A Discussion of the Baldwin Hills Reservoir Failure 1987 Engineering Geology

8/16/2019 A Discussion of the Baldwin Hills Reservoir Failure 1987 Engineering Geology

1/15


2/15


3/15

5001

GTOWE

CL

WE

RM

E

RM

.

J

~

Y

P

i1

/

~

~

B

3

~'

L

~

SO

HCR

O

LNA

NETU

(L

n

No

h

I+50

2+50

3+50

4+50

5+50

5

4

3

2

I+

S

CR

O

TATIO

N

S

IN

T

S

O

4 4

,>

3

d

0 0 0 0 0 0

~

O.T-

5oB

~0

m

~

IO I 1 1

314

L

AFTER IRST

l

L-~

~

~

.N,TIAL ~

TION

N

1

JU1

|

(07

s

IN

T

t02

N

1

M

1

~

|

(64y

~

~

,

O,X~

.

-

03

'

N

S

P

E

C

T

'

O

~

LOUT

t0

4

C

M

O

,NSE

TOR.

1

/

L'

C

M

Z~ ~

-

-~

LO

E

1

~

1

]~

I

~

~

A

F

LU

/

,=,

~4C

~~

C

OO

i

OB

NA

~

7

FO

~GWS

L

r

09

~

-

~

I 1 1 14

(AeS

8WLSO1O

15

Fg1

S

emen

p

eao

so

hcruao

a

ne

o

e

u

s

"4


4/15

EAST

COMPACT

D

EART

LNING

i 0

4

P

G

DRAIN

4

$

4

~4

4

~

-

-

-

-

.

T

n

~

1

S

-

-

-.

_SM-

S

.

.

.

.

.

.

-

-

1

.

.

.

.

.

.

.

_

_

-

S

~

-:~

:

_

.

:

.

.

.

.

.

.

~

.

.

.

.

.

.

,_

-

._

:

_

S

C

L_M

L: ...............

1

.

.

.

.

.

T

~

O

CL

o

ML

C

L

•

SM-SP

.

.

.

.

-

::'

-

~

T

n

o

~

e

o

y

b

.

,

•

S

,

ML

o

C

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

F

UL

V

.

.

.

.

.

.

.

.

•

.

.

•

-

_

-

.

.

.

:

.

SM

-

P

-

:

.

.

.

.

.

.

.

_

_

_

T

n

mfeb

CL

o

ML

.

.

.

.

_-

_-

_

.

.

.

.

.

.

.

.

SP

'C

On

ML

I

i

I

I

I

I

5

I0

S

ALE

OFF

Fig2

S

o

h

s

h

fa

o

Tr

No

3a

Fa

V.

A

O~

h

O

InO hs

COC

o

4

5

=

=

ML

/

ML

S

.

S

~

-

-CL

-

_

;_~

.

.

.

.

.

.

.

.

i

,

$

/

:

.

:

5M

T~

b

CLML

•~

SM

-

M

L

-

C

L

MASS;VE

CL

ML

o

WEST

-ASPHAL

MEMBRANE

i4

Z

d

neb

io

ome

Ieb

io

on

l~

e

I

I

I

I

I 1

(Afe

SHANNON

•

WIL

ON,

1

4 4 4 4

~ b L L

4OO

Z Z

3

_

3

IJ

3 5 3

C~


5/15

131

Baldwin Hills Reservoir is not an isolated case, bu t since the soils of theInglewood for mat ion are particularly susceptible to loosening, these effectsare unusually severe with respect to other know n instances.

At the same time as the reservoir was tilting due to regional subsidence,

and differential compression settlements were developing across Faults I andV, the reservoir was elongating in the NE--SW direction . The t ota l change inlength from 1950--1964 (13.24 yrs) was 0.86 ft., or 0.065 ft. per year. Thetota l elongation divided by the tota l length, assuming the strains were uni-form th rou gho ut, was 0.071%. In the inspection chamber, based on straingauge measurements, it was determin ed tha t the inspection chamber elon-gated appr oximat ely 0.6 inches over a tota l length of about 100 ft. , which isan average of 0.051% strain. This strain is compar able t o average strains of0.057% in E--W lines across the reservoir and indic ates th at the strains wereuniform, rather than being concentrated at the faults.

The measured horizonta l elongation or stretch is most likely directlyrelated to the regional subsidence which caused the tilting of the reservoir. Itis equally obvious that the magnitu de of the max imu m average strain (0.071%)is so small, that if it were distributed uniformly across the reservoir, would inno way adversely affect a well designed earth structure such as a dam orreservoir. In fact, the strain measurements along the crest of the south d amshowed elonga tion strains of th e order of only 0.04% strain, which was com-parable to average strains of 0.036% mea sured across the reservoir in thesame direction. The max imu m tensile strains in the nor th dam were larger asa result of settle ment of the e mba nkm ent itself. Strains in the nort h embank-ment itself, however, were not critical because the original failure developedin the floor of the reservoir, and the final breach developed in the east abut-ment adjacent to the main dam but not in the e mbank ment i tself.

It is clear that at the Baldwin Hills Reservoir site there was a combinationof factors that contr ibute d to the sharp offsets along the f aults tha t ulti-mately led to failure. There was certainly regional subsidence which led totilting of the reservoir. There was regional elongation which, a lthough o fsmall magnitude, may have decreased the lateral earth pressure even if it didnot result in tension cracking. There was reservoir loading which resulted in

settle ment of the floor of the reservoir. There was infiltration into the under-lying softs which increased the compressibility of th e fou nda tio n softs and inparticular increased the differentia l compression across the faults.

One questio n that remains unanswere d even toda y, however, is whet her ornot there were tectonic m ovemen ts (fault slippage) which activated the actualfailure. It is possible that re-pressurization of the producing oil field to thewest co ntrib uted to the lateral spreading and to gradual opening of thesurface fault nor th and sout h of the reservoir. It is also certain that waterfro m the reservoir, once it infiltrated the fault un der full reservoir pressure,opene d up the fault to a width of perhaps several inches. When this happened,

the vertical shear stresses across the fault surface were reduced to zero andthe block o n the east side of the fault reboun ded about 0.16 ft. as shown inFig.3. The theoretic al rebou nd, as com pute d by Dr. R.F. Scott, was based


6/15

5 4 4 4 3 3 5

G

TO

-

P~

~

FOM

O

~tgt:~

;: RA R

~

1

C~SER

i+~

0

o+ooa+5o

z+oo

2+50

3+oo

3

4+00

4+~0

5+00

5+~U

=

i

i

.

.

.

.

.

.

..

. ~

i

i

i

i N

T

SAO

IN

OCA~

STmO

P

L

INLET

T

8

INSPECTION

C

+

" ,~>~

.~

-~

•

B

E

~O

~E

~o~

B

+0~50-

~

y

(R

RFLU

D

M

1

k

-

+0100~

~

~

T

C

~+

0

50

~

--

R

C

oeRES

N

1

00

¢

6

sb

,6

,o

2

z

3bo

~

46

4

s

~

o

oo

_

2

oo

~o

S

NF

,~F~~4_4#CHLORNATNG S

O

L

•

M

e

1

0

M

e

~

1

6

+

T

~

wae

S

OISTaNCE ROM ~LT

-+

(AeS

~WL

1

6

7

7

8

02

0 00

o

~

Fig3

Re

e

o

Fa

I

• 500

,4 440

,410 ~

-350

3

~


7/15

133

u p o n a s s u m e d v a l u e s o f m o d u l i a n d e x i st i n g s h e a r st re s se s , w h i c h w e r er e d u c e d f r o m a m a x i m u m o f 5 0 0 0 p s f at d e p t h 2 0 0 t o 4 0 0 f t . t o z e ro f o ll o w -in g f a il u re . T h e e x c e l l e n t a g r e e m e n t b e t w e e n t h e t h e o r e t i c a l a n d m e a s u r e dr e b o u n d s i n d i c a t e s t h a t t h e r e b o u n d w a s t h e r e s u lt o f r el i ef o f s h e ar s tr e ss e s

a n d n o t a t r u e f a u lt m o v e m e n t .T h e r e is n o q u e s t i o n b u t t h a t t h e r e w a s r e g io n a l s u b s i d e n c e , r e g io n a l t il t-

i ng , l a t er a l s t r e t c h i n g o f t h e g r o u n d s u r f a c e , a n d a c t u a l c r a c k i n g o f t h e g r o u n ds u r f a c e n o r t h a n d s o u t h o f t h e r e s e r v o i r a lo n g F a u l t I . T h e s e a c t iv i t ie su n d o u b t e d l y c o n t r i b u t e d t o t h e s t at e o f s tr es s i n t h e f o u n d a t i o n o f t h er e s e r v o i r t h a t u l t i m a t e l y r e s u l t e d i n v e r t ic a l o f f s e t s a c r o s s t h e f a u l t s. T h i s o f f -s e tt in g c r a c k e d t h e a s p h a lt m e m b r a n e a n d r u p t u r e d t h e d r a in p i p e s a n d t h u sp e r m i t t e d w a t e r f r o m t h e r e s e rv o i r t o s a t u r a t e t h e f o u n d a t i o n s o il s a d j a c e n tt o t h e f a u l t s, w h i c h i n t u r n i n c r ea s e d t h e d i f f e r e n t i a l c o m p r e s s i o n . I n t h ef i n a l an a l y s is , t h e c o l l a p s e o f t h e l in i n g i n t o o p e n i n g s c r e a t e d b y i n t e r n a le r o s i o n w a s t h e i m m e d i a t e c a u s e o f t h e f a il u re . H o w e v e r , t h e i n t e r n a l e r o s i o nh a d u n d o u b t e d l y b e e n d e v e l o p i n g o v e r t h e l if e o f t h e r e se r v oi r.

Question 2 S h o u l d t h e f a u l t m o v e m e n t s h a v e b e e n a n t i c i p a t e d i n t h e d e s ig ns ta g e ? Wo u l d t h e y n o w b e a n t i c i p a te d i f a si m il ar c a s e a ro s e ?

T h is q u e s t io n a s s u m e s t h a t t h e r e w e r e " f a u l t m o v e m e n t s " . T h e L e t t e rR e p o r t f r o m t h e S t a t e C o n s u lt in g B o a r d ( 1 9 6 4 ) s t a te s t h a t :

" M o v e m e n t o c c u r r e d at t h e fa u l t o n D e c e m b e r 1 4 , r u p t u r in g t h e i m p e r v io u sb l a n k e t a n d a d m i t t i n g f u ll re s e rv o i r p re s s u re t o t h e f a u l t a n d t o t h e d r a i n a g es y s t e m f o r t h e f i rs t t i m e . "

T h e a u t h o r , o n t h e b a si s o f t h e i n v e s t ig a t io n s r e p o r t e d b y S h a n n o n a n dWi ls on ( 1 9 7 0 ) w o u l d h a v e w o r d e d t h i s s ta t e m e n t s o m e w h a t d i f f e r e n t ly,p e r h a p s a s f o l l o w s :

" D i f f e r e n t i a l s e t t l e m e n t o f t h e f o u n d a t i o n s oi ls o n t h e d o w n t h r u s t s id e o ft h e f a u l t s u n d e r r e s e r v o i r l o a d i n g r u p t u r e d t h e d r a i n p i p e s a t a n e a r l y s t a g eo f t h e r e s e r v o ir li fe , p e r m i t t i n g w a t e r t o i n f il t ra t e t h e f o u n d a t i o n s o il s a n d t of l o w a l o n g p a t h s a d j a c e n t t o t h e f a u l t s. T h i s s e e p a g e le d t o i n t e r n a l e r o s i o na n d t h e d e v e l o p m e n t o f c a v e r ns a n d p i p es . T h e c o l la p s e o f t h e l in in g i n t o

t h e s e o p e n i n g s o n D e c e m b e r 1 4 , 1 9 6 3 , a d m i t t e d f u ll r e s er v o i r p r e s s u re t o t h ef a u l t w h i c h i n t u r n r e l ie v e d t h e e x i s t in g v e r t i c a l s h e a r s tr e s s es a n d r e s u l t e d i nu p w a r d r e b o u n d o f th e e a st si de o f t h e f a u l t ."

I t is n o t r e a s o n a b l e t o c o n c l u d e t h a t f a u l t m o v e m e n t s s h o u l d h a v e b e e na n t i c i p a t e d i n t h e d e s i g n st a g e w h e n t h e r e is n o t u n a n i m i t y o f o p i n i o n , a f t e rt h e f a i lu r e , a s t o w h e t h e r o r n o t t h e r e w e r e f a u l t m o v e m e n t s p r i o r t o t h ef a il u re . T h e e x i s t e n c e o f fa u l t s in t h e f o u n d a t i o n a r e a w a s k n o w n a t t h e t i m eo f d es ig n b u t t h e a m o u n t o f f u t u r e a c t i v i t y w a s a n t i c ip a t e d t o b e m i n o r.E v e n a f t e r f a i lu r e t h e t o t a l v e r t i c a l o f f s e t a c r o s s F a u l t I w a s l e ss t h a n 0 . 5 f t . ,a n d a b o u t 0 . 2 5 f t . a c r o ss F a u l t V. I n a n y e v e n t i t is t h e a u t h o r ' s o p i n i o n t h a t

i n te r n a l e r o s i o n a l o n g th e f a u l t w h i c h r e s u l t e d f r o m i n f i lt r a ti o n o f w a t e r w a st h e c a u s e o f f a il u re , a n d n o t " f a u l t m o v e m e n t " . P r o b a b l y f a il u re w o u l d h a v eo c c u r r e d w i t h o u t a n y f a u lt d i sp l a c e m e n t h a d t h e s a m e a m o u n t o f w a t e r b e e np e r m i t t e d t o i n f i lt r a te t h e f o u n d a t i o n s oil s a t th e l o c a t i o n o f t h e e x i st in g f a u lt s .


8/15

134

D i f f e r e n t ia l s e t t l e m e n t o f t h e d o w n t h r u s t b l o c k a l o n g e x is t in g f a u l ts h a su n d o u b t e d l y o c c u r r e d o n o t h e r p r o j e c ts , a l th o u g h t h e a u t h o r is u n a w a r e o fa n y r e p o r t e d i n t h e l i te r a t u r e . T h i s is a m a t t e r o f i n t e r e s t t o d a m e n g i n e e r s ,h o w e v e r , a n d c e r t a i n ly t h e p o s s i b i li t y o f d i f f e re n t i a l s e t t l e m e n t s h o u l d b ei n v e s t i g a t e d if e x i s t in g f a u l t s a r e l o c a t e d b e n e a t h l o a d e d s t r u c t u r e s . A t t h eB a l d w i n H i ll s R e s e r v o i r i t w a s p a r t i c u l a r l y c r i ti c a l b e c a u s e o f t h e h i g h l ye r o d i b l e n a t u r e o f t h e f o u n d a t i o n s oi ls .

Ques t ion 3 .W o u l d m o r e ca r e f u l s t u d y a n d i n t e r p r e t a t i o n o f c r e st s e t t le m e n t s ,s e e p a g e f l o w r a t e s , e t c . , h a v e a l e r t e d t h e d e s i g n e r s t o i m p e n d i n g f a i lu r e b e f o r eit w a s t o o l at e? Wo u l d m o d e r n m e t h o d s o f i n te r p re t in g t h e d a t a d o a b e t t e rj o b t h a n w a s p o s si b l e i n 1 9 6 3 ? I f s o , h o w ?

T h e r e w e r e t h r e e b it s o f i n f o rm a t i o n d e v e l o p e d b y t h e m o n i t o r i n g p r o g r a mt h a t , i n h i n d s ig h t , a p p e a r t o g iv e a n i n d i c a t io n o f a w o r s e n i n g c o n d i t i o na l t h o u g h n o t n e c e s s a r il y o f i m p e n d i n g f a il u re .

H o r i z o n t a l a n d v e r t i ca l m o v e m e n t s w e r e m e a s u r e d b y t h e D W P in t h ed r a i n a g e i n s p e c t i o n c h a m b e r, w h i c h i s a r e l a t iv e l y ri g id , r e i n f o r c e d , c o n c r e t eb o x u n d e r t h e r e s er v o i r f l o o r a n d w h i c h c r o s s e d F a u l t I . T h e f i r s t c r a c k int h e i n s p e c t i o n c h a m b e r w a s o b s e r v e d o n M a y 4 , 1 9 5 1 , d u r i n g t h e f i rs t p a r ti a lf il li n g, a t S t a t i o n 0 + 7 0 9 j u s t e a s t o f F a u l t I . O n O c t o b e r 2 9 , 1 9 5 1 , s t r a i ng a g e p o i n t s w e r e e s t a b l i s h e d a c r o s s t h i s c r a c k .

T h e d i f f e r e n t i a l s e t t l e m e n t a c r o ss F a u l t I i m p o s e d b e n d i n g s tr e ss e s i n t h er e la t iv e l y r ig id c o n c r e t e i n s p e c t i o n c h a m b e r w h i c h d e v e l o p e d a t e n s i o n c r a c ki n t h e r o o f a b o u t 1 0 f t . e a st o f t h e f a u l t ( S t a t i o n 0 + 7 0 ) . A s s e t tl e m e n t si n c r e a s e d t h i s c r a c k g r a d u a l l y w i d e n e d u n t i l i n l a t e 1 9 5 7 a n e w t e n s i o n c r a c kd e v e l o p e d s u d d e n l y i n t h e f l o o r a t S t a t i o n 0 + 8 9 , a b o u t 9 f t . w e s t o f F a u l t I .T h e r e s u l t i n g t il t in g o f t h e r ig id c h a m b e r a c r o s s t h e f a u l t r e s u l t e d i n a s u d d e ni n c re a s e in o p e n i n g o f t h e r o o f c r a c k 1 9 f t . t o t h e e a st .

T h e o v e ra ll t r e n d o f in c r ea s in g r a t e o f c r a c k o p e n i n g w a s s o m e w h a to b s c u r e d b y t h e e r r a t ic c h a n g e s r es u l ti n g f r o m s e a so n a l t e m p e r a t u r e c h a ng e s.A c o m p a r i s o n o f r ea d i ng s o n a p a r t ic u l a r d a t e w i t h t h o s e o f t h e 2 o r 3p r e v i o u s r e a di n g s w o u l d s h o w o n l y e r r at ic r e a d in g s o f n o s i g n if ic a n ce . H o w -

e v e r, a l o n g - t e r m p l o t , s u c h a s F i g . 4 , r e v e a l s q u i t e c l e a r l y a r a p i d i n c r e a s e i nr a t e o f c r a c k o p e n i n g s t a rt in g i n a b o u t 1 9 5 9 . O n e s t a n d a r d c r i te r i o n f o re v a l u a t i o n o f m o v e m e n t d a t a i s t o b e a le r t t o a n y i n c r e a se in th e r a t e o fm o v e m e n t w i t h t i m e , i r r e sp e c t iv e o f t h e a c t u a l r a t e. C u r v e s A , B, an d C( F i g. 4 ) a r e t y p i c a l l a b o r a t o r y u n c o n f i n e d c o m p r e s s i o n t e s t s o n a cl a y- s h al ef r o m P a n a m a w h i c h s h o w e d s u c h a n in c re a se i n ra t e o f d e f o r m a t i o n w e l l ina d v a n c e o f a c t u a l f ai l u re . T h i s p a r t i c u l a r s e t o f d a t a b y i t se l f is n o t n e c e s s a r i l yr e l a t e d t o t h e u l t i m a t e f a i l u re b u t i t m i g h t h a v e s e r v e d as a r e d f l a g h a d i tb e e n p l o t t e d a n d r e v i e w e d .

I n A p r i l a n d M a y, 1 9 5 1 , w h i l e t h e r e s e r v o i r w a s b e i n g f i l l e d f o r t h e f i r s t

t i m e , e x c e s si v e l e ak a g e o f u p t o 7 5 g p m d e v e l o p e d t h r o u g h t h e l in in g a d j a c e n tt o t h e i n t a k e t o w e r. T h e r e s e rv o i r w a s d r a i n e d a n d a f t e r e x t e n s iv e re p a ir s w a saga in f i l l ed .


9/15

TIME

t YEARS

1 3 5

F i g .4 . C r a c k o p e n i n g i n B a l d w i n H i ll s i n s p e c t i o n c h a m b e r.

F i g .5 is a s u m m a r y p l o t o f t h e t o t a l m e a s u r e d l e ak a g e o v e r t h e o p e r a t i o n a ll if e o f t h e r e s e r v o i r. I n J u l y 1 9 5 1 s h o r t l y a f t e r t h e r e s e r v o i r w a s p l a c e d i n t or e g u l a r s er v ic e , t h e t o t a l m e a s u r e d l e a k a g e p e a k e d a t a l m o s t 2 4 g a l lo n s p e rm i n u t e , w i t h t h e p o o l r a is e d t o w i t h i n 6 ft . o f t h e s p i l l w a y li p. S i n c e t h i sp e a k f l o w w a s o n l y 1 / 3 o f t h a t m e a s u r e d d u r i n g t h e f ir s t fi ll in g , t h e r e p a irw o r k w a s j u d g e d t o h a v e b e e n s a t i s f a c to r y. H o w e v e r , d u r i n g t h e l if e o f t h er e s e r v o i r t h e r e w e r e l arg e v a r i a t i o n s i n t h e r a t e o f f l o w f r o m c e r t a i n d r a i n sw h i c h a p p a r e n t l y b e c a m e p l u g g e d a n d t h i s c a u se d i n c r ea s e d f l o w in a d j a c e n td r a i n s .

D u r i n g t h e f i rs t 2 . 5 y e a r s o f r e s er v o i r o p e r a t i o n , t h e t o t a l m e a s u r e d l e ak a g es te a d il y d r o p p e d f r o m t h e m a x i m u m r a te o f 2 4 g p m t o a b o u t 1 0 g p m . H o w -

e v e r , i t w a s d u r i n g t h i s p e r i o d , o n O c t . 3 0 , 1 9 5 1 , t h a t t h e D . W. P. f i rs t p r o v e d ,b y m e a n s o f a d y e t e s t , t h a t w a t e r f r e e ly f lo w e d t h r o u g h t h e a s ph a lt " m e m -b r a n e " f r o m t h e s o u t h e a s t t o e d r ai n t o t h e t o w e r b a se d r ai n. T h u s w a t e r w a sa b l e t o f l o w i n t o t h e f o u n d a t i o n a n d f a u l ts e a r ly i n t h e l i fe o f t h e r e s e rv o i r.T h e n , f o r t h e n e x t n i n e y e a r s , f r o m e a r ly 1 9 5 4 t h r o u g h e a r ly 1 9 6 3 , t h e l ea k -a g e r a t e f l u c t u a t e d b e t w e e n 7 an d 1 0 g p m , e x c e p t in ea r ly 1 9 5 7 w h e n t h er e s e r v o i r w a s d r a i n e d f o r c l ea n i n g , i n s p e c t i o n , a n d r e p a ir. W h i l e e m p t y, t h et o t a l m e a s u r e d f l o w d r o p p e d t o a b o u t 2 g p m , b u t r e t u r n e d t o a n av e ra g e r a teo f a b o u t 8 g p m a f t e r re f il li n g. D u r i n g t h e l a s t s ix m o n t h s o f t h e r e s e r v o i r 'sl if e , t h e t o t a l m e a s u r e d s e e p a g e in c r e a s e d s h a r p l y f r o m a n a v e ra g e r a t e o f

a b o u t 9 g p m t o o v e r 1 3 g p m j u s t p r io r t o f a i lu r e ; h o w e v e r , t h e m e a s u r e d r a teo f l e ak a g e w a s s ti ll w e l l b e l o w t h a t m e a s u r e d i n 1 9 5 1 a n d 1 9 5 2 .T h e s i g ni f ic a n t d e c r e a s e i n r a te o f l e a k a g e m e a s u r e d i n t h e e a r l y y e a r s o f

r e s e r v o i r o p e r a t i o n w a s b e l i e v e d b y D . W. P. t o b e t h e r e s u l t o f t h e s e lf - se a l in g


10/15

2 2 2 2

1

'5

D 1:~0 8 6 4 2 0

TO

LE

GM

1

1

1

1

1

1

R

RF

L

1

1

6

R

RD

N

FO

N

O

C

N

A

MIN

R

R

D

N

MA

1

(A

e

S

eWIL

N

9

I

I

1

I

I

I

1

1

1

1

1

1

1

•2

•2 -2 -2 -

2 o

-1 -2

5

-8

~

-6 -

Fg5

Me

e

v

me


11/15

1 3 7

n a t u r e o f th e c o m p a c t e d e a r t h l in in g . In t h e a u t h o r ' s j u d g m e n t t h e t o t a ll e ak a g e t h r o u g h t h e e a r t h l i ni ng m a y a c t u a l l y h a v e i n c re a s e d w i t h t i m e , a n dt h e d e c r e a se o f r a te m e a s u r e d i n th e i n s p e c t i o n c h a m b e r m a y h a v e b e e nc a u s e d b y g r a d u a ll y i n c re a si n g s e e p a g e i n t o t h e f o u n d a t i o n , p a r t ic u l a r ly i n t of a u l t s. T h e p r e - e x i s ti n g s y s t e m o f fa u l t s a n d j o i n t s p r o v i d e d a r e l a t iv e l y e a s yp a t h f o r w a t e r to f l o w d o w n w a r d i n to m o r e p e r m e a b l e s t ra t a a t d e p t h . O i lc o m p a n y r e c o r d s s h o w t h a t d e v e l o p m e n t w e l ls d ri ll ed a f te r 1 9 2 4 i m m e d i a t e l ys o u t h o f th e r e s e rv o i r e n c o u n t e r e d l o s t c i r c u la t io n o f t h e d r il li ng f l u id s w i t h i nt h e u p p e r s e v e ra l h u n d r e d f e e t . T h e s e s a m e c o n d i t i o n s m u s t h a v e e x i s t e du n d e r t h e r e s e rv o i r , e x p l a i n i n g h o w l arg e q u a n t i t i e s o f s e e p a g e w a t e r a n d f i n es o il p a r t ic l e s c o u l d e a s il y h a v e b e e n c a rr ie d t o d e p t h a n d w o u l d n o t h a v ea p p e a r e d a s s e e p a g e at t h e t o e o f t h e d a m o r e l se w h e r e a r o u n d t h e r e s e rv o i r.T h e f a c t t h a t l ar g e e r o s i o n a l c av i ti es a n d p i p e s w e r e f o u n d i n t h e f o u n d a t i o nf o l l o w i n g f a il u re s u b s t a n t i a t e s t h e a u t h o r ' s o p i n i o n t h a t f o u n d a t i o n d e t e r io r a -t i o n h a d b e e n d e v e l o p i n g s i n c e f i r st fi ll in g . S u c h e r o s i o n p i p e s t a k e y e a r s t od e v e l o p a n d g r e a tl y i n cr e as e t h e c a p a c i t y o f t h e f o u n d a t i o n t o a c c e p t l e ak a g ef r o m t h e r e s e r v o i r. T h u s , t h e s i g n i fi c a n c e o f t h e o b s e r v e d d e c r e a s e i n le a k a g ec a n b e e a s il y m i s i n t e r p r e t e d a n d l e a d t o g r o s s ly m i s l e a d i n g c o n c l u s i o n s . I t isp r o b a b l e t h a t t h e s h a r p i n c r e a se in th e f l o w m e a s u r e d i n t h e i n s p e c t i o nc h a m b e r d u r i n g t h e s e c o n d h a l f o f 1 9 6 3 ( F ig . 5 ) r e fl e c t e d a su b s t a n t ia li n c re a s e i n t h e r a t e o f f l o w p e n e t r a t in g t h e e a r t h l in in g , w h i c h e x c e e d e d t h ec a p a c i t y o f t h e v a ri o u s r u p t u r e s i n t h e a s p h al t " m e m b r a n e " t o a c c o m m o d a t et h e a d d i t i o n a l s e e p a g e .

S e t t l em e n t m e a s u r e m e n t s w e r e m a d e a r o u n d t h e c r es t o f t h e d a m p e ri od i -c a U y t h r o u g h o u t i ts l if e. T h e i n t e r p r e t a t i o n o f t h e s e t t l e m e n t d a t a w a s c o m -p l i c a te d b y t h e f a c t t h a t t h e r e w a s r e g io n a l s u b s i d e n c e , v ar ia b le c o m p r e s s i o no f t h e e m b a n k m e n t s , v a r ia b l e c o m p r e s s i o n o f t h e f o u n d a t i o n s oi ls , a n d e r r o r sd u e t o b e n c h m a r k s u b s i d e n c e . C a r e f u l p o s t - fa i l u re a n a l y s is o f t h e d a t a , h o w -e v e r, p e r m i t s o n e t o c o n c l u d e t h a t s h a rp d i f f e r e n t i a l s e t t le m e n t s o f 0 . 2 t o0 . 3 f t . d e v e l o p e d a c r o s s F a u l t I u n d e r l y i n g t h e m a i n d a m d u r i n g t h e l if e o ft h e r e s e r v o i r, a s s h o w n i n F i g . 6 .

T h e a u t h o r d o e s n o t c o n s id e r a n y o f th e a b o v e d a t a to b e s o o b v i o u s t h a ti t s h o u l d h a v e a l e r t e d t h e d e s i g n e r t o a n i m p e n d i n g f a i l u r e . N o r d o e s h e

b e li ev e t h a t m o d e m m e t h o d s o f in t e rp r e ti n g t h e d a t a w o u l d h a v e le d t od i f f e r e n t c o n c l u s io n s .

Ques t i on 4 . C o u l d a b e t t e r d e f e n s e a g a in s t s e e p ag e e r o s i o n h a v e b e e nd e s i g n e d ? I f s o, h o w ?

T h e a n s w e r t o t h i s q u e s t i o n i s o b v i o u s l y y e s B u t t o a c t u a l l y d e s ig n ab e t t e r d e f e n s e is n o t s o ea s il y a c c o m p l i s h e d . L e o n a r d s ( 1 9 8 4 ) p r o p o s e s t om o v e t h e d r a in a g e s y s t e m f r o m b e l o w t h e c l a y l in in g t o t h e s u r f a c e, a n d t oa d d a 3 - in c h p r o t e c t i v e a s p h a l t l in i n g o n t o p . T h i s i s p e r h a p s a n i m p r o v e m e n t

b u t i n t h e a u t h o r ' s o p i n i o n n o t a c o m p l e t e s o l u t i o n . M a k i n g a w a t e r t i g h t s ea lw h e r e t h e l in in g j o i n s t h e i n t a k e t o w e r w o u l d h a v e b e e n v e r y d i f fi c u l t. D i f fe r -e n t ia l s e t t l e m e n t a c r o s s t h e f a u lt s , c o m b i n e d w i t h h o r i z o n t a l e l o n g a t i o n , q u i t e


12/15

1 3 8

0 . 0

O i

0 2

0 . 3 -

0 4

~u¢ ' 0 . 5

O

Ld_ o z

I - -

O,B

£ 0 9

L O

l. i

1 2

4 + O 0|

390 8ERM STATIONS( L o o k i n g N o r t h )

6 + 0 0 8 + 0 0 I O + O 0 1 2 + 00 1 4 + 0 0I I I I I I I i I I

\ / ;

/ ,========================================================================= ..¥ ~ " I===================== , i

~ i i iiii i :: .. CO MP ACT ED i~ '~ ,~, BR E A C H• -:-::::':-:. ix~::::::::::::;: F IL L ~ i

" "::i: :i: : ~ ,

uRIGINAL \ ~ - - - EXCAVATION LINE

~, ~ ,~ / 0.210,

/TOTALETTLEMENTl O ( M S L )

" ~ [ / D E C 1 948 - DEC1 9 6 3~ # (15y e a r s )

( A f t e r SHANNON ~ WILSON, 1970)

F i g .6 . S e t t l e m e n t o f m a i n d a m .

1 6 + 0 0i

• 3 9 0

• 3 7 0

• 3 5

• 3 0 ~

.3,o ~w

• 2 9 0

2 7 0

p r o b a b l y w o u l d h a v e ru p t u r e d t h e a s p h a l ti c m e m b r a n e s a s w e l l a s t h e a s p h a ltp a v in g a n d th u s s t i l l l e d t o c o n c e n t r a t e d i n f l o w s t o t h e f o u n d a t i o n f a u lt s .A l s o th e 4 - i n c h c l a y t i l e d ra i n s w o u l d h a v e h a d i n s u f f i c i e n t c a p a c i t y t o h a n d l ethe l a rge in f lows f rom c racks in the a spha l t l i n ing .

A t t h e L u d i n g t o n P u m p e d S t o r a ge R e s e r vo i r i n M i c h i g a n W h i t e h ea d a n dR u o t o l o , 1 9 7 3 ) , w h e r e t h e e m b a n k m e n t s w e r e a ls o o f h i g h ly e r o d i b lem a t e r i a ls , t h e s c h e m e a d o p t e d c o n s i s t e d o f t h e f o l l o w i n g :

H y d r a u l i c a s p h a lt c o n c r e t e l i n i n g w a s s e l e c te d f o r t e c h n i c a l e c o n o m i c

a d v a n ta g e s. F r o m b o t t o m t o t o p , t h e a s p ha l t p a v e m e n t c o n s i s t s o f a 3 - i n c h7 6 m m ) l a y er o f h y d r a u li c a sp h a l t c o n c r e t e ; 1 8 i n c h 4 6 0 m m ) o f w e l l -graded l l /~-inch 38 m m )- m ax ) aggregate , w hi ch serves as drain ; a 21/~-inch6 4 m m ) l a y e r o f b i n d e r c o u r se , se rv in g a s b i n d e r b e t w e e n o v e r l y i n g i m p e r v i o u s

aspha l t and d ra in ma te ri a l ; and two 21/~-inch 64 m m ) l ayer s o f im perv io ushydrau l i c a spha l t concre te .

A t th e t o e o f t h e a s p h al t f a c e a n d i n s i d e t h e 1 8 - i n c h 4 6 0 m m ) d ra i n l a y e ri s a 1 0 -i n c h 2 5 4 m m ) s l o t t e d f i b erg la s s p i p e r u n n i n g a ro u n d t h e e n t i r e i n n e rp e r i m e t e r o f t h e e m b a n k m e n t . To th i s p i p e are c o n n e c t e d 8 - i n c h 2 0 3 m m )d iam . f ibe rg lass cas ing no s lo t s ) a t 150 f t . 45 .7 m ) in te rva l s; t hes e run up

t h e s l o p e a n d t e r m i n a t e i n a m a n h o l e a t t h e t o p o f t h e e m b a n k m e n t . A n yw a t e r f i n d i n g i t s w a y i n t o t h e d r a i n a ge z o n e b e t w e e n a s p ha l t la y e rs i sc o l l e c t e d i n t h e t o e d r a i n a n d p u m p e d t o t h e s u r f a c e b y s u b m e r s i b l e p u m p sins ide the 8 - inch cas ings .


13/15

1 3 9

T h e s a f e t y o f th e B a l d w i n H ills R e s e r v o ir d e p e n d e d o n p r e v e n t in g w a t e rf r o m t h e r e se r v o ir e n t e r in g t h e u n d e r l y i n g s t r a ta a n d f a u l ts ; b u t i r o n ic a l ly,c e r t a in f e a t u r e s o f t h e d e s i g n w h i c h w e r e i n t e n d e d t o a c c o m p l i sh th i s m a yh a v e s u b s ta n t i al ly c o n t r i b u t e d t o t h e p r o c es s es w h i c h c u l m i n a t e d i n f a il u re .I f t h e b r i tt l e d r a i n a g e s y s te m b e n e a t h t h e c l ay l in in g h a d n o t b e e n u s e d , t h es e ep a g e w o u l d h a v e b e e n m o r e d i s p e rs e d a n d i t m a y h a v e t a k e n y e a r s l o n g e rt o p r o d u c e t h e d e g r e e o f u n d e r g r o u n d e r o s i o n w h i c h l e d t o f a il u re ; a l t h o u g he v e n t u a l l y t h e r e s e r v o i r m i g h t s t il l h a v e f a i le d e s s e n t ia l ly i n t h e s a m e m a n n e r.

I t is d e b a t a b l e w h e t h e r a s a f e r e s e r v o ir c o u l d h a v e b e e n d e s i g n e d f o r t h e s ef o u n d a t i o n c o n d i t i o n s . P r o b a b l y o n l y a s t e e l l in i ng c o u l d h a v e g i ve n r e a s o n -a b l e a s s u r a n c e o f s a f e t y. O t h e r m e a s u r e s , s u c h a s o v e r e x c a v a t i o n a n d b a c k f il l-i n g o f t h e f a u l t s w i t h c l a y, u s e o f a m o r e p l a st ic c l a y f o r t h e l in i n g, a n dp a r t i cu l a r l y e l im i n a t i o n o f a n y s u b d r a in a g e s y s t e m , w o u l d h a v e e x t e n d e d t h el if e o f t h e r e s e rv o i r, b u t w i t h o u t e n s u r i n g t h e d e g r e e o f s a f e t y t h a t m u s t b ed e m a n d e d o f s u c h a r es er v oi r. A m o n g t h e m a n y d i f f e re n t t y p e s o f e n g in e er -i ng p r o b l e m s w h i c h t h e d e s ig n e r s o f s u c h f a c il it ie s m a y e n c o u n t e r , t h e m o s tf e a r e d c o n d i t i o n s a re t h o s e t h a t c o u l d l ea d t o u n d e r g r o u n d e r o s i o n a n dp i p in g f a i lu r e . M o s t o t h e r r i sk s c a n b e e f f e c t i v e l y m o n i t o r e d b y a p p r o p r i a t eo b s e r v a t io n s , b u t p i p in g i s a n i n s id i o u s p h e n o m e n o n w h i c h u s u a l ly e s c a p eso u r a t t e n t i o n u n t i l it is t o o l a te .

Question 5 C o u l d a b e t t e r m o n i t o r i n g s y s t e m h a v e b e e n d e s i g n e d ? I f so ,h o w ?

T h e m o n i t o r i n g s y s t e m i n s t a ll e d a t B a l d w i n H i ll s R e s e r v o i r w a s w e l ld e s i g n e d , c a r e f u l l y i n s ta l le d , m o n i t o r e d a t f r e q u e n t in t e rv a l s, a n d t h e d a t aw e r e p r o m p t l y r e v i e w e d a n d a n a l y z e d . B e c a u se t h e d e s ig n e r s c o n s i d e r e dc o n t r o l o f l e a ka g e t o b e o f p r i m e i m p o r t a n c e , t h e y t o o k e l a b o r at e s te p s tor e c o r d a l l m e a s u r a b l e fl o w s a t f r e q u e n t in t e rv a l s. H o w e v e r, t h e e f f e c t i v e n e s so f t h e d r a in a g e s y s t e m a n d t h e r e l ia b i li ty o f t h e s e ep a g e m e a s u r e m e n t sd e p e n d e d e n t i r e l y o n t h e w a t e r t i g h t n e s s o f t h e 1 A -in ch -th ic k a s p h a lt m e m -b r a n e w h i c h s e p a r a t e d t h e p e a g r a v el -d r ai n ag e s y s t e m f r o m t h e f o u n d a t i o ns oils. A n y r u p t u r in g o f th is m e m b r a n e , o r i m p e r f e c t i o n in t h e m e m b r a n e

i ts e lf , w o u l d o b v i o u s l y p e r m i t w a t e r t o i n f i l tr a t e t h e f o u n d a t i o n s oi ls , ac o n d i t i o n t h e d e s i g n er s k n e w w o u l d n o t b e a c c e p t a b l e .I t is d i f f ic u l t t o c o n c e i v e o f a m o n i t o r i n g s y s t e m t h a t w o u l d d e t e c t s e ep a g e

a t c o n c e n t r a t e d p o i n t s o f t h e a s p h al t l in in g , c o n s i d e ri n g t h e a r e a o f t h em e m b r a n e i nv o lv e d.

C o n s i d e r i n g t h e t h i n n e s s o f t h e a s p h a l t l in i n g a n d t h e p o s s i b i li t y o f f a u l tm o v e m e n t ( o r d i ff e r e n t i a l s e t t l e m e n t a c ro s s t h e f a u lt s) i t w o u l d h a v e b e e nd e s i r a b le t o h a v e i n s t a ll e d r e m o t e s e n s in g d i f f e r e n t i a l s e t t l e m e n t g a ge s a c r o sst h e f a u l t s . S u c h g ag e s w e r e n o t r e a d i l y a v a il a bl e a t t h e t i m e o f c o n s t r u c t i o na n d p r o b a b l y a c u s t o m d e s ig n e d i n s t r u m e n t w o u l d h a v e b e e n r e q u i r e d . T h e

i n s p e c t i o n c h a m b e r i t s e lf c r o s s e d t h e f a u l t b u t i t w a s so r ig i d t h e s e t t l e m e n t sw e r e m o r e o r le ss u n i f o r m l y d i st r i b u t e d o v e r a b o u t 5 0 f t. a n d c o u l d h a v eb e e n i n t e r p r e t e d a s b e in g a g e n e r a l p a t t e r n o f s e t t l e m e n t .


14/15

140

Q u e s t i o n 6 . S h o u l d a d a m h a v e b e e n b u i l t a t th i s s it e i n 1 9 5 1 7 To d a y ?

I t m u s t b e e m p h a s i z e d t h a t t h e d a m i ts e lf p e r f o r m e d s a t is f a c to r i ly d u r i n gt h e e n t i r e l i fe o f t h e r e s e r v o ir a n d i n n o w a y c o n t r i b u t e d t o t h e f a il u re o f t h er e s e r v o i r.

I t w a s k n o w n b e f o r e c o m p l e t i o n o f t h e r e se r v o ir t h a t t h e B a l d w i n H i ll sw e r e e x p e r i e n c i n g s u b s i d e n c e , a c c o m p a n i e d b y t e n s i le s t r ai n s a t th e r e s er v o irs it e. T h e s e m o v e m e n t s p e r s i s t e d a t a s t e a d y r a t e t h r o u g h o u t t h e l if e o f t h er e se r v o ir a n d s ti ll c o n t i n u e . E v e n s o , t h e m a g n i t u d e o f t h e s e m o v e m e n t s i s s os m a l l , i n c l u d i n g t h e d i f f e r e n t i a l s e t t l e m e n t a c r o s s F a u l s I a n d V, t h a t i tw o u l d n o t h a r m m o s t o f t h e e a r t h d a m s a n d r e se rv o ir s w i t h w h i c h t h e a u t h o ris f a m il ia r , e x c e p t i n g t h e B a l d w i n H i ll s R e s e r v o ir . O n e m a y q u e s t i o n w h e t h e ra n y i m p o r t a n t d a m a n d re s e r v o ir s h o u l d b e p e r m i t t e d t o b e c o n s t r u c t e d i f i tc a n n o t w i t h s t an d w i t h p e r f e c t s a f e t y m o v e m e n t s o f s u c h m a g n i tu d e .

T h e r e a r e a t le a s t se v e r a l e x i s t in g d a m s t h a t a r e b u i l t a c r o s s f a u l t s w h i c ha r e e i t h e r k n o w n o r s u s p e c t e d o f b e i n g a c ti v e. C e d a r S p r i ng s D a m i n s o u t h e r nC a l if o rn i a, c o n s t r u c t e d i n 1 9 7 1 , w a s d e s i g n e d t o w i t h s t a n d s u d d e n f a u l tm o v e m e n t s o f 3 t o 5 f t . e i th e r v e r t i c a l ly o r tr a n s v e r s e t o t h e c o r e . T h e d e s i g no f t h e s a f e t y f e a tu r e s i n c o r p o r a t e d i n t h is 7 6 m h ig h d a m w a s re v i e w e d i nt h e e a r l y 1 9 8 0 ' s a n d t h i s r e v i e w v e r i f ie d t h e o r i g in a l d e s i g n a s s u m p t i o n s w i t hr e g a rd t o t h e s e i s m ic e f f e c t s o n t h e d a m .

T h e L o s A n g el e s D a m , w h i c h w a s b u i l t t o r e p l a c e t h e L o w e r S a n F e r n a n d oD a m w h i c h w a s se v e re ly d a m a g e d b y t h e S a n F e r n a n d o E a r t h q u a k e , w a s

d e s i g n e d t o w i t h s t a n d f a u l t m o v e m e n t s o f u p t o 1 5 f t . i n a n y d i r e c t i o n a n da t u n s p e c i f i e d l o c a t i o n s .A l t h o u g h t h e v a l i d i ty o f t h e d e s i g n h a s n o t b e e n v e r i f ie d b y a c t u a l f a u l t

m o v e m e n t s a t e i th e r o f t h e a b o v e s i te s , i t is c l ea r t h a t d e s ig n e r s t o d a y c o n -s id e r i t f e a si b l e t o d e s ig n e m b a n k m e n t d a m s t o r e si st f a u l t m o v e m e n t s o fm u c h l ar g er m a g n i t u d e t h a n d e v e l o p e d a t B a l d w i n H i lls . H o w e v e r , h e r e t h ep r o b l e m w a s t o p r e v e n t s e e p a g e t h r o u g h t h e b l a n k e t f r o m i n f il tr a ti n g t h ef o u n d a t i o n s o il s. W i t h t h e b e n e f i t o f h i n d s i g h t i t is e a s y t o o u t l i n e , i n p r i n -c i p le , a f e w b a s i c s t e p s t h a t m i g h t h a v e m a d e t h e l in i n g s af e .

( 1 ) E x c a v a t e t h e f a u l t t r a c e s t o a d e p t h o f a t le a s t 5 f t a n d r e p l a c e w i t h a

p l a s t i c m a t e r i a l .( 2 ) D o n o t c a r r y t h e p e a - g r av e l d r a i n a c r o s s t h e f a u l t s , b u t s t o p i t at l e as t5 f t. a w a y f r o m t h e f a u lt s.

( 3 ) P l ac e t h e d r a i n p i p e s o n l y i n N - - S d i r e c t i o n s s o a s t o a v o i d r u p t u r e d u et o o f f s e t t i n g s e t t l e m e n t a l o n g t h e f a u l t , a n d a l so t o a v o i d e l o n g a t i o n .

( 4 ) R e p l a c e t h e 1A -inch a s p h a l t m e m b r a n e w i t h a t h i c k e r l a y e r o f a m o r ep l as ti c m a t e r i al , p o s s i b l y e ve n a p la s ti c o r r u b b e r m e m b r a n e .

( 5 ) P l a c e a n i m p e r v i o u s p l a s t ic m a t e r i a l s e v e r a l f e e t i n t h i c k n e s s b e t w e e nt h e m e m b r a n e a n d t h e f o u n d a t i o n s oils .

CONCLUDING REMARKS

T h e B a l d w i n H il ls R e s e r v o i r d u r i n g i ts s e r v ic e l if e o f 1 3 y e a r s w a s s u b j e c t e dt o t i l ti n g , s u b s i d e n c e , e l o n g a t i o n , a n d v e r t i c a l o f f s e t t i n g a c r o s s t w o p r e - e x i s t in g


15/15

Date post:	05-Jul-2018
Category:	Documents
Upload:	edy-tejeda
View:	217 times
Download:	0 times

A Discussion of the Baldwin Hills Reservoir Failure 1987 Engineering Geology

Documents