Feasibility Level Evaluation of Seismic Stability for Remedy

Selection

Feasibility Level Evaluation of Seismic Stability for Remedy

Selection

Senda Ozkan, Tetra Tech Inc.Gary Braun, Tetra Tech Inc.

Presentation OutlinePresentation 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 NorthwestSeismic 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 NorthwestSeismic 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 DeltaSeismic 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 <40 m to >300 m

and in length

<100 m to >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 <40 m to >300 m

and in length

<100 m to >500 m.

Seismic Stability Concerns in Lower Duwamish DeltaSeismic Stability Concerns in Lower Duwamish Delta

Liquefaction-induced deformation is major cause of the landslides.

Liquefaction-induced deformation is major cause of the landslides.

Site-Specific Seismic AnalysisSite-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 AnalysisSite-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 CriteriaSeismic 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 CriteriaSeismic 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-year 475-year 2,475-year

2002 USGS 0.16 0.34 0.7

2008 USGS 0.14 0.31 0.61

Liquefaction Susceptibility and InitiationLiquefaction 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 AnalysisLiquefaction 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 EvaluationSeismic Slope Stability Evaluation

SLOPE/W program was used. WSDOT Geotechnical Design Manual was followed

SLOPE/W program was used. WSDOT Geotechnical Design Manual was followed

Area of concern (removal or capping)

Enhanced natural recovery area

Seismic Slope Stability EvaluationSeismic 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.

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 AnalysisEarthquake-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).

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.

Seismically induced slope deformation was estimated to be in the range of 1 to 20 feet for the analyzed seismic events.

ConclusionsConclusions

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

ConclusionsConclusions

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.