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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
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
