Strength Determinations on Hogarth Mine Clay Barrier

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STRENGTIJ IjBTERFIIIVATIOII35 ON HOGAHTH MIIiU CLAY BAlifiIEF?

W.

J. Eden

ANALYZED

IS

P r e p a r e d for S t e e p Rock

Iron

K i n e s L 5 r c i t e d "

R e p o r t >To.

53

of' t h e

D i v i s i o n of Building Research

OTTAWA

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STRENGTH CETERMINATIONS ON HOGARTH

MINE

CLAY BARRIER

'rl. J. Eden

The s t r i p p i n g of l a k e bottom s i l t s from t h e Hogarth and "G" o r e b o d i e s a t Steep Rock Lake, by h y d r a u l i c dredging, i s t o be c a r r i e d o u t i n two s t a g e s . The i n i t i a l s t a g e , now almost complete, i n v o l v e s t h e s t r i p p i n g of 50 _{m i l l i o n c u b i c}

y a r d s from t h e a r e a of Hogarth Mine. The b a r r i e r i s a s l o p e c u t i n t h e varved c l a y south of t h e Hogarth Mine, d i v i d i n g t h e two s t a g e s . The s l o p e of t h e b a r r i e r h a s a maximum h e i g h t of about 160 f e e t . When t h e dredging n o r t h of t h e b a r r i e r i s complete, t h e dredges w i l l be moved over t h e b a r r i e r t o s t a r t t h e s t r i p p i n g i n t h e a r e a of t h e "G" o r e body. As t h e second

s t a g e of dredging p r o c e e d s , t h e b a r r i e r w i l l g r a d u a l l y be removed. Thus, t h e b a r r i e r i s t o have a s h o r t l i f e , b u t i t

must perform a s a s t a b l e s l o p e and a c t a s a n a t u r a l e a r t h

dam f o r t h i s p e r i o d i n o r d e r t o r e t a i n t h e dredge p o o l f o r t h e second s t a g e of s t r i p p i n g ,

The o v e r a l l s l o p e of t h e b a r r i e r i s t o be 8:1, made up of a s e r i e s of

3:l

s l o p e s , each r i s i n g 20 f e e t and i n t e r s p a c e d w i t h l e v e l berms 100 f e e t wide. T h i s d e s i g n i s based on an assumed shear s t r e n g t h of t h e varved c l a y of

4

p. s.i., and on p a s t experience. I n o r d e r t o check t h e s a f e t y of t h e b a r r i e r , 2%-inch diameter Shelby tube samples were o b t a i n e d a t roughly 10-foot i n t e r v a l s i n A p r i l 1954, A s e r i e s of t r i a x i a l com- p r e s s i o n t e s t s were conducted on t h e s e samples 'by t h e E i v i s i o n of B u i l d i n g Research. The purpose of t h i s r e p o r t i s t o d i s c u s s t h e r e s u l t s of t h e t r i a x i a l t e s t s i n r e l a t . i o n t o t h e s a f e t y of t h e b a r r i e r .

A block sample was t a k e n from e l e v a t i o n 1060, i n a d d i t i o n t o t h e tube samples o b t a i n e d by Steep Rock I r o n Mines Limited. S e r i e s of t e s t s were conducted on b o t h s e t s of samples so t h a t

i t h a s been p o s s i b l e t o check t h e e f f e c t of t.he tube-sampling o p e r a t i o n . A s e r i e s o f t e s t s was made t o determine whether t h e r e was a v a r i a t i o n i n s t r e n g t h due t o o r i e n t a t i o n of t h e varve s

2. The T r i a x i a l Compression T e s t

A c y l i n d e r of u n d i s t u r b e d s o i l ,

1.4.

i n c h e s i n d i a m e t e r and

3

i n c h e s long i s covered w i t h a t h i n r u b b e r membrane. It

i s p l a c e d i n a c e l l and s u b j e c t e d t o a c o n s t a n t f l u i d p r e s s u r e i n a l l d i r e c t i o n s . No d r a i n a g e from t h e sample i s allowed t o t a k e p l a c e .

With. t h e f l u i d p r e s s u r e s t i l l a c t i n g , tb.e sample i s s u b j e c t e d t o a x i a l compression a t a c o n s t a n t r a t e of s t r a i n .

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The a x i a l l o a d i s c o n t i n u e d u n t i l f a i l u r e i s r e a c h e d . The maximum a x i a l l o a d r e p r e s e n t s t h e compressive s t r e n g t h under t h e p a r t i c u l a r c o n f i n i n g p r e s s u r e . By c o n d u c t i n g a

s e r i e s o f such t e s t s , each one under a d i f f e r e n t c o n f i n i n g p r e s s u r e , t h e s h e a r s t r e n g t h may be d e r i v e d t h r o u g h u s e o f a " ~ o h r diagram", i n t e r m s of c o h e s i o n and a n g l e of i n t e r n a l f r i c t i o n . F i g u r e 1 i s a t y p i c a l Mohr diagram, o b t a i n e d f o r t h e t e s t s conducted on sample No. 2-135.

3.

T e s t R e s u l t s

( a ) Tube Samples

Shelby t u b e s 2% _{i n c h e s i n d i a m e t e r were used i n t h e} sampling o p e r a t i o n s . They were e i t h e r pushed i n by hand o r d r i v e n w i t h a heavy hammer. J u d g i n g from t h e c o n d i t i o n o f t h e samples, t h e d r i l l e r t o o k r e a s o n a b l e c a r e i n o b t a i n i n g t h e samples, and i t i s t h o u g h t t h a t t h e r e s u l t s p r e s e n t e d a r e a f a i r l y r e l i a b l e i n d i c a t i o n o f s h e a r s t r e n g t h .

Each sample t u b e was a s s i g n e d a number, e,g., 2-135,

A number o f t e s t s were conducted on each t u b e , w i t h a t e s t number a s t h e l a s t d i g i t , e.g,, 2-135-2. Depth was t a k e n from

i d e n t i f i c a t i o n t a g s on sample t u b e s , s u p p l i e d by S t e e p Rock I r o n Mines Limited. Samples 2-125 t o 2-129 were o b t a i n e d

from a h o l e marked

~ ~ 4 1 9 ,

Samples 2-130 t o 2-138 were o b t a i n e d from a b o r e h o l e w i t h t h e f o l l o w i n g c o - o r d i n a t e s N 26, 978, E 23, 254, w i t h a c o l l a r e l e v a t i o n o f 1062, Water c o n t e n t s , e x p r e s s e d a s p e r c e n t of d r y weight of s o i l , a r e t a b l e d u n d e r d a r k , l i g h t , and a v e r a g e , The d a r k r e f e r s t o a s m a l l sample t a k e n from a d a r k l a y e r o f v a r v e d c l a y , l i g h t from a l i g h t l a y e r of v a r v e d c l a y . The a v e r a g e w a t e r c o n t e n t was o b t a i n e d by weighing t h e whole t e s t s p e c i - men, and so i t i s t h e t r u e a v e r a g e f o r the varved c l a y . It

i s n o t t h e a v e r a g e o f t h e w a t e r c o n t e n t s o f t h e d a r k and l i g h t l a y e r s

,

D e n s i t i e s quoted i n t h e t a b l e a r e wet d e n s i $ i e s i n a l l c a s e s ; qc i s t h e a x i a l compression s t r e s s e x p r e s s e d i n pounds p e r s q u a r e i n c h , C o n f i n i n g p r e s s u r e i s t h e f l u i d pressi.ire a p p l i e d t o t h e specimen d u r i n g t h e t e s t . The v a l u e o b t a i n e d f o r c o h e s i o n from a t e s t s e r i e s i s r e p r e s e n t e d by

"c"

.

It i s d e r i v e d t h r o u g h u s e of t h e Mohr diagram ( F i g . l ) , and i s e x p r e s s e d i n pounds p e r s q u a r e i n c h , Tan

6

i s t h e t a n g e n t o f t h e a n g l e of i n t e r n a l f r i c t i o n r e - v e a l e d by a t e s t s e r i e s , and i s a l s o d e r i v e d fro13 a Mohr diagram.

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SUTJIF~I?RY OF TES'I' Illi4iL'.LtS ON TUBE SAPIPLES

-

.--- 2-130-1 I 307-10"

i

69.4

23.6

54.8

2-130-2

'

31!-.0" 1

63.7

,

--

' 9- 2-130-3 319-4" i

--

;

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TABLE I

continued

2-136-1 110 1 -

(7)

- 5

-

TABLE I c o n t i n u e d

( b ) Block Sample

A s e r i e s o f t e s t s were conducted on a b l o c k sample

o b t a i n e d from t h e 1060-foot berm. The o b j e c t o f t h e t e s t s was t o d e t e r m i n e whether t h e s t r e n g t h v a r i e d w i t h o r i e n t a t i o n of t h e v a r v e s . Specimens were c u t so t h a t t h e i r l o n g a x e s were o r i e n t s d a t v a r i o u s a n g l e s t o t h e d i r e c t i o n of t h e v a r v e s . T a b l e I1 i s a summary of t h e r e s u l t s .

I n t h e c o l - m n t i t l e d " t y p e of f a i l u r e f ' .'

-

a s h e a r f a i l u r e i s one which d e s c r i b e s f a i l u r e t a k i n g p l a c e on one o r more w e l l d e f i n e d p l a n e s , whereas a b a r r e i l i n g f a i l u r e

---

i s a

d e s c r i p t i o n a p p l i e d t o a f a i l u r e wliich t a k e s p l a c e when no w e l l d e f i n e d f a i l u r e p l a n e s a p p e a r a s t h e sample u n d e r g o e s

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MM! G F: 4 4 r l d

G

k k k k cd cd a

a

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SUMNARY OF TABLE I1

a n g l e v a l u e s

4.

D i s c u s s i o n of T e s t R e s u l t s ( a ) Tube Samples

The two main c o n c l u s i o n s which can be drawn from t h e t e s t i n g of t h e t u b e samples a r e : ( a ) The c o h e s i o n of t h e varved c l a y remains n e a r l y c o n s t a n t wfth d e p t h , a t about

5

p.s.i. (Fig. 2 ) ; and ( b ) The varved c l a y e x h i b i t s a s l i g h t a n g l e of i n t e r n a l f r i c t i o n , which i n t h e a v e r a g e of 10 de- t e r m i n a t i o n s was

4 '

-

40

1 .

It w i l l be noted from F i s . 2 _{t h a t t h e r e i s a c o n s i d e r a b l e}

s p r e a d i n t h e v a l u e s o b t a i n e d f o r cohesion. S e v e r a l f a c t o r s could c o n t r i b u t e t o t h e spread. The most i m p o r t a n t i s t h a t o f sample d i s t u r b a n c e , which i s i n e v i t a b l e t o some degree i n t u b e samples. Badly d f s t u r b e d samples were n o t t e s t e d , b u t i t was

n o t e d i n some samples t h a t t h e upper p o r t i o n of t h e t u b e was b a d l y d i s t u r b e d , and t h i s suggested t h a t t h e whole t u b e had

s u f f e r p a more t h a n normal d i s t u r b a n c e . G e n e r a l l v , t h e samples were good. ?be f a c t t h a t t.bp ~ e s u l t s from tiL& t;L= s = : y l e s agrez c l o s e l y w i t h t h o s e o b t a i n e d from t h e b l o c k sample s u p p o r t s t h i s impression.

It i s probable t h a t t h e r e l a t i v e amounts o f d a r k and l i g h t m a t e r i a l i n t h e t e s t specimens caused a v s r i a t i o n i n t h e r e s u l t s . It was p o s s i b l e d u r i n g t h e s e r i e s of t e s t s t o o b t a i n one specimen made up of one d a r k l a y e r . I n t h i s c a s e

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conducted on mixed d a r k and l i g h t l a y e r s from t h e same sample t u b e . T h i s t u b e however c o n t a i n e d e x c e p t i o n a l l y l a r g e d a r k l a y e r s , and cannot be c o n s i d e r e d r e p r e s e n t a t i v e . It was n o t e d d u r i n g t h e t e s t s t h a t t h e dark l a y e r s were made up of a much more b r i t t l e m a t e r i a l t h a n t h e l i g h t l a y e r s , and f a i l e d by s h e a r i n g a l o n g w e l l d e f i n e d p l a n e s . The l i g h t l a y s r s u s u a l l y f a i l e d by b a r r e l l i n g , i n d i c a t i n g a more p l a s t i c n i a t e r i a l . The average v a l u e of i n t e r n a l f r i c t i o n f o r 10 d e t e r - m i n a t i o n s was

4 '

-

401. S i n c e t h e 10 d e t e r m i n a t i o n s i n c l u d e some t e s t s on s l i g h t l y d i s t u r b e d samples, i t i s t h o u g h t t h a t t h e t r u e average rnay be somewhat h i g h e r t h a n

b0

-

LCCl V e c a u s e t h e more d i s t u r b e d samples showed no a n g l e of i n t e r n a l f r i c t i o n . I f t h e c l a y i s t r e a t e d a s a m i f o r a m d e p o s i t t h r o u g h o u t , t h e n t h e a v e r a g e o b t a i n e d f o r each s e t of t e s t s conducted a t a

c o n s t a n t l a t e r a l p r e s s u r e i s h 0

-

3 l .

T a b l e 111 l i s t s t h e d a t a p e r t a i n i n g t o t h i s c o n d i t f o n and Ffg.

3

_{shows t h e Mohr envelope} o b t a i n e d .

TABLE; I11

-

SUIQ:AHY OF TEST HESULTS AT COIJSTANT LAlERAL PRLISSURE

_-

S t r e n g t h ( p s i )

( b ) E f f e c t of O r i e n t a t i o n of t h e Varves

o r i e n t a t i o n of t h e varved p l a n e s w i t h r e s p e c t t o t h e a x i s of s t r e s s a p p e a r s t o a f f e c t tb.e s h e a r s t r e n g t h , a s h-as t h e c a s e w l t h unconfined compression t e s t s on e a r l y samples. F i g u r e

4

i l l u s t r a t e s t h e v a r i a t i o n i n one b l c c k sampie o b t a i n e d from t h e 1060-foot berm, t h e n u m e r i c a l r e s u l t s b e i n g t a b u l a t e d i n Table 11, With t h i s sample, t h e maximum s h e a r s t r e n g t h

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a p p l i e d p e r p e n d i c u l a r t o t h e varved p l a n e s , a n t i t h e minitnum when t h e d i r e c t i o n of s t r e s s i s p a r a l l e l t o t h e Trarves. Judging from e a r l i e r t e s t s r e p o r t e d i n DhH Report ~ 2 4 : 11 _{Laboratory Study of Varved Clay from S t e e p}_Rocl: _{L a k e " , t h e} r e l a t i v e amounts of d a r k and l i g h t m a t e r i a l i n t h e t e s t

specimens w i l l d e t e r n i n e t h e shape of t h e diagrar:; o b t a i n e d , T h i s i s a t t r i b u t e d t o t h e d i f f e r e n t c o n s i s t e n c y of t h e d a r k

and l i g h t l a y e r s .

Test R e s u l t s w i t h lief e r e n c e t o S t a b i l i t y of B a r r i e r

5'.

-.*

P r o f e s s o r

D.W.

T a y l o r evolved i n 1937 a mathematical s o l u t i c n t o t h e problem of s t a b i l i t y of uniform homogeneous e a r t h s l o p e s which e n a b l e s a f a c t o r of s a f e t y t o be determined when t h e f o l l o w i n g v a l u e s a r e k ~ o w n : ( a ) a ~ p a r e n t c o h e s i o n of t h e s o i l , and ( b ) a n g l e of i n t e r n a l f r i c t i o n o f ;;re soil,, . T a y l o r h a s drawn c u r v e s frorn which a nufiber h e t e r m s " s t a b i l i t y number" can be determined when t h e two f a c t o r s Listecl and d a t a on t h e s l o p e a r e a v a i l a b l e , simple c a l c u l a t i o n s w i l l produce a f a c t o r of s a f e t y .

T a y l o r r s s t a b i l i t y number i s a s follows::

c n a p p a r e n t c o h e s i o n of t h e s o i l a s deteramined by t h e t r i a x i a l

t e s t a.nd Mohr diagram F

=

f a c t o r of s a f e t y

W

a wit weight of t h e s o i l

H

=

h e i g h t of t h e slope

C o n s i d e r i n g a 20-foot bench, t h e known v a l u e s a r e c

=

5

p s i

n. 720 l b / ~ q e f t e

d

=

5"

bC:

108 l b / c u , f t , H z 200

For a s l o p e of

3:1,

T a y l o r ' s s t a b i l i t y number frorn t h e z i l r v e s e q u a l s 0.081, Then 0 , 0 8 1

=

_

C o r solvirig f c r F, W x H x F t h e s a f e t y f a c t o r y i s

4.1,

S i m i l a r c a l c u l a t i o n s f o r an o v e r s 1 1 s l o p e of

8:

1 r i . s i n g 160 f e e t show a f a c t o r o f s a f e t y of 2.1.

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T a y l o r 7 s s o l u t j - o n f o r s l o p e s t a b i l i t y c a l c u l . a t i o n s f s

based on an i d e a l c a s e i , e . , uniform homogeneo.us e a r z h s b g e s ,

which r a r e l y occur i n p r a c t i c e , The method i s , houevep,

g e n e r a l l y c o n s i d e r e d s a t i s f a c t o r y f o r a p p r o x i n n t e ca::.r:ulations, S i n c e only an i n d i c a t i o ~ of s h e a r s t r e n g t h h a s been obkssined

frorn t h e t r a i a x i a l t e s t s , t h e u s e of T a y l o r y s m.ettcd i s j u s t i f i e d , 6. Summary and Conclusions

i i ) The t e s t d a t a p r e s e n t e d i n t h i s r e p o r t shoultl bs r e g a r d e d merely a s g i v i n g an j n d i c a t i o n of t h e v a l u e s of shear

s t r e n g t h , s i n c e r e l a t i v e l y few samples were z e s t e d . Samples t a k e n from d i f f e r e n t e l e v a t i o n s f n o t h e r bore h o l e s could p r e s e n t a somewhat d i f f e r e n t p i c t u r e .

( i i ) The t r i a x i a l conlpression t e s t s t h u s f a r condusted i n d i c a t e t h a t :

( a ) c o h e s i o n I s c o n s t a n t w i t h d e p t h , w i t h a v a l u e of' about

5

p s i ; and

( b ) varved c l a y p o s s e s s e s a s l i g h t a n e l e of' i n t e r n a l f r i c t i o n , which on t h e a v e r a g e was

4.'

-

) l o 7 .

Becai:.se o f smtple

d i s t u r b a n c e , t h e r e i s some i n d i c a t i o n t h a t t8hs t r u e v a l u e may be h i g h e r t h a n t h i s f i g u r e .

(iff) T r i a x i a l compression t e s t s conducted on one b l o c k sample i n d i c a t e t h a t o r i e n t a t L o n of t h e v a r v e s has some e f f e c t on s 3 e a r s t r e n g t h .

j i v ) Based on a n a p p a r e n t cohesion of

5'

p s i and an ~ ~ l 2 l . e 3f i n t e r n a l f r i c t i o n of

5',

t h e o v e r a l l s l o p e of t h e b a r r i e r h a s a s a f e t y f a c t o r of appi70xirnate%y 2, T h i s f i g u r e i s