The Teton Dam Failure a Discussion 1987 Engineering Geology

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Engineering Geology,

24 (1987) 207--215 207

Elsevier Science Publishers B.V., Amsterd am -- Printed in The Netherland s

THE TETON DAM FAILUR E -- A DISCUSSION

E.M. FUCIK

57 South Deere Park Drive, Highland Park, IL 60035 U.S.A.)

(Accepted for publication December 1986)

This discussion will deal with the "we t sea m" which was discovered in

the spring of 1978, after the rep ort on the failure of Teton Dam, prepared

by the Independent Review Panel, had been submitted. The members of the

Inde pend ent Panel visited the site in June 1978 and had an opp ort un ity to

inspect the excavated key trenc h on the left abu tmen t, and to see the exposed

wet seam in its original condition. After this inspection, each Panel member

submit ted his comment s to R.B. Jansen, then Assistant Commissioner of

the Bureau of Reclamation. Altho ugh most of the Panel member s believed

that if the wet seam had been present in the right abu tmen t, i t would have

been a contr ibutin g factor in causing the failure to occur earlier than it

would have otherwise, the present writer is, as far as he knows, the only

memb er who believes that the we t seam may well have been the primary

cause of the failure of Te ton Dam.

The presence of the "wet seam" in the left remnant of Teton Dam came

as a complet e surprise to everyone conne cted with th e proj ect. It was found

when the excavation of the left abutment had been carried down to about

elevation 5115 MSL, abo ut 215 ft . below the crest of the dam. The excava-

t ion of the lef t abutme nt was undertaken in an effort to uncover any evidence

which might have supported, or refuted, the conclusions regarding the prob-

able failure causes, as put for ward in the repo rts o f the I ndep ende nt Panel

and the Interior Review Group. The excavation had been completed down to

the lowest level of the key trenc h in the left abu tme nt when the presence of

a wet seam, running the full width of the core, and about 3 ft. to 5 ft. thick,

was uncovered. This seam was then carefully explored, both by driving a

100 ft. long adit into the seam and drilling some 30 vertical bore holes in the

left remnant to d etermine the extent of the wet seam.

The wet seam was determine d to be abo ut 3--4 ft . thick, and to cover an

area of at least 5 acres. It was not completely continuous, as seen from the

exposed face, but lensatic in nature. The material in the seam was very soft

and not as dense as the surrounding dry material. Free water was present in

the seam, as evidenced by the fact that when test pits were dug in the bo tt om

of the adit, the y filled with water overnight And this was some two years

f t e r

the placing of the fill in this area, and in a location perhaps 100 ft.

above the ground watertable.

0013 -795 2/87 /$03 .50 © 1987 Elsevier Science Publishers B.V.


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Fig.1 shows a cross-section of the dam, indicating the location of the wet

seam, where it was uncover ed. It can be seen tha t t he seam occur red just

above the level at which the dam work was suspended during the winter o f

1974--7 5. Explor ation o f the seam by bore holes indicated that t he seam

existed just above the 1974--75 winter shut


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Fig.2. Wet seam as photog rap hed in 1977.

Fig.3. Adit entrance into and above the wet seam.

the r ock jointing was abo ut the same in bot h trench es, although the vertical

joints ran

toward

the river on the right {failure) abu tme nt, and

away

f rom

the river on the left abutme nt. If piping into the ro ck joints in the right key

tren ch was the immediate cause of failure one might ex pect to have found

some evidence of the beginnings of piping action either in the re mna nt of the

right abutment, or in the left abutment, where the entire key trench was

intact, and subjected to intense scrutiny when the fill in it was excavated in

1978. Howeve r, the re was no evidence whatsoeve r of any piping of fill


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F i g . 4 . O vern igh t accum ula t i on o f w a te r i n bo t tom of adi t .

F i g . 5 . Excava ted f ace o f le f t a b u t m e n t s ea m a t b o t t o m o f p h o t o g r a p h is seen as a wet

sof t mater ia l in cont ras t to the upper , dry , uni form nature of the f i l l .

material at the bottom or sides of the key trench, in either the remnant on

the right abutment or the entire key trench on the left abutment. Thus, i t

must be conclud ed tha t the only evidence as to the cause of failure as de-

scribed in the reports of the Ind epen den t Panel and the first report o f the

Interior Review Group is circumstantial. This is not surprising since both

reports were written prior to the discovery of the wet seam in the spring of

1978.


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TABLE I

Causes of wet seams (fr om Duck et al., 1979) .1

Cause I.R.G. Final Report

(Interior Review

Group)

I .P . R e v i e w . 2

(Independent

Panel)

1. High percentages of an CC

amorphous mineral and

calcium carbonate

equivalent

2. Earthwork placement CC

practices

3. Insu ffic ient removal of NC

material from winter

shut-do wn surface

4. Frost action on surface NC

of soil during spring

start-up period

5. Frost lenses created NC

during winter shut-down

6. Froz en material from borrow NC

placed in fill

7. Snow and rain during May of CC

1975

8. Placement of fill wet of NC

optimum

9. Mixing of wet and dry CC

material on fill surface

10. Reservoir water ent ry into CC

low-density, high-

permeabilit y layer

11. Piping through emb ank men t NC

at El. 1558+-

12. Solutioni ng of calcium NC

carbonate

13. Excessive capillary NC

pressure

14. Hydrau lic fracturing NC

15. Differen tial mov eme nt NC

cracks

16. Structural movem ent during NC

failure

CC

NC

CC

NC

NC

NC

NC

* lc c = contrib utin g cause; NC = non- cont rib utin g cause.

*2Prevalent opin ion as determined by authors.

I t is t he w r i t e r ' s op in i on t ha t t he w e t s e a m ma y w e l l ha ve be e n t he p r im a ry

c a use o f t he fa i l u re o f t he Te ton D a m. The t e s t s a nd obse rv a t i ons ma d e o f t he

w e t s e a m c l e a r ly i nd i c a t e t ha t i t mus t ha ve e x i s t e d a t t he t ime o f f a i l u re ,

s i nc e no w a te r ha s be e n p re se n t a t t he e l e va t i on o f t he w e t s e a m s inc e t he

da y t he re se rvo i r wa s e mpt i e d by t he fa i l ure . If t he w e t s e a m e x i s t e d e i t he r


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Fig.6. Excavated right abutm ent key trench.

Fig.7. Excavated left abut men t key trench.


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in the same co ndit ion as when it was excavated, or as a soft, low densit y

seam which permitted the easy entrance of water from the reservoir edge of

the seam, it can readily be imagined that reservoir pressure may have been

transmitted through the seam, to the downstream edge of Zone 1 (core}

material. If this had occu rred, and if the wet seam was present at the right

abutm ent, a blow-out might have started at the dow nstream edge of the

seam, due to the presence of headwater pressure which had been transmitted

thr oug h the free water in the w et seam. It has been seen tha t free water was

present in the wet seam when it was uncovered in 1978, and it is hard to

imagine that this free water was not in existence at the t ime of failure.

The wet seam has been sho wn to exist at or just above the level of the fill

as it was during the wint er shut -down of 1974--75. Fig.8 is a sketch showing

the elevation of the fill during the 1974--75 shut< lown, and it can be seen

that winter surface intersects the right abutme nt fo unda tio n at abou t the

horizontal location of the first observed blow-out in the face of the dam,

alth ough at a lower elevation tha n th e blow -out level. Thus, it would seem

possible that, if the wet seam, with free water in it, was present in the right

abu tme nt at the time o f failure, it could indeed have had an im portan t role

in causing the failure.

One can imagine a soft, wet layer in Zone 1, the core of the dam, being

filled with water f rom the rising reservoir, and the n as the reservoir rose, the

pressure from the reservoir, gradually increasing at the downstream contact

of Zone 1 with more pervious material in Z one 2, the shell, until erosion

occurred in the shell material and piping took place through the wet seam

and the downs trea m shell.

One piece of evidence from the original excavation of the remnant on the

right abutment may have a bearing on whether or not the wet seam did exist

in the right abutm ent at the time of failure. The following excerpt is taken

from the Indepe ndent Panel Report *l: "Near Sta. 13+15 at El. 5215, the

emba nkme nt for the first time was found extr emely wet cont inuous ly across

the w idth of the key trench. Some free water was encountered. The fill was

extrem ely mud dy over the surface of the grout cap. Between the grout cap

and the upstream key-trench wall, the backhoe sank up to the axle. Even

under the lighter ground pressure of the small dozer, the fill was spongy and

quick. The in-place emb an kment remaining at this elevation was very limited

in axial extent , being about 15 ft. A transverse vertical face was cut by hand

3 to 4 ft. to t he key-t rench invert rock. By probing over this vertical surface,

a softer, wetter horizon was detected. Penetration resistance readings were in

the 170-psi range while readings above were in the 400-psi range and those

below averaged 330-psi. Because this horizon was everywhere within 15 in.

of the rock, and in such close proxi mity to the face of the breach it was not

possible to d etermine if the w etter horizon existed pre-failure or was created

during the f ailure."

1 R e p o r t t o U . S . D e p a r t m e n t o f th e I n t e r i o r a n d S t a te o f I d a h o o n F a i lu r e o f T e t o n

D a m , I n d e p e n d e n t P a n e l t o R e v i e w C a u s e o f T e t o n D a m F a i lu r e , D e c e m b e r 1 9 7 6 .


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5 3 0 0

5 2 0 0

5 1 0 0

5 0 0 0

~ - p . . A

ST A 2 5 + 0 0 S TA . 2 0 + 0 0 ST A . 1 5 + 0 0 ~ o ~ ^, ~r ,, J,P , T

• TOP OF DAM E 533 ~ .. .. .. .. .. .. 0

\ T U N N E L m , ( ~ - ~ \ \ \ \ \ \ ~ \ R O C K ~ 52 00

.

A P , R O X . , 9 , , - , 5

i / , 5o 0 o

' - - z o , ,, , E o F

I I N I T I A L B R E A C H

A R E M A I N I N G

E M B A N K M E N r

E L E V A T I O N L O O K I N G D O W N S T R E A M

W E T S E A M

V I E W A-A

Fig.8. Cross-section of dam showing 1974--75 shut-down.

E l 5 1 1 0

Note the referenc e to "free water " in the wet area, which existed "con-

t inuously across the width o f the key trench ". The presence of "free water"

was a notable characteristic of the wet seam in the left ab utme nt. The wet

area found in the right abutment remnant was at an elevation considerably

above the winter shut-down surface of 197 4--75 , so its existence is certainly

not a positive proof of the presence of a wet seam in the area of the failure.

It seems to the present writer that the discovery of the wet seam, its loca-

tion and its physical condition must be acc ount ed for in any final conclusion

regarding the causes of the Teton Dam failure. Its presence, and the reasons

for its very existenc e are certain ly not self-evident. It seems reason ably certain

that the wet seam was caused by events that too k place during and/ or

immediately following the winter shut~lown of 1974--75, but a complete

expl anat ion of why the material in the wet seam is so soft and light in weight,

and wh y free water would flow into the pits dug in the bo tt om of the

exp lor ator y adit a year after the failure is beyo nd the writer 's compreh en-

sion. Several rece nt occa sions of very so ft layers having been fou nd in fills

that were placed during winter cond itions have been brought to the writer 's

attention, but in these cases very little care was taken to prevent freezing of

the fill before or during the plac ement. However, at Tet on Dam t he reco rd

indicates that the danger of placing fill on a frozen surface seems to have

been taken into acco unt when placing operations were resumed in the spring

of 1975.


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In a paper delivered at the Inte rnat ional Conference on Case Histories in

Geotechn ical Engineering in May 1984, G.A. Leonards and L.W. Davidson

presented a hypothesis concerning the cause of the failure of Teton Dam in

which the y concluded that a horizontal layer of high permeability existed in

the right abu tmen t key trench at the point of failure. This very permeable

layer was caused by over-compaction at very low water con ten t. The perme-

able layer rapidly filled with water fro m the reservoir and collapsed. The

water flowing into the interstice caused a horizontal fracture allowing water

under pressure to flow from open joints upstream to open joints downstr eam

in the key trench walls. Fill material was then eroded into the open rock

joints in the key trench and failure resulted by piping. The L eonards--

Davidson paper seemed to imply th at t he cause of the we t seam was due to

the same general mechanism as the hypothesis for the right abutment failure,

namely a series of layers compacted very dry of optimum, which collapsed

upon wet ting and became very permeable, transmitting water throu ghou t the

layers to form the wet seam. This hypothesis would mean that a very large

and thick part of the dam was uniform ly compacted in a drier cond ition

tha n the rest of the dam. Since the wet seam was 4 ft. thic k, at least, and at

least 5 acres in extent it seems unlikel y tha t most or all of this seam would

have been compact ed in such a way tha t all of it would be in a condi tion to

absorb water and become so very wet. Also the excavated face of the left

abutment remnant, as seen in Fig.5, shows one or two wet spots, but nothing

to compare with the wet seam itself. It seems hard to understand why the

wet seam is the o nly section, and a large one, to have been comp acte d in such

a way as to have collapsed and absorbed water as per t he Leonards--Davidson

hypothesis. Thus the writer concludes that a more likely cause of the wet

seam is somehow connected with the effects of the winter shut

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