσv=Δu)
Testing stages:
1) Consolidation
2) Shearing under
monotonic or cyclic
conditions
Slide 3
Contour diagram
Input
Output
from a minimum of
3 stress-controlled
tests
Contour diagram of shear strains
τmax/
Su
failure envelope*
τcyc
τave
τmax
γmax γcyc
γave
* failure criterion in literature usually the same for symmetrical and
non-symmetrical tests
Slide 4
Testing objectives
• Produce contour diagrams for NC kaolin clay – not available in public domain
• Define shear strain criteria at failure
• Validate the criteria for both symmetrical and non-symmetrical cycling loading
Slide 5
Results from symmetrical tests
5 symmetrical cyclic
DSS tests on NC kaolin:
σv’0=150kPa,
f=0.1Hz,
up to N=1000
τmax/su=0.4 τmax/su=0.5
Failure in symmetrical cyclic DSS tests
• Failure is defined where shear strain increases rapidly
• Failure achieved through cyclic degradation
Slide 6
Failure
γmax=4% γmax=5.3%
Shear strain contour diagram
Comparison with published data shows:
• Good agreement with limited published data on nc kaolin
• As expected below shear strain contour for natural Drammen clay
Zografou et al (2016)
Slide 7
ax/S
u
failure envelope
τ max/S
u
Results from non-symmetrical tests
τmax/su=0.5 τmax/su=0.6
Failure in non-symmetrical cyclic DSS tests
• Failure achieved through accumulation of shear strains
• No clear point where failure starts, it can be defined by shear strain reaching a specified value.
• Higher shear strain failure criteria could be used for types of foundations under non-symmetrical
cyclic load with high tolerances to movement.
For all tests:
τave/su=0.2
3 non-symmetrical cyclic
DSS tests on NC kaolin
σv’0=70kPa,
f=0.1Hz,
up to N=1000
Slide 8
Symmetrical vs non-symmetrical cyclic loading
Significant
cyclic stiffness
degradation
No cyclic
stiffness
degradation
Some cyclic
stiffness
degradation
τmax/su=0.5
τave /τcyc=0.67
τmax/su=0.6
τave /τcyc=0.5
τmax/su=0.4
τave /τcyc=0
Non-symmetricalSymmetrical
Different failure mode for tests under symmetrical and non-symmetrical cyclic load.
Use of different failure criterion appears more appropriate.Slide 9
N=1
N=506 N=2 N=200N=2 N=20
Conclusions
• Is the use of a common failure criterion for symmetrical and non-symmetrical cyclic DSS tests appropriate?
• Is there potential for optimising the current design practice for foundations of subsea structures under certain cyclic loading
conditions?
Slide 10
This exercise is a research project, funded by the Australian Research Council, for the purpose of advancing geotechnical knowledge and practice for the benefit of all.
@SusanGourvenecJamesDohertySusanGourvenec
