slide1 n.
Download
Skip this Video
Loading SlideShow in 5 Seconds..
Soil Properties, Strength and Analysis PowerPoint Presentation
Download Presentation
Soil Properties, Strength and Analysis

Loading in 2 Seconds...

play fullscreen
1 / 38

Soil Properties, Strength and Analysis - PowerPoint PPT Presentation


  • 201 Views
  • Uploaded on

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. A few questions.

loader
I am the owner, or an agent authorized to act on behalf of the owner, of the copyrighted work described.
capcha
Download Presentation

PowerPoint Slideshow about 'Soil Properties, Strength and Analysis' - nhi


An Image/Link below is provided (as is) to download presentation

Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author.While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server.


- - - - - - - - - - - - - - - - - - - - - - - - - - E N D - - - - - - - - - - - - - - - - - - - - - - - - - -
Presentation Transcript
soil properties strength and analysis
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

a few questions
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

impact of csz on geotechnical analysis
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

site response example
Site response example

Soil Properties, Strength, and Analysis

current state of seismic geotechnical analysis
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

range of available nda methods
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

deterministic vs risk based
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

key factors in nda
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

discussion on soil parameters
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

site characterization
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

site characterization1
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

stratigraphy
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

liquefaction triggering
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

liquefaction triggering1
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

liquefaction triggering2
Liquefaction triggering

Epistemic uncertainty in CRR relationships

Idriss and Boulanger (2010)

NCEER Workshop (1997)

Cetin et al. (2004)

Soil Properties, Strength, and Analysis

crr estimates for duncan dam
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

crr estimates for duncan dam1
CRR estimates for Duncan Dam

(Boulanger and Idriss 2012)

Soil Properties, Strength, and Analysis

uncertainty in crr relationships
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

importance of fines and density on c n
Importance of fines and density on Cn

Soil Properties, Strength, and Analysis

residual strength
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

residual strength1
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

residual strength3
Residual strength

Soil Properties, Strength, and Analysis

residual strength4
Residual strength

Soil Properties, Strength, and Analysis

residual strength5
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

lateral spreading
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

lateral spreading1
Lateral spreading

Example prediction of cumulative strains

Soil Properties, Strength, and Analysis

post liquefaction settlement
Post-liquefaction settlement

Common design chart

(Ishihara and Yoshimine, 1992)

Settlements from Tohoku Earthquake

(EERI, 2011)

Soil Properties, Strength, and Analysis

cyclic failure clay like
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

cyclic failure clay like1
Cyclic failure (clay-like)

Calibration example for NDA

Cyclic triaxial tests on cohesive soil for Tuttle Creek Dam

Soil Properties, Strength, and Analysis

cyclic failure clay like2
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

cyclic failure clay like3
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

intermediate soils
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

cracking of dams and levees
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

ground improvement
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)

risk analysis
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

concluding remarks
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