1 Scoggins Dam Overview of Seismic Risk July 18, 2012

Overview of Seismic Risk Facility Description Seismic Risk Identified Risk Evaluation –Seismic Loading –Field Program –Lab Testing –Embankment Analyses –Risk Analyses –Conclusions 2

Facility Description 3

Scoggins Dam Scoggins Dam is an earthfill embankment located on Scoggins Creek about 25 miles west of Portland, Oregon Dam construction was completed in 1975 Reservoir has a capacity of 53,323 acre-ft

Scoggins Dam

Embankment Dam Dam has length of 2,700 ft Maximum structural height of 151 ft (Crest El. 313) Zoned embankment

Scoggins Dam CFR Concluded that latest (2004) earthquake loadings were higher than used in previous engineering analyses, and that seismic risks may exceed guideline values, justifying additional actions to better define risks Resulted in one new Safety of Dams (SOD) recommendation 2004-SOD-A: After the study to update the potential seismic hazards has been finalized, evaluate the need to perform additional investigations and dynamic analyses 7

Scoggins Dam – 2010 IE 2010 Issue Evaluation (IE) studies included the following activities: –Updated PSHA and development of ground motions –Extensive field program –Evaluation of in situ and laboratory testing –Detailed engineering analyses –Facilitated, team risk analysis 8

Seismic Loading Increments were chosen to bracket the 5 return periods developed in the ground motion study 9 Basic Return Period Loading IncrementProbability of Load Approximate Ground Motion Range < 300-yr %<.23g 500-yr300- to 800-yr0.208 %.23 to.42g 1,000-yr800- to 3,000-yr0.092 %.42 to.76g 5,000-yr3,000- to 8,000-yr0.021 %.76 to 1.05g 10,000-yr8,000- to 25,000-yr0.008 %1.05 to 1.42g 50,000-yr> 25,000-yr0.004 %> 1.42g

Field Program 10

Laboratory Testing Tests included: –1-D consolidation –U-U triaxial shear (peak undrained strength of clays) –C-U triaxial shear –DSS (both peak and remolded undrained clay strengths) –Lab vane shear (peak/remolded clay strength) –Cyclic triaxial –Cyclic DSS 11

Assignment of Foundation Strengths 12 Peak Undrained StrengthsRemolded Undrained Strengths Low Estimate Su (psi) Best Estimate Su (psi) High Estimate Su (psi) Low Estimate Sur (psi) Best Estimate Sur (psi) High Estimate Sur (psi) Right Side of Valley (Approximate Dam Stations 6+00 to 10+00) Beneath Crest Beneath D/S Shell D/S Toe Beneath U/S Shell Center and Left Side of Valley (Approximate Dam Stations to 22+00) Beneath Crest Beneath D/S Shell Beneath U/S Shell

“Squashed Dam” Results 13

FLAC Analyses 14

Most Plausible/Critical Embankment PFMs Dam overtopping due to slope failures caused by significant strength loss in foundation soils Dam overtopping due to Newmark-type displacements (without significant strength loss in foundation soils) Internal erosion resulting from cracking due to partial slope failures caused by significant strength loss in foundation soils Internal erosion due to cracking caused by Newmark-type displacements (without significant strength loss in foundation soils) 15

16 Probability of Failure Probability of failure was estimated by using an event tree that included: –Type of ground motion model –Probability of the earthquake loading –Probability of widespread foundation strength loss –Probability that deformations would exceed freeboard

Predicted Deformations (CSZ EQ) 17 Expected Deformation (feet of Vertical Crest Loss) - Cascadia Subduction Zone Earthquake using USBR approach Load IncrementType of EstimateSS1SS2SS3 300-yr to 800-yr Upper Bound60*5*4* Best Estimate33*3*2* Lower Bound10*1* 800-yr to 3,000-yr Upper Bound Best Estimate40205 Lower Bound1052 3,000-yr to 8,000-yr Upper Bound Best Estimate Lower Bound ,000-yr to 25,000-yr Upper Bound Best Estimate Lower Bound20157 > 25,000-yr Upper Bound Best Estimate Lower Bound201512

Embankment Risks Dam overtopping failure mode –Annual probability of failure estimated at 6x10 -4 Internal erosion failure mode –Annual probability of failure estimated at 1x10 -4 Relatively insensitive to variables associated with geotechnical analysis Driven by long-duration shaking 18

Scoggins Dam – 2010 IE Concluded that estimated mean seismic risks from dam overtopping or internal erosion brought about by earthquake-induced slope failures and cracking justify further risk reduction measures Concluded that estimated mean seismic risks from failure of spillway wall, which could lead to an erosional failure of the embankment, justify further risk reduction measures 19

Questions ? 20