Session I. Liquefaction-induced flow slides that are governed by the residual shear strength of liquefied soil

Transcription

1 U.S. New Zealand Japan International Workshop Liquefaction-Induced Ground Movements Effects The Faculty Club, UC Berkeley, California Session I Liquefaction-induced flow slides that are governed by the residual shear strength of liquefied soil

2 Session I Schedule 9:15 Invited Presentations Steve Kramer University of Washington Roland Orense University of Auckland Y. Tsukamoto Tokyo University of Science Ross Boulanger U.C. Davis 10:15 Additional Presentations Scott Olson Gabriele Chiaro Adda Athanasopoulos-Zekkos Akihiro Takahashi Bruce Kutter Les Harder University of Illinois University of Canterbury University of Michigan Tokyo Institute of Technology U.C. Davis HDR, Inc. 10:45 Break 11:15 Discussions of identified key challenges/issues 12:45 Lunch

3 Residual Strength of Liquefied Soil Steve Kramer University of Washington Seattle, Washington

4 Stress-strain and stress path behavior - Castro (1969) q Dilation q Limited Liquefaction Limited Liquefaction Dilation Liquefaction Liquefaction γ p Steady state of deformation - Constant volume Constant effective stress Constant shearing resistance Constant velocity

5 Steady state??? Stress-strain and stress path behavior - Castro (1969) Steady state q Dilation q Limited Liquefaction Limited Liquefaction Dilation Liquefaction Liquefaction γ Steady state p Steady state of deformation - Constant volume Constant effective stress Steady state strength, S su, is function of density alone Constant shearing resistance Constant velocity

6 Normalized strength concept e If consolidation curve is parallel to SSL, then S su is proportional to σ vo. If so, then S su /σ vo = constant Consolidation curve SSL log σ vo

7 State Parameter e ψ ψ Consolidation curve SSL ψ Constant state parameter Constant level of contractivness Steady state strength increases at same rate as effective stress log σ vo

8 Normalized strength concept Typical behavior

9 Normalized strength concept Typical behavior

10 Normalized strength concept Typical behavior

11 Normalized strength concept Typical behavior

12 Normalized strength concept Typical behavior Negative state parameter dilation Increasingly positive state parameter increasing contractiveness State parameter increases with increasing effective stress Steady state strength doesn t increase as quickly as effective stress

13 Laboratory-Based Approach Common laboratory tests Triaxial test Simple Shear test Ring Shear test All have limited ability to achieve strains large enough to reach steady state of deformation: - Stresses are nonuniform, unknown - Strains are nonuniform, unknown Resulting strengths are questionable

14 Laboratory-Based Approach Peak strength Steady state??? Quasi-steady state

15 Laboratory-Based Approach Steady state???

16 Ring Simple Shear Device (RSSD) University of Washington Outer rings Inner rings

17 γ = 0% γ = 8%

18 γ = 41% γ = 69%

19 γ = 141% γ = 102% γ = 163% Single failure plane

20 Stress-Strain and Stress Path Normal Stress (psf) ~2% % % Test 8 Shear Strain (%) Shear Stress vs. Normal Stress Shear Stress (psf) Normal Stress (psf)

21 Effects of Specimen Preparation Large strain behavior similar Small strain behavior very different

22 Back-calculated residual strength Original approach Based on soil mechanics Accounts for dilation at low confining pressures Predicts same residual strength for same density S r = atm

23 Back-calculated residual strength Original approach Based on soil mechanics Accounts for dilation at low confining pressures Predicts same residual strength for same density Normalized strength approach Recognizes increased density with depth Predicts high residual strength at great depths Can predict very low residual strength at shallow depths, even for relatively high blowcount material High blowcounts, low strength

24 Back-calculated residual strength Newer approaches Based on soil mechanics Recognize increased density with depth Recognize increasing contractiveness with depth Account for lateral spreading at low confining pressures Weber (2016) Kramer and Wang (2015)

25 Back-calculated residual strength - Newer approach Comparison of predicted strengths Higher strengths at low confining pressures Kramer-Wang Similar strengths at intermediate confining pressures Lower strengths at high confining pressures

26 Issues in Residual Strength Model Development and Application Definition of residual strength Ultimate vs. quasi-steady state Stress path effects Soil fabric effects Initial stress effects None Linear dependence (proportionality) Nonlinear dependence (non-proportionality) Dynamic effects Inertial effects influence final displacements Viscosity of liquefied soil

27 Issues in Residual Strength Model Development and Application Flow slide case history investigation/documentation Variable quantity of available information Variable quality of available information Characterization of uncertainty In input parameters In predicted residual strengths Selection of penetration resistance Effects of fines content Void redistribution effects Effects of mixing and hydroplaning Extrapolation beyond bounds of data Denser soils Greater depths

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