Cougar_Crest_Road_1977_Dec_Test_Pits.pdf

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1-il>PEN-FH-1

Chief, Proj Operations Div

Trip Report to Couear Dam , 29-30 November 1977

Chief , Engrg Div 5 Dec 77 Irvine/nb/6456

1. A report of the investigation of cracking on the crest of Cougar Dam conducted by David Irvine on the 29th and 30th of November 1977 is attached for your infor mation.

2. It is requested that the .following temporary repairs be made to the crest road at Cougar Dam:

a. Pit-2 through Pit-6 should be re-excavated and backfilled with 3/4-inch minus gravel or crushed rock. This material to be obtained from a local quarry or gravel pit. A hand operated power tamper should be used to compact the backfill material in 6-inch lifts. The top 3 inches of the holes should be paved. Compaction to be accomplished with the power tamper. On the surface the center of the backfill pit should be slightly mounded to allow for further compaction by traffic crossing the dam.

b. Pit-1 should be excavated with the project's backhoe forming a trench 2 feet wide and no deeper than 4 feet. This excavation is to extend both east and west from Pit-1 to a point where open voids are no longer encountered or anticipated.

c. If the crack extends deeper than 4 feet , the crack should be backfilled up to the bottom of the trench with the same rock as was used in Pit- 2 through Pit-6 and compacted with a shovel handle. The trench should be backfilled and paved in the same ~~nner and using the same materials as was specified in Pit-2 through Pit-6.

1 Incl as

L. J . STEIN

Chief~ Engineering Division

NPPEN-FM-1 2 December 1977

MEMORANDUM FOR: FILES

FROM: David J. Irvine

SUBJECT: Trip Report to Cougar Dam on the 29th and 30th of

November 1977

PURPOSE: To investigate cracking and subsidence of the dam's crest road.

PERSONNEL CONTACTED: Walt Johnson, Cougar Dam Ray Haney, Cougar Dam

BACKGROUND:

Cougar Dam began filling in October of 1963. During the first filling and drawdown cycle cracks appeared along the crest of the dam.

Most of the cracks were longitudinal cracks located along the center line or downstream of the centerline. There was also one transverse crack located near the left abutment. Early studies related to the cracking, settlement and downstream deflection showed that the cracks were related to differential settlement between the upstream and down· stream zones of the dam. The cracks did not threaten the integrity of the structure and were obiliterated. In 1967 a contract was let to restore the crest road and guardrails. Two inches of asphalt was placed along the crest. Since then the road has been resurfaced bringing the total average depth of asphalt to 0.7 feet. During

September 1977 while attempting to tar seal surface cracks in the road, it was noted that excessive amounts of tar was being absorbed into the cracks and that they would not seal. Based on this fact it was decided that several pits should be dug down through the asphalt to determine whether or not continued settlement was reopening the cracks and causing voids to form under the asphalt. This report deals with that investigation.

I arrived at Cougar Dam at about 1130 on 29 November 1977.

Walt Johnson and Ray Haney had already opened two pits along the crest road where the surface cracks and subsidence of the asphalt appeared to be at its worst. Crack width is about 3/8 inch and subsidence is about 1 inch. These two pits were designated PIT-1 and PIT-2. (See attached map and field book for location of pits, surface crack mapping and field notes.) While jackhammering through the asphalt for PIT-1 the jackhammer broke through exposing a void which the asphalt was bridging. The void was one foot wide directly under the asphalt and tapered down to about an inch at a depth of about 4 or 5 feet. Caving caused by the jackhammer obstructed the crack below that depth. A steel rod 1/2-inch by 10-feet could easily

NPPEN-FM-1

SUBJECT:

2 December 1977

Trip Report to Cougar Dam on the 29th and 30th of

November 1977 be pushed full length into the crack. Although the void was irregular it generally ran parallel to the dam ' s centerline in both directions and sloped in the upstream direction at about the slope shown for the various zones on the dam ' s typical section. The downstream face ot the cracks was a silty sandy gravel . The larger materials were angular rock fragments to about 4 or 5 inches. The upstream face had the same material as the downstream to a depth of 1.7 feet. Below that depth the material appeared to be 1/2-inch minus crushed rock, described as silty gravelly coarse sand. The material on the downstream face of the crack and the material down 1 . 7 feet on the upstream face is believed to be part of the spall zone shown on the typical section. The 1/2-inch minus material appears to be leveling coarse material that was either used to fill an original crack or has settled down into the crack .

Directly downstream of PIT-1 the guardrail is warped. The warping occurred last summer when temperatures in the area exceeded 100 degrees .

The guardrail slide bolts in this area are atornear full travel which seems to verify downstream movement of the structure at this point.

PIT-2 is located about 92 feet east of PIT- 1 . They are located along the same surface crack. The surface crack at this site is about

1/4-3/8-inch wide and the upstream asphalt is about 1/2-inch lower than on the downstream side of the crack . No open void was found below the asphalt but a very loose zone of material 6-7 inches wide clearly defines the crack. The 10-foot rod could not be worked in to full depth because it was obstructed by the larger gravel material . The loose material defining the crack extended both east and west from the pit and the slope of the crack was in the upstream direction as it was in

PIT-1 .

The rest of the day was spent mapping the cracks and depressions along the crest road. After mapping, four other sites were selected for exploration. Site exploration was based on location and extent of cracking visible on the surface .

On the 30th of November PIT-3 through PIT-6 were dug.

PIT-3 was selected as the western most pit and is 3.7 feet down stream of the road centerline . The surface crack at this point runs for about 40 feet and is about 1/4-inch wide . After removing the asphalt no open void was found. The crack is defined by a 2-inch wide zone of loose material which tapers to about l-inch at a depth of 2.4 feet.

Tar which had been poured through the surface crack was also visible to a depth of 1.9 feet. The crack appeared to slope in an upstream direction as it had in PIT-1 and PIT-2. At the bottom of this pit

(2.4 feet) a 7 by 2-inch piece of asphalt was found which indicates that this was also the site selected for one of the earlier trench explorations.

NPPI.::J-FH-1 2 December 19 77 ··· SUBJECT: Tr i port t o Cougar Dam on t he 29tl d 30th of

November 1977

PIT-4 was selected as a centerline crack adjacent to the piezometers at Station 18+50 . The s urface crack is about 1/4-inch wide. From the centerline a circular depression slopes toward the piezometer manhole .

No open void was found below the asphalt . The crack below the surface is displaced 0.8 feet upstream from the crack in the asphalt . It is again defined by a loose zone of material. In this case the loose zone was 1-foot wide to a depth of 2.8-feet which was the bottom of the hole.

The 10-foot steel road could easily be pushed to a total depth of 12-feet.

At the bottom of the pit the material in the loose zone was very wet .

This is believed to be the result of water seeping down through the crack from recent rains . In most other pits the loose zones which defined the cracks was wet at an average depth of 2-3 feet. This seemed to be the result of water seeping into the cracks during the rain storm that occurred about 48 hours before the pits were dug .

PIT-5 was dug to verify that the open crack seen in PIT-1 was in fact closed east of PIT-2. PIT-5 is located about 14-feet east of PIT-2 and is on the same crack. The crack and depression in the asphalt at the site of PIT-5 appeared more open and more depressed than at PIT-2. Beneath the asphalt the materials and condition of the crack appeared much the same as it did in PIT- 2 . The loose zone which defined the crack was about 6-inches wide to a depth of 3-feet which was the bottom of the pit . Near the bottom the materials in the crack became very wet. Tar which had been poured through the surface crack was visible along the sidewalls of the pit all the way down to the bottom of the pit (3 . 0 feet) .

PIT-6 is the eastern most pit and is the only pit dug on the up stream side of the road centerline. Below the asphalt the crack was very hard to follow because all the materials were loose. No tar was visible below the asphalt. At a depth of 1.9 feet the 2-4-inch gravel common to all the pits disappeared. From 1 . 9 feet to the bottom at

2.9 feet the material was a silty gravelly coarse sand which was loose and had a maximum size of 1/2-inch. The crack was located with some difficulty using the steel rod . The rod could only be worked into the crack about 1 . 3 feet below the bottom of the pit . At 2 . 9 feet the crack is estimated to be only l-inch wide .

With the exception of PIT-1 , all the pits were backfilled with the materials that had been dug out. They were not compacted and will undoubtedly settle under the traffic load and when it rains they should be watched and filled as necessary since the edges left by the jackhammer were rather sharp and could damage a vehicles tires.

PIT-1 was left open with a barricade and flashing light to warn traffic.

NPPEN-FM-1

SUBJECT:

2 December 1977

Trip Report to Cougar Dam on the 29th and 30th of

November 1977

CONCLUSIONS:

After reviewing the data gathered during this investigation along with details of construction and old investigative reports, it appears that the cracks and voids noted along the crest road are the result of continuing differential settlement of the various zones of the dam.

Most cracking is likely related to differential settlement between the impervious core and the downstream gravel transition zone.

This investigation did not produce any evidence of serious crack ing that threatens the integrity of the structure. The void found at the location of PIT-1 does present a hazard to log trucks that cross the dam regularly and to other heavily loaded vehicles. The following temporary repairs are therefore recommended as soon as possible. A more general restoration of the road is recommended when weather permits during 1978.

TEMPORARY REPAIRS:

PIT-2 through PIT-6 should be re-excavated and backfilled with

3/4-inch minus gravel or crushed rock. This material to be obtained from a local quarry or gravel pit. A hand operated power tamper should be used to compact the backfill material in 6-inch lifts. The top 3 inches of the holes should be paved. Compaction to be accomplished with the power tamper. On the surface the center of the backfill pit should be slightly mounded to allow for further compaction by traffic crossing the dam.

PIT-1 should be excavated with the projects backhoe forming a trench 2 feet wide and no deeper than 4 feet. This excavation is to extend both east and west from PIT-1 to a point where open voids are no longer encountered or anticipated.

If the crack extends deeper than 4 feet, the crack should be back filled up to the bottom of the trench with the same rock as was used in PIT-2 through PIT-6 and compacted with a shovel handle. The trench should be backfilled and paved in the same manner and using the same materials as was specified in PIT-2 through PIT-6.

Civil Engineer Soils Engineering Section

Pertinent Do.ta

Dra i nage area, sq mi • • • • • • • • • • • • • • •

Pool eleva -ti'on*

Minimum power • • • • • • • • • Minimum flood control • 1 •

Maxim~~ conservation Maximum • • • • • • •

Reservoir storage, acre-ft

Total storage, maximum pool • • • • • • • • Inactive sto1~ge Power storage • • • • • • • Flood control stora8e • • • • • • • • • • • • •

Ty-pe • • • • • • • Length • • • • • Crest elevation • • •• Crown width, ft . • • • • • • • ~~ximum ht, f t. (above lowest foundation) Height above r;r0 md level • • • • • Freeboard (above maximum pool), ft

Spillway

Type • • • • . . . • .

Gross crest length, ft • Gates • • . • •

Size of gates • • • • • C~est elevation • • • • Top of gates elevation • Design discharge, ~fs

Outlet vrorks Ty-pe Ty-pe gates Gate size , ft Invert eleva tion

Maximum discharge , pool elevation 1,699 cfs

Power plant

Penstock . . . . . . . . . • . . • .

Number of gene1~ ting units . . .

Rated cap:1ci ty, kilowatts , total Completion date

* All elevations are in feet above mean sea l evel.

1 -

Cougar Dam

10.5

1,516 1,532 1,690 1,699

219,000 54,000 10,000

155,000

Rockfill 1,6oo 1,705

Gated chute

2,tainter 40 X !~3.3

1656.75 1,699

76,140 l3.5'tunne1 2 - slide

6.5 X 12.5

llq8.75 1~,050 ft diameter

25,000

S4Jt.. lC • CO.O~ P.C .

TD t2

•0"/

.... ·;..-r .,. / c;,/ · /

UPSTRtAM ftT-S fjf- 2

DOW ' :J 'l'Rl:AM

P.rollel to AxJa 100 lOC 200 300 Normol to Axla •~f~._~L------1~0----~r_o ____ ~I?O

Scolo • Foot

I STA. lS.Sl.U l'.T.

TO t2

I "' ....!'~e·::;~ .,oe .

D?:?aESSION

CRACK

• EXfLofZATION Plr

Mo)(. Pool

Mia. F. C. PoolE/. 1532 S'SelectRock

Min. Power Pool £/. /516

/.8

S'Select Rock

SECTION

STATION 18+ 00

SCALE IN FEET

100 200 fiG. 4 COUGAR DAM, .TYPICAL SECTION

Rock l.i l7tO'

10. 10.

MO)( . .Section Shewn

60' to' 1 lr----+~-t ',. J f'or £/. 1260~

DETAIL OF CrtEST

WITH OVERBUILD FOR SETTLEMENT

Scale: i"~10'

Crest £1.1710.00

Elevcdion Vor~e.t, Minimum D. 168J.JO ot Sto. 18+00 cou&Afl 011m ~12csr EoAo ..

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