undrained shear strength in the assessment of slope ... shear... · undrained shear strength in the...

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14 november 2016 Undrained shear strength in the assessment of slope stability of water defenses Geotechnical Lectures Evening - TU Delft

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Page 1: Undrained shear strength in the assessment of slope ... shear... · Undrained shear strength in the assessment of slope stability of water defenses ... • For slope stability introduction

14 november 2016

Undrained shear strength in the assessmentof slope stability of water defenses

Geotechnical Lectures Evening - TU Delft

Page 2: Undrained shear strength in the assessment of slope ... shear... · Undrained shear strength in the assessment of slope stability of water defenses ... • For slope stability introduction

Context

• New safety standards and assessment rules for water defenses(WBI 2017)

• From high water exceedance frequencies to flood risk approach• Risk to loss life by flooding in whole NL: 10-5 per year• Safety standards per dike section (was before dike ring)• New hydraulic conditions• New insights for different failure mechanisms• For slope stability introduction of Critical State Soil Mechanics

model and SHANSEP model

14 november 2016

Page 3: Undrained shear strength in the assessment of slope ... shear... · Undrained shear strength in the assessment of slope stability of water defenses ... • For slope stability introduction

Water defenses and slope stability

14 november 2016

Page 4: Undrained shear strength in the assessment of slope ... shear... · Undrained shear strength in the assessment of slope stability of water defenses ... • For slope stability introduction

14 november 2016

Slope instability Lekdijk Streefkerk 1984

Page 5: Undrained shear strength in the assessment of slope ... shear... · Undrained shear strength in the assessment of slope stability of water defenses ... • For slope stability introduction

Slope instability Zuiderlingedijk 2006

14 november 2016

Page 6: Undrained shear strength in the assessment of slope ... shear... · Undrained shear strength in the assessment of slope stability of water defenses ... • For slope stability introduction

14 november 2016

Slope instability field test Bergambacht 2001

Page 7: Undrained shear strength in the assessment of slope ... shear... · Undrained shear strength in the assessment of slope stability of water defenses ... • For slope stability introduction

14 november 2016

Slope instability field test IJkdijk 2008

Page 8: Undrained shear strength in the assessment of slope ... shear... · Undrained shear strength in the assessment of slope stability of water defenses ... • For slope stability introduction

Field test Uitdam – Dijken op Veen 2013

14 november 2016

Page 9: Undrained shear strength in the assessment of slope ... shear... · Undrained shear strength in the assessment of slope stability of water defenses ... • For slope stability introduction

Shear strength from lab tests

14 november 2016

Dutch Cell test Triaxial test

Page 10: Undrained shear strength in the assessment of slope ... shear... · Undrained shear strength in the assessment of slope stability of water defenses ... • For slope stability introduction

14 november 2016

Stress paths from triaxial testMohr-circles from cell test

Shear strength from lab tests

Page 11: Undrained shear strength in the assessment of slope ... shear... · Undrained shear strength in the assessment of slope stability of water defenses ... • For slope stability introduction

Current practice in NL

c' tan

sterkte in stabiliteitsberekening

'n 'v

Mohr Coulomb model = current practice in NL

Page 12: Undrained shear strength in the assessment of slope ... shear... · Undrained shear strength in the assessment of slope stability of water defenses ... • For slope stability introduction

14 november 2016

Current practice in NL

klei

zand

5% rek (triaxiaalproeven)

2% rek (triaxiaalproeven)

1 à 1½ % rek (celproeven)

rek [%]

veen

schu

ifste

rkte

[kPa

] organische klei

Shear strength determined for chosen strain level -c’ and ’ depend on strain level

Page 13: Undrained shear strength in the assessment of slope ... shear... · Undrained shear strength in the assessment of slope stability of water defenses ... • For slope stability introduction

14 november 2016

Current practice in NL

CSLa

b gedraineerdu

K0

ongedraineerd

s' = ( v h kN/m2

t=(

vh

kN/m

2

Excess pore water pressure due to undrainedshearing not in slope stability analysis

Page 14: Undrained shear strength in the assessment of slope ... shear... · Undrained shear strength in the assessment of slope stability of water defenses ... • For slope stability introduction

Critical State SoilMechanics(Schofield and Wroth, 1968)

Coupling of stress, voidratio, ‘state’,compression and shearstrength

(Figure from Wroth, 1984)

Critical state soil mechanics model

14 november 2016

Page 15: Undrained shear strength in the assessment of slope ... shear... · Undrained shear strength in the assessment of slope stability of water defenses ... • For slope stability introduction

14 november 2016

Critical State Soil Mechanics model

Critical State Soil Mechanics (Schofield and Wroth, 1968)

Critical State LineTension cut off

residuelesterkte

pieksterkte

K0

OCR = 1OCR = 1,5

OCR = 3

Kur OCR = 4

s' = ( v h kN/m2

t=(

vh

kN/m

2

Page 16: Undrained shear strength in the assessment of slope ... shear... · Undrained shear strength in the assessment of slope stability of water defenses ... • For slope stability introduction

Shear strength model SHANSEP

su = ’vi S OCRm

OCR = ’vy / ’vi en POP = ’vy ’vi

su undrained shear strength(kN/m2)’vi in situ effective vertical stress (kN/m2)

S normally consolidated undrained shear strengthratio = (su ’vc)nc (-)

OCR overconsolidation ratio (-)m strength increase exponent (-)

’vy yield stress (kN/m2)POP pre overburden pressure (kN/m2)

SHANSEP (Stress History And Normalized Soil Engineering Properties)(Ladd et al 1974 en Ladd 1991)

Page 17: Undrained shear strength in the assessment of slope ... shear... · Undrained shear strength in the assessment of slope stability of water defenses ... • For slope stability introduction

Shear strength model SHANSEP

14 november 2016

S

’vy

su = ’v S OCRm with OCR = ’vy / ’v

Page 18: Undrained shear strength in the assessment of slope ... shear... · Undrained shear strength in the assessment of slope stability of water defenses ... • For slope stability introduction

14 november 2016

Direct Simple Shear test

• Samples supported by membrane or stacked rings• Stress conditions during test not fully clear• Different possible interpretations of the test results• Test with constant height assumed as undrained test• No measurement of pore water pressures (no back pressure)• Apply DSS test for peat (fibrous soils)

Page 19: Undrained shear strength in the assessment of slope ... shear... · Undrained shear strength in the assessment of slope stability of water defenses ... • For slope stability introduction

Undrained shear strength from CPTu

14 november 2016

Empirical correlation Lekdijk Streefkerk

Page 20: Undrained shear strength in the assessment of slope ... shear... · Undrained shear strength in the assessment of slope stability of water defenses ... • For slope stability introduction

Undrained shear strength from CPTu

14 november 2016

Page 21: Undrained shear strength in the assessment of slope ... shear... · Undrained shear strength in the assessment of slope stability of water defenses ... • For slope stability introduction

Interpretation lab tests

14 november 2016

Triaxial tests single stage with anisotropic consolidation

Page 22: Undrained shear strength in the assessment of slope ... shear... · Undrained shear strength in the assessment of slope stability of water defenses ... • For slope stability introduction

Interpretation lab tests

14 november 2016

su ‘vi increases with increasing OCRAt 25% axial strain no clear critical stateAt critical state theoretically no effect of OCR

Page 23: Undrained shear strength in the assessment of slope ... shear... · Undrained shear strength in the assessment of slope stability of water defenses ... • For slope stability introduction

Interpretation lab tests

14 november 2016

su ‘vc from triaxial tests

OCR from oedometer testsand CRS tests

su ‘vc increases forincreasing OCR

SHANSEP (Ladd et al1974 en Ladd 1991)

Page 24: Undrained shear strength in the assessment of slope ... shear... · Undrained shear strength in the assessment of slope stability of water defenses ... • For slope stability introduction

Interpretation lab tests

14 november 2016

Friction angle increases with decreasing soil unit weight

Page 25: Undrained shear strength in the assessment of slope ... shear... · Undrained shear strength in the assessment of slope stability of water defenses ... • For slope stability introduction

Interpretation lab tests

14 november 2016

0

0,1

0,2

0,3

0,4

0,5

0,6

0,7

0,8

0,9

1

9 10 11 12 13 14 15 16 17 18 19 20 21

s u/

' vc(-)

wet [kN/m3]

cau txc piek

dss bij T-piek

cau txc ult

dss ult

Undrained shear strength ratio S increases with decreasing soil unitweight

Page 26: Undrained shear strength in the assessment of slope ... shear... · Undrained shear strength in the assessment of slope stability of water defenses ... • For slope stability introduction

Shear strength in WBI 2017

• Application of Critical State Soil Mechanics model andSHANSEP-model

• Distinguish between drained and undrained soilbehaviour

• Take into account the state of the soil (yield stress,OCR) and distinguish between normally consolidatedand overconsolidated behaviour

• Use the shear strength at failure (ultimate state)because of different strength mobilisation in active andpassive zone and differences in stiffness

14 november 2016

Page 27: Undrained shear strength in the assessment of slope ... shear... · Undrained shear strength in the assessment of slope stability of water defenses ... • For slope stability introduction

When apply su?

• When su results in the most critical analysis (minimumSF)

• Apply su for low permeable soil layers• su most critical for higher stress levels and normally

consolidated and light overconsolidated soils(contractant behaviour: OCR < 2,5 à 3,0)

• Drained shear strength most critical for lower stresslevels and overconsolidated soils (dilatant behaviour:OCR > 2,5 à 3,0)

• Apply drained shear strength above phreatic level(especially in the top of the dike material with high qc)

14 november 2016

Page 28: Undrained shear strength in the assessment of slope ... shear... · Undrained shear strength in the assessment of slope stability of water defenses ... • For slope stability introduction

Validation

14 november 2016

Comparison of the ‘operational’ undrained shear strengthat slope failures with shear strength from lab tests(Jardine and Hight, 1987)

Page 29: Undrained shear strength in the assessment of slope ... shear... · Undrained shear strength in the assessment of slope stability of water defenses ... • For slope stability introduction

00,20,40,60,8

11,21,41,61,8

22,22,42,62,8

33,23,43,63,8

4

' en ' ' ' ' su su su su

celproef triaxiaalproef dss proef klei txc veen dss triaxiaalproef dss proef klei txc veen dss ADP

max rek bij tpiek max rek max rek max rek max rek max rek

Fmin

[-]

Bergambacht (macrostabiliteitsproef) Deltares Lekdijk west (TAW-proefvak) Witteveen+BosLekdijk west (TAW-proefvak) Deltares Heinoomsvaart (deformatie kade) TauwIJkdijk (macrostabiliteitsproef) Deltares Spijk Zuiderlingedijk afschuiving DeltaresGorinchem Wolpherensedijk Haskoning Streefkerk afschuiving 1984 Deltaresnorm norm incl 3Dbeoogde aanpak

Validation

Dutch Cell testoverestimates‘operational’shear strength

With CSSMand SHANSEPmore realisticestimate of thesafety factor

Celproeven

Results based on bast estimates of the schematization and shear strength

38 mm monsters en66 mm monsters

Page 30: Undrained shear strength in the assessment of slope ... shear... · Undrained shear strength in the assessment of slope stability of water defenses ... • For slope stability introduction

Validation

Dikes that withstand high water levels are (just) stable withundrained shear strength

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Thank you for your attention

Questions and discussion

14 november 2016