Soil Properties, Strength and Analysis

2. Soil Properties, Strength and Analysis OBJECTIVE Review concerns and highlight challenges for geotechnical seismic analysis of Cascadia Subduction Zone events with a particular focus on soil properties and strength. Mt. Hood, Oregon Soil Properties, Strength, and Analysis

3. A few questions What needs to be improved for geotechnical analyses of CSZ events? How does this list differ from the analysis needs for shallow, crustal events? How does increasing the number of load cycles impact the reliability and conservatism of our analyses? Do our current characterization methods capture all of the soil properties that we need for a CSZ analysis? Soil Properties, Strength, and Analysis

4. Impact of CSZ on geotechnical analysis? In many cases, it seems pretty simple DURATION • number of load cycles & impact on soil properties • pore pressure migration and void ratio distribution during shaking • coincidence of kinematic loads from lateral spread and inertial loads from structure (e.g., deep foundations) • will conservatisms compound with each loading cycle? • will something be missed (e.g., strength loss in moderately sensitive clays? fabric disintegration in lightly cemented soils?)? Will analyses properly capture long period effects on deformation? Effect of non-stationary characteristics of ground motion Soil Properties, Strength, and Analysis

5. Site response example Soil Properties, Strength, and Analysis

6. Current state of seismic geotechnical analysis For critical infrastructure (focus on dams):  Nonlinear deformation analysis (NDA) [FLAC, etc.]  Typically 2D plane strain Perlea and Beaty (2010) Soil Properties, Strength, and Analysis

7. Range of available NDA methods Simple -- Few properties, simple stress-strain models, residual strengths imposed. Typically undrained (total stress) analysis. Often considered conservative compared to most likely behavior, but depends. Moderate -- Somewhat more sophisticated stress-strain, triggering analysis performed, no pore pressure dissipation. Often needs additional soil properties to describe degradation of stiffness and strength. Often undrained or simplified effective stress. Often considered conservative compared to most likely behavior, but depends. Complex -- Effective stress analyses considering volumetric strains due to contraction and dilation as well as pore pressure migration. Requires sophisticated parameters to define soil behavior. Conservatism is less clear due to complexity. Soil Properties, Strength, and Analysis

8. Deterministic vs Risk Based Analysis requirements for dams shifting from deterministic approach to risk-based approach Deterministic Objective is to confidently determine when a dam is “safe” • safe is not defined Risk informed Objective is to make best estimate of actual response, with an understanding of the uncertainty in the prediction Risk-informed evaluations raises standard on analysis quality Soil Properties, Strength, and Analysis

9. Key factors in NDA Earthquake history Need to develop appropriate suite • Should they all look like Tohoku 2011? • What non-stationary aspects are important for CSZ? Post-cyclic strength Residual strength? Strain-softening of clays? silts? Degradation of loading stiffness Soil Properties, Strength, and Analysis

10. Discussion on soil parameters Site characterization Liquefaction (sand-like soils) • triggering or initiation • residual strength • lateral spread • settlement Cyclic mobility (clay-like soils) • stiffness and strength loss with cycles Intermediate soils (e.g., low plasticity silts) Organic soils Foundation Inspection Trench (B.F. Sisk Dam) Soil Properties, Strength, and Analysis

11. Site characterization Sand-like soils CPT, SPT, Vs, Becker hammer Disturbed samples for classification Clay-like soils CPT, Vs, Vane shear, full flow penetrometers Disturbed samples for classification Undisturbed samples for consolidation, strength, cyclic response, and post-cyclic response (Boulanger, 2012) Soil Properties, Strength, and Analysis

12. Site characterization Difficulties the in situ challenge large particles aged, cemented, &/or residual organic soils soil strata and heterogeneity Instrumented Becker Penetration Test Device Joint UC Davis – CA DSOD (Boulanger, 2012) Soil Properties, Strength, and Analysis

13. Stratigraphy At range of scales (vertical and horizontal) Impact on response and uncertainty? How to best incorporate into analysis? How address variability within strata? Pronounced layering of dam shell(photo from Upper San Fernando Dam) Response of Lower San Fernando Dam (EQIIS, Steinbrugge collection) (Boulanger, 2012) Soil Properties, Strength, and Analysis

14. Liquefaction triggering For sand-like soils sands, low plasticity silts and silty sands, gravels (etc.) with low organic Typical analysis approach Estimate cyclic strength from penetration resistance • difficult to obtain undisturbed samples • freezing techniques? • representative? • CPT and SPT most common • Vs and BPT NCEER Workshop (1997) Soil Properties, Strength, and Analysis

15. Liquefaction triggering Earthquake Magnitude Adjustments factors for CRR Static Overburden Stress Idriss and Boulanger (2010) Static Shear Stress Idriss and Boulanger (2010) Soil Properties, Strength, and Analysis

16. Liquefaction triggering Epistemic uncertainty in CRR relationships Idriss and Boulanger (2010) NCEER Workshop (1997) Cetin et al. (2004) Soil Properties, Strength, and Analysis

17. CRR estimates for Duncan Dam Cyclic testing of frozen samples from unit 3c provides unique case for CRR comparison Initial study: Pillai and Byrne (1994); BC HydroCRR Comparison: Idriss and Boulanger (2010) Soil Properties, Strength, and Analysis

18. CRR estimates for Duncan Dam (Boulanger and Idriss 2012) Soil Properties, Strength, and Analysis

19. Uncertainty in CRR relationships Probability relationships Also consider uncertainty in (N1)60cs and CSR Implications to risk-based evaluations Cetin et al. (2004) Idriss and Boulanger (2010) Soil Properties, Strength, and Analysis

20. Importance of fines and density on Cn Soil Properties, Strength, and Analysis

21. Residual strength The minimum shear strength (Sr) that can be mobilized after liquefaction Potential factors: • critical state at large strains • void ratio redistribution and pore pressure migration • mixing of soil layers at large strains • initial overburden stress • theoretical arguments uncertain • Sror Sr/ σvo can influence deformation pattern for dams • high overburden stress (?) • silt content (?) Soil Properties, Strength, and Analysis

22. Residual strength Reliance upon empirical curves based on case histories Difficult to obtain undisturbed samples • and difficult to perform disturbance corrections Undrained laboratory tests may not capture all field behavior in some cases Soil Properties, Strength, and Analysis

23. Residual strength

24. Residual strength Soil Properties, Strength, and Analysis

25. Residual strength Soil Properties, Strength, and Analysis

26. Residual strength Rate of strength loss Some processes may require time to develop • void ratio redistribution Some failures have occurred after shaking • Lower San Fernando dam (1971) [inferred from limited information] • Mochikoshi Tailings Dam No. 2 (1978) [24 hour delay] Importance? May be more significant for denser, liquefiable Less uncertainty for very long duration(?) Lower San Fernando dam (NISEE, 2012) Soil Properties, Strength, and Analysis

27. Lateral spreading Deformations accumulate even if residual strength provides stability Considerations: Estimating strength of all materials involved in deformations • sand-like soils with high pore pressures • liquefied soils (residual strength, or dilative behavior?) • clay soils (with localization effects?) • anisotropy (compressive versus DSS versus extension) Progressive deformation through unload-reload cycles • ratchet behavior Soil Properties, Strength, and Analysis

28. Lateral spreading Example prediction of cumulative strains Soil Properties, Strength, and Analysis

29. Post-liquefaction settlement Common design chart (Ishihara and Yoshimine, 1992) Settlements from Tohoku Earthquake (EERI, 2011) Soil Properties, Strength, and Analysis

30. Cyclic failure (clay-like) Accumulation of strain with load cycles • softened loading modulus • static shear stress bias Soil properties for NDA • How does stiffness and strength of clay degrade with increasing number of cycles? • For critical structures, cyclic testing is imperative for NC and low OCR clays Soil Properties, Strength, and Analysis

31. Cyclic failure (clay-like) Calibration example for NDA Cyclic triaxial tests on cohesive soil for Tuttle Creek Dam Soil Properties, Strength, and Analysis

32. Cyclic failure (clay-like) Strain localization? e.g., Tuttle Creek Dam • Cyclic laboratory tests showed modest degradation of undrained strength • Vane shear tests showed significant strength degradation at large strain • Transition poorly defined • Potential for strain localization requires special consideration Soil Properties, Strength, and Analysis

33. Cyclic failure (clay-like) Sensitivity? When will strains accumulate sufficiently to induce strength loss in moderately sensitive soils? 4th avenue slide, Anchorage (1964) (Hansen, 1971) Assumed strength reduction (Idriss, 1985) Soil Properties, Strength, and Analysis

34. Intermediate Soils Silts and clays with PIs of ~ 5 to 18 Boulanger and Idriss (ASCE 2006) Bray and Sancio (ASCE , 2006) Soil Properties, Strength, and Analysis

35. Cracking of dams and levees Transverse cracking (dams) Analysis tools are very limited and empirical Effect of duration uncertain Assumption on depth can be significant to risk analysis due to erosion potential (EERI, 2011) (EERI, 2011) Soil Properties, Strength, and Analysis

36. Ground improvement Jet grout columns (John Dillon 2009) Piles (Sardis Dam) Analytical research (e.g.,Goughnour & Pestana 1998) Tuttle Creek Dam, USACE Stone columns (Hayward Baker, 2012)

37. Risk analysis Need best estimate of response Conservative bias may cloud usefulness of analysis result Should develop and consider probability distribution of predictions Questions for CSZ • How good are the analyses? • How does increasing the number of load cycles affect conservatism Possible fragilitycurve for liquefiable site Soil Properties, Strength, and Analysis

38. Concluding remarks Improve analysis tools (constitutive models, etc.)? A range of analysis tools are available, but there is plenty of room for continued advancement (complex load path, void ratio redistribution, 3D, etc.). Improve analysis approach? Recognize importance of sensitivity studies, transparent documentation, and model validation for NDA. Include in budget planning and analysis scope. Improve laboratory test and in situ methods? Continued adoption of cyclic DSS for clay-like and transition soils. Improve collection, documentation, and use of case histories? May be most significant current need. Soil Properties, Strength, and Analysis