SITE RESPONSE SENSITIVITY ASSESSMENT COMPLETED FOR … · October 18,2016. Click to edit Master title style 2 Presentation Outline •Objective of Site Response Analysis (SRA) Sensitivity

SITE RESPONSE SENSITIVITY ASSESSMENT COMPLETED FOR THE

PANTEX PROBABILISTIC SEISMIC HAZARD ANALYSIS CONCEPTUAL PLAN

J. K. Kimball, K. M. Cline, R. C. QuittmeyerRIZZO Associates, Pittsburgh, Pennsylvania

Presentation at DOE Technical NPH MeetingOctober 18,2016

Click to edit Master title style

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Presentation Outline

• Objective of Site Response Analysis (SRA) Sensitivity Assessment.

• Site Response Models.• SRA Logic Tree.• SRA Results.• SRA Impact on PSHA.• Summary and Insights.


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Objective of the SRA Sensitivity Assessment

• Help prioritize technical issues related to site response and site investigation recommendations.

• Assess the impact on hazard of epistemic (data) uncertainties in site response inputs – do larger epistemic uncertainties result in higher hazard?

• Technical issues include:

Thickness of the soil and sedimentary rock geologic layers. Shear wave velocity (Vs) profile for geologic layers down to “hard rock”. Strain degradation properties for the geologic layers. Site response methodology for combining the hard‐rock hazard curves

and the site response amplification factors.


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Site Response Model

• Site profile models are intended to “look like Pantex.”

Model 1 represents where the hard‐rock boundary is very deep – 550 ft of soil over ~4,000 ft of sedimentary rock.

Model 2 represents where the hard‐rock boundary is at the base of the soil at a depth of 550 ft.

• Each model includes 3 cases to assess epistemic uncertainty in site response.

Case 1 full epistemic uncertainty – 3 profiles and 3 damping values. Case 2 reduced epistemic uncertainty ‐ 1 profile and 3 damping values. Case 3 minimum epistemic uncertainty ‐ 1 profile and best estimate damping value.

• To assess the impact on hazard, the hard‐rock PSHA curves for the Topeka test site are used – CEUS SSC Model and the EPRI Ground Motion Update Model.


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Site Response Model

• Upper portion of the profile consists of 2 layers over a stiffer layer.

• Layer thickness considers cross‐sections in central plant area.

• Best Estimate (BE) Vs values taken as 1,000 fps for the top layer, 1,750 fps for the second layer, and 4,000 fps for the stiff layer.

• Vs epistemic uncertainty modeled using sigma ln(Vs) = 0.5, [a factor of 1.65 on Vs].


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Site Response Model 1

• For rock layers, BE Vs selected from reviewing literature for bedded salt and anhydrite.

• Vs uncertainty selected considering experience with sonic logs within Paleozoic sedimentary rocks at other sites.

• Vs gradient with depth based on guidance from EPRI.


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Site Response Model 2

Model 2 is the same as Model 1 for the top 550 ft, but truncates the column at that depth (hard rock is located at a depth of 550 ft.).


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Site Response Logic Tree Case 1


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Site Response Analysis Results

At each end branch of the logic tree the mean and standard deviation of the amplification factor (AF) is calculated. This plot shows the mean AF for all 11 input motions for Profile 1. The large peak near 1 Hz is the resonance frequency of the soil column.

[ # 1]

[ # 11]


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The full epistemic case (Case 1) results in generally lower mean AFs.


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By including epistemic uncertainty in profiles (i.e., Case 1 in which P1, P2 and P3 are included) the position of the resonance frequency moves relative to using Profile P1 only (Cases 2 and 3), resulting in a general lowering of the mean AF.


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Site Response Analysis Impact on PSHA

The sensitivity of damping uncertainty on amplification.

C1 = Curve Set 1 dampingC2 = Curve Set 2 dampingC3 = Curve Set 3 damping


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Assigning the hard rock boundary to the base of the soil (Model 2) results in increased mean AFs for spectral frequencies above ~ 5 Hz. Including the thick section of sedimentary rocks (Model 1) lowers ground motions by lowering the impedance between the rock and soil, and including damping over the thick sedimentary rock section.


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The pga hazard curve is lower for the case of larger epistemic uncertainty (Case 1). This is counter‐intuitive to expectations set by current guidance. This suggests that the current methodology for incorporating site response into the PSHA needs adjustment.


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If the hard rock boundary is placed at the base of the soil (Model 2) the mean pga hazard curves increase as shown by the green curves. The same general trend holds – larger epistemic uncertainty in site response inputs (Case 1) results in somewhat lower mean hazard.


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The 1 Hz hazard curve is significantly lower for the case of larger epistemic uncertainty (Case 1). Similar to pga, this is counter-intuitive to expectations set by current guidance.


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Spectral frequencies near 1 Hz represent the resonant frequency of the ~550 ft soil column. As epistemic uncertainty is reduced, there will be an increase in ground motion at 1 Hz. Understanding site amplification at that spectral frequency is important. Modeling epistemic uncertainties should not result in ground motion amplitudes being under‐assessed.


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Summary and Insights

• Method to integrate site response into PSHA needs to be improved. The intention that sites with large epistemic uncertainties will result in higher probabilistic ground motion than well characterized sites does not hold at the resonant frequency of the column; this finding needs to be addressed by NRC and in revision to ANS 2.29.

• From a site investigation perspective, the highest priority is to reduce epistemic uncertainty in the VS profile over the top ~550 to 650 ft of the site, with the goal of being able to justify use of only one base‐case site profile over this depth range or two to three base‐case profiles but with a significantly reduced range of epistemic uncertainty.


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Summary and Insights

• The second priority is to reduce epistemic uncertainty associated with dynamic properties for the top ~550 ft to 650 ft. While the use of generic dynamic properties may be supportable, this approach may be controversial.

• The rock layers below the top ~550 ft to 650 ft are of secondary importance. While one option would be to assume that the hard‐rock boundary is located at this depth (similar to Model 2), this assumption would result in over‐estimated ground motions. Compiling available velocity data from sonic logs will help to define the VS profile for these deeper units; however, it is unclear whether current sonic log data will be sufficient to accurately define the interface velocities between soil and sedimentary rock.

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SITE RESPONSE SENSITIVITY ASSESSMENT COMPLETED FOR … · October 18,2016. Click to edit Master title style 2 Presentation Outline •Objective of Site Response Analysis (SRA) Sensitivity