ML19317E784

Summarizes Discussions,Conclusions & Recommendations Resulting from Jocassee Hydro Consulting Board 750515 Meeting to Insp Data Re Settlement,Seepage Readings & Dam Performance

ML19317E784
Person / Time
Site:	Oconee
Issue date:	08/15/1975
From:	Conn W, Cullum C, Sowers G JOCASSEE HYDRO CONSULTING BOARD
To:	Dail L DUKE POWER CO.
References
NUDOCS 7912180973
Download: ML19317E784 (8)

Text

{{#Wiki_filter:, y e, . - , Mr. L. C. Dail August 15, 1975 Duke Power Company C arlot e, t orth Carolina 28201 Do N

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RE: Jocassee Hydro Consulting Board Meeting g_f g . Fi1e No. J-4 ..

Dear Mr. DaII:

The Board met on Thursday, May 15, 1975 at the Jocassee si te. The dam and abutments were inspected. The data on settlement, seepage readings, and . other aspects of dam performance were studied. The following summarizes our discussions, conclusions and recommendations:

1. Jocassee Embankment .

a. Horizontal Movement of the embankment crest has been measured at surf ace monuments on the downstream f ace of the dam, 20 feet from the base line or about 3 feet below the theoretical crest.

Measurements by transit since June, 1973 have shown a maximum movement in a downstream direction of approximately 1.5 feet. More accurate measurements by theodolite have shown a maximum downstream movement of 0.46 feet (at Sta.13+00) accumulated between July 12, 1974 and May 9, 1975. Movements at stations 11+00 and 15+00 (within the riverbed and the. highest reach of

                          'he dam) were 0.42 feet.      It was noted that during the period when horizontal movement of surf ace monuments was made both by transit and theodolite that the movements were comparable. Also, these movements are of similar magnitude as on other ccmparable rock fill dams. As expected, there is a gradual decline in the rate of movement.           ,

b. Vertical Settlement of the embankment has been measured by three j

separate arrangements: Cross-arm devices (2) mainly in the core, 4 surf ace monuments (noted in a), and pins set for measurements across the crack at the upstream f ace (deflection devices, so- _ called). The following is a comparison of the settlement (in feet) . Column (1) is for the period 9-27-72 (start of highest cross-arm readings) to 7-1-73 (start of surf ace monument readings). Column (2) is for the period 7-1-73 to start of deflection device 18121809 7 3

Consulting Bo,1rd Report Page 2 readings (10-20-73 for Pin "L" and 1-10-74 for Pin "R") . Column (3) is from start of deflection device readings to 7-12-74. Column (4) is from 7-12-74 to 5-9-75. l (1) (2) (3) (4) Device #1, Arm #23 (Sta. 7+00) 1.49 0.49 0.43 0.33 Defl . Device, Pin "R" (Sta. 7+03) -- -- 0.15 0.22 Monument #3 (sta. 7+00) -- 0.23 0.12 0.07 Device #6, Arm #29 (sta.15+90) 1.07 0.77 0.57 0.30 Defl . Device, Pin 'L" (sta. 15+50) -- -- 0.27 0.18 Monument #7 (Sta. 15+00) -- 0.20 0.23 0.15 1 0.26 0.16 Monument #8 (Sta. 17+00) -- 0.31 Settienent as measured by these devices show a gradual reduction in the rate of settlement which is normal for rock fill dams,

c. The Upstream Crack - Horizontal measurements across the upstream crack (between upstream and downstream deflection device pins) continue to increase. i t appears defini te, however, that this does not Indicate an upstream slippage of the shell, and that

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such possibility is precluded by greater settlement of the core than of the shell . (in columns 3 and 4, para, b, tne settlement of Vertical Settlement devices #1 and #6 is greater than settle-ments of Pin "R" and Pin "L" respectively. The Monuments also experienced less settlemen t as compared to devices 1 and 6.) ~ Although the device pins are not referenced to a fixed line, it

;                               can be assumed with reasonable accuracy that the downstream pins j                                have moved downstream the same amount as the nearby crest monuments (para a) . Under this assumption, the upstream pins moved downstream
           ~

between 7-12-74 and 2-28-75 as fol lows : s ta. 9+00 - 0.15 f t. , Sta. 13+00 - 0.13 ft., Sta. 15+50 - 0.12 ft., sta. 17+00 - 0.11 ft. _ d it is believed that the attached sketch, with its notes, is a reasonable rationalization of the movement. I 3 l l

Consulting Board Report Page'3 Downstresm movement of rock fill dams has been reported in the li terature and similar cracking has occurred. Information was found regarding Cherry Valley, Mud Mountain, Wautauga and South Hols ton in the USA plus Gepatsch in Austria - all center core dams; also, regarding the seven sloping core dams built in western North Carolina by Alcoa. Cracking was generally attributed to differential settlement between shell and core but, since no measurements of shell settlement were made, it is possible that this explanation given above for Jocassee is more real i s ti c. The settlement di f ferential might, however, have been greater at Jocassee than at the others as a result of compaction, by vibration, of the shell materials in Jocassee. There has been no significant difficulty with any similar dams and the Jocassee movement, considering heights and designs, is consistent wi th them. Moreover, a stability check alcag an assumed circular slip surfaces in the upstream rock fill, coinciding with the crack near the crest surface, with its deepest point at Elev.1070 and assuming sudden drawdown to Elev.1080 shows a satisf actory safety f actor of about 1.31. The Board concludes: l

i. Hovements of the embankment are not abnormal . Their causes and the changed geometry of the structure can be rationalized.

11. Apprehension regarding a serious slip in the embankment has been largely dissipated but continued moni toring of move-ments should be carried out as recommended below.

ill. The slope indicator readings are consistent with this conclusion, merely assuming the core of the dam as the ' moving mass and the upstream shell as stationary, or moving less, rather than the reverse.

g. -

Consulting Board Report

            .Page 4 The Board recommends:

I. The longitudinal crack should be repaired by regrading of the crest and/or filling with gravel / filter material. ii. 4nstall permanent monuments upstream of the crack in the crest to take the place of the deflection device pins. These monuments should be placed at stations 7+00, 9+00, 13+00 and 17+00.

iii. Provide for frequent inspection of dam structure and abu tmen ts . These should be periodic Inspections by qualified personnel, inspections should also be made af ter severe storms and following a significant rapid drawdown of the reservoir.

iv. The following instrumentation should be observed at three

  ,                       month Intervals.

1) Vertical Settlement Devices i

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2) Surface Monuments

a. Vertical Settlement

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b. Horizontal Movement

{ 3) Slope Indicator Readings l These readings should also be scheduled af ter severe storm and significant rapid drawdowns of the reservoir. I v. Judgment should be made af ter one year as to whether the instrument observation interval might be increased to six months. After 2 additional years, judgment should be made as to whether the observations might be discontinued altogether.

I Consulting Board Report Page 5

2. West Abutment.

The seepage as measured through the west abutment was observed to have decreased since January 1, 1975 Weir F reduced from approxi-mately 1200 GPM to '950 GPM, although the reservoir elevation during this period remained relatively constant. It was noted that seepage flow in other weirs registered increases af ter heavy rainfall, but Weir F did not respond to these rainfall events. There is a strong l possibility of some seepage flow passing under the heavy rock fill between Weir F-1 and Wei r F; thus not being recorded at Weir F. l It does not appear possible to monitor this possible diversion. Also, other seepage may be passing under the dam directly to the downstream toe. During this period of time, sorne of the observation wells in the west abutment (notable wells W-12 and W-18) have shown a considerable decrease. Well W-18 went from elevation 1081.2 on 1-2-75 to eleva-tion 1072.5 on 4-15-75 (The reservoir elevation was about the same for both dates.) ,. . The Board inspected the west abutment on the upstream side, with the reservoir elevation at 1100+; joints in the weathered rock may be i potential seepage paths. The Board recomends: f! 1. Install a weir at a suitable location between Weirs F and F-1 to eliminate possibility of diversion of seepage into the rock fill ._ A good monitoring device, like Parshall flume, should be installed at uns point to facilitate continuous long-term monitoring.

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I l Consulting Board Report Page 6

11. Re-establish observation.well W-6. .

111. Install an observation well between existing observation wells V-12 and W-13 on station -0+50. Consideration should be given to grouting through the cracks , Iv. in the bluf f rather than through drilled holes. Efforts [ should be made by underwater inspection to locate rock joints or cracks in the bluff, which are responsible for seepage. This could be achieved by scuba diving up to about The diver should release 30 feet below the reservoir level. dye near the face of the bluff by a squirt bottle and watch Joints to be for dye movement into the cracks or rock joints. grouted can be identified by means of wooden pegs or steel pins. If the above proves unsuccessful, the reservoir may then be lowered in one or two feet increments and the dye injected along the water line. Two types of grout should be considered. Where a definite flow is observed, a sIIIcate grout, which sets up rapidly and becomes insoluble, should be used. For cracks in which there is no general suspicion of flow, but which might suggest Internal neat cement grout could be used. connection wi thin the abutment, Grouting should be performed through barge-mounted equipment. v. At the next meeting of the Board, the lake level should be pulled down to permit a visual inspection of the cracks by boat.

       -   -         Information-on underwater inspection should be made available at this time.

i i l l e

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l l 1 Consulting Board Meeting ) Page 7 l 3 The Board also inspected the recent erosion between intakes A and B. Board recommends that fu.-ther erosion should be checked by building a concrete wall above the cleared ledge at elevation 1102+ and  ; backfilling with riprap.

4. The Board was informed that the pins installed on the rock face upstream of Jocassee powerhouse did not show any movement. Board does not see any need to continue this monitoring.

Yours very truly, l [[ /d 4. m em C. C. Cullum WF. V. Conn

                 -/       rsl

[ F. Mowers

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