Failure of Teton Dam 1976 Invited Discusser s Responses to Prepared Questions 1987 Engineering Geology

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Engineering Geology, 24 (1987) 217--22 0 217

Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands

F A I L U R E O F T E T O N D A M , 1 9 7 6 -- I N V I T E D D I S C U S S E R ' S

R E S P O N S E S

TO PREP RED

Q U E S T I O N S

THOM AS M. LEPS

Con sult ing Civil Engineer, P.O. B ox 22 28, Me nlo Park, Calif . 940 26 U.S .A.)

(Acce pted for publ ica tion D ecember 1986)

T h e q u e s t i o n s r e f e rr e d t o a b o v e w e r e p r e s e n te d i n a m e m o r a n d u m p r e p a r e d

b y D r . G . A . L e o n a r d s , a n d t r a n s m i t t e d v i a h is W o r k s h o p P r o s p e c t u s d a t e d

A u g u s t 2 0 , 1 9 8 4 . T h e r e s p o n s e s o f f e r e d a r e b a s e d o n t h e w r i t er ' s o v e ra ll

e x p e r i e n c e s i n c e 1 9 3 9 i n d a m i n v e s t i g a t i o n s , d e s i g n , c o n s t r u c t i o n a n d p e r -

f o r m a n c e m o n i t o r in g , a n d h i s p r o f e s s io n a l i n v o l v e m e n t in t h e i n v e s ti g a ti o n o f

t h e f a i lu r e o f T e t o n D a m b y t h e I n d e p e n d e n t P a ne l, J u ly t o D e c e m b e r 1 9 7 6 .

Q u e s t i o n 1 . I s t h e c u r r e n t s t a t e - o f - t h e - a r t o f a n a ly s i s a n d / o r i n s t r u m e n t a -

t i o n a d e q u a t e t o d e t e r m i n e r e l i a b l y t h e s t a te o f s t re s s a n d s t r a in w i t h i n e a r t h

d a m s ?

R e s p o n s e 1 . C l e ar ly , th e a n s w e r t o t h i s q u e s t i o n n e e d s t o b e s u b d i v i d e d

i n t o t h e t w o c a t e g o r i e s : ( a ) s t a t i c l o a d i n g , a n d ( b ) d y n a m i c l o a d i n g . I n t h e

d i s c u s s e r ' s e x p e r i e n c e , s t a t i c l o a d i n g a n a l y s i s , b o t h f o r a p p r o x i m a t e s t a b i l i t y

a n a l y se s a n d f o r d e t a i le d f i n it e e l e m e n t a p p r o a c h e s , a r e r e a s o n a b l y r e l ia b le

i n t h e i r c u r r e n t s t a g e o f d e v e l o p m e n t , w h e n o n e r e c o g n iz e s t h a t n e i t h e r c a n

b e s c i e n t if i c a ll y p r e c i s e g i v e n t h e i n h o m o g e n e i t i e s t h a t a r e in e v i t a b l e i n

m a t e r i a l s u t i l i z e d a n d i n c o n s t r u c t i o n a c t i v i t i e s .

I n t h e d y n a m i c l o ad i n g c a t e g o r y , b o t h f o r a p p r o x i m a t e a n a l y t i c p r o c ed u r e s

a n d f o r f in i t e e l e m e n t a n a l y s e s , t h e a n a l y t i c p r o c e d u r e s c u r r e n t l y b e i n g u s e d

a r e a c c e p t a b l e o n c e o n e r e c o g n i z e s t h e i n e v i t a b l e u n c e r t a i n t i e s i n v o l v e d i n

e s t im a t i n g o r d e t e r m i n i n g t h e d y n a m i c r e s p o n s e c h a r a c t e ri s ti c s o f a n a l m o s t

i n f i n i te v a r i e t y o f e a r t h m a t e r i a l s i n a s i m i la r l y v a r y in g p h y s i c a l a r r a n g e m e n t

o f c ro s s -s e c ti o n s , d a m s i t e s h a p e s, f o u n d a t i o n c o n d i t i o n s , a n d d y n a m i c i n p u t

p a r a m e t e r s . A g a i n s t t h is s c e n a r i o , i t s e e m s r e a s o n a b l e t o c o n c l u d e a t t h is

t i m e t h a t t h e a v a i la b l e b a s i c a n a l y t i c p r o c e d u r e s a r e m o r e s t r a i g h t - f o r w a r d

a n d r e l ia b l e t h a n o n e ' s a b i l i ty t o p r o p e r l y q u a n t i f y a l l t h e n e c e s s a r y i n p u t

p a r a m e t e r s .

I n r e g a rd t o t h e c u r r e n t a d e q u a c y o f a va i la b le i n s t r u m e n t a t i o n f o r d e t er -

m i n a t i o n o f s t a t ic l o a d i n g s t re s s e s a n d s t ra i n s in e a r t h e m b a n k m e n t s , i t is t h i s

d i s cu s s e r' s i m p r e s s i o n t h a t t h e b e s t e q u i p m e n t a v ai la b le , t h o u g h n o t i n e x p e n -

s iv e , i s f u l l y c o m p e t e n t t o p r o v i d e r el i a b le m e a s u r e m e n t s . O u r a b i l i t y t o in t e r -

p r e t s u c h m e a s u r e m e n t s i n t e r m s o f th e i r c o n f o r m a n c e t o d e si g n a s s u m p t i o n s

is a d e q u a t e ; h o w e v e r , o u r a b i li t y to i n t e r p r e t t h e m in t e rm s o f c o n c l u d i n g

w h e t h e r o r n o t s e r io u s l y d a m a g i n g s t re s se s o r s tr a in s h a v e b e e n d e v e l o p e d

0013-7952 187] 03.50 © 1987 Elsevier Science Publishers B.V.


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commonly receives little attention, and is probably far short of the develop-

ments of instrumentation.

With referenc e to the current adequacy of dynamic ins trumenta tion of

earth dams for stresses and strains, the subject is so specialized, and records

of dynam ic response to eart hquakes are so scarce, that this discusser is un-

able to offe r an opinion. Certainly this is an area f or very selective research,

and not one which, for the foreseeable future, need be urged as an essential

item of instrumenta tion f or all earth dams in earthquake country.

Question 2 To what ext ent can borehole data assess the pote ntial for

hydraulic fracturing under the saturating flow conditions extant in an (earth)

dam?

Response 2 Hydraulic fracturing has been at least partially understood

and described for over forty years, probably beginning with petroleum en-

gineers and thei r experie nces with drilling mud loss in the drilling and develop-

men t of oil wells. .1 Concur rentl y, it has been well know n to f ound ati on

grouting engineers and con tra cto rs for muc h longer. The use of it as a tech-

nique for evaluating in situ stress conditions in a compa cte d earth e mbank-

ment, however, is believed to be relatively recen t, and to some e xt ent its use

in the investigations of Teton Dam was somewhat pioneering. As such, that

use can be said to have been some what crude and inconclusive. Suffice it to

say that, in retrospect, something was learned but not much. The most im-

porta nt item th at emerged was the fact that the field procedure requires the

use of precise contr ols and great care, or else fracturing will occur prema ture ly

and at locations which are not necessarily useful in diagnosis.

In review, for earth dam investigation purposes, it may be too uncontrol-

lable and imprecise an investigative proc edur e to be of value exc ept in the

most special of cases, and woul d p roba bly never be conclusive because the

actual nat ure of the fra cturing is virtually impossible to observe directly or

prove except by use of dyes and careful excavation to locate fractures.

Question 3

What is you r prese nt opinio n regarding the most plausible

mechanism that initiated the failure of Teton Dam?

Response 3 The fundamental mechanism of course was piping. Its occur-

rence was physically possible because the Zone 1 core material was non-

cohesive and fine grained, with up to 74% finer than a No. 200 screen

(0.003 in. opening), and over a large area its conta ct surface with the down-

stream wall of the core trench (excavated in intensely jointed rhyolite) con-

tained many, nearly vertical, open joints in rock ranging from 0.1 to 4 in.

wide. The joints extended downstream beneath the embankment core zone

diagonally towar ds the can yon wall, to fre e exits along the right groin of the

Dam. Hence, with no filter control on the downstrea m face of the core-to-

rock contac t, occurre nce of piping was just a matter of time, undoub ted ly

.l, ,Comp osition and Properties of Oil Well Drilling Fluids, Walter F. Rogers, 1947,

Chapter 12, Gulf Publishing Co., Houston.


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speeded by the effects of critically low, intergranular, stress conditions in the

core zone near the breach location at about axis Sta. 14+ 00.

From the analytic work on embankment stresses that has been done, and

from purely intuitive considerations, there is little doub t that (a) arching

transverse to the axis of the dam was inevitable, deep in the narrow, steep

sided, cuto ff trench; and t hat (b) tensional horizonta l strain of an apprec-

iable degree occurred in Zone I due to differ entia l sett lements along the axis

of the dam down the steep right abutment. These combined actions, if not

enough to produce an actual crack through the core at about Sta. 14, throug h

which erosive leakage could commence, were certainly enough to so lower

the hor iz ont al stresses parallel to the axis as to permit transverse hydraulic

fract uring at ab out Sta. 14 as soon as the reservoir rose suffic iently. The fact

tha t fatal internal erosion occurred so quickly would seem to argue more

strongly in favor of a pre-existing or rapidly formed transverse crack, than

the alternative of con centra ted leakage beginning just above or just below

the gr out cap, a limited area of the fou ndat ion which was generally in tight

rock, was easily inspected, and was grouted.

Question 4 Should the key tre nch have been omit ted ? Would you have

preferred shotcreting of open joints to the use of slush concrete?

Response

4. A k ey trench, but of a more conservative design, was a pru-

dent requ irement. It would have been muc h more acceptable if the following

requirements had been met:

(a) The side slopes should have been no steeper t han 1:1. For this require-

ment, and including the following concepts, the tr ench need not have been

excavated to such large depth.

(b) The entire rock surface of the trench, sides and bottom, should have

been paved with a concrete slab of about 18 in. thickness.

(c) The entire paved surface should have received a patter n of co nsolida tion

grouting to a depth of say 50 ft.

(d) At least one deep grout curtain was needed.

In review of wh at was kn own during the design stage of the exceptional

regional and local perviousness of the grossly jointed and perforated bedrock,

it is concluded tha t the use of localized shotcrete a nd/or slush concrete to

cover obviously open joints should have been viewed as little more than hit

or miss, cosmetic treatme nts. Furthe rmore, it seems not unlikely that the

downdrag caused by gross differential settl ement of the dam emb ank men t

down the steep right abutme nt tended to further open the tops of the nearly

vertical bedrock joints, which would have damaged the integrity of any thin

surface seal such as shotcrete.

In regard to the placement of slush grout or concrete, as shown on fig.9-1

of the Inde pend ent Panel's report, extensive volumes were placed on the right

abu tme nt in open joints, by gravity, totaling 1830 cub. yrd. This work was

commen dable as far as it went, but must have been highly subjective in

regard to decision as to where to deposit the slush grout, appare ntly did no t

follow an engineered patter n, by its rand om n ature could hardly have been


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expected t o complete ly fill the ope n joints, was nowhere followed by pres-

sure grouting to refusal, and was totall y discontinued in the upper 120 ft. of

elevation of the right a butme nt. Accordingly, there was no reason to believe

tha t it would have adequately sealed bedrock adjac ent to the cut off trench.

Question 5 What is the origin of the we t seams (or low dens ity, high

permeability zones) in the dam? Are they likely to be present in other dams,

but have gone unno ticed ? What should be done in design and/or construc-

tion stages to avoid them in the futur e?

Response 5 The wet seams were intensively investigated and tested, and

the findings were thor oughl y reported by the Interior Review Group (IRG)

in its Final Repo rt dated January 1980. The probable causes as summarized

therein may be paraphrased as follows:

Subst andar d earth work placement and control practices. This criticism

applies chiefly during May 1975, following the wint er s hut dow n of fill place-

ment, but also less frequ ently in June, July and August.

The ke y deficiency was placement at moisture content s which were exces-

sively dry of optimum, resulting in low density horizons, wherein dry den-

sities in situ of as low as 80 pcf were discovered as comp ared to the average

of 99 pcf for all Zone 1 comp act ed fill. In-situ dry densities of as low as 85%

of laboratory optimum were measured. Hence, it is clear to this discusser

tha t horizons of such low density material were proven to exist, and that

their exis tence was inevitable given the comb ined effect s of (a) permission to

place Zone 1 as dry as 3.7% dry of optimum and (b) the reported inefficien-

cies in moisture conditioning and blending borrow from excessively dry

borrow sources, an inadequate construction procedure which guaranteed

tha t sizeable areas of placed fill were to some degree even drier than the lim-

ited test data indicated (drying by wind and solar effects).

Incid entall y, it is curious tha t USBR pe rmit ted Zone 1 fill placemen t at

moisture content s as dry as 3.5% below opt imu m when its own la boratory

research, performed as long ago as 1942, showed that placement of imper-

vious fill at moisture conten ts drier than about 2% below opt imu m would

result in abrupt consolidation upon subsequent saturation (La borat ory

Report No. E.M. -- 18.5).

In summary , this discusser concurs tha t the wet seams resulted from fill

placeme nt at unacce ptably low moisture contents, a nd the free water subse-

quently collected in those seams was contrib uted by rainfall during the con-

struc tion period (freq uent), a nd seepage during reservoir filling.

In the futur e, closer control of moisture cont ent should be specified and

administered, although it certainly is not clear that these occasional seams

had any thing to do with the failure of Teton Dam; and, although such seams

are probably inevitable to some degree in all impervious core zones, they

would be entirely unlikely to constit ute a serious defect if appropriate filter

zones and materials are incorporated in the designs.

Documents

Failure of Teton Dam 1976 Invited Discusser s Responses to Prepared Questions 1987 Engineering Geology