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Geotechnical Investigations for ODOT Seismic Design
Tova R. Peltz, PE, CEGODOT Region 1
Geo-Hydro-Hazmat Unit
2009 ODOT Geo-Hydro-Hazmat ConferenceSeptember 23, 2009
Topics:
Brief overview of Oregon seismicity.
Review of seismic design codes in Oregon.
Seismic hazard information that should be considered for every project.
How to make a preliminary assessment of seismic hazards for a project.
What data needs to be collected in the field to conduct the necessary analyses for project design.
PNW Seismicity:
Cascadia Subduction Zone – Megathrust earthquakes occurring on the subducting Pacific Plate. Paleoseismic investigations in the PNW, historical documents describing tsunami inundation of Japanese harbors, and trans-Pacific tsunami modeling have demonstrated the occurrence of Holocene earthquakes in the region prior to the written historical record. Last recorded in 1700. Estimated 300-750 year recurrence interval between events.
Intracrustal - Typically M5-7 earthquakes, occur deep (40 to 60 km) and over 200 km from the deformation front of the subduction zone. Recorded in WA (Puget Sound) and N. CA, but historically absent in OR.
Local Crustal – Occur on shallow crustal faults throughout OR. Recurrence intervals on events largely unknown.
Tectonic map of the PNW, showing orientation and extent of CZS (from Dragert + others, 1994)
Largest Historical Earthquakes Felt in OregonDate Location Magnitude
11-23-1873
Near Brookings, OR 6.8
10-12-1877
Portland, OR 5.3
7-15-1936 Milton-Freewater, OR 6.4
4-13-1949 Olympia, WA 7.0
11-5-1962 Portland, OR 5.3
4-29-1965 Puget Sound, WA 6.8
1968 Adel, OR 5.1
4-12-1976 Maupin, OR 4.8
4-25-1992 Cape Mendocino, CA 7.0
3-25-1993 Scotts Mills, OR 5.6
2-28-2001 Nisqually, WA 6.8
6-14-2005 Near Crescent City, CA
7.0Notes:1. Data from Advanced National Seismic System (ANSS), USGS, and Johnson A. and Madin, I, 1994, Earthquake Database for Oregon, 1983 through October 25, 1993: DOGAMI Open File Report 0-94-4.2. Magnitudes are Ms, ML, mb or based on felt area of Modified Mercalli Intensity. Maximum reported magnitudes are listed on the table.
Wong, I., 2005, Low Potential for Large Intraslab Earthquakes in the Central Cascadia Subduction Zone: Bulletin of the Seismological Society of America, V.95, No.5.
Background on the Design Code:
Uniform Building Code (1997 and earlier) – Used Seismic Zones, old Site Class designations.
International Building Code (2000 and later) – Adopted new USGS hazard maps that considered seismic sources more discreetly. No more Zone 3. Site Classes A-F adopted.
AASHTO LRFD first adopted the USGS National Hazard Maps for seismic design of bridges in 2007.
The 2009 ODOT Geotechnical Design Manual (GDM) and Bridge Design and Drafting Manual (BDDM) use the AASHTO method and 2002 USGS hazard maps for design.
Which documents provide guidelines for ODOT seismic design?
Geotechnical Design Manual (June 2009)Chapter 6, Seismic Design - plus chapters for each structure type.
AASHTO LRFD Bridge Design Specifications (2009)
ODOT Bridge Drafting and Design Manual (2009)
Seismic hazard information that should be considered for every wall and/or bridge project:
1. Distance to earthquake sources, and properties of these sources.
2. Potential for: Ground accelerationSurface RuptureLiquefactionSlope instability / lateral spreadTsunamiSeiche
3. What risks do these hazards pose to the project?
4. What is the Site Class for the project?
5. What is the design response spectrum for the structure/project?
How to make a preliminary assessment of seismic hazards for a project:
Collect and review available existing geologic and subsurface information:
Boring logs from nearby projects
As builts for existing nearby projects
Well logs (http://apps2.wrd.state.or.us/apps/gw/well_log/Default.aspx)
Geologic maps and publications (DOGAMI and USGS)
Other geo-experts in the Agency
USGS Seismic Hazard Mapping Project website
Find out what faults contribute to the earthquake hazard at your site. Use USGS Earthquake mapping project website:
Find out what faults contribute to the earthquake hazard at your site. Use the USGS Earthquake Hazards Program website:
Fault Maps – will show you faults and structure believed to be sources of M>6 earthquakes in the last 1.6 million years.
Database is the source for faults used in the 2002 National Seismic Hazard Maps and regional probabilistic seismic hazard analysis (PSHA). This study developed the ground motion maps currently referenced in the Bridge Design and Drafting Manual and used for ODOT seismic design.
Link to USGS fault maps:http://earthquake.usgs.gov/regional/qfaults/index.php
Link to 2002 USGS ground motion maps:http://earthquake.usgs.gov/hazmaps/
Good place to start finding fault information – USGS website:
For Salishan, at south end of Depoe Bay:
Closest faults (within 50 km) are all related to the Cascadia Subduction Zone.
Plus:Cape Foulweather FaultYaquina FaultsCorvallis Fault Zone
According to the USGS Quaternary Fault Database:
The Cape Foulweather fault is a down-north, northeast-striking fault that offsets marine-terrace platforms at Whale Cove, and inland, offsets Miocene through Eocene volcanic and sedimentary rocks in the Oregon Coast Range. Vertical offsets of about 20 m of the approximately 80 ka Newport marine terrace and about 80 m of the approximately 125 ka Yachats marine terrace across the projected trace of the Cape Foulweather fault indicate repeated displacements in the late Quaternary. As with other folds and faults located in the Cascadia forearc, it is unknown if coseismic displacements on this fault are always related to great megathrust earthquakes on the subduction zone, or whether some displacements are related to smaller earthquakes in the North American Plate.
Use USGS hazard mapping project website to identify the principal sources for the design earthquakes – and their contribution to overall hazard.
For 2% in 50 year earthquake at Salishan. This equates to a 2,500 year return period earthquake.
Source Category: % contribution and R(km)
Cascadia M8.0-M8.2 Floating 6.53% R =25.3 km Cascadia M8.3-M8.7 Floating 32.29% R = 25.4 kmCascadia Megathrust 57.39% R = 23.9 km
Individual fault hazard details if its contribution to mean hazard > 2%:
Goal before writing the SOW for subsurface explorations:
Know the distance to the nearest earthquake sources.
Know how they contribute to the overall hazard.
Know whether the site may be liquefiable (anticipated soil type, groundwater level).
Know your hazards, so you can plan for them.
Know your hazards, so the Project Design Team can plan for them.
Planning subsurface explorations:Candid discussion between geologist and geotech re: what analyses will be conducted and what data is needed.
Planning explorations and laboratory testing to collect that data.
Example:Defining Site Class in accordance with AASHTO (Table 3.10.3.1-1) is based on the soil properties (SPT, Vs, Undrained Shear Strength) of upper 100 ft of site. Need enough data from the field to make this assessment.
Liquefaction analysis requires N-values, grain size, plasticity for soil column. CPT data preferable.
If project demands, site specific site response analysis, consider geophysical investigation for Vs.
Hollow stem augering should not be employed when assessing liquefaction potential. (GDM Chapter 3.6.2.1)
How AASHTO defines Site Class:(Considers average property measurements over upper 100 ft of soil profile – see AASHTO Table C3.10.3.1-1 for equations and steps)
Site Class Soil Type and Profile
A Hard rock with measure Vs > 5000 ft/sec
B Rock with 2500 ft/sec < Vs < 5000 ft/sec
C Very dense soil and soil rock with 1200 ft/sec < 2500 ft/sec
D Stiff soil with 600 ft/sec or with either 15<N<50 bpf or 1.0<su<2.0 ksf
E Soil with Vs<600 ft/sec or with either N<15 bpf or su<1.0 ksf or any profile more than 10 ft soft clay defined as soil with PI>20, w>40% and su<0.5 ksf
F Soils requiring site-specific evaluations, such as:Peats or highly organic claysVery highly plasticity claysVery thick soft/medium stiff clays (H>120 ft)
At the end of explorations, you need data for:Assessment of Site Class, design code site coefficients
Determination of Peak Bedrock Accelerations (PGA), 0.2 and 1.0 second spectral accelerations for 500 and 1000 year return periods, which correlate to the Serviceability and No-Collapse design criteria, respectively.
Liquefaction analysis
Slope stability
Foundation design
Earthquake induced earth pressures + stiffness values
Mitigation options for seismic hazards
And finally, is site specific seismic study necessary for site response?
Two options for site response:
1. AASHTO general procedure using the 2002 USGS maps and site coefficients.
2. Ground Response Analysis using 2002 USGS maps with site specific ground response analysis.
Should be considered for most sites with competent soils (i.e. no liquefaction, sensitive or weak soils).
Determination of whether site / project demand site specific analysis is up to engineering judgment of appropriate members of the Design Team, and ODOT Bridge Section.
Next presentation to delve further into this topic…
How to of geotechnical seismic design
“Keen observation is at least as necessary as penetrating analysis.”-Karl Terzaghi, Father of Soil Mechanics
“A desk is a dangerous place from which to view the world.”-John le Carre, Author
