[PPT] - 3 rd ISSMGE McClelland Lecture Cyclic soil parameters for offshore PowerPoint Presentation

SLIDE 1

3rd ISSMGE McClelland Lecture Cyclic soil parameters for

ffshore foundation design

Knut H. Andersen Norwegian Geotechnical Institute

SLIDE 2

Cyclic soil parameters for

ffshore foundation design

Main goals

Cyclic contour diagram framework Data base with contour diagrams and correlations of required parameters

SLIDE 3

Presentation

When do we need cyclic soil parameters? What parameters do we need? How does soil behave under cyclic loading? Cyclic contour diagram framework

─ Construction ─ Important parameters ─ Data base (diagrams and correlations with index parameters)

Application of contour diagrams in design Calculation procedures Slope instability due to cyclic loading Verification by prototype observations and model tests

SLIDE 4

Presentation

When do we need cyclic soil parameters? What parameters do we need? How does soil behave under cyclic loading? Cyclic contour diagram framework

─ Construction ─ Important parameters ─ Data base (diagrams and correlations with index parameters)

Application of contour diagrams in design Calculation procedures Slope instability due to cyclic loading Verification by prototype observations and model tests

SLIDE 5

Presentation

When do we need cyclic soil parameters? What parameters do we need? How does soil behave under cyclic loading? Cyclic contour diagram framework

─ Construction ─ Important parameters ─ Data base (diagrams and correlations with index parameters)

Application of contour diagrams in design Calculation procedures Slope instability due to cyclic loading Verification by prototype observations and model tests

SLIDE 6

Wave loads: The Ekofisk Oil Storage Tank - 1973

SLIDE 7

Wave loads, Frigg TCP2, 1977

H100~ 30?m

(Beryl A & Brent B Aug 1975) Drammen clay JIP 1974/75

SLIDE 8

Snorre tension leg platform (1991)

SLIDE 9

Anchoring of floaters

2004: 485 suction anchors, 50 sites 2000m water depth

SLIDE 10

Offshore wind power structures

5 4 3 2 1 5 4 3 2 1

10 m 67 m

5 4 3 2 1 5 4 3 2 1

By Per Sparrevik

SLIDE 11

Sea protection; Oosterschelde

1986

SLIDE 12

Wave loads on harbour structures - Amalfi, Italy

SLIDE 13

Ice loading on bridge pillars; Great Belt bridge, Denmark

Low frequency 10 sec High frequency 1 sec

1990

SLIDE 14

Arctic; ice loads

Drawing: Per Sparrevik

SLIDE 15

Earthquakes

Photo: Amir Kaynia

SLIDE 16

Earthquakes, slope instability

Earthquake induced slide, El Salvador, 600 dead

SLIDE 17

Foundation design topics

Cyclic bearing capacity Cyclic displacements Soil stiffness in global dynamic analyses Permanent displacements (settlements) due to cyclic loading

─ Dissipation of pore pressure due to cyclic loading ─ Increased average shear strains

Soil reactions Static capacity reduction due to cyclic loading

SLIDE 18

Presentation

When do we need cyclic soil parameters? What parameters do we need? How does soil behave under cyclic loading? Cyclic contour diagram framework

─ Construction ─ Important parameters ─ Data base (diagrams and correlations with index parameters)

Application of contour diagrams in design Calculation procedures Slope instability due to cyclic loading Verification by prototype observations and model tests

SLIDE 19

Cyclic soil parameters needed in design

Cyclic shear strength Cyclic shear modulus Permanent shear strain due to cyclic loading Pore pressure generation Recompression modulus Damping Static strength reduction due to cyclic loading

SLIDE 20

Cyclic soil parameters needed in design

Cyclic shear strength Cyclic shear modulus Permanent shear strain due to cyclic loading Pore pressure generation Recompression modulus Damping Static strength reduction due to cyclic loading

SLIDE 21

Presentation

When do we need cyclic soil parameters? What parameters do we need? How does soil behave under cyclic loading? Cyclic contour diagram framework

─ Construction ─ Important parameters ─ Data base (diagrams and correlations with index parameters)

Application of contour diagrams in design Calculation procedures Slope instability due to cyclic loading Verification by prototype observations and model tests

SLIDE 22

τ

Monotonic Cyclic

τcy τa

up

Cycle N Cycle 1

σ’

τ

Monotonic Cyclic

τcy τa

up

Cycle N Cycle 1

σ’

Monotonic Cyclic

τcy τa

up

Cycle N Cycle 1

σ’

Cyclic loading generates pore pressure

τa τa τcy time τ0

τ

τcy

SLIDE 23

Pore pressure & shear strain increase with no. of cycles

Cyclic and average shear stresses Pore pressure generation Cyclic, average and permanent shear strains up time

u

time

γ

γp γcy τa τa τcy time τ0

τ

ucy ucy τcy γcy γa

SLIDE 24

Shear strain definitions

τcy τa τ0

τ

γcy γp

γ

Cycle 1 Cycle N

γcy

γa

γcy -> Cyclic displacements and soil stiffness for dynamic analyses γa+γcy-> Total displacements γp -> Displacements after storm

Model to follow behavior during a cycle: Kaynia & Andersen (2015)

SLIDE 25

Shear strains depend on test type and τa

DSS, τa=0

τ

Time

Triaxial, τa= 0

τ

Time

τ

Time

Triaxial, τa= τcy

τcy τcy τcy τcy τcy τcy τa

SLIDE 26

Shear strains are not governed by τmax

Test

τmax τa τcy

Result

A 50 50 Failure (γ=15%) 10 cycles B 50 25 25 γp=0.8%, γcy=0.3% 2500 cycles C 50 42.5 7.5 γp=0.03%, γcy=0.02% 2500 cycles

τ (kPa)

Tid +50

50

A B C

Triaxial

SLIDE 27

Soil elements follow different stress paths

τ H W’ h

Triax ext. DSS DSS

Time

τa τa τa τ τ τ

Time

Triax comp.

SLIDE 28

Presentation

When do we need cyclic soil parameters? What parameters do we need? How does soil behave under cyclic loading? Cyclic contour diagram framework

─ Construction ─ Important parameters ─ Data base (diagrams and correlations with index parameters)

Application of contour diagrams in design Calculation procedures Slope instability due to cyclic loading Verification by prototype observations and model tests

SLIDE 29

No. of cycles to failure depends on τa and τcy

τ

cy /s u DSS

τ

a/s u DSS

0.25 0.5 1.0 0.75 0.75 0.25 0.5 1.0 = N f γ

p / γ cy

τ

cy /s u DSS

τ

a/s u DSS

0.25 0.5 1.0 0.75 0.75 0.25 0.5 1.0 = N f γ

a / γ cy

τ

a/s u DSS

τ

cy /s u DSS

0.25 0.5 1.0 0.75 0.75 0.25 0.5 1.0

N f=10 100 1000

= γ p / γ cy

τ

a/s u DSS

τ

cy /s u DSS

0.25 0.5 1.0 0.75 0.75 0.25 0.5 1.0

N f=10 100 1000

= γ p / γ cy

τ

cy /s u DSS

0.25 0.5 1.0 0.75 0.75 0.25 0.5 1.0

N f=10 100 1000

= γ a / γ cy

DSS

time

γ

γcy

γa γcy

τa

τcy

time

τ0

τ

τcy

Drammen Clay, OCR=1

SLIDE 30

10000 1.5 1.0 0.5 0.0 1 10 100 1000 τcy/su

DSS

log N

Test 1 Test 2

τa = 0

3 3 0.5 1 15 15

3 15 1 0.5 = γcy (%)

DSS

time

γ

γcy

γa γcy

τa

τcy

time

τ0

τ

τcy

Drammen Clay, OCR=1

No. of cycles to failure depends on τa and τcy

τ

a/s u DSS

τ

cy /s u DSS

0.25 0.5 1.0 0.75 0.75 0.25 0.5 1.0

N f=10 100 1000

= γ p / γ cy

τ

a/s u DSS

τ

cy /s u DSS

0.25 0.5 1.0 0.75 0.75 0.25 0.5 1.0

N f=10 100 1000

= γ p / γ cy

τ

cy /s u DSS

0.25 0.5 1.0 0.75 0.75 0.25 0.5 1.0

N f=10 100 1000

= γ a / γ cy

SLIDE 31

= γa / γcy

Nf=10 100 1000

15/0.1 15/0.5

15/0.5
15/15

15%/15%

0.5/15

0/15

τa/su

C

0.25 0.5 1.0 0.75

τcy/su

C

0.5

0.25
0.5

0.25

τ0

Triaxial

time

γ

γcy

γa γcy

τa

τcy

time

τ0

τ

τcy

Drammen Clay, OCR=1

No. of cycles to failure depends on τa and τcy

SLIDE 32

Cyclic shear strength is τf,cy= τa+τcy

τa τcy

time

τ0

τ

τf,cy=τa+τcy

τa/su

DSS

τcy/su

DSS

0.25 0.5 1.0 0.75 0.75 0.25 0.5 1.0

Nf=10 100 1000

= γp / γcy

τa/su

DSS

τcy/su

DSS

0.25 0.5 1.0 0.75 0.75 0.25 0.5 1.0

Nf=10 100 1000

= γp / γcy

τcy/su

DSS

0.25 0.5 1.0 0.75 0.75 0.25 0.5 1.0

Nf=10 100 1000

= γa / γcy

DSS

τf,cy/su

DSS

τa/su

DSS

0.25 0.5 1.0 0.75 0.75 0.25 0.5 1.0 1.25 1.5

Nf=1 100

0/15 0.5/15 3/15 15%/1% 15/15 15/0.25 15/0

10 1000

τf,cy/su

DSS

τa/su

DSS

0.25 0.5 1.0 0.75 0.75 0.25 0.5 1.0 1.25 1.5

Nf=1 100

0/15 0.5/15 3/15 15%/1% 15/15 15/0.25 15/0

10 1000

= γa / γcy

SLIDE 33

Cyclic shear strength is τf,cy= τa±τcy

τf,cy

C=τa+τcy

τa τcy

tid

τ0

τ

τf,cy

E=τa-τcy

τcy

τf,cy

C=τa+τcy

τa τcy

tid

τ0

τ

τf,cy

E=τa-τcy

τcy τa τcy

tid

τ0

τ

τf,cy

E=τa-τcy

τcy

τa,f/su

C

0.25 0.5 1.0 0.75

τf,cy/su

C

0.5

0.25
0.5

1.5 1.0

Extension Compression

15 / 0

Static Ext 15 / 0 Static Comp

15/0.1
15/0.5
15/15
0.5/15

0/15 0/15 15/15 0.5/15 15%/0.5% 15/0.1

γa / γcy=

Nf=1 10 100 1000 Nf=1 1000 100 10

τ0

= γp / γcy

Nf=10 100 1000

15/0.1 15/0.5

15/0.5
15/15

15%/15%

0.5/15

0/15

τa,f/su

C

0.25 0.5 1.0 0.75

τcy,f/su

C

0.5

0.25
0.5

0.25

τ0

= γa / γcy

Nf=10 100 1000

15/0.1 15/0.5

15/0.5
15/15

15%/15%

0.5/15

0/15

τa,f/su

C

0.25 0.5 1.0 0.75

τcy,f/su

C

0.5

0.25
0.5

0.25

τa,f/su

C

0.25 0.5 1.0 0.75

τcy,f/su

C

0.5

0.25
0.5

0.25

τ0

Triaxial

SLIDE 34

Shear strains, DSS tests, N = 10

DSS

Drammen Clay, OCR=1

τcy τa τ0

τ

γcy γp

γ

Cycle 1 Cycle N

γcy

γa

Static test data Cyclic test data N=10

= γa/γcy

0,00 0,25 0,50 0,75 1,00

τa /su

DSS

0,00 0,25 0,50 0,75 1,00

τ

cy /su DSS 0,52 0,38 0,25 0,78 0,1 0,08 0,25 0,58 4 0.03/ 1.07/

0.06/

2.87/ 0.35/ 0.64/ 1.04/ 5.76/ 27/ 0,25 2 0,5 3 1 5 15

Failure envelope

N=10

γa (%) =

= γcy (%)

0,00 0,25 0,50 0,75 1,00

τa /su

DSS

0,00 0,25 0,50 0,75 1,00

τ

cy /su DSS

0,1 1 0,25 3 0,5 15 0,25 3 0,5 5 1 15 2

SLIDE 35

Shear strains; DSS tests, N = 10 & 100

DSS

N=100 = γcy (%)

γa (%) =

Failure envelope 0.00 0.25 0.50 0.75 1.00

τa /su

DSS

0.00 0.25 0.50 0.75

τ

cy /su DSS

0.25 3 1 0.1 0.5 15 3 5 0.25 1 2 0.5 15

SLIDE 36

N=100 = γcy (%)

γa (%) =

Failure envelope 0.00 0.25 0.50 0.75 1.00

τa /su

DSS

0.00 0.25 0.50 0.75

τ

cy /su DSS

0.25 3 1 0.1 0.5 15 3 5 0.25 1 2 0.5 15

Shear strains; DSS tests, N = 10 & 100

N=100 N=10

10000 1.5 1.0 0.5 0.0 1 10 100 1000 τcy/su

DSS

log N

Test 1 Test 2

τa=0

3 3 0.5 1 15 15

3 15 1 0.5 = γcy (%)

DSS

SLIDE 37

τcy/su

DSS

τa/su

DSS

1.0 1.0 0.5 0.0 0.5 0.0 N=1, 10, 100

γcy=15%

3 1 0.5 0.25 0.1

γa=0.25%

0.5 1 2 3 5 15

Nf=1000 100

10

1.0 0.5 0.0 0.5 1.0 0.0

τa,f/su

DSS

τcy,f/su

DSS

3D representation, DSS

SLIDE 38

Shear strains; Triaxial tests, N = 10 & 100

γa (%) =

γcy (%) =

N=10 Failure envelope

0,5

0,5 1

τa/su

C

0,25 0,5

τ

cy/su C

0,5 0,05 1 0,1 5 0,25 15

15
4

0,25

1,5

1

0,5

15

0,25

γa (%) =

γcy (%) =

N=100 Failure envelope

0,5

0,5 1

τa/su

C

0,25 0,5

τ

cy/su C

0,5 0,05 1 0,1 5 0,25 15

15
4

0,25

1,5

1

0,5

15

0,25

Triaxial

τcy τa τ0

τ

γcy γp

γ

Cycle 1 Cycle N

γcy

γa Drammen Clay, OCR=1

SLIDE 39

Pore pressure due to cyclic loading, DSS, N = 10

τ H W’ h

Triax ext. DSS DSS

Time

τa τa τa τ τ τ

Time

Triax comp.

τ H W’ h H W’ h

Triax ext. DSS DSS

Time

τa τa τa τ τ τ

Time

Triax comp.

SLIDE 40

Use of contour diagrams

Used directly in design
Basis to develop constitutive models
Check of constitutive models
Framework for specification and interpretation
f site specific cyclic laboratory tests

SLIDE 41

Prepared by Ana Page

NGI contour diagram - Cake version

SLIDE 42

NGI contour diagram – Basket ball version

Linda Hårvik

SLIDE 43

Presentation

When do we need cyclic soil parameters? What parameters do we need? How does soil behave under cyclic loading? Cyclic contour diagram framework

─ Construction ─ Important parameters ─ Data base and (diagrams and correlations with index parameters)

Application of contour diagrams in design Calculation procedures Slope instability due to cyclic loading Verification by prototype observations and model tests

SLIDE 44

Some important parameters for contour diagrams

Stress vs. strain controlled cyclic loading Load period Preshearing Drained vs. undrained ∆τa

SLIDE 45

Stress controlled and strain controlled tests give different strain contours

DSS tests with τa=0

SLIDE 46

Effect of load period - clay

Nf = f (T) for given τcy

SLIDE 47

Effect of preshearing – Sand and silt (OCR=1)

Effect may be negative (loosening of dense sand) if preshearing causes large γ

SLIDE 48

Effect of preshearing - Clay

25 50 75 100 up (kPa) 200 400 600 800 1000 Number of cycles 0.1 0.2 0.3 γcy (%)

Drammen Clay OCR=1 σvc'=400 kPa

20

20 40 60 up (kPa) 100 200 300 400 Number of cycles 1 2 3 4 γcy (%)

Drammen Clay OCR=4 σvc'=100 kPa

DSS τa=0 τcy~0.5∙su

SLIDE 49

Cyclic shear strength - Drained vs. undrained ∆τa

Sand & silt:

∆τa can be drained or undrained, depending on

variation of ∆τa with time
drainage path
consolidation characteristics

Clay:

∆τa is undrained

SLIDE 50

Cyclic shear strength - Drained vs undrained ∆τa - DSS

Undrained ∆τa

0.2 0.4 0.6 0.8 1

τa/σvc'

0.0 0.5 1.0 1.5 2.0

τ

cy/σvc'

Nf=1 10 25 100 1000

Drained ∆τa

0.2 0.4 0.6 0.8 1

τa/σvc'

0.0 0.5 1.0 1.5 2.0

τ

cy/σvc'

Nf=1 10 25 100 1000

Effect will be opposite for loose sand

Dense sand, OCR=1

SLIDE 51

Cyclic shear strength - Drained vs undrained ∆τa - Triaxial

Undrained ∆τa

3
2
1

1 2 3

τa/σvc'

0.5 1 1.5 2

τ

cy/σvc'

Nf=1 1 10 10 25 100 1000

2.6
0.2

1.2

Drained ∆τa

0.4

3
2
1

1 2 3

τa/σvc'

0,5 1 1,5 2

τ

cy/σvc'

Nf=1 10 25 100 1000

Dense sand, OCR=1

SLIDE 52

Cyclic shear strength - Drained vs. undrained ∆τa

Undrained ∆τa , N=10

τf,cy

C /σvc’ = (1.47+1.83)=3.3

τf,cy

E /σvc’ = (0.5+1.2)=1.7

τf,cy

DSS/σvc’ = (0.47+0.71)=1.2

Drained ∆τa ,N=10

τf,cy

C /σvc’ = (1.05+1.25)=2.3

τf,cy

E /σvc’ = (0.12+0.58)=0.7

τf,cy

DSS/σvc’ = (0.4+0.58)=0.98

τcy/∆τa = 1.5

Drained/Undrained Average ratio ~65%

Compression: 0.70 Extension: 0.41 DSS: 0.82

τcy/∆τa = 1.5

SLIDE 53

Cyclic shear strength - Drained vs. undrained ∆τa

P

C E DSS

P

DSS C E

Average τf (Drained ∆τa/Undrained ∆τa)= 1/3∙(0.7/1.7+1/1.2+0.55/3.3) = 0.3 Average τf (Drained ∆τa/Undrained ∆τa)= 1/3∙(2.2/3.3+1/1.2+1.7/1.7) = 0.83

Example: Dense sand, N=10, τcy/∆τa=1.5

Failure defined as γ=5%

SLIDE 54

Presentation

When do we need cyclic soil parameters? What parameters do we need? How does soil behave under cyclic loading? Cyclic contour diagram framework

─ Construction ─ Important parameters ─ Data base (diagrams and correlations with index parameters)

Application of contour diagrams in design Calculation procedures Slope instability due to cyclic loading Verification by prototype observations and model tests

SLIDE 55

Strategy to establish cyclic contour diagrams

τa,f/su

DSS

τcy,f/su

DSS

0.25 0.5 1.0 0.75 0.75 0.25 0.5 1.0

Nf=10 100 1000

= γp / γcy 0/15 . 5 / 1 5 3/15 15%/1% 15/15 15/0.25 15/0

τa,f/su

DSS

τcy,f/su

DSS

0.25 0.5 1.0 0.75 0.75 0.25 0.5 1.0

Nf=10 100 1000

= γp / γcy 0/15 0.5/15 3/15 15%/1% 15/15 15/0.25 15/0

1 DSS 2 3 4 5

= γp / γcy

Nf=10 100 1000

S t a t i c C

m

p 1 5 / 15/0.1 15/0.5

15/0.5
15/15

15%/15%

0.5/15

0/15 Static Ext

15 / 0

τa,f/su

C

0.25 0.5 1.0 0.75

τcy,f/su

C

0.5

0.25
0.5

0.25

τ0

= γp / γcy

Nf=10 100 1000

S t a t i c C

m

p 1 5 / 15/0.1 15/0.5

15/0.5
15/15

15%/15%

0.5/15

0/15 Static Ext

15 / 0

τa,f/su

C

0.25 0.5 1.0 0.75

τcy,f/su

C

0.5

0.25
0.5

0.25

τa,f/su

C

0.25 0.5 1.0 0.75

τcy,f/su

C

0.5

0.25
0.5

0.25

τ0

1 C E 3 2 4 5

Find contour diagrams for similar soil from database or establish contour diagrams from correlations Perform monotonic test(s) and 3 cyclic tests to see if results match with contours in existing data base Supplement with more cyclic tests if necessary

(depends on match, consequence and if full diagrams are needed)

SLIDE 56

Contour diagrams - Sand & silt - Examples

1 10 100 1000 log N 0.0 0.1 0.2 0.3

τ

cy/σ ref'

1 15 5 0.25 7.5 0.5 2.5 0.1

Labels: γcy(%)

(τ

cy,f /σ'ref)N=10=0.19

Cyclic shear strain, γcy (%)

1 10 100 1000 log N 0.1 0.2 0.3

τ

cy/σ ref'

1 0.025 0.05 0.25 0.1 0.5

(τ

cy,f /σ'ref)N=10=0.19

= up/σ

ref'

Failure line; γcy=15%

Permanent pore pressure. up/σ’ref

γ and up = f(τcy, N) DSS, Dr~65%. DSS and triaxial with various Dr in paper. σref’=pa∙(σvc’/pa)n

SLIDE 57

Contour diagrams - Sand & silt - Examples

Cyclic shear strength, Dr~65%. Various Dr in paper.

Undrained ∆τa (%) Drained ∆τa (%)

0.6
0.2

0.2 0.6 1.0

τa/σvc'

0.0 0.2 0.4 0.6

τ

cy/σvc'

1000 25 10 100 Nγ=10%=1

0.4

0.0 0.4 0.8

τa/σvc'

0.0 0.2 0.4 0.6

τ

cy/σvc'

10 100 25 1000 Nγ=10%=1

Triaxial DSS

SLIDE 58

Contour diagrams - Sand & silt - Examples

Average and cyclic shear strains, γa and γcy , Dr~65%, N=10. Various Dr and N=1, 10 and 100 in paper.

Undrained ∆τa (%) Drained ∆τa (%)

DSS Triaxial

0.4

0.0 0.4 0.8

τa/σvc'

0.0 0.2 0.4 0.6 0.8

τ

cy/σvc'

10
1-0.25

1 2.5 5 10 10 0.5 0.1 0.05

Average Cyclic

γcy(%)= γa(%)=

0.6
0.2

0.2 0.6 1.0

τa/σvc'

0.0 0.2 0.4 0.6 0.8

τ

cy/σvc'

Average Cyclic

10
5 -2.5 -0.25

0 0.25 0.5 1 2.5 5 10 10 2.5 10.5 0.25 0.1 0.05

γa(%)= γcy(%)=

SLIDE 59

Static shear strength correlation - Clay

400 800 1200

σvc' (kPa)

0.1 0.2 0.3 0.4 0.5

τ

f/σvc'

Ip<25 Ip>25 Ip=25%

σref’=pa∙(σvc’/pa)n

pa=100kPa n=0.9: static strength of clay n=0.1 - 0.9: static strength of sand & silt n=0.9: cyclic strength of sand, silt & clay

DSS OCR=1

20 40 60 80

Ip (%)

0.1 0.2 0.3 0.4

τ

f /σ ref'

Static DSS Shear Strength >10% clay content n=0.9

For low Ip, also consider Figures 10.1 and 10.2 High Mean Low

For low Ip, also consider correlation for sand/silt

SLIDE 60

Cyclic shear strength correlation - Clay

20 40 60

Ip (%)

0.4 0.6 0.8 1.0 1.2

τ

f,cy/su DSS

Labels: OCR Best fit: 0.41*Ip**0.224

~1 1 1 ~1 1 1 4 1 1 p0' 40 1 11 p0' 4 p0' 1 1 1 ~1 1 1 1 1 ~6 3.5

DSS N=10 τa = 0

SLIDE 61

Static shear strength correlations - Sand & silt

DSS OCR=1

20 40 60 80 100 120

Dr,after (%)

0.1 1 10

τ

f/σ ref'

21 5 2 1 39 20 3 39 20 8 39 6 6 6 6 6 6 8 9 9 7 8 7 7 2 10 2 18 2 10 1 11 1 7 1 11 26 26 5

DSS <5% fines DSS 20% fines DSS 35% fines

σ'ref=pa(σvc'/pa)n

pa=100kPa

Static shear strength <10% clay content τ

f/σ ref' n

<0.75 0.9 0.75-1.5 0.7 1.5-5 0.4 >5 0.1

Labels: Fines Content 10 20 30 40

wafter (%)

9 8 8 1 3 2 8 7 39 2 7 7 39 20 18 50 6 20 11 50 6 2 11 30 6 2 26 31 6 10 26 15 6 10 5 23 6 7 5 30 21 45 39 1 9 1 1

DSS <5% fines DSS 20% fines DSS 35% fines

SLIDE 62

Static shear strength correlation - Anisotropy

Similar anisotropy ratios for cyclic strength

10 20 30 40

wafter (%)

0.1 1 10

τ

f/σ ref'

3 2 20 20 2 9 1 2 3 20 20 1 2 2 2 9 2 1 45 9 2 2 46 2 24 25 40 45 2

DSS <5% fines DSS 20% fines DSS 35% fines

CAUC Static <10% clay CAUE Static <10% clay

σ'ref=pa(σvc'/pa)n

pa=100kPa

Static shear strength <10% clay content τ

f/σ ref' n

<0.75 0.9 0.75-1.5 0.7 1.5-5 0.4 >5 0.1

Labels: Fines Content

SLIDE 63

Cyclic shear strength correlation - Sand & silt

20 40 60 80 100 120

Dr,after (%)

0.1 1 10

τ

f/σ ref'

9 23 1 9 1 7 3 1 5 11 5 1 5 21 12 10 20 7 20 2 1 1 2

DSS 35% fines DSS 20% fines DSS <5% fines

Cyclic shear strength n=0.9 Labels: Fines Content

DSS N=10 <10% clay σ'ref=pa(σvc'/pa)n

pa=100kPa

10 20 30 40

wafter (%)

1 5 30 23 1 5 30 28 1 12 30 28 1 11 3 21 81 9 5 7 9 29 20 10 2 38 1 7 2 45 1 20 45

DSS 35% fines DSS 20% fines DSS <5% fines

Cyclic shear strength n=0.9 Labels: Fines Content

DSS N=10 <10% clay σ'ref=pa(σvc'/pa)n

pa=100kPa

DSS OCR=1 τa=0 N=10

SLIDE 64

Correlations for effect of N

DSS OCR=1 τa=0

SLIDE 65

Shear strength correlation – Effect of OCR

SLIDE 66

Presentation

When do we need cyclic soil parameters? What parameters do we need? How does soil behave under cyclic loading? Cyclic contour diagram framework

─ Construction ─ Important parameters ─ Data base (diagrams and correlations with index parameters)

Application of contour diagrams in design Calculation procedures Slope instability due to cyclic loading Verification by prototype observations and model tests

SLIDE 67

Equivalent number of cycles

Contours show behavior as function of N with constant τa and τcy In a storm τa and τcy vary from one wave to the next Storm can be transformed to an Equivalent number of cycles of the maximum wave, Neq , that gives the same effect as the irregular load history by

Pore pressure accumulation
Strain accumulation

SLIDE 68

Cyclic strength and stress-strain relations

Determine stress path and anisotropy by assuming:

For limit equilibrium & finite element:

− τcy/∆τa = Pcy/Pa − Strain compatibility at failure (Andersen & Lauritzsen, 1988)

Special finite element (UDCAM & PDCAM; Jostad et al, 2014 & 2015)

τcy/∆τa =1

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τcy/∆τa =1

Cyclic strength and stress-strain relations

SLIDE 70

τcy/∆τa =1

Cyclic strength and stress-strain relations

Determine stress path and anisotropy by assuming:

For limit equilibrium & finite element:

− τcy/∆τa = Pcy/Pa − Strain compatibility at failure (Andersen & Lauritzsen, 1988)

Special finite element (UDCAM & PDCAM; Jostad et al, 2014 & 2015)

SLIDE 71

Presentation

When do we need cyclic soil parameters? What parameters do we need? How does soil behave under cyclic loading? Cyclic contour diagram framework

─ Construction ─ Important parameters ─ Data base (diagrams and correlations with index parameters)

Application of contour diagrams in design Calculation procedures Slope instability due to cyclic loading Verificationby prototype observations and model tests

SLIDE 72

Verification - Brent B Condeep platform

Hs=10.3m Calculated/Measured =1.06 (rotation) and 0.71 (horizontal)

0.4 0.8 1.2 Cyclic horizontal displacement, st. dev. (mm) 10 20 30 40 Cyclic horizontal load, st. dev. (MN) 0.6 1.2 1.8 2.4 Cyclic moment, st. dev. (MNm) 0.08 0.16 0.24 Cyclic rotations, st.dev. (10-4 rad)

Brent B displacements

Measured rotations Calculated rotations Measured horizontal disp. Calculated horizontal disp.

SLIDE 73

Verification - Centrifuge tests of GBS on very dense sand

Ekofisk Tank characteristic

horizontal load: 786MN

Displacements may govern rather

than capacity

Significant negative pore pressure

underneath heel

Cavitation during Storms 3 & 4

Centrifuge tests performed by Delft Geotechnics

End Storm 3

0.4 0.8 1.2 Cyclic horizontal displacement (m) 1000 2000 3000 Cyclic horizontal load (MN)

Centrifuge test. GBS on very dense sand.

Measured seabed displ. Calculated seabed level Measured actuator level Calculated actuator level

End Storm 1 End Storm 2 End Storm 4

SLIDE 74

Verification - Snorre TLP anchor field model tests

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Verification - Snorre TLP anchor field model tests

Test Test type Predicted/measured No. failure load 1 Monotonic 1.00 2 Cyclic 1.05 3 Cyclic 1.06 4 Cyclic 1.01

Photo: Rune Dyvik

SLIDE 76

Verification - Snorre TLP anchor field model tests

1 2

Cyclic rotation (10-2 rad)

20 40 60

Cyclic load, Pcy (kN)

0.5 1

Cyclic horiziontal displacement (cm)

0.5 1

Cyclic vertical displacement (cm)

Predicted low Predicted high Measured Test 2 Measured Test 3

Predicted

SLIDE 77

Summary and conclusions

Cyclic soil behavior depends on

─ Stress path ─ Average and cyclic shear stresses

Contour diagrams

─ Convenient presentation form ─ Provide parameters for capacity, displacements and stiffness ─ Verified by backcalulated prototype and model test behavior ─ Basis to formulate and verify constitutive models ─ Framework to specify and interpret site specific laboratory tests

Data base

─ Contour diagrams for various soils and densities ─ Correlations with index parameters

Static and cyclic shear strengths
Stress-strain relations
Gmax, friction angles, consolidation characteristics

SLIDE 78

Acknowledgement

Based on more than 4 decades of research and project work at NGI Cooperation with NGI colleagues Colleagues in industry

─ Insipiring and rewarding cooperation ─ Identifying needs and challenges

Funding from Industry and Research Council of Norway

─ Project work and JIPs

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