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

14 november 2016

Undrained shear strength in the assessmentof slope stability of water defenses

Geotechnical Lectures Evening - TU Delft

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

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Water defenses and slope stability

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Slope instability Lekdijk Streefkerk 1984

Slope instability Zuiderlingedijk 2006

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Slope instability field test Bergambacht 2001

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Slope instability field test IJkdijk 2008

Field test Uitdam – Dijken op Veen 2013

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Shear strength from lab tests

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Dutch Cell test Triaxial test

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Stress paths from triaxial testMohr-circles from cell test

Shear strength from lab tests

Current practice in NL

c' tan

sterkte in stabiliteitsberekening

c´

'n 'v

Mohr Coulomb model = current practice in NL

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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

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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

Critical State SoilMechanics(Schofield and Wroth, 1968)

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

(Figure from Wroth, 1984)

Critical state soil mechanics model

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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

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)

Shear strength model SHANSEP

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S

’vy

su = ’v S OCRm with OCR = ’vy / ’v

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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)

Undrained shear strength from CPTu

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Empirical correlation Lekdijk Streefkerk

Undrained shear strength from CPTu

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Interpretation lab tests

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Triaxial tests single stage with anisotropic consolidation

Interpretation lab tests

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

Interpretation lab tests

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su ‘vc from triaxial tests

OCR from oedometer testsand CRS tests

su ‘vc increases forincreasing OCR

SHANSEP (Ladd et al1974 en Ladd 1991)

Interpretation lab tests

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Friction angle increases with decreasing soil unit weight

Interpretation lab tests

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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

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

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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)

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Validation

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

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

Validation

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

Thank you for your attention

Questions and discussion

14 november 2016