Feasibility Level Evaluation of Seismic Stability for Remedy Selection Senda Ozkan, Tetra Tech Inc. Gary Braun, Tetra Tech Inc.

Presentation Outline  Background information on regional seismicity  Site-specific Analyses Design criteria (recurrence events, PGAs) Liquefaction Analysis Liquefaction potential and initiation Deformation analysis Slope Stability Evaluation Slope stability analysis Displacement analysis  Summary  Background information on regional seismicity  Site-specific Analyses Design criteria (recurrence events, PGAs) Liquefaction Analysis Liquefaction potential and initiation Deformation analysis Slope Stability Evaluation Slope stability analysis Displacement analysis  Summary

Seismic Stability Concerns in Pacific Northwest  Liquefaction and seismic slope stability are major geotechnical concerns in contaminated sediments remediation projects  Washington State seismicity: Cascadia Subduction Zone and Seattle Fault  A deep intra-plate earthquake > M9.0 and a shallow crust earthquake > M6.5 with PGAs of 0.5g to 0.75g are predicted  Liquefaction and seismic slope stability are major geotechnical concerns in contaminated sediments remediation projects  Washington State seismicity: Cascadia Subduction Zone and Seattle Fault  A deep intra-plate earthquake > M9.0 and a shallow crust earthquake > M6.5 with PGAs of 0.5g to 0.75g are predicted

Seismic Stability Concerns in Pacific Northwest  Elliott Bay/Harbor Island marine Terminals, and other developments In Puget Sound are built over liquefaction susceptible deposits  Elliott Bay/Harbor Island marine Terminals, and other developments In Puget Sound are built over liquefaction susceptible deposits

Seismic Stability Concerns in Lower Duwamish Delta  Lower Duwamish Delta near Elliott Bay is susceptible to liquefaction induced flow slides. A series of relatively large landslides occur on the Duwamish River delta front (box C)  The landslides range in width 300 m and in length 500 m.  Lower Duwamish Delta near Elliott Bay is susceptible to liquefaction induced flow slides. A series of relatively large landslides occur on the Duwamish River delta front (box C)  The landslides range in width 300 m and in length 500 m.

Seismic Stability Concerns in Lower Duwamish Delta  Liquefaction-induced deformation is major cause of the landslides.

Site-Specific Seismic Analysis  Former Shipyard located in Elliott Bay.  Regional studies categorizes the soils in the vicinity of Harbor Island, the former shipyard, and PSR as Class E, which is the most prone to liquefaction.  USGS and WSDOT recommend a site-specific investigation to assess the actual geologic conditions and the potential for liquefaction.  Former Shipyard located in Elliott Bay.  Regional studies categorizes the soils in the vicinity of Harbor Island, the former shipyard, and PSR as Class E, which is the most prone to liquefaction.  USGS and WSDOT recommend a site-specific investigation to assess the actual geologic conditions and the potential for liquefaction.

Site-Specific Seismic Analysis  Seismic Analysis Design Criteria (recurrence events, PGAs) USGS probabilistic seismic hazard analysis  Liquefaction Analysis Liquefaction potential and initiation Deformation analysis  Slope Stability Evaluation Slope stability analysis Displacement analysis  Seismic Analysis Design Criteria (recurrence events, PGAs) USGS probabilistic seismic hazard analysis  Liquefaction Analysis Liquefaction potential and initiation Deformation analysis  Slope Stability Evaluation Slope stability analysis Displacement analysis

Seismic Design Criteria  Typical design criteria for contaminated sediment capping projects is 100- yr to 500-yr return period events.  Landfill design criteria 2,500-year return period (10% probability of exceedence in 250 years).  A return period of 500 years is the minimum hazard used by the WSDOT and AASHTO for the seismic retrofit of ordinary structures. A return period of the order of 2500 years is commonly used for the design and seismic retrofit of critical structures (buildings, bridges, viaducts).  WSDOT states that the design standard in the rare 2,500 year event is to prevent collapse and loss of life.  Typical design criteria for contaminated sediment capping projects is 100- yr to 500-yr return period events.  Landfill design criteria 2,500-year return period (10% probability of exceedence in 250 years).  A return period of 500 years is the minimum hazard used by the WSDOT and AASHTO for the seismic retrofit of ordinary structures. A return period of the order of 2500 years is commonly used for the design and seismic retrofit of critical structures (buildings, bridges, viaducts).  WSDOT states that the design standard in the rare 2,500 year event is to prevent collapse and loss of life.

Seismic Design Criteria  Probabilistic Seismic Hazard Analysis 2002 and 2008 USGS National Seismic Hazard Mapping Program (NSHMP) Hazard Maps were used to estimate site- specific PGAs for nominal 100-, 500-, and 2,500-year events  Probabilistic Seismic Hazard Analysis 2002 and 2008 USGS National Seismic Hazard Mapping Program (NSHMP) Hazard Maps were used to estimate site- specific PGAs for nominal 100-, 500-, and 2,500-year events Peak Ground Accelerations 108-year475-year2,475-year 2002 USGS USGS

Liquefaction Susceptibility and Initiation  A computer program, WSLiq developed by UW and WSDOT was used.  Liquefaction analysis is highly dependent on the geotechnical properties of soil, particularly standard penetration test (SPT) results, initial water content, plasticity index, and liquid limit.  A computer program, WSLiq developed by UW and WSDOT was used.  Liquefaction analysis is highly dependent on the geotechnical properties of soil, particularly standard penetration test (SPT) results, initial water content, plasticity index, and liquid limit.

Liquefaction Analysis  Top 10 feet of loose silty sediments form a homogenous liquefiable layer. The liquefaction will likely be initiated during the analyzed seismic events. Below 10 feet, the zones of liquefaction are sporadic because the liquefied sediments are interbedded within denser, less liquefiable materials.  Liquefaction-induced deformation analysis includes lateral spreading, post-liquefaction settlement, and flow slides due to residual strength of liquefied soil.  Lateral spreading could be up to 5 ft for 100-yr; 8 ft for 500-yr; 9 ft for 2,475-year event The damage is expected to be confined within non-liquefiable layers  Post-liquefaction settlement is estimated to be in the range of 0-3 ft  Top 10 feet of loose silty sediments form a homogenous liquefiable layer. The liquefaction will likely be initiated during the analyzed seismic events. Below 10 feet, the zones of liquefaction are sporadic because the liquefied sediments are interbedded within denser, less liquefiable materials.  Liquefaction-induced deformation analysis includes lateral spreading, post-liquefaction settlement, and flow slides due to residual strength of liquefied soil.  Lateral spreading could be up to 5 ft for 100-yr; 8 ft for 500-yr; 9 ft for 2,475-year event The damage is expected to be confined within non-liquefiable layers  Post-liquefaction settlement is estimated to be in the range of 0-3 ft

Seismic Slope Stability Evaluation  SLOPE/W program was used. WSDOT Geotechnical Design Manual was followed Area of concern (removal or capping) Enhanced natural recovery area

Seismic Slope Stability Evaluation  Slope stability analysis results indicate that the analyzed slopes are stable under existing conditions and in a 108-year seismic event. Slope stability failure is predicted during 500- and 2,500-year seismic events.

Earthquake-induced Displacement Analysis  Displacement analysis was performed using Newmark-based displacement charts (Makdisi and Seed, 1978; Seed et al., 1984; Bray and Travasarou, 2007) as referenced in WSDOT GDM (2010) and NCHRP (2008).  Seismically induced slope deformation was estimated to be in the range of 1 to 20 feet for the analyzed seismic events.

Conclusions  Deformation and ground movement intersecting contaminated sediments is unlikely in the project site footprint for 100-yr events, and likely for 500-yr and 2,500-yr events;  When ground movement intersecting contaminated sediments is predicted, the liquefaction-induced hazards and slope displacements are such that corrective measures, such as cap repair and/or replacement, are feasible  Deformation and ground movement intersecting contaminated sediments is unlikely in the project site footprint for 100-yr events, and likely for 500-yr and 2,500-yr events;  When ground movement intersecting contaminated sediments is predicted, the liquefaction-induced hazards and slope displacements are such that corrective measures, such as cap repair and/or replacement, are feasible

Conclusions  Based on the methods and assumptions used for this analysis, no contaminant releases are predicted based on the 100-year and 500-year events evaluated;  Predicted seismic activity at this site does not preclude remediation of contaminated sediments by dredging and/or capping.  Based on the methods and assumptions used for this analysis, no contaminant releases are predicted based on the 100-year and 500-year events evaluated;  Predicted seismic activity at this site does not preclude remediation of contaminated sediments by dredging and/or capping.