ML19308A747

Jocassee Development Rept on 790825 Earthquake & Effects on Jocassee Structures.

ML19308A747
Person / Time
Site:	Oconee
Issue date:	09/27/1979
From:	DUKE POWER CO.
To:
References
NUDOCS 7912110709
Download: ML19308A747 (38)

Text

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l.0 INTRODUCTION On August 25, 1979 a local magnitude 3.6 ear 9 quake was recorded in the vicinity of Jocassee Development of the i%w.0-Toxaway Project. The purpose of this report is to present in vemation on the seismic event and to discuss the effect of the event e. the Jocassee Dam and appurtenant structures.

For additional details on the Jocassee Development and a discussion of previous seismic activity refer to " Duke Power Company, Jocassee Development, 31, 1977, Sumary Jocassee Dam, Seismic Report" submitted Investigation, to the Nuclear October Regulatory 1975 - May(NRC) on July Commission 19, 1977 and to the Federal Energy Regulatory Commission (FERC) on August 2,1977.

Also refer to "A Report on the Seismic Activity at Lake Jocassee between October 1975 and December 31, 1978" prepared by Dr. Pradeep Talwani of the University of South Carolina submitted to the NRC on February 15,1979 and to the FERC on March 7, 1979.

Presently Duke has an agreement with the University of South Carolina (USC) to monitor seismicity using three semi-permanent stations Duke has previously installed in the Jocassee area. USC maintains the three field stations and Duke maintains the recording equipment located in the Jocassee Powerhouse.

Duke provides the seismograph charts to USC and USC reports unusual activity to Duke and provides Duke with research reports.

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U 2.0 AUGUST 25, 1979 EARTHQUAKE At 9:31 p.m., on August 25, 1979, a local magnitude 3.6 earthquake was re-corded in the Jocassee area. The earthquake was located about 2 miles southwest of the Jocassee Dam and was followed by a ser'es of much smaller aftershocks.

Available particulars for these events are given below:

Mag. Location Depth Date Time (local) Lat/Long (km) 8/25/79 9:31 pm 3.6 34 56.84' N/82 56.69'W 2 to 3 -

0 8/25/79 9:45 pm <0 34 56.56' N/82 56.38'W 2.4 U

8/26/79 5:48 pm <0 34 56.25' N/82 56.21'W 4.05 8/27/79 1:07 am 2.0 34 55.95' N/82 56.15 'W 1.74 From August 27, 1979 to September 4, 1979, 16 events had been recorded with a maximum magnitude of about 0.50.

The magnitude 3.6 event on August 25 was recorded by the three semi-permanent seismograph stations in the Jocassee area. After the event, USC (ov) deployed a series of 4 portable stations in the Jocassee area to monitor aftershocks.

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Jocassee Development September 27, 1979 Page: 2 The event did not trigger the strong motion instrument, located in the Oconee Nuclear Station which is set to be triggered by .0lg acceleration. The Oconee Station is approximately 12 miles from Jocassee.

3.0 EFFECTS OF EARTHQUAKE 0N'J0CASSEE STRUCTURES 3.1 GENERAL Immediately following the August 25, 1979 earthquake, the Jocassee Dam and appurtenant structures were inspected by plant personnel to detect any detrimental effects caused by the event. No significant chances were.

observed in the instrumentation or the structures themselves. Dn August 31, 1979, the plant structures were again inspected by plant personnel, engineers from Duke's Design Engineering Department and Professor George F.

Sowers and Mr. C. E. Sams of Law Engineering Testing Company.

3.2 POWERHOUSE Some minor cracking was observed for the first time in the Jocassee Power-house. In the Powerhouse Control Room, hairline cracks were observed in the glazed tile north wall. In the Cable Room, cacks were observed in the

, a concrete block partition wall. These cracks formed a zig-zag pattern in the mortar joints of the non-structural walls and extended -to se'veral. blocks on each wall. They have no significantly adverse effect on the powerhouse structure.

Four hairline cracks were also observed, for the first time, across the pipe gallery floor near the west end of the powerhouse. They may have been caused by the event, but could have existed prior to the event. These cracks are not considered to present any safety problems.

3.3 SEEPAGE As reported in previous referenced reports, seepage through both abutments is collected and monitored. Some minor i nreases in the seepage readings were observed after the seismic event of -Wgust 25, but such variations are within the normal changes caused by minfall and reservoir fluctuations.

Almost three and one half inches of rainfall were recorded in the Jocassee area for the period August 22 through August 26, 1979. The Jocassee Reservoir level was increased from Elevation 1106.7 at midnight on August 24 to Elevation 1109.0 at midnight on August 26.

A Parshall Flume is used to mearure cummulative seepage on the west abutment.

A recording chart is used to monitor the flow instantaneously. Attached as Figure A is a copy of the chart for tae day of August 25, 1979 showing an p almost indiscernable increase in the seepage following the seismic event.

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,m Jocassee Development September 27, 1979 (v) Page: 3 3.4 SURFACE MONUMENTS There are thirteen monuments on the crest of the Jocassee Dam, nine monu-ments on the downstream side of the crest and four monuments on the upstream side. The monuments are monitored twice yearly for horizontal deflection and vertical settlement.

Readings of the monuments had been made on August 23, 1979 and were taken again, following the event, on August 27, 1979. The maximum vertical settlement recorded was .06 ft. However, the closure error for the vertical measurement was .08 ft. which is considered excessive. A resurvey on September 5, 1979 showed a maximum vertical settlement of .01 ft. with the closure error of .03 ft.,which is within the usual accuracy. The movement due to the earthquake, if any, is very small, when compared to the total settlement recorded to date.

Graphs showing settlement versus time are attached as Figures B-1 for 1973-77 and B-2 for 1978-79. The graphs were plotted using data taken on August 23, 1979 and September 5, 1979.

The readings showed a maximum horizontal deflection of .03 ft, which is very (m

small when compared to the readings to date. Graphs showing horizontal V) deflection versus time are attached as Figures C-1(a) through C-9(a) for 1973-77 and C-1(b) through C-9(b) for 1978-79. Measurements taken on August 23, 1979 and August 27, 1979 were incorporated in the above plots.

3.5 SLOPE INDICATORS As discussed in the referenced reports,a longitudinal crack had been noted along the upstream edge of the Jocassee Dam crest in 1973. Two slope in-dicator tubes were installed at station 14+46 and 14+92 to monitor movement of the crack. The tubes were installed just upstream of the crack and extended through the rock and filter material and into the core.

Prior to the August 25, 1979 oarthquake, the slope indicator tubes had shown a maximum deflection in the upstream direction of about 6.5 inches, recorded i

on July 12, 1979. This deflection was measured at the 10 ft. depth of the tube which i: about four feet into the embankment slope.

Readings taken on August 27 and September 4, 1979 show the movement of the l tubes at 10 ft. depth to be about 1 inch in the downstream direction, re-l lative to the July 12, 1979 reading, indicating that the shell material moved downs?. ream and closed the crack to some degree.

l There appeart.d to be no significant changes in the lower portion of tube extending inte the core material.

Figure D-1(a) ind (b) and D-2(a) and (b) are plots of Slope Indicator Tube

1. 1 and #2 respectively and show deflection versus time for selected depths L

Jocassee Development September 27, 1979 Page: 4 below the top of the slope indicator tube. Readings of July 12, August 2?

and September 4, 1979 were plotted.

The report of Law Engineering Testing Company, the independent consultant attached as Appendix 1 concludes that there were no significant changes as a result of the August 25, 1979 event which could affect the safety of structures.

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4.0 CONCLUSION

Based on a thorough inspection of the structures and an evaluation of the dam instrumentation, no detrimental effect of the August 25, 1979 earthquake j has been noted on the Jocassee Dam and the appurtenant structures.

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l APPEllDIX 1 Law Engineering Testing Company Report on Effect of Earthquake of August 25, 1979 on Integrity of Jocassee Dam l

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A i U

September 17, 1979 Mr. S. B. Hager, Chief Engineer Civil-Environmental Division Duke Power Company 422 South Church Street i Charlotte, North Carolina 28242

Subject:

Effect of Earthquake of August 25, 1979 on Integrity of Jocassee Dam

Dear Mr. Hager:

,[ '\,' At your request Professor George Sowers and Mr. Clay Sams,

('" accompanied by engineers from the Duke Power Company, Charlotte, made an inspection of Jocassee Dam on Friday af ternoon and evening of August 31. An earthquake of local magnitude 3.6 occurred Saturday evening, August 25, 1979. As with other

,. earthquakes in the region where shocks are very infrequent, people were alarmed by the sound and motion associated with the event. Their accounts were magnified in person-to-person transmission to persons outside the area into alarming rumors that questioned the safuty of the dam.

Three high magnification seismometers continue to monitor ground motion. These have been maintained by the University of South Carolina and are the subject of separate reports. According to Dr. Talwani, the epicenter was downstream and southwest of the dam about 2-3 km.

There were two objectives in our inspection: 1) to make an independent inspec 'on of the dam including a review of the data already collected L. Duke Power as a part of their regular mon-

! itoring program and, 2) to examine specific phenomena that had

! been objects of rumored public concern.

i

Mr. S. B. Hager, Chief Engineer l

l l Civil-Environmental Division Jocassee Dam September 17, 1979 Rumored Damage In addition to specious reports regarding abutment leaks, there were rumors regarding damage to a school and movement of a boat.

Dr. Talwani's team examined the school at Salem where cracks had been reported. They found that there were no cracks caused by the earthquake.

At the time of the earthquake, Mr. Clyde Pelfrey was fishing in a boat about 1/4 mile downstream and somewhat west of the dam.

He was probably within a mile of the epicenter. He told us that he felt a sharp upward force or thump on the bottom of the boat.

l The sensation was like the boat bottom rising one inch. This -

l sensation is consistent with the sensation of the bottom of one's -

foot being " slapped" when standing directly above a tunnel in  !

l which blasting occurs. l l  :

Unfortunately his account became distorted by person to j l I person transmission. The story that reached us was that his boat j rose 1 inch above the water surface. I Evaluation of Instrumentation i

The on going instrumentation and data gathering provide  !

i quantitative data on the continuing performance of the dam. The l records through the years have shown slow settlement and deflection of the dam as well as continuous seepage from the abutments. All are influenced by reservoir fluctuation. The abutment seepage is also influenced by rainfall.  !

i

In the two week period commencing one week before the  ! I earthquake, the reservoir fluctuations were only a few feet. '

Rainfall, however, was heavy throughout the period; nearly 6 inches total with about half the first week and half the second.  ! l In general the instrumentation has disclosed some small l i changes following the earthquake. Most of the changes that have .

I

! been measured are small compared to the long-term changes--so -

l small that they probably would not have been subject to special  ! l scrutiny except for the earthquake. l l

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Mr. S. B. Hager, Chief Engineer l l Civil-Environmental Division Jocassee Dam September 17, 1979 The observation wells have exhibited no changes except for W-4. The level in W-4 dropped about 5 feet following the earth-quake and has remained at that lower level since. The well is on l

the right abutment downstream of the dam axis in the area in which the greatest abutment seepage occurs. A drop in piezo-metric level downstream of the axis is not an indication of impending trouble. We conclude that the piezometric data reveal nothing hazardous to the dam.

The weirs either indicate no flow change or a very slight increase following the earthquake. The chart recording for flow through the Parshall flume on the right abutment showed practi-cally no increase in flow - less than the width of the recorder

pentip, if any, which would correspond to less than 10 gpm out of  ;

l a total between 700 and 800 gpm. The fluctuations are well  !

l within the range'previously observed. There is nothing in the j i weir records to suggest any significant change in the abutment  ;

i seepage patterns. According to Duke's technician who monitors ,

l these readings, there has been no show of silt in any of the l weirs. We conclude that there has been no significant change in  ;

{ j abutment seepage caused by the earthquake.

l l Surface Monuments l

\

l Observations of crest settlement and horizontal movement are 3 l made at regular intervals by theodolite and precise levels. Such  ;

! readings had been made on August 23, 2 days before the earth-  !

quake. New measurements were ordered by Duke immediately after the event. They were made on August 27 and indicate small set-tlement of the crest, negligible on the right or west abutment and increasing toward the left abutment. The greatest indicated l settlement was 0.7 inches adjacent to the left abutment in the

, same area the greatest crest settlement has occurred since the  ;

dam was constructed. There was also an indicated slight movement l of the crest in a downstream direction, amounting to 0.36 inches near mid-length of the dam.  !

I The closing error in the August 27 level traverse was 0.96 li inches, greater than the usual 0.36 inches or less. A re-survey 1

of elevation was made on September 5, and indicated settlement I

i since August 23 of 0.12 inches at practically all points along J l the crest. The September 5 survey had a closing error within the l l usual range, and is thus believed to be more representative of i

actual settlement than the August 27 survey. Thus, this suggests the crest settlements were even less than indicated by the August f l

{ }

27 level traverse. In either case,.both the settlement and the l l

l I

, t LAlf DdGDE0500 TI3fDdG COMM

Mr. S. B. Hager, Chief Engineer Civil-Environmental Division

{ Jocassee Dam September 17, 1979 deflection are very small compared to the continuing small dam movements. Interestingly enough, the area on the upstream face that previously exhibited surficial movement showed no abnormal movement. We conclude that the observed movements are only what should be expected and that they indicate no unusual behavior nor distress.

I Examination l

The dam had already been examined by Mr. Raleigh Martin, the l technician who is responsible for the continuing weir and piezo-  !

meter records. He noted no changes in areas of previous surface rock movement: the right abutment berm and the upstream surface near the left abutment. He did feel that there might have been a i j slight subsidence of the dumped rock on the right abutment bench j )

near well W-4; however, he could point to no specific change.  : )

! f We examined the right abutment, its rock-covered bench, the .

l crest of the dam, the lef t abutment and made a traverce down the  !

downstream f ace and along the berm at elevation 925 on the l downstream face. We could find no evidence of any movement of i

{ }  !'

rock or change in the rock fill surface on the west abutment bench, the crest, or the toe. Some rocks loosely piled on one- l another at angles approaching the rock friction angle were not i displaced. Surprisingly the surficial rock slides previously j observed had not been re-activated. We did find 3 scattered  ; .

rocks weighing 20 to 75 pounds on the downstream berm that had i l fresh impact spalls, suggesting that these had rolled downward 2 to 5 feet. Considering the millions of rocks loosely dumped on the dam surface at a slop'e approaching the angle of repose, it is surprising that more had not been dislodged. Although there were i ,

several large rocks on the berm, an examination disclosed intact l 1 l moss and other evidence that these rock.c had been in their pres- j i ent locations for a long time. Moreover, Mr. Martin specifically j remembered seeing them previously. ,

Two inclinometers spaced 46 f t apart, extending 40-45 ft

! below the crest of the dam and located at the top of the upstream l l slope were installed in November, 1973, to evaluate the depth of movement causing longitudinal cracking along the upstream edge of j  ;

the dam crest. These inclinometers have verified the Consulting l i Board's conclusion that the movements causing the crack were sur- 1 l

ficial in nature, and involved only the upstream shell material. i (The Consulting Board concluded that the longitudinal tension crack was the result of unequal strains between the core and up-stream shell produced by the normal deflection and settlement of I I the dam.) These inclinometers were read on July 12, 1979, and i

I -1 LAW DmDETEmm TE5DC COWEMr

Mr. S. B. Hager, Chief Engineer l } Civil-Environmental Division Jocassee Dam September 17, 1979 again on August 27, 2 days after the earthquake. They indicated the upper 12 to 14 ft of upstream slope shell material in the vicinity of the inclinometers moved " downstream" by up to about l an inch. We interpret this to simply be the shell material

" settling" back into the tension crack. The inclinometers showed no appreciable change in readings in the casing length within the  ;

core. We conclude that the inclinometer readings are perfectly consistent with previous data concerning this longitudinal crack, and are of no consequence to the integrity of the dam.

We conclude that the embankment exhibits no change as a result of the earthquake and that the few surface rock ' movements that occurred are of no significance.

Reservoir I l l

Mr. Martin examined the reservoir rim adjacent to the dam by  ;

boat. The rim appears unchanged. He did note a few loose rock j i in the rim of the old quarry that might have rolled or slid down-ward. It is our opinion that such loose rock movements are to I gg be expected either from earth tremors or severe weather.

! Power House l After the earthquake, the Duke Power operating staff made a .

I I thorough examination of the power house for any signs of dis-tress. Cracts were noted in two areas: 1) in the control room  !

i walls and 2) in the floor of the level of the pipe gallery, j particularly of the east end. Although these cracks had not been

previously noted there had been no regular inspection for cracks and therefore it is not certain if they were quake-related. l l We examined these cracks in detail, (including using a j magnifying glass). The control room cracks are in thin walls ,

that are not structural (load supporting) . There are zig-zag cracks in mortar joints in glazed tile on the north side of the i control room. This wall rests on the floor and is separated from l' the structural wall about 6 inches, but with spaced metal ties. l The cracks are as wide as 1/16 inch. They could have been caused

by long-term structural deflection; they could have been enlarged l by vibration. They are not characteristics of structural damage

, i caused by earthquake motion. There are also mortar joint cracks

{

in a concrete block partition, generally less than 1/16 inch I

wide. These could be shrinkage but could have been aggravated by

.. vibration. They are not characteristic of structural damage

{ [ caused by earthquake motion.

f l t  :

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Law omsteme usmo cownwr

• h e- w e- - - - --N- . - - -- -- -  %- - . ---

Mr. S. B. Hager, Chief Engineer Civil-Environmental Division

\ Jocansee Dam September 17, 1979 Several cracks in the floor of the pipe gallery extend upstream from the turbine pit walls to the downstream face of the structure. They are uniformly about 1/32 inch wide, relatively straight, and appear to extend through the slab. Two exhibit water staining, showing that they have been present for some time, probably years. Although there are no stains in the others a close examination shows all of them to be identical. It is our opinion that these cracks have been present for an indeterminate period. Their orientation suggest that they are temperature or shrinkage cracks. They are not characteristic of structural distortion produced by earthquakes.

We recommend that gage points to set on all of these and that they be observed for the effects of temperature and reservoir loading.

i Conclusion l We conclude that there is no evidence of any impairment of i l

function or reduction of safety in Jocassee Dam as a result of j the earthquake on Augus t 25, 1979.

l Very truly yours, j 1

LAW ENGINEERING TESTING COMPANY I l 9 l

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[ Mh  !

' Clay . Sams, P. E.

Geotechnical Consultant Registered, S. C. 3667 i

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George F. wers, P. E. i

' Sr. Geotechnical Consultant .

Registered, S. C. 6231 l

, CES/GFS/tmc I I