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

e: dimitra.zografou@research.uwa.edu.au

Outline of Presentation

• Direct Simple Shear (DSS) Tests background

• Testing objectives

• Discussion of results

• Conclusions

Slide 1

dimitra.zografou@research.uwa.edu.au

Direct Simple Shear (DSS) Tests

Direct simple

shear

from NGI website

Example of

application

Non-symmetrical DSS

τave

Symmetrical DSS

0

Slide 2

dimitra.zografou@research.uwa.edu.au

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

dimitra.zografou@research.uwa.edu.au

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

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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

dimitra.zografou@research.uwa.edu.au

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

dimitra.zografou@research.uwa.edu.au

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

dimitra.zografou@research.uwa.edu.auτ m

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

dimitra.zografou@research.uwa.edu.au

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

dimitra.zografou@research.uwa.edu.au

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

dimitra.zografou@research.uwa.edu.au

e: dimitra.zografou@research.uwa.edu.au

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