ML18046B176

Suppls Draft ,Developing Basis for Recommendations for Changes to Sssp Spectrum to Account for Soil Column

ML18046B176
Person / Time
Site:	Palisades
Issue date:	11/20/1981
From:	Bernreuter D LAWRENCE LIVERMORE NATIONAL LABORATORY
To:	Cheng T Office of Nuclear Reactor Regulation
Shared Package
ML18046B116	List: ML18046B115 ML18046B117 ML18046B175 ML18046B176 ML18046B177
References
EG-81-39, NUDOCS 8112290316
Download: ML18046B176 (20)

Text

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Lawrence Livermore National Laboratory b

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NUCLEAR SYSTEMS SAFETY PROGRAM EG-81-39 November 20, 1981 Mr. T. Cheng Systematic Evaluation Program Branch Division of Licensing U.S. Nuclear Regulatory Commission Washington, D.C.

20555

Dear Tom,

Recommendations for Changes to the Spectrum Developed for the Palisades Site as part of the SSSP to Account for the Palisaides Site 6 s Soil Column.

1.

letter (EG-81-29) from O.L. Bernreuter to T.

Cheng dated August 12, 1981.

2.

Letter. (EG-81-35) from J.C. Chen to T. Cheng dated August 21, 1981.

3.

"Assessment of the Need for Correction to Spectra Developed as Part of the SSSP for Influence of Sited Geological Column," D.L.

Bernreuter and J.C. Chen, Report to NRC~ 1980.

4.

letter (EG-81-22) from O.L. Bernrueter to Mr.

W.T. Russell, dated June 12, 1981~

5.

"Seismic Hazard Analysis:

Application of Methodology, Results and Sensitivity Studies,"

Vol. 4, D.L. Bernreuters NUREG/CR-1582, Vol. 4.

At your and B i ~ 1 Russ *e 1 l 1 s request we ( J

• C. Chen and I )

have attempted to develop a basis for recommendations for chang~s to the spectra at the Palisades site.

This letter is an expanded (with recent work) version of the draft letter I

.sent to you on October 6~ 1981.

This question of local site amplification has been of concern to us for a long time.

As discussed in some detail in Reference 3 it is a very difficult question to resolve.

We (LLNL) are scheduled to spend considerable effort attempting to shed light on the question of site amplification effects in FY82 as part of the SSMRP.

As any results "from SSMRP in this area will be too late to be of v a 1 u e t o y o u i n t h e s h o rt r u n i'I e h a v e t a k en a 11 q u i c k a n d d i rt y 11 approach to arrive at recommendations for the _Pal_isades site.

RtGUlATO.RY DOCKET f ilE C.OPY-.

An Equal Opportimity Err.p:cyer

• Universityoi Caiifcmia *P.O. Box 808 Livermore, Caiiiomi::i 94550

• Te!ephone (415) 422-1100

• Twx 910-386-8339 UCLLL LVMR

F*

I 81-SS-39 November 20, 1981 Our analysis in Reference l~ using a refined soil column model (as compared to the soil column model used in Reference

3) confirmed that we should expect large amplification effects at the Palisades site.

However, the amplification factors developed in Reference l may not be the appropriate amplification factors to use for several reasons:

1.

As discussed in Reference 4 there may be some EUS site amplification included because we used attenuati-0n of intensity for the EUS as one element of our ground motion modeL

2.

Large values for the uncertainty of the ground motion model were used, which in part, would account for some site amplification.

3.

The spectra already contain some site amplification consistent with Western US (WUS) soil sites.

4.

The analysis in References 1 and 2 were based on computer generated time histories which are typically richer in all frequencies than actual time histories which could contribute to the computed amplification factors.

5.

The analysis were carried out using the SHAKE computer program, and as discussed in Reference 3 such analysis can lead to artifically high computed amplification factors.

It is very difficult to sort out these factors.

We judge (1) is not too significant of a problem, i.e. 9 the effect is small.

Our basis here is the comparisons that were made to Nuttli's theoretical model discussed in Reference 5 as well as the analysis performed using Nutli's model.

The comparisons made in Reference 5 show that for the same magnitude earthquake that Nuttli's theoretical model gave higher estimates for the ground motion out to about 100 km than the ground motion model selected by NRC to arrive at its final recommendations.

In developing his model Nuttli assumed that the ground motion out to 20-30 km was the same.in the East ~nd in the West and used WUS data to calibrate his model.

The attenuation of ground motion observed in the WUS was corrected to the ELIS using theoretical considerations* and measured attenuation of the deep rock layers in the EUS.

Thus local site amplification effects are limited to those observed in the WUS and do not include the special local site conditions found in parts of the EUS such a~

at the Palis~des site.

J 1

i 81-SS-39 November 20, 1981 The second item is very complex.

As discussed in Reference 3, much of the uncertainty in the ground motion model is solely due to source and travel path effects.

A large value of uncertainty was also used in part to account for the difference between the background and no background cases (See Reference 5).

Because of these considerations we judge that we can neglect this factor at this stage.

There are several possible approaches that could be used to assess item 3.

One approach would be to treat the Palisades site as a rock site and compute the uniform hazard spectra using the SSSP methodology etc. outlined in Reference 5.

This would result in the same PGA but a slightly different spectral shape.

Corrections to account for the soil column at the Palisades site could then be developed by selecting a set of real time histories and using say SHAKE to correct them for the local soil column.

The main drawback to this apporach is item 5

• The effect of item (4) was assessed by using a set of real earthquake records in the analysis.

The effect was small.

To assess item (5) we modeled.several WUS sites.

Then using the same set of time histories (all scaled to the same PGA} recorded at rock sites input into the SHAKE computer program we*computed the surface ground motion.

From the set of runs a smoothed averaged Palisades and a smoothed averaged WUS spectra was obtained.

The correction to be applied to the SSSP spectra for the Palisades site would be roughly multiplicative factor needed to raise the WUS smoothed average sepectrum to the smoothed Palisades spectrum.

This would allow us to filter out some of the artifically high amplification factors that might be introduced by the SHAKE analysis and still account for the local site soil column.

There are many problems with such an approach.

One major problem is the selection of WUS soil columns.

In the limited time we had available we could only consider three WUS sites.

We chose to model the Ferndale site, the Taft site and the SONGS site.

Taft was chosen because we have a boring log (NUREG/CR-1643) and it is a stiff shallow site -- in a certain sense a WUS counterpart of the Palisaides site. The Ferndale site was chosen as typical of a deep stiffer WUS and also because we have a boring log (NUREG/CR-1643).

The SONGS site was chosen because we already had it modeled and thus saved considerable effort in modeling.

We agree that it would be useful to model other sites.

We selected five earthquakes, listed in Table l, recorded on rock sites from one somewhat larger set contained in Reference 5.

Once again including addition~l earthquakes would

81-SS-39 November 20, 1981 be useful; however, the results we obtained using real records are not significantly different from the results reported in Reference l.

We scaled all records to 0. lg (only Temblor #2) required significant scaling.

All represent nearby smaller earthquakes of the type we might ex~ect to occur in the CUS.

Some considerable subjectivity is also required to model the dynamic properties (shear modulus and soil damping) of the soil column at the Palasides site.

Our intial choice of soil properties is discussed in Reference (1).

There we selected three models for the value of soil shear mo~ulus for each layer which we labeled high bound, low bound and mixed bound.

We felt ~hat the model which we labeled as mixed best represented the soil column.

We used this model to perform the analysis reported on in this letter.

In Reference (l) only the nominal curve was used for damping (Table 3 of Reference (1)).

In this study we ran the following three cases:

o Case l -

Nominal damping curve for all layers.

o Case 2 -

Upper bound curve for all layers.

o Case 3 Upper bound curve for upper sand layers and nominal curve for other layers.

Figure l shows the two damping curves used for the Palisades site.

For the WUS sites we used damping recommended by Seed and Idriss.

See Reference (3).

Only the results for Case l were discussed in our draft letter as our analysis for the other two cases was just getting underway at the time the draft letter was written.

A summary of the results of our analysis for Case l soil damping values for the four sites and five earthquakes in terms of PGA* is given in Table l.

The factor between anyone of the WUS sites and Palisades site varies from about 1.6 to 3 with an overall average of 2.07.

The spectral amplification factors vary over a wider range than the PGA as can be seen from Figure 2~

For frequencies greater than 1.0 Hz the rock outcrop spectra are amplified (between the rock outcrop and the soil surface).

Figures 3-5 shows a comparison of the transfer functions (the transfer functions are computed between the rock outcrop and the soil surface) between Palisades site and the three WUS sites.

Examination of the Palisades transfer function explains 'the behavior shown on Fig.. 2.

Two major modes are seen where the amplification factor is higher than for the PGA.

• PGA = Peak ground acceleration

81-SS-39 November 20, 1981

.In order to work out approximate correction factors the peak spectral levels were tabulated in the 2-3 Hz range and at the peak spectral level which varied between 4-lOHz except for the Temblor record where the peak was in the 2-3Hz range.

These. values are tabulated in Table II.

Comparisons between the various tabulated values would suggest that the Sa for Palisades is about a factor of two higher than for the WUS sites.

As can be seen from Figures 2-5 the damping seems somewhat low for the Palisades Site.

To explore the effect of damping on our results we ran damping Cases 2 and 3 for two earthquake records (Golden Gate and Temblor).

Figure 6 shows a comparison of the transfer function (between the rock outcrop and surf ace) for the Palisades site between the nominal damping and upperbound damping cases.

Table III gives the computed damping range taken from the SHAKE analysis.

Table IV gives a comparison of the computed PGA for the three damping cases and Figures 7 and 8 compare the spectra for the three damping cases for the two earthqu~kes used.

The higher damping cases reduces the amplifiction somewhat.

The above analysis leaves much to be desired.

Thus it is worthwhile to examine other avenues.

For example, we might ask what sort of amplifications are observed between "regular soil sites" and "shallow soil" sites?

This question is generally difficult to address because we can seldom compare two such sites for the same earthquake located the same distance away.

Some data does exist for the Oroville aftershocks and is reported on in Reference (3).

Table V developed from the data in Reference (3) summarize the results.

The average amplification factor is 2.7 which is certainty consistent with the results reported in Tables I, II, and IV.

In Reference (3) other data was presented showing the observed amplification between a rock site and a nearby soil site.

Figure 9 taken from Reference (3) provides this comparison.

The average amplification was based on four earthquakes.

The mean amplification for the E-W component varies from about 2.5 to 4.5 for periods between 0.04s (25 Hz) to 0.5s(2 Hz) and the mean amplifiction factor on the PGA (E-W component) is 2.5.

These values are consistent with the computed values.

The same sort of behavior was observed for the Lake Hughes Array during the San Fernando earthquake.

Station 4 located on rock was about a factor of two lower than Station 12 located on thin soil over rock.

Clearly any conclusions reached from the above analysis mLct be considered as tenative to a large extent subjective.

After careful consideration of the above (and other data) and

I I 81-SS-39 November 20, 1981 subjectively weighting the different results, I recommend that the spectra developed as part of the SSSP (Reference 5) for the Palisades Site should be multiplied by a factor of 2 at all frequencies greater than 3Hz.

At about lHz no correction is required and less than lHz we expect the WUS spectra to be higher.

Below 2Hz the correction factor* should rapidly approach one, as can be seen from a comparison of the transfer functions shown o~ Figs. 3-5.

One other factor could be considered.

As discussed Reference 5 smaller earthquakes are most likely to occur near the Palisades site.

Smaller earthquakes have less long period content than larger earthquakes.

This can be seen from the large variation of Sa in Table II in*the 2-3Hz range.

Both the Temblor and Helena records were from larger earthquakes and the peak Sa is either at 2-3Hz range or the value of Sa in the 2-3 range is near the peak.

However, for the otber smaller earthquakes this is not the case and we see that the Sa value is the 2-3Hz range is significantly smaller.

Thus some consideration could be given to using the mean spectra from actual earthquakes developed in Reference (5) scaled to 1000 year return period-PGA times a factor of 2 as the appropriate spectrum to use.

DLB/vj cc:

p

• 0

• J.C.

T.

T.A.

Smith Chen Lo Nelson Sincerely, d2~

Don L. Bernreuter, Leader Engineering Geosciences Group

ML EQ Site Palisades SONGS Taft Ferndale TABLE I - Cqmparisory of Peak Ground Ac~eleration (PGA)

Between Palisades Site and Three WUS Sites for O.lg Input Motion at "Rock Outcrop" -. Nominal Damping Case for Palisades Site 5.3 6.0 S. F..

Helena Golden Gate

.26

.24

• 1 5

. 1 1

• 1 5

• 13

• 1 1

. l 1

• 14

. 1 1

l. 9

2. 2 5.5 Temblor

.34

• 1 6

• 18

• 1 3

• 16

2. 1 4.6 Oroville 9/27/75

.30

• 1 5

• 1 5

• 1 0

• 13 2.3 4.9 Oroville 8/8/75

.28

• 1 8

• 1 5

• 1 3

• 1 5

1. 9 Average factor f = 2.07

• Factor =

(PGA) Palisades (PGA) Av. of WUS Sites

TABLE II - Comparison of Spectral Levels (Peak & Levels in 2-3 Hz Range)

Between Palisades Site and WUS Sites -

Nominal Damping Case Golden Gates Helena Temblor Orv. 9/27 Orv. 8/8 Site

--sa--r--w Sa f

Iv Sa f

w Sa f

'w'I Sa f

Iv Pa l1sc.des

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. 7 2-3

1. 15 2-3

. l 5

-- 2-3

. 3 3 2-3

.95 4-6

.83 6-7

. 6 6-7 1.4 10

. 7 4

SONGS

.22 2. 1 2-3

.28 2.5 2-3

. 5

2. 3 2-3

.07 2. 1 2-3

• l 2 2.8 2-3

. 6 2

l. 5 4-5

.43 1. 9 6-7

. 3 5

l. 7 4-5

. 8

1. 8 10

. 5 5 l. 3 4

I aft

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2-3

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1. 7 2-3

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• 17 1. 9 2-3

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4-7

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. 3 2

6-7

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. 4 l L-7 4

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2-3

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.52

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• 08 I. 9 2-3

• 2 I

• 7 2-3

• 4 5 2. 1 4-5.26 3.2 6-7

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. 5 2.8 10

• 4

1. 8 tl Average f 2-3 Hz

= 2.09 Average f (max)

= 2.06 f

= (Sa) Palisades (Sa WO S Site

0 0

TABLE III Computed Damping Values from SHAKE Analysis for the Three Damping Cases Studied Case l -

Case 2 -

Case 3 -

Using nominal damping curve for iteration process.

Damping in Palisades site for temblor EQ:

Dune sand 2.4 /".-.

4.7%

(25 ft)

Lacustine deposit 3.5 ~ 4.2%

(28ft)

Glacial till 3.3 ~ 3.5%

(97ft)

Using upper bound damping curve (see attachment l from earlier report.

Damping in Ralisades site for Temblor EQ:

Dune sand 4.5 ~ 8.7%

Lacustine deposit 6.5 t"J 8.0%

Glacial till 6.2 r...-

6.6%

Using higher damping factors for Damping occurs in Palisades site Dune sand 6

l"--

Lacust i ne deposit 5

r-Glacial till 3.3%

upper sand layers.

for temblor EQ~

l l %

6%

f

=

TABLE IV Computed PGA and Correction Factor at the Palisades Site for the Three Soil Damping Cases Studied Golden Gate Temblor Damping Case PGA f

PGA f

Nominal

• 2 6

l. 9

• 3 4 2

• I Upper Lim it

. 2 2

l. 6

.29 LB Mixed 023

l. 6

.32 2

(PGA) Palisades (PGA) Average of WUS Sites j

A v f a c t o r *~ l

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TABLE v Observed Amplification Factors Between a "Regular EQ Date 8/6 8/6 8/8 9/ 27 site II ML 4.7 3.6 4.9 4.6 (Oroville R

Airport 13 l 3 13 15 Average Factor = 2.7 Airport)

R Ranch 13 l 3 13 17 and Shallow Site (Johnson Ranch)

PGA PGA Airport Ranch Factor*

.25

. 7 2.8

.05

. l 6 3.2

.08

• 2 l 2.6

.33

. 5 8 2.2*

l 7 1

• 8

• a correction factor of (15) was used to correct for attenuation difference due to different distances.

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G. G. SBOE, ?AL. SITE, ~llX DA\\.l?ING fACTORS, HIGHER IN Ol'i'lE St.NO REC=

3 LP=

1 1

1 1

ROCK OuTCROP I HG \\.DT l ml -

I ~IPUT 0

z 0

1.0 0.8 0.7 0.6 Cl::'.

w w

()

0.5

~

w t-

l 0.4

-1 0

(/)

r:::o

.q:

FREQUENCY c.

.;;;1'~

GENERATED 9-25--81 D=.050 GEHE?.ATED 10-i3-31

"/,'-tA_

D=.050 ---------

GEMEP.ATEO 10-1!3-131 D=.050 ---------

CDIEP.AT:::O 10--1 S-B<

D=.050

---~-----

i i I i

l:..:.

• j I i I I.

• • '"'".-..... u-****-***-** _. **** * -.... -.._ *.,_, __.,_.,..__,,__. _____.....,,_....,..... -

.. -..... -*.-***""--**-*--*-*~****.-*~** *.-.~** ~w-*** ** *

-***~**-*-*-*-**-* -**--*- -**-- ***'"'*-*-**-****

100~

• 10 *.

1.0 0.1 0.01 0.1 Period "' Sec.

LO 10.0 Fig. 9 Mean: Spectral Amplification Soil Site (C. Forgoria) to Rock Site (S. Rocco)

_