WA ST Dept. of Ecology Dam Safety Office Draft Seismic Practice Jerald LaVassar 1, Lead Engineer July

Portfolio Mentality Required Pend Oreille Mines Tailings Facility City of Marysville Stormwater Pond Asamera

WA’s Consequence Dependent Risk Based Design Approach Identifying a suitable minimum design performance goal

Worksheet to Quantify Risk Selection of Design/Performance Goals for Critical Project Elements, Appendix A, Technical Note 2 Controlling factor typically

Design Performance Goal Where there is the potential for loss of life – design is at a minimum of Step 3

Associating ground motions with design performance goal at a specific dam

Annual Exceedance Probability Total mean hazard Cascadia Megathrust hazard Deep Intraplate hazard WUS Crustal hazard 1.0 second Spectral Acceleration (g) USGS PSHA Dam Safety Office USGS event of interest is not generally representative of DSO event of interest. USGS is interested in predicting IMs at a given exceedance probability The DSO is interested in predicting the annual probability of a dam failing. To do that we need the individual hazard curves for the various contributing source zones. Total Mean Hazard vs. Individual Source Zone Hazard Curves

Annual Exceedance Probability Total mean hazard Cascadia Megathrust Deep Intraplate WUS Crustal 1.0 second Spectral Acceleration (g) USGS PSHA Dam Safety Office Idealized Process to Generate Suites of Ground Motion Time Histories S A - g Period – sec -> Accel - g Time - sec Cascadia Megathrust Scenario S A - g Period – sec -> Accel - g Time - sec WUS Crustal Scenario Function of Individual Source Mean M w, R, & Epsilon and θ

Dam Safety Office practice in conducting our independent seismic analyses

Minckler Dam Project Locale

Minckler Dam 1) Model problem geometry (LIDAR data & surveyed dam cross-section) 2) Chose a constitutive model (effective stress model tracks seismic induced pore pressure changes) 3) Select suite of EQ time histories representative of principal source zones contributing to the seismic hazard at a consistent exceedance level

Line 3000 Line V S m/sec >300 Lower Pond Upper Pond V S m/sec >300 Minckler Dam B Cross-section JML 12/21/ ? ? Unweathered Till N 1,60 > >50 Weathered Till N 1,60 > 50 Less dense fill N 1,60 ~ More dense fill N 1,60 ~ All distances in feet Note: Line 3000 was run along the downstream face. Raleigh wave solutions assume the ground surface is level so that the wave direction is in a vertical plane and that the vertical plane coincides with a principal stress plane. This is not the case for Line 3000 and thus, the shear wave velocities (V S ) are more suspect. I have assumed that the level of compaction would be less near the dam slopes where there is less horizontal confining stress to assist in the compaction process and operators would have been reluctant to get too near the edge for fear of rolling the hauling equipment.

Lower Pond Upper Pond 11 ft 24 ft 58 ft 70 ft 82 ft 84 ft 20 ft Existing dam Drained buttress Filter/Drained Buttress JML 3/2011 Note: Assumed engineered fill has same properties as Fill between 12 to 16 feet in Table 1 of Model Soil Properties. Wide drain at interface of old dam section with buttress not shown. Assume similar properties for both buttress fill and that of drain.

Input earthquake record: 2001 Nisqually at DNR building in g. The record was scaled to PGA 0.58g and only the first 30 sec was run in the dynamic analysis.

Vertical displacement after buttress addition. zdis maximum was reduced from 1.3 ft to 0.4 ft along central reach of the dam crest.

Minckler Dam High Downstream Hazard Setting Case where multiple source zones contribute in roughly equal fractions to the hazard and thus, the recurrence intervals for each of the larger single source zone hazard curves would be significantly larger than the return period for the Intensity Measure cited in the PSHA.

EZ-Frisk cuts the Gordian Knot

Mean Spectral Acceleration at 0.5 second 1% chance of exceedance in 50 years Cascade Interface Composite g PNW Deep Gridded g Strawberry Pt g Devil’s Mtn. Fault g Total Mean Hazard 1% chance of exceedance in 50 years - SA 0.5 sec 1.04 g

1% chance of exceedance in 50 years - SA 0.5 sec 0.57 g

Not USGS 2008 Deagg! – Case where Young et al attenuation relationship given zero weight Total Mean Hazard 1% chance of exceedance in 50 years - SA 0.5 sec 0.90 g Mean Hazard Pacific Deep Gridded Source Zone GMPE Weights Zhao 50% 67% 50% A & B 50% 33% 25% Youngs % M w R (km) ε SA 0.5 sec (g)

Purchase 3 month/25 site license for EZ-Frisk once BC Hydro & Zhao GMPEs are updated and we can get our hands on them Setup & run USGS analysis to demonstrate it yields USGS results Rerun analysis for a single source zone to yield SA hazard curve Select SA value at desired exceedance probability for that single source and deaggregate to obtain largest R/M/ε bin contributor Compile uniform hazard spectra/conditional mean spectra from GMPEs Select suite of time histories to cover the spectra and run model If dam survives, move on to next source zone and repeat above If dam fails, find minimum SA/time histories necessary to fail the dam. Determine the exceedance probability for that scenario. Strategy

Anxiously awaiting Processed subduction zone time histories appropriate for PNW use Attenuation relationships (GMPE) that reflect the dramatic increase in record numbers and at greater magnitudes in both the Maule & Tohoku Earthquakes A study of how actual dams performed in the above mentioned earthquakes/verses how our engineering methods would have predicted them to have performed.

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