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EARTHQUAKE HAZARDS , .Fdy Co;m '/ |

.- ,, , PROPOSED EODEGA ESAD REACTOR , , , ;. Karl V. Steinbrugge

' i!

GEOLOGIC CONSIDERATIONS

Proximity to the San Andreas Fault:

4 The earthquake engineering design has to consider the proximity of the San . Andreas fault in addition to potential movement on auxiliary faults (such as the Shaft fault). The location of the facility is generally agreed to be about 1,000 feet west of . . the west edge of the San Andreas fault zone;' this zope is about 1-1/2 miles wide *here.

The surface faulting in the 1906 earthquake took place near the eastern side of the zone. -

There is more than ample evidence from observations made in many earth- *

. quakes that certain types of man-made structures can survive with little or no damage I when not astride the fault break. The author has extensively field studied the 1959

- Hebgen Lake carthquake, and other earthquakes wis surface faulting, having this j ' )

problem in mind. There are differences of opinion among engineers regarding the . . . . , f 1 * , amount of the earthquake design force, but not whether a structure can success' fully I resist earthquake forces adjacent to a fault.

. Shaft Fault:

, . . . The numerous extensive reports by the U. S. Geological Survey and the ! l Pacific Gas and Electric Company dealing with the Shaft fault and the site bedrock are ' \

based on the same observed data, but these studies reach rather opposing conclusions. |

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.. . . - .' ' , . The U. S. Geological Survey concluded, in a draft abstract dated May 7,1964,

. .that the expected total displacement in any direction in the event of an 8. 5 magnitude .

. earthquake with its epicenter at Bodega would be in terms.of the following: , < s ., . . Displacement of a Fault Probability In Granitic Rock of Shaft

2 inches or less Moderate to high

.

- Approximately 1 foot Low .

Approximately 2 feet Low, lower than above, but still a possibility. *

' Approximately 5 feet Remote

The U. S. Geological Survey states in conne etion with the foregoing *

conclusions that these conclusions were "perhaps somewhat subjective. " They also - state regarding the Shaft fault: "The maximum measured displacements in the

. sediments is 14 inches vertically and in the grantric rocks is at least 24 feet horizon- tally. " The U. S. Geological Survey traced the Shaft fault on the surface for about | 230 feet along a strike of N 40 E.

.

Pacific Gas and Electric Company, in their Amendment #8, concludes on ~ ^

page 1 as follows: ". . . . . within the foreseeable future and well beyond the life of the

' i Plant, any movement exceeding a fraction of an inch is extremely unlikely, and any ' movement exceeding one foot is practically impossible, on any of the fractures (or

, so-called minor faults) in the granitic rock at the site of the Plant. " They further

- state on pages 3 and 4, "The rock of the Bodega Head site has not experienced any { | differential ground movement exceeding a fraction of an inch during at least the last -

.- 40,000 years, during which there have been hundreds of large ground movements in the San Andreas fault zone. "

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. The Curtis and Evernden letters to Pacific Gas and Electric Company . (February 26, 1964) and to Dr. Glenn Seaborg (April 25, 1964) are important. Professors Curtis and Evernden are respected members of the University of California faculty in Berkeley. Their participation originated from purely professional interest.

By their own admission, prior to their visit, both of them were emotionally biased ' against the use of the site by the Pacific Gas and Electric Company. However, they concluded after their visits to the site, ". . . . we believ'e that tha total amount of dis- placement of any faults passing through the reactor shaft that has occurred since

Pleistocene sedimentary beds were deposited is in the order of one foot. " Also,. based on a major carthquake frequency of 65 years, they believe that, ". . . . the Shaft fault has moved more than once during this time interval with displacements of, say, 4 . inches each time, then the chances are about 1/500 for disruption of the site during the next 200 years. "

. Professor Clyde Wahrhaftig, also of the University of California, was asked to resolve, if possibic, the differences of professional opinion between the U. S ' . Geological Survey and Curtis and Evernden. Wahrhaftig's conclusions are at some variance to the others: "The Shaft fault probably has a displacement with a total net

. . . . strike-slip component of approximately 24 feet, has moved more than once, and has , moved once in the last 200,000 years with a dip separation of about 1 foot and a strike- slip component of displacement of between 4 inches and 2 feet. "

He also states, "There - , is no good statistical basis for making an estimate of future breakage along the Shaft

. fault, but a reasonable guess is that the probability lies between 1/50,000 and 1/50, and may be 'about 1/1,000. " (From Wahrhaftig Report, dated April 28, 1964.)

* ! Each consultant's basis for the foregoing summarized professional opinion l came from a study of the site, and in some cases included an evaluation of the 1906

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. ground breakage found elsewhere in and near the San Andreas fault. The reactor site i

. has been thoroughly studied by competent geologists, and it seems doubtful if additional ' study by them would change their opinions.

As mentioned in the previous paragraph, an influencing factor on the judge- ment decision regarding the possibility of movement on the Shaft fault, or other faults in the reactor pit, is ground breakage and faulting located away from the main 1906 fault rupture as a, result of that earthquake. The writer has experience in this field,

having examined faulting and ground breakage from the following earthquakes shortly .

, after their occurrence: -

, , July 21,1952 White Wolf fault, California July 6, '1954 Raillbow Mountain fault, Nevada

. August 23, 1954 . Rainbow Mountain fault, Nevada

December 16, 1954 Dixie Valley and Fairview Peak * , |' faults, Nevada . August 17, 1959 Hebgen Lake, Montana 1 .

In addition, major sections of the San Andreas fault have been visited and | ' - - . t fa'ulting from other earthquakes has been inspected. These field examinations were

, made as a part of a team consisting of geologists, seismologists, and engineers. The

resulting studies were sufficiently thorough in several cases to allow the publication ) of scientific papers which included maps; enclosed are three maps. It will be noted

;, on these maps that the ground breakage has not been firmly classified as being either ' faulting or a surficial feature. This was because in some instances there were : . - differences of professional opinion as to what constituted faulting and what could be j, - attributed to surficial movements. Therefore, even soon after the event it is not

always possibic for competent and experienced observers to properly catalog all

i ground breakage; this raises some question regarding the interpretation given to '

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, present day field studies of obliterated 1906 ground breakage. When examining

fresh seround breaks as a result of earthquakes, the inspection teams which the author i

has been on usually noted that breakage associated with faulting usually followed features - indicative of previous movements.

. ^ In view of the foregoing, the writer interprets Table 1 in Schlocker and Bont11a's Report, " Summary'of Engineering Geology of the Proposed Nuclear Power

Plant on Bodega Head, Sonoma County, California" (no date) to indicate that the ! { incidence of reasonably well established faulting away from the fault zone is quite i I small. Of the 42 cases of " Branch Surface Ruptures" cited in Table 1, only a small * ) | fraction of the 42 instances are located out of the San Andreas fault zone and are associated with old scarps or sags. In view of the at least 190-mile length of surface

breakage on the San Andreas fault in 1906, the knovin number and total length of

. auxiliary faulting outside of the San Andreas fault zone is quite small. Undetected

cases would, of course, increase the number, but it is very doubtful if many large * displacements were missed. .

In conclusion, a 1/500 chance of a fault movement at the shaft site during the next 200 years as stated by Curtis and Evernden is, in the writer's opinion, a ... . reasonable estimate for engineering design considerations to be discussed in later paragraphs.

In the writer's opinion, the three-foot unobstructed radial clearance between

. the outside of the reinforced concrete containment structure and the inside of the con- tainment, pit meets the requirements of all credible fault movements through the site. It is the function of the engineering design to provide a safe structure which can

, accommodate the maximum movements provided by the three-foot radial clearance. ! !

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* , ENGINEERING CONSIDERATIONS-

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The Pacific Gas and Electric Company, in their Amendment #8, has -| proposed a special type of foundation design to solve the engineering problems stemming from an assumed 3-foot maximum displacement in any direction beneath the containment structure. It states that their revised design would allow up to

' 6-feet of total h'orizontal fault displacement along a diameter before local spalling and crushing of concrete of the containment structure would occur. This revised-

foundation design will also affect the earthquake forces on and within the containment

' structure. - > ,

- Sand Base Beneatli the Containment Structure: , ,

It is proposed that the containment structure be founded on a one-foot - - minimum layer of carefully selected sand of known characteristics. Dames and . Moore, in Appendix III of Amendment #8, specify the desirable sand characteristics

. during a design earthquake to be as follows:

1. The minimum transmitted shear force from the rock , base through the sand to the containment structure ~'

should be such that the structure does not move relative to the bedrock during a design earthquake. In other

. words, friction should not be overcome up to a design

, earthquake.

' 2. ' The maximum transmitted shear force is designed to be between 60 and 80 percent of the weight of the structure. };

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, | Sand meeting the second criterion is in the study stage. Dames and Moore state that 1 ' { i they "believe that studies now in progress will permit further limitation of the maxi- ] mum shear force between 60% and 80% of the structure's weight. " It is of some ) importance that the 60% value be,obtained if Pacific Gas and Electric pompany is to maintain their important argument that the containment structure wul slide at

approximately the value of 1/2 G, and certainly slide at greater than 2/3 G (Amend- ment #8, page 55).

The planned study of sand characteristics may have certain difficult 1 {

' l problems to solve. Design spectrum curves for vertical motions are to be 2/3 those ) ! of horizontal motions (Amendment #8, pages 24 and 25). This means that the hori- ] 1 - zontal force necessary to cause incipient sliding will vary as a function of vertical ] and horizontal motions. Maximum and minimum va' lues of the incipient sliding force

. wul occur when peak values of horizontal and vertical motions essentially coincide.

Simultaneous peak values probably wul be so rare and of such short duration that '

the maximum and minimum values of the force to cause incipient sliding will bc , f negligible from a design standpoint. However, the force to cause incipient sliding wul be a variable. Since this force probably will vary appreciably, then the basis for the arguments given for slippage occurring at greater than 2/3 G may be open ~' )

to some question (Amendment #8, page 35). i } | The foregoing is complicated by expected horizontal displacements of the

bedrock as a result of a design earthquake with accompanying faulting on the San

, a| . Andreas fault. Many square mues will be horizontally displaced, including all of i

| Bodega Ficad. Lawson (Carnegie 1908, I:149) states that "For the middle half of

the extent of the fault trace from Point Arena to Crystal Springs Lake, the maximum

4' measurements are very commonly from 15 to 16 feet, and these figures may thus be

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. taken as a minimum expression for the amount of displacement on the fault of this

- segment. " In other'words, a fence or road crossing the fault at right angles at Bodega Head could be reasonably offset by 15 to 16 feet in the event of a repetition of the 1906 shock. Assuming that the bedrock on each side of the fault shifts half

. , of this amount, or from 7 to 8 feet, then the bedrock beneath the reactor site must also be displaced some distance up to 7 or 8 feet. The containment structure must

displace a like amount to the northwest with respect to the continent unless friction

beneath the containment structure is overcome. Benloff (Amendment #5, page 3)

' believes that the duration of a fault slippage would be about 10 seconds in a major ~ shock. This figure by Bentoff seems much too small based on the writer's carth-

. quake experience, but Pacific Gas and Electric Company may accept this very restrictive figure by their consultant. The complication arises from the potential

. effects of the previously discussed variable horizontal force required to cause

incipient sliding. The displacement of Bodega Head bedrock with accompanying - vibration.will tend to cause the containment structure to slip in a single direction

' with respect to the bedrock. This slippage of the containment structure in a design earthquake appears to be much more likely than envisioned from a reading of

Amendment #8. It follows that great reliability must be placed on the flexibility of - critical items from the containment structure to nearby structures. While compli- cating the design, it is not a cause for rejecting the site.

It should also be added that the possibly unreliable nature of the frictional

, force may allow forces greater than 2/3 G on the containment structure and its

contents; this will be discussed in later paragraphs. '

' Careful review will have to be given to the results of the Dames and Moore r sand study, particularly to the treatment of vertical accelerations. If their study is

successful, then ACRS can conclude that the sand base foundation will be acceptable. 1

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, . Peripheral Ringwall for the Sand Base:

Pacific Gas and Electric Company proposes to place a peripheral ringwall

of concrete or cement-stabnized sand around the containment structure to eliminate the " cutting edge" from overturning moments induced by earthquake forces. A sketch of this is shown in Appendix III of Amendment #8

, In view of the possible difficulties in relying upon friction beneath the containment structure to prevent sliding, it seems desirable to increase the width of

. the peripheral ring. This is neither a design nor a cost problem. '

.

Structural Isolation of Plant Structures: .4

. Pacific Gas and Electric Company has sta'ted in their Amendment #8 that ~

. "the Plant wn1 b,e designed with no rigid structural interconnections between any major component" (page 5). They also indicate that piping and other sim nar items

wul accommodate relative movement of 3-feet without fauure. .

. It would be well to repeat Newmark's comment that base movements of

less than 3-feet on a fault beneath the containment structure could result in move- ments greater than 3-feet at a location higher in the containment structure (ACRS

Memo on June 17, 1964 meeting, page 3). It is this attention to detail during design I that too often dictates the earthquake performance of a structure. This latter point cannot be overemphasized.

..

. It is probable that at least a portion of the 3-foot isolation will be required in the event of a design carthquake, and therefore structural isolation must be of a

, high degree of reliabuity. j |

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s- n ' . Earthquake Design Spectrum:

. Earthquake resistive design of structures may involve the use of diagrams ' known as design spectrum curves. The basis for these curves is the records taken from strong motion seismographic instruments which have recorded the motions of

a strong shock. The 1940 El Centro earthquake is the strongest shock to have been recorded to date, and the records of this earthquake are the basis for much of today's design data. '

.

The 1940 El Centrp spectrum was for an earthquake having a magnitude of - 7,1; this is considerably less than the 8.3 magnitude for the 1906 San Francisco

, shock. El Centro is on deep alluvial soil. The recording instrument in the 1940 shock was located about 7 miles from the epicenter and about 4 miles from the .

. nearest point on the surface trace of the fault (USC & GS MSS-9, plate 14,1941). ' Richter, howev$r, states that the epicenter location is not accurately known and he

indicates that it may have been closer than 7 miles from the recording instrument (" Elementary Seismology", page 489). However, the June 3,1964 USC & GS report to DLR states "no recordings have been made within 7-8 miles of the epicenter so

-- . there are no experimental data for accelerations or displacements of ground motions i within a mile or two of an earthquake epicenter" (page 5). '\ Comparing El Centro with Bodega Head, Bodega Head is closer to a major

fault, the maximum credible earthquake at Bodega has a greater magnitude than the | l . El Centro shock, and rock is the foundation material at Bodega instead of alluvial j 1 material. It is quite reasonable to expect some differences of opinion regarding

spectrum values at Bodega under these circumstances, l r

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' Incidentally, the epicenter of an earthquake may be given too much importance in discussions of this type. The epicenter is defined as~the point on . the earth's surface directly above the position of the initial rupture. The epicenter need not be in the middle of the fault tr2ce; the epicenter of the 1952 earthquake on the White Wolf fault in Kern County, California, has been placed on the southwest end of the faulting. It also follows that the epicenter need not be in the center of the energy release. However, the epicenter of the 1906 shock is mapped near the

~ center of the 190'6 fault trace.

The spectra from other earthquakes are of limited help since they were of smaller earthquakes or the instrument was located relatively fcr away. This - includes the 1964 Alaskan earthquake from which only aftershock records have

~ been obtained from the Anchorage area; Anchorage is about 75 miles from the . epicenter of the main shock.

Pr.cific Gas and Electric Company states, on page 19 of their Amendment #8, that it has not basically changed its previously proposed design principles and

| criteria. Amendment #8 states on page 21, "that all critical structures, equipment

and systems will be capable of withstanding earthquake ground motions corresponding "

. to spectrum displacement, velocity and accelerations two times as great as that \ shown in Figure I without impairment of functions necessary for containment and

; safe Plant shutdown. " This corresponds to 0. 66 G at the low frequency end o'f the scale. (It is noted that the damping factor for the containment structure has varied: , 7. 5 in Amendment #3,10 in Amendment #6, and 7. 5 in Amendment #8,)

' Newmark, in his report of June 25, 1963, states on page 5: "On bedrock &' the accelerations would be higher, (in the range of 0. 75 to 1. O G), but the displace- ments possibility lower with about the same maximum velocity. " He also states on

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_ * ' w - f ( . . : - - i %' j i ' - .the same page: ". . . the estimate is made that the maximum acceleration in } . the maximum credible earthquake at or near the site would be of the order of about 1 '

4 twice that measured in the El Centro earthquake record. . . f" Newmark has verbally stated at DLR-ACRS meetings that a concern of his is in the low freduebAy end of . .the spectrum.

4

The U.i S. Coast and Geodetic Survey states in their report of June 3,1964,

on page 11, as follows: " ....' m ximum probable ground accelerations of 2/3 G at periods of 0.'2 - 0.'6 second should be expected at Bodega Head and that ground ,

, . accelerations as high as 1/0 G in the same period could occur and should be taken into account in the design of the facilityf" |. a Pacific Gas and Electric Company has countered these high values by ,

. stating: "For ground motions more intense than 2/3 G maximum acceleration, the slip in the sand layer would effectively clip peak accelerations the stresses of which

would be additive to the shear stresses produced in the sand layer by the fundamental

mode of the building" (Amendment #8, pages 35-36). .

The foregoing is valid only if the current studies regarding the characteris- - . j tics of sand are successful.' If not, it is then recommended that the low end of the

spectrum be raised from 0. 33 G to a higher value, possibly not less than O. 50 G for i ; all ductile or ductile acting members and structures. It must be stressed that this

is a judgement decision based on extensive experience plus instrumental data.

* s - , : | Desien for Ductility: j

.

Williamson, on page 4 of the Appendix to his letter of April 21, 1964, b recommends that " strains should be maintained within the elastic range of 66% G"

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' havin in mind radiation embrittlement as well as other factors. If there are any m1:er'..Ci ;ied. : es-tp2: '::::'.~.; :: :t:: : die i.2,'a:t t: i'.;-W.::nt eirt92e stresses and which are brittle, or may become brittle, then study need be given to it.

' Housner, in Appendix II of Amendment #8, states the case for structural

I members yielding when forces exceed those called for in the design e' arthquake. 'His j statements are quite true for m' aterials such as mild steel in bending. It might not be true for the connections of steel beams or pressure vessels anchored to concrete, for examples. To make reinforced concrete " ductile" requires a careful design to be sure that the reinforcing steel will yield before the concrete fails. Earthquake over- turning forces may cause high compressive stresses in any material, and ductility . which is equivalent to that usually available in bending may not be available in com- pression members. If ductility is used to provide a factor of safety, then the choice .~ of materials arid the method of using them imposes another limitation on the designer in their practical application.

*

. Cesign for ductility has been done in conventional structures by engineering organizations skilled in earthquake resistive design. The practical details, however, are much more complex than implied in Amendment #S. ~~

SUMMARY, CONCLUSIONS, AND RECOMMENDATIONS - | |

. The proximity of the San Andreas fault is not, in itself, an adequat'e reason

, for prohibiting the construction of the proposed Bodega Head facility.'

The provisions for 3-foot radial clearance around the walls of the containment structure is satisfactory, in the writer's opinion, for any credible fault displacement e, bener.th the structure.

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- The sand base beneath the containment structure probably can be designed , i to act as a horizontal shear limiting mechanism within reasonable limits. The effect ' . of vertical accelerations make this type of foundation somewhat unreliable with respect to exact values of the horizontal force at incipient sliding. The horizontal displacement of the bcdrock 1 encath the containment structuro at the time of a design carthquake with . faulting on the San Andreas fault complicates the problem by tending to shift the con- tainment structure on its sand base. If the Dames and Moore study is successful, then

. ACRS can conclude that the sand base foundation will be acceptable.

The containment ring for the sand base is necessary, and probably should - . - M" ' he wider than presently planned. This is neither a design nor cost factor.

Structural isolation of the plant structures as proposed by the Pacific Gas .

. and Electric Company, is adequate when used with judgement which is experienced in carthquake engineering.

It is recommended that the low end of the spectrum be increased from 0. 33 G to a higher value, possibly not less than 0. 50 G for all ductile or ductile acting members and' structures. It must be stressed that this is a judgement decision . . . . 1 based on extensive experien'c e plus some instrumental data.

\ It would be reasonable to request that the detailed earthquake design be

left in the hands of a firm specializing in earthquake engineering.' *

. The foregoing conclusions and recommendations have been directed towards

specific problems at Bodega Head.~ The findings are believed to be conservative, and

are consistent with the present knowledge regarding the state of the art of earthquake

' engineering. It seems mandatory that a broader outlook be given to the problems of

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| - - 1 ' * - . , ( . . ( ! * . i ' 1' - , , | ' earthquake resistive design since these problems will undoubtedly reoccur in many | other nuclear facilities. It is also in the public interest to keep costs down by reducing carthgake requirements if this should prove to be feasible. Therefore,

, the folloviing program is recommended:

1.~ It is recommended that strong motion recording equipment be installed in the Bodega Head containment structure, on the bed- rock nearby, and at some distance away in order to determine the effect of fault proximity on spectrum in the next earthquake. , Geologic maps show that California has many faults and no doubt tiase problems will rise again in other reactors and designs. . Additionally, a minor or moderate shock in the vicinity of

' Bodega Head, or any other reactor, will give excellent design - , ' data.' In one' sense, the Bodega Head reactor is an experimental plant from an earthquake standpoint.

. 2 Strong motion recording equipment should be located in all

re'accors west of the Rocky Mountains, and probably in some east of the Rocky Mountains. ~'

4 ! 3.' The ACRS should actively support earthquake damage studies ) | i after a major shock in which earthquake resistive structures. are affected.' Tnis may be best accomplished by supporting * the U. S. Coc.st and Geodetic Survey which is doing excellent work in this ficid.

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' l, 4. Earthquake design criteria should, at ,least in c. general way, | |

be cc.,d!fied for nuclear reactor design. ! .- i i 1

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Karl V. Steinbrugge Structural Engineer | .

August 14, 1964

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