Undrained shear strength in the assessment of slope stability of water defenses

Transcription

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

2 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

3 Water defenses and slope stability

4 Slope instability Lekdijk Streefkerk 1984

5 Slope instability Zuiderlingedijk 2006

6 Slope instability field test Bergambacht 2001

7 Slope instability field test IJkdijk 2008

8 Field test Uitdam Dijken op Veen 2013

9 Shear strength from lab tests Dutch Cell test Triaxial test

10 Shear strength from lab tests Mohr-circles from cell test Stress paths from triaxial test

11 Current practice in NL c' tan sterkte in stabiliteitsberekening c ' n ' v Mohr Coulomb model = current practice in NL

12 Current practice in NL veen schuifsterkte [kpa] zand klei 5% rek (triaxiaalproeven) organische klei 2% rek (triaxiaalproeven) 1 à 1½ % rek (celproeven) rek [%] Shear strength determined for chosen strain level - c and depend on strain level

13 Current practice in NL t = ( v h kn/m 2 b ongedraineerd u a gedraineerd K 0 CSL s' = ( v h kn/m 2 Excess pore water pressure due to undrained shearing not in slope stability analysis

14 Critical state soil mechanics model Critical State Soil Mechanics (Schofield and Wroth, 1968) Coupling of stress, void ratio, state, compression and shear strength (Figure from Wroth, 1984)

15 Critical State Soil Mechanics model t = ( v h kn/m 2 pieksterkte Tension cut off Critical State Line K 0 residuele sterkte OCR = 1 OCR = 1,5 K ur OCR = 4 OCR = 3 s' = ( v h kn/m 2 Critical State Soil Mechanics (Schofield and Wroth, 1968)

16 Shear strength model SHANSEP s u = vi S OCR m OCR = vy / vi en POP = vy vi s u undrained shear strength(kn/m 2 ) vi in situ effective vertical stress (kn/m 2 ) S normally consolidated undrained shear strength ratio = (s u vc ) nc (-) OCR overconsolidation ratio (-) m strength increase exponent (-) vy yield stress (kn/m 2 ) POP pre overburden pressure (kn/m 2 ) SHANSEP (Stress History And Normalized Soil Engineering Properties) (Ladd et al 1974 en Ladd 1991)

17 Shear strength model SHANSEP S vy s u = v S OCR m with OCR = vy / v

18 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)

19 Undrained shear strength from CPTu Empirical correlation Lekdijk Streefkerk

20 Undrained shear strength from CPTu

21 Interpretation lab tests Triaxial tests single stage with anisotropic consolidation

22 Interpretation lab tests s u vi increases with increasing OCR At 25% axial strain no clear critical state At critical state theoretically no effect of OCR

23 Interpretation lab tests s u vc from triaxial tests OCR from oedometer tests and CRS tests s u vc increases for increasing OCR SHANSEP (Ladd et al 1974 en Ladd 1991)

24 Interpretation lab tests Friction angle increases with decreasing soil unit weight

25 Interpretation lab tests 1 0,9 0,8 0,7 cau txc piek dss bij T-piek cau txc ult dss ult 0,6 s u / ' vc (-) 0,5 0,4 0,3 0,2 0, wet [kn/m 3 ] Undrained shear strength ratio S increases with decreasing soil unit weight

26 Shear strength in WBI 2017 Application of Critical State Soil Mechanics model and SHANSEP-model Distinguish between drained and undrained soil behaviour Take into account the state of the soil (yield stress, OCR) and distinguish between normally consolidated and overconsolidated behaviour Use the shear strength at failure (ultimate state) because of different strength mobilisation in active and passive zone and differences in stiffness

27 When apply s u? When s u results in the most critical analysis (minimum SF) Apply s u for low permeable soil layers s u 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 stress levels 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 q c )

28 Validation Comparison of the operational undrained shear strength at slope failures with shear strength from lab tests (Jardine and Hight, 1987)

29 Validation Fmin [-] 4 3,8 3,6 3,4 3,2 3 2,8 2,6 2,4 2,2 2 1,8 1,6 1,4 1,2 1 0,8 0,6 0,4 0,2 0 Celproeven 38 mm monsters en 66 mm monsters ' 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 Dutch Cell test overestimates operational shear strength With CSSM and SHANSEP more realistic estimate of the safety factor Bergambacht (macrostabiliteitsproef) Deltares Lekdijk west (TAW-proefvak) Deltares IJkdijk (macrostabiliteitsproef) Deltares Gorinchem Wolpherensedijk Haskoning norm beoogde aanpak Lekdijk west (TAW-proefvak) Witteveen+Bos Heinoomsvaart (deformatie kade) Tauw Spijk Zuiderlingedijk afschuiving Deltares Streefkerk afschuiving 1984 Deltares norm incl 3D Results based on bast estimates of the schematization and shear strength

30 Validation Dikes that withstand high water levels are (just) stable with undrained shear strength

31 Thank you for your attention Questions and discussion