2011 Shakey Board and Revised Ground Motion Estimation

Procedures

Bill FraserChief, Geology BranchDivision of Safety of Dams

Background• Ground motion estimation procedure (2002)

posted on our website…WHY?• Internal consistency

• Technical discussions with owner’s people

• Guidance for less experienced owners

• Two major issues required revision

• NGA formulas

• Time histories and their modification

Issues to be Discussed (1)

• Implementation of NGA– Selection and use of formulas

– Vs30 issues

• Methods of measurement

– Downhole, OYO, refraction surveys

– SASW, REMI

• Considering site variability

– Multiple geologic units

– Topographic considerations

• Limiting values

– Special problem for dams on rock

• Estimation without measurement

Issues to be Discussed (2)

• Need for durational target parameter to constrain Time Histories

• Common targets (PGA and SA) are peak parameters

• Seed record magnitude insufficient control

– Options include:

• bracketed duration

• significant duration

• Cumulative Average Velocity (CAV)

• Arias Intensity

Issues to be Discussed (3)

• Methodology for selecting and modifying time histories for engineering analyses

– Quality control procedure is a must

• Evaluate conservatism

• Reject unrealistic time histories

• Use and acquisition of synthetic time histories for under-represented scenarios

– Magnitude 8

– Near field sites

– Hard rock site condition

Issues to be Discussed (4)

• Directivity considerations– Occurrence expectations

– Fault normal vs. non-fault normal adjustments

• Adjustments to target response spectra

• Adjustments to time histories

• Empirical NGA predictions vs. site response analyses to account for site condition

– Especially important for hard rock sites

Issues to be Discussed (5)

• Modifications to the Consequence Hazard Matrix

• Modifications to Minimum Earthquake Parameters

Guideline Revision Process

• Staff-proposed practices completely vetted internally

• Convene an expert panel

• Staff formally presents to the Board

– Workshop meeting to obtain expert advice

– Follow-up meeting if necessary

• Published revised document on our website

Internal Vetting

• Draft procedure paper containing:– Older procedures continued from past practice

– New procedures since 2002

• Obtain Geology Branch Consensus

• Obtain Design Branch Consensus

• Formal briefing for Division Chief approval

Internal Vetting

To facilitate internal discussion:

• Identified about 60 individual procedural items were embedded in the draft document

• A level of confidence assigned to each– Continue prudent past practice

– Propose new practice

– Discuss a tentative position with the experts

Selection of Safety Evaluation Earthquake (SEE) using the Consequence-Hazard Matrix

• For dam safety evaluations continue to use a maximum of 84th percentile level of design and a minimum of 50th percentile level of design from known active and conditionally active faults.

• Continue to use the Consequence-Hazard Matrix to guide risk-informed selection of the SEE ground motion

• In regions without known active faults, continue pre-specified Minimum Earthquake loading parameters

• Where the matrix allows flexibility in choosing the appropriate level of design, continue practice of achieving return periods of several thousand years whenever possible. If a level of design represents a return period of greater than 5000 years, a lower level of design is allowable.

• Propose minor revisions to three matrix scenarios to providing additional flexibility in selecting appropriate level of design

• Propose that all flood control dams with ungated outlets and limited duration of storage be analyzed at the 50th percentile regardless of TCW

• Propose that (upon request) additional ground motions be provided above and below the SEE for sensitivity analyses

Vs30 issues

• Propose empirically predicting ground motion for Vs30 greater than 180m/s

• Propose limiting Vs30 input to that value recommended by its author

• Propose Vs30 estimates (without measurement) be based on geologic characterization of the site, using Wills and Clahan (2006) as the default source of information typical for that geology

• Propose that Vs30 estimates are chosen to be representative of the site conditions as identified by geologic information in the geotechnical report and file.

• Propose that if a single Vs30 measurement cannot confidently address site variability it should be discouraged. Propose a single estimate can at best provide guidance as to where the site falls within the range of typical Vs30 values for that geology unit from the published data sources

• Propose that multiple measurements be required to determine an appropriate single Vs30 value for larger dam sites and for sites with complex site geology.

• Discuss dam site topographic considerations for obtaining Vs30 measurements

• Propose that “short interval” (Vs10 for example) measurements can be used in conjunction with conservative extrapolation to accomplish multiple measurements

• Propose that boring-based down hole and interval suspension logging are preferred methods for Vs30 measurement

• Propose spectral analysis of surface wave techniques (SASW, MASW, REMI) generally be avoided for dam site characterization if borings will be performed. Avoid cross hole surveys

• Propose that the combination of seismic refraction and boring-based measurements are useful for large sites

• Discuss if an overburden-type correction is needed for Vs30 measurements taken within an existing dam’s foundation

Arias Intensity as a Target Parameter

• Propose use of Arias Intensity as the preferred target parameter for controlling the durational aspects of a time history

• Estimation of Arias Intensity– Propose to use the TBA and WLA relationships

with equal weighting to estimate Arias Intensity with the following exclusion:

• Weighted average for hanging wall sites

• Weighted average for magnitude 6.5 and less

• Weighted average for C and D sites

– Propose a boundary site condition convention for TBA

– Propose Vs30 and Rrup limitations for WLA

– Discuss predicting 84th percentile values using the WLA formula

Design ground motion development

• Propose to modify selected seed records by spectral matching to the target response spectra and meeting the target Arias Intensity

• Propose 3 time histories as modified as above are adequate in SEE engineering analysis

• Propose developing additional time histories for sensitivity analysis with known AI both above and below the target AI

Internal Vetting

• Using this breakdown (affectionately known as the script) consensus effort began

– Numerous informal Geology Branch meetings

– Eight formal (2 hour) meetings with Design Branch

– One (1.5 hour!) meeting with Division Chief for final approval of Division position

Contracting Background

• Technical discussions and contracting for expert panel occurred in parallel

• A&E contracting process– No sole source contracting allowed

Re-advertisement possible!

“Shakey” Board History

• The name “Shakey” Board…..an urban legend

• Board used for State Water Project design between 1962 and 1979

• DSOD convened its first Shakey Board in 1988.

– 9th Meeting of the DSOD Board

Notable Issues Addressed

• 1988: Subduction Zone Hazards

• 1991 and 1997: Blind Thrust Faults

• 1994: Fault Activity Guidelines

• 1997: Minimum Earthquake Policy

• 2002: Consequence Hazard Matrix

• 1997 and 2002: Hanging Wall and Directivity

• 2001, 2002, 2005: Uses of PSHA

2011 Shakey Board

• Historically Board a “full range of expertise” panel:

– Quaternary geologist

– Engineering seismologist

– Geotechnical engineer

– Structural engineer

• 2011 Board reflects the special focus of this meeting:

– Engineering seismologist

– Modeling engineering seismologist

– Geotechnical engineer

Meeting Participants • Board Members

– Dr. Ralph Archuleta, UC Santa Barbara

– Dr. Yousef Bozorgnia, PEER Berkeley

– Dr. Jonathan Stewart, UC Los Angeles

• Presenting DSOD Staff

– David Gutierrez, Division Chief

– Bill Fraser, Geology Branch Chief

– Sharon Tapia, Design Branch Chief

– Jeff Howard, Senior Engineering Geologist

– Marvin Woods, Senior Engineering Geologist

– Chris Tracy, Engineering Geologist

– Erik Malvick, Design Engineer

– Richie Armstrong, Design Engineer

Contracting Details

• Three $15,000 2-year contracts awarded

• 2 day meeting/pus 16 hours allotted for report writing

– First meeting a $25,000 package

• Financial disclosure requirement became an issue

– Originally required to report all holding and affiliations

– Successfully argued a restricted disclosure requirement

• Holdings in engineering/geology companies

• Affiliations with Jurisdictional dam owners

• Certain undeveloped property holding

Meeting Preparation

• Each presentation rehearsed in front of team.– 2-3 hours of intense discussion followed each 1

hour presentation

• Stick to the script

• Improve the clarity

• Improve the persuasiveness of the presentation

Selected Technical Discussions

• DSOD earthquake analyses capability

• The Safety Evaluation Earthquake (SEE)

• Risk-Informed Deterministic Hazard Analysis

• NGA formula selection

• Vs30 calculation and measurement

• Time Histories in Engineering Analysis

• Arias Intensity as a Target Parameter

Dam Inventory by Type

14% Concrete

3% Rockfill

2% Other

26Source: Sharon Tapia

Click to edit the outline text format

Second Outline Level

Third Outline Level Fourth Outline

Level Fifth

Outline Level

Sixth Outline Level

Seventh Outline Level

Eighth Outline Level

Ninth Outline LevelClick to edit Master text styles

0

100

200

300

400

500

600

Years Old

> 100 Yrs

50 - 100 Yrs < 50 Yrs

Dam Inventory by Age

• Aging infrastructure

• Increased downstream development

– Greater risk

• Improvement in engineering techniques

• Better understanding of seismic hazard

27Source: Sharon Tapia

Types of Analyses Performed

• Simplified Analysis– Pseudo-static analysis (pga based coefficient)

• F.S. and Ky of Critical Slip Surface

– Liquefaction triggering assessment (pga & M)

– Newmark-Type Rigid Block Deformation Analysis (TH)

– Response Spectra Analysis of Concrete Dams (SA)

• Advanced Analysis- (TH)– Numerical Modeling - QUAD4, FLAC, SAP 2000, LS-

Dyna

28Source: Sharon Tapia

Pseudo-static Engineering Analysis

– Select Critical Section(s)

– Select Parameters:

• Geometry

• Shear strength

• Phreatic surface

• Foundation conditions

– Estimates FS and Fragility (yield acceleration) along circular failure surfaces

29Source: Sharon Tapia

Use of Advanced Engineering Analysis

• Provides information not obtainable from a simplified analysis

– Non-linear behavior of materials due to the variation in the EQ’s energy content

– Reduction of strength loss due to development of excess pore water pressure

30Source: Sharon Tapia

FLAC Results

•

(Kobe JMA 090 SM2)

Source: Richard J. Armstrong31

Safety Evaluation Earthquake (SEE)

• Historically referred to the “design earthquake”

– Sometimes referred to as:

• Maximum Design Earthquake (MDE)

• Maximum Credible Earthquake (MCE)

• As “performance-based” engineering evaluations consider multiple loadings-

– precise terminology about the loading is essential

• Propose the Safety Evaluation Earthquake (SEE) is the loading level at which the dam is analyzed to judge acceptability

Current GM Practice

• “Risk-Informed” deterministic approach

1. Use Active Fault Guidelines to determine seismic sources

2. Calculate ground motion parameters

3. Use of PSHA Comparisons and Consequence Hazard Matrix to select appropriate level of design

4. Report parameters for 1 or more controlling fault sources at the chosen level of design

5. Select appropriate seed time histories

6. Modify time histories to meet all target parameters

DeterministicLoad Selection

Risk-InformedLoad Selection

ProbabilisticLoad Selection

Performance-BasedEngineering

Risk-Based Engineering

“Risk-Informed” Deterministic

• Deterministic but….considers:• likelihood of earthquake

• consequence of dam failure

– in selecting ground motion level

Why Deterministic?

– Historically used in dam industry

• Emphasis on the fault scenario….to better predict the expected character of ground motion

– Especially important for geotechnical engineering

– Practical Issues

• 20 seismic hazard analyses per year

• Impractical to review of owner-submitted PSHA

– No agreement on return period appropriate for dams

• 3,000 -10,000 yr RP… just not feasible for California

DSHA –PSHA used together

• Simplified PSHA performed to evaluate the return period associated with the deterministic parameters

• USGS website

• EZFRISK

• A check on appropriate conservatism

2008 Formula Selection

• Use the geometric mean

• 5 formulas with equal weighting for Vs30 ≥ 450 m/sec

• 4 formulas with equal weighting for Vs30< 450 m/sec

• No Idriss

• Average better represents the epistemic uncertainty in modeling

40Source: Chris Tracy

2008 Formula Selection

Exceptions

1. Do not use Idriss on hanging wall sites

• No hanging wall term

1. Do not use Idriss for blind fault sources

• Lacks a rupture depth term

41Source: Chris Tracy

42

30 m

300

2000

500

5 m

10 m

15 m

Travel Time 5 m / 300 m/s = 0.0167 sec [38%]

10 m / 500 m/s = 0.0200 sec [45%]

15 m / 2000 m/s = 0.0075 sec [17%)]

Total = 0.0442 sec

VS30 = 30 m / 0.0442 sec

= 679 m/s

VS30 is a time-weighted average

Failure to appreciate that VS30 is a travel-time-weighted average typically leads to overestimation Source: Marvin Woods

43

Summary – VS Measurement• OYO – very reliable, but samples very small volume;

vertical travel path

• Downhole – reliable, lower resolution than OYO, but samples larger volume; vertical travel path

• Refraction – multiple line azimuths needed to account for anisotropy, non-horizontal layers; good complement to OYO and Downhole

• MASW, SASW, ReMi – based on horizontally migrating Rayleigh waves; should be confirmed with OYO or Downhole

• Crosshole – horizontal travel path inherently emphasizes thin high-V layers & overlooks horizontal discontinuities

Source: Marvin Woods

Time Histories and Dams

• 1970’s and 1980’s - a small set of synthetic time histories used for the most important analyses

– Spliced composites of existing recording

• 1990’s – well-recorded California earthquakes resulted in increasing use of the natural time histories for important analyses

• Current – Many engineering analyses use time histories. Records easily available in useable format

Evolution of Time History Modifications

– Limit modification to preserve character

• But few natural records meet design level objectives

– Simple scaling

• To PGA target

– Too conservative

• To portions of the target response spectra

– Multiple records needed

– Spectral matching (time domain)

• Satisfies spectral considerations

• Maintains character of the original record

• Reduces the number of records needed

Evolution of Time History Modification

• Historic approaches have been concerned with:

– spectral considerations

– character of a record

• Historic approaches have been much less concerned with energy considerations

– Indirectly constrained by duration

– Directly constrained by Arias Intensity

Arias Intensity

A measurement of the energy content of a time history

The integral of the acceleration time history squared over time

What is Arias Intensity?48

Ia = 5.6 m/s

Ia = 1.7 m/s

Earthquake Damage Potential

-1

-0.5

0

0.5

1

1.5

0 2 4 6 8 10 12 14

time (sec)

ac

ce

lera

tio

n (

g)

1983 M5.77 Coalinga aftershock Transmitter Hill

Earthquake Damage Potential

-1

-0.5

0

0.5

1

1.5

0 2 4 6 8 10 12 14

time (sec)

ac

ce

lera

tio

n (

g)

1994 M6.7 Northridge Newhall FireStation

Source: Jeff Howard

Spectral and AI Matching49

Source: Jeff Howard

Summary of Proposals/Actions

• Continue to develop a minimum of three design ground motions satisfying spectral and AI targets

– Provide additional w/ AI above/below target on request for sensitivity analysis

50

Thank You