ML19312C609

Summarizes Observations & Recommendations Resulting from Jocassee Hydro Consulting Board 751201 & 02 Meeting to Review Data Collected Since Previous Meeting & to Inspection Crack Along Upstream Face

ML19312C609
Person / Time
Site:	Oconee
Issue date:	12/10/1975
From:	Conn W, Cullum C, Sowers G JOCASSEE HYDRO CONSULTING BOARD
To:	Dail L DUKE POWER CO.
References
NUDOCS 7912190802
Download: ML19312C609 (5)

Text

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December 10, 1975 Mr. L. C. Dall Duke Power Company P.O. Box 2168 Charlotte, North Carolina 28242 _ g )4f/7/4/Jf7 RE: Jocassee Hydro Consulting Board Meeting to $ 6"l December 2, 975 /8* />L 7b -

Duke File No. J-4

Dear Mr. Dall:

The Jocassee Hyuro Consulting Board met on the. evening of December 1, 1975 to review data collected since the previous meeting, and on December 2 at the job si te. Mr. Phil McEride Ill, of the Federal Power Commission, at-tended the December 2 meeting. The top of the dam was inspected for evidence of propagation of the crack along the upstream face, previous-

. ly noted. The upstream face of the right abutment was inspected by boat.

This letter summarizes the Board's observations and recommendations.

1. Review of the Data on Dam Deflection

a. As suggested at the previous Board Meeting of May 15, new deep concrete monuments have been established on the dam crest.

Measurements of horizontal and vertical movement have been made at three month intervals, of the bench marks and the new monuments, as recommended by the Board. The movements recorded so far have been identical.

, b. The settlement of the dam crest is continuing at a decreasing

. rate: 0.2 feet since January 1,1975. The rate of settlement

! is comparable to that of other rock fill dams. A plot of settlement as a function of log-time shows the cate to be approaching a straight line as for other rock fill dams.

c. The vertical settlement device cross arr.s also indicate a decreasing rate of movement. There is an Irregularity in the reading of vertical settlement device #1, that is the same at all levels, suggesting a local shift in the top

section of the cross arm casing. All vertical settlement l

holes but six are now partially blocked by broken tape and j lost reading torpedoes. The Board recommends that a lead -

or iron plug be forced down these blocked cross arm holes so that readings can be continued on the upper sections _

of the davices where practical . However, all of the data available Indicates that the amount and rates of settlement of the dam are normal .

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Mr. L. C. Dall December 10, 1975 Page 2

d. The horizontal movements have been in the downstream direction, a naximum of 0.55 feet at Monument 7 at about the The rate of movement is decreasing with time.

center of There the dam. f.:

has been no dif ference between the movement of the upstream All the 6~

and downstream f aces during the past three months.

movements are normal and comparable to that of other rock fili ,

dams.

h?

The Board, as previously done, recommends readings at three If unusual conditions such as high rains, month intervals.

sudden large drawdowns, or seismic activity shall occur, the measurements should be made as soon as possible af ter the event in order to determine if there had been any changes I'n movement.

2. Upstream Crack A crack in the crest of the dam, on the upstream f ace, parallel to the axis, had been noted previously. The Board, in the May 15 meeting, recommended that the crack be filled with fine gravel and that it continue to be observed,

a. The upstream and downstream horizontal movement measurements during the past six months Indicate that there has been no a.

movement across the crack. f

b. The slope indicator readings confirm no significant con-  ;

tinuing movement across the crack.

c. There is a IIttle erosion of fines into small localized

" maize holes" along the crack. There is no visible movement along the crack.

d. The Board recommends that the crack continue to be observed at three month intervals and af ter unusual events, as out-lined in Section 1 of this report. There is nothing in the crack movement to suggest a weakness or abnormality in the dam. Instead, it appears to be the result of the unequal strains between the core and the upstream shell produced by the normal deflection and settlement of the

~ d am.

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Mr. L. C. Dall December 10, 1975 Page 3 3

Seepace Observaticis Data on seepage quantities being monitored downstream of the right v.

abutment were presented.

Flow at Weirs A, C, D, E, H and G is *n '

The. weirs accord with previous readings and is considered normal.

monitsring powerhouse and diversion tunnel drainage produces only p minor amounts of drainage.

The Parshall flume which was originally below Weir F has been A weir moved upstream to about midway between Weirs F and F-l. Readings was suggested for the location in our previous report.

- at these two locations are generally parallel, with Weir F The presently reading 1000 GPM and the fiume reading 1250 GPM. '

present procedure of reading the F weir dally and continuously Comparison recording the Parshall fiume should be continued.

of the present readings with those recorded since completion of the grouting program in December,1973 show good correlation with respect to rainfall and reservoir level. An addi tional f actor which appears to be constant for all previously recorded years is that lower quantitles of water are recorded during the late winter or early spring months.

Preliminary studies have discounted the possibility that increased water viscosity during the winter months could be responsible. Continued observation of this feature will be ofWe interest, although it has little effect on overall seepage. '

recommend that observation of the abutment seepage should be continued at the present frequency. The quantities do not indicate any change in the condition of the abutment, and no re-medial work is recommended at this time.

25, 1975, slight turbidity Af ter the minor earthquake of NovemberThis discoloration had dis-I of water in the fiume was noted.

appeared by the following morning, However,and we event in the do notofconsider future it to be ur"sual or significant.

earthqJakes, the amount and condition of the seepage should be closely observed, in addition to soy other effect on the project.

Profiles across' the abutments were presented, showing the phreatic line plotted from piezometer observations. In accord with our i

previous suggestion, piezometer W-6 has been repaired, an The profiles in clarifying the section along station -0+50. -

confirm that the seepage lines through the abutment continue to reflect satisfactory abutment operation. We recommend that aa additional piezometer be installed at the embankment toe along the line at station 4+00, to more fully define the piezometric Readings of these piezometers should surf ace at this location.

continue at the present intervals.

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Mr. L. C. Dall December 10, 1975 Page 4 4 Seismic Events

a. Four seismic events have been felt in the vicinity of the dam

• since reservoir filling. Three have been very small, and were k.

felt by only a few people. The fourth, on November 25, 1975, "

was larger and was felt over a radius of more than 10 miles. ,

The attached table provides a preliminary summary of the 4 characteristics of all four events. Data on the last three h which occurred within a period of six weeks, were obtained by observations by the University of South Carolina and Georgia Tech Seismograph Stations supplemented by portable instruments placed one-half to three miles from the dam following the October 18, 1975 event.

The largest event, November 25, had a magnitude of between 3 2 and 3.5, and an epicentral intensity of more than t il and less than IV. The corresponding epicentral accelera-tion was probably no more than approximately 0.019

b. In the opinion of the Board, the largest event would have no detrimental effect on the dam. The slope indicator on the crack showed no change. There was no changes in the abutment leakage or other seepage. There was a discharge of si t ty water from the right abutment springs for a few hours af ter -

the event of November 25 but the water cleared up by the next ,

morning. Significantly, there was no appreciable accumulation -

of slit in the weir boxes.

d. The Board recommends that Duke Power coordinate the investigative efforts of the University of South Carolina, Georgia Tech, and *{

j Law Engineering in evaluating the significance of these events and establishing _a program for seismic monitoring.

• - m V.' v. C'onn kk //u/A C. C. Cuilum

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/ G. F. Sowe

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r PRELIMINARY EVALUATION OF SEISMIC EVENTS JOCASSEE DAM Time Ep i cen te r Hypocenter Ma,qnitude Acc Intensity 1973 12:10 EST Felt 3.5 mi. NE of - - - -

Aug. 12 Jocassee Dam

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1975 00:31 EST Aftershocks moni-  ? 1.5-2.0 mb -

Il MM Oct. 18 tored by Taiwani, lo- (es t. )

cated under Reservoir just upstream from dam.

5:30 EST Aftershocks approx. Aftershocks 1.5-2.0 mb -

Il MM Nov. 6 3000' south of dam. less than (e s t. )

4000' 10:17 EST Wi thin 2 km of dam Less than 3.2 -

lil-IV Nov. 25 2 Km (3.5) MM (es t.)

Note: All data subject to verification from Instrumental studies-.

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