Stripping of clay from the south end of the "B" Orebody at Steep Rock Iron Mine, Ontario

(1)

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Stripping of clay from the south end of the "B" Orebody at Steep Rock

Iron Mine, Ontario

(2)

http://irc.nrc-cnrc.gc.ca

S t r i p p i n g o f C l a y f r o m S o u t h E n d o f t h e “ B ”

O r e b o d y a t S t e e p R o c k I r o n M i n e , O n t a r i o

D B R - R 1 2

S u t h e r l a n d , H .

A u g u s t 1 9 4 8

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(3)

4 Hugh

B,

Sutherhml

TCJ:

PI,

S, Fot,l~erLnghm

Re:

STUPPING

OF

CLAY

PW1.1

THE

S

UUTH &ND

OF TIG, ''8'' OREEODH

The following mtes have been written a f t e r study of

a

plan presented

by

M

r

,

W.

Samuel regarding the clay stripping from t h e South end of the l'B'l Orebody, I+, Samuel proposes to reduce the varved clay to a l e v e l of appro-

xhn*,ely

U1+5

feu&

i n two operations, the clay s o removed flowing t o Falla y e The slopes $0 the p i t are then to be formed by sZul.c5ng and pumping the material t.o l%lls 'day,

llodifica-ki.ons ko this plan are suggested, rnodificntions which c a l l

for less removal ?f material. Lirnltlng he5ghL.s of banks for particular slopes

of banks are also suggested.

The Analysis of t h e Stabil3.-3.-1r 0.C Earth S l o ~ e g ,

If

an earth rnass csrlrriats of a granular material such as sand or

gravel and it is devf red to cut a slope i n i t , then the mass w i l l always be

h

stable despite the $eight of t he cut slope, provided the

s h o e

ia less than the

angle

of repose of %he

material,

the angle of repose being equal to the

angle of i n t e r n a l f r i c t i o n of t h e materid. i n its loosest s t a t e , Such a cond- i t i o u i i.s not true

Tor

a

cohesive material, !:F~kh such

soil,

a cut

40

feet high

at a s ? . o p of 1 i n 3 nay be stable wMle a s i m l l n r slope

ir!.

the same material

80 f e e t h-lgh may -Tail, The soil. at Steep b c k Lake comes i n t o the cohesive

ca.tegoxy. 'The problge of stabilizing the slopes therefore does not become one of f'ixing one given irlcLlnatLon and s t i c k i n g Lo it throughout but is one of

addusting slopes i n t h e l i g h t of t h e depLh to which they are cut, and imposing

h

lLi11t wallleer on the geight to which ia given slope should be cut.

(4)

no. S l ; r i p p u of Clay fzom the

South End

of tth w"8

Orgr,&.

:

Ia&;e 2 )

first ie faiLure by a l i d l n g of the e o i l o n

a

olrouler arc, ThB aecond by the

structure

of

the a011 breaking down and a

failure

by rgreacl ar Plow resultingo

Eaoh

of

ttmm typ08

or

r ~ i i u ~

i a

dmorlbed in detail in tha

i o ~ o w i n g

notoo

and

an

at-%

is

made to present amdy%ioal methods

for

deallog with eaoh

type(. The seoond t y p of failure, by

flow

and spread, peeants a d i f f i c u l t problan of u 4 ~ 8 i s due to the unknomr quantity v l z , the water pressure b t b

parea of the 0011., ThiB

form of

fail-

is

that whioh ha8 oawed most trouble at Steep Rook and

all

p o a s i b b saf'egumds m u a t be taken t

o

prevent a

TBClWFWlC8 o

l a1

Failure by slid in^ along a Ckroular

mc

The f b a t analyses of the aawee of failure of earth la pea

werep

undertaken

es

a rseult OX a number of diaaatroua failures

In

rail soad

mbaPhnenta b 3.poeden' Inveetlgatiom of the surfam

ale%

oelhiah failure rook plaw ahowad that it approximated t o a oircralar a r o , and as a result tbb Circular Arc Method of Analysis waa developed, Referenoa t o F l g o 1 ahow

suoh

a form of failure, The surfaoe A B C of f a i l w e 18 presumed t o be circu- lar an8 failure takes place by the ser$h ma88 A

B

C D movbg about the aantre of mtat 10x1 0 , disturbiag

moment

oauelng failure is therefore the woigha

of the earth a~arse t h e e the distanoe between perallel. line0 &&mi though i*

oentroid and point 9 .

i,a, the dis%urbing moment e

M

;c

Failure is resisted by t h e shear strength of the eofl that is mobilized along

the m a A

B

C a d the reslating mawent is t h i s t o t a l

shear

strength t-a the

radiue of the uiraular ara along whioh failure tekes plaaefi The e o i l m y have

v w i n g ehear strength along a r o A B C and this ehcrriLa be taken acoomt of i n

the ana1y;ajs..

(5)

R e : Strippins o f Clay

f r o m

the South

End

of the _{RBw Ombody {page 3)} Whare B Tb.e ahear cr trengbh of the s o i l , assuming it t o b e homogeneow

A 1 L e m h of the ax0 A

B

0 PI

r

JJ Radiue of

f

allure airole

The factor

of

Safety agalnet failure

Q TheReeiatlngmoment 2 a o l u r t ,

TBs

Disturbing moment

mK'-

While a f aoeor of 8afei;y of 3 or 4 ie usual in Struo.torrel Engheerlng deai*, PdLiias greater than 1,,5 care

unusual

i n the design of earth Blopee,

e=th

Qama

and eimilm etruoturee,

Earth

dame tha% ham

been

designed recently

in t h e United States have had factors of safety o f l , 8 f a 1,3,

fn

such

etruotures rigid control o f the compaction of the placed 8011 bxeercised during uoaatructlon,

When checking the stabllllty of' exiating earth slopert, the two faut-

osa which m u e t be determined are the radius of the oircle of f a i l m e and t h s shear etrength of

the

Boilo

The radius of the failure oirole mwt be determined by trial and error 80 a8 t o give the smallest value of faetor of saf'aty,

(6)

Re

o S t r i a i n g o m c w z a y from the SoutkEnd o f the "13" O r e b o s

i

p a ~ r 4

1

A fspsptdish 'IEp)ginoerQ F e l l c r s i ~ ~ who invsertiga%sd the early slope

failures which occurred

i n

his couzltry, has gublirshed a table which a s e i e t s in

a more r a p i d determination sf the oen3re of %he oircle of f a i l u r e

where %M

a o i l

ooncesned is hcrm~geneoua and unifom,

D l

W , Taylor bas published la grephi,ocal form a method whereby %he stability of an a a t h rslopa o f homogeneous maeerba2. quickly be determined, ;Professor Tay%or Goes not

c.h&

that his method, and the assumptions made,

give@ a exmot; m e : conpleteXy acczarate method of mabysia- Hovmver, the very

na'tupe of e o i l , m2.a the varlatioms, j33, it, i s such @B t o make my theoxeticaX aaakfais ~tabjact Lo error when applied t o a practicab p r ~ b l @ $ & ~ I%% intelligent

w e

o f tSs T a y l o l ~ nethold, I n conguxrction with thorough inveatigntion8 of a a i t g 8howBw aio e s e e n t i d l y homo~eneow s o i l , sbouLd give infox-mtion to Chct s a i l engineer whierb cw:~ aerm aa a very uoaful guide

Jn

design, The !S'qilar curves

have been wad ir! $ha sl.ops analyefa carried Q U ~ i~ t h i s mmorandm, t h l o

am2.yais heilcng ca; ceruedl on3.y of courercs w i t h :stability agalast failure along a

ulscular a r c

lFha 8k@axn strength of a oil can bo determined

i n

a number of weye,

the two mast important a d frequently ursed be-big the dfrec.9; action shear

box

and the f r i - a x i d oomyression alpparat- T'kaa ehem box and the nature

of " t h e tests performed with i t E h a w been cliscwsed in a previous memorandum t o

MF

K LC

MoRorie W i t h tha tri-2axlaff. cmprension apparatus, the sgacimn i a pbaced

kn~idgs ca lucite cylinder, a cylindsx xhich an aAx GOT water preeuwe can b e

b u i l t ug to m y d f a h e d value, fi'hexl this lateral preosurcr is b u i l t up,

arn

ezid.

pressure i a s~lgp1it.d to the cylindrical. specimen by means of a platon and the

epecimea l.8 2.0adaC %O friiXureo The mia.b preuaure, c d l e d the mgor principel

aOrsss corresponding t o the lateral pressure, callecl the minor p r i n o i p d e t * 0 ~ 8 ~ l a determined For a number of l a t e r a l gressures,

(7)

P&;m2&"frarm the South End o l the "Bfl O r e b o z [ p a g e 5)

v

The major and ninor prinoipal stresaea are p i ~ t t e d and a r e l a t i o n s h i p can

than be ob"Lb6~..~ f o r the shear s t r e n g t h of the s a i l ,

axid\ ) > * r g 4 ~ ~ f ~

2aEE-L

(hap; p r i ~ c ; p o J stress)

il

i49r-2 ; c 5 keQci. 3 _I__._ 1CI, Uncon6ir\e i = o v - + ~ c ~ s 1 c . e

-

tes? ; c0rre5ponding m a j o r princi

pa

~ W S S ( a * ; a \ p r t % ~ r e )

Ef t::re slpec?inaen fs not d l m d t o conslolidate under the lateral

or €ixI@, E ~ T ~ E I E ~ ~ , a w&~k t e a t R kaalalts,

If

f u l l consolidation is allovred

under tihe %ateas1 a-&rme alone, a Wcoeerqlidatled quick tseSPs results, Sf

full. oonaaslidatlun l e allow@b m d a r bath XatsraP and -2.191 stresass t h e result- ing tost l.es c a l ;sd a t e s t w , The signiffieanca of the t e a t rc;sultu has 39en o q l a f n e d l u a preview Fnsmo on %he shear box,

Botil the ahem box and the t r i - m i a b @ompression apparatus are ezpeneivs pieoas of agpliratw and are not a v a i l a b l e a t Steep RackJ A mod-

i f i e d farm of the tri-axial, compreeeion e,pparatus oan easily b e conatruoted

6 0 that, "quiak t e a t e m crm be

run,

Thie is known a8 t h e unaonfined compreesion

(8)

Ro : 6 t b - o f C l a y from the South End of 1;he _{"Rr: Orebodx f}_page_{6 )}

presswe arad o i l y

an

axial pressure is applied to give failurs, Born the plote, a f the dt.fferent typos of teats aham .in Figme 2, it can b e seen that the shear strength equaba one half the compressfon stress i n the

Wquiok* t e a t ~ , e , the plot 0% shear strength Is a line parallel t o _the

baae and with :5

=

o.

Qbaer~ations of iailuros of earth allopes and d a m in Great Britain and the U n i t e d S t a t e s have shorn that w h o m such f a i l u s 8 occurred In c l a y

materlal that :he ahem strength along tho arc s f fallura wae e c p i l "c one half tho maocl'ined compreasslon tost i , a , equal to the shear s?,reng.th value obtained from the t v q ~ ~ i c k r 9 tost,

A n unconfined campress~.on device he8 beon c o n ~ t m c t e d in the machine

a'

p ~ at Stoep o r'iock, m d the values of s%ne,ar s t r e n g t h B Q U ~ P t o one half the

i;ncor,%ined com:>ressdon a9Jreagtlb have been used i i z atxhil.iLty computations,

Taw& Weeults

Clg tc the time of" writing (late Ju.ne 1948) ,, osa1.y r e l a t i v s l y f e w

uzcanfined co~~l:,rasslon t e s t s have been mad@, These t e s t s , about, 30 in

nrmber, give3 ~ ~ ~ 1 3 average value of sh@~a? ~ t r e n g t h of about 4 Ib, psr spa in,

"

556 lb per ,3q, ft, =I(? t h i a vaiu(3 has been used thrauqghout In the c d -

ohilations+ Re ~laatsd checks 8hod.d be mado cf t h i s valm a8 the test fr.s a simple one to ,erfom, a;l some modif l a a t i o n may be nsc0sae.q to t h i s ~ a l \ l w e

A f a o ~ o r of safalty of X,25 haaabeen &oped throughout

in

t h t p oa%culat ions,

(9)

AG-

a Str-8 c f' C : l ~ f ' ~ ~ l ~ $A@ South End 0% Pill@ *ISm O r e b a d g ~

--

-,-

:

b

1

Balbmaa

bg--ad

P'bow of %he P a n e d Clax

The fc~m of f a i l u r e desoribad under IaeadSsg (a)

viz,

the f a i l -

w along a circular a r c , l a the tyge of failwe met o o m n in homo-

geweala slays aohesive nxlerials, With materials 8uch as varved cBags

where thorn ax2e.G layers of silt o~ similar relatively ooaxse material

a

which i a water tomiag, %&@:re oocwu different type o f faiPurs,, Psnet- rnti0.a of water ;Worn an adjaoent higher water tabla into these layers a l l o m a water pesaure to be b u i l t

zap

in. the pores of the material, The s t a b i l i t y

of 'tihe, soil, masa i s depe~denl, an the shear strength of %be aoiPo T ~ ? A 8bcmr

atrength Is a funotion of the e f f e a t l v ~ pressure i , , s , the p r s s a u e trm,.emitted

From g r a u t o g x s l n In %Go eoi.7, naaBo A PGFB preesrwe a e t a opposite to %he

effective prsaawo asld ao rsdc~oes i t , thm r@duoIng the @hem strength 0 %

%he sol% aad tba a.;cab:'ilA-t-.:= of.' ihs slope, A chmacterie$ie of t h i s

fom

of

falaP;~a i e the stddamosr? with which I t takes plaoe and if %he material in

whish it occurs has a ssu!:~Ltlvs slra,ctwe ( w oonditian wuaZly associated

w i t h high n . a % ~ ~ 1 water aalo.%ent), then a flow of viscoue material.. sesrults,

f

The mat nevore o f the 8Rope failures Bshich have ocuurred at Stsep Rook ham

corn ,in20 thA8 category, and have beeu associated with prioUe 09 high rain-

*

fall or aP%h pexantiration of ~ u T a c c drainage into the e o i l mes, Both %hem uondftiom build, pore water preseure in the water bearing lay8r6, Big, 3

i l l w t r a t e s the

Porn

of failure,

fai \dfcr by C\OW

(10)

s%,-.ripping

h e South

End

of the Orebok (page 8 )

The qufistion has been raised a t time8 as t o the effect o f t b natural d i p

of

tSis varved clay, The direction of the d i p oannot ,&%csct the mschanioa of a stlsbiPPliy analyois as neithsr the t o t a l mass (oonstituting the

distiurbixq elenmrsXn (;he s l i d i n g cirale f ailura) nor the pore water preesme (the di~turbing e.Lemen% i n the f l o w or spreaa failure) i s influenced,

However, the mxtt nt o f the s l i d e where f l o w or spread occurs i s effected b y

the d l r e c t l o n 02 bhe srarvaa as ahom in Figure 4;

Flowaw~q of mdi er9aZ

$ss?Ji6&4bcd try dope Natesfal em

U s ,

a d o ~ o t resiatosroe fLowanag t o e

so

Pn

Figare 4, ( i 1 w i t h the V B R V ~ B lying w i t h the slope the materiel

oaoe failed f l o w ~ Q W E the alope a d lgada %o pragressivs failure by sv.ttipe5

ba.a,lk of the a i o p s . :l'f a p.l.t of open sxcava$ion Riea some distanoe down tbe

slope, then the r ? 3 o o w g l a t a ~ l a l uan flow i n t o %he p i t Zf as

in

[ii) above

.tb slops 0% the irmves is ogposits io t h e earth

m m s

elope,, then the spread

ma%srial tsnds t~ f l o ~ "kipn the v-ves and b u i l d s up a tog which 0- oifer

soae reaistwue f a further f l o w aad progressive breaking back of the material

in t h e slope.

no

r e l l s b l a opinion can be R o m d concerning the iaoeor of safety

o f %ha slope with sesgac% t o flow and spreading mleas the pore water pressure

(11)

Re:: S%r=%sf G k ~ " r a ? a t h e South Pad of the "8" Oreb~Q (page 3 )

---

-

This gresome can only be determined

In

the f i e l d by pressure gauge obser-

vation@, h r i o i f i o obsematian of the gauge48 oould give warning of a s l l d e , If the obeervo'tions ovsr a period indigatedl a low faator ef erafety, 6rainqe

o o d d be installed to keep the pore water gressuro down to safe lilQit8,

Both ~ h . e a e prsoautione w e expansive a d t ime consuruin&,

W o r t -

unatew

this type of f a i l w e appears to ba that whioh has oocurred most

Rr~1quenafily at. :;keep Hook,,

T%r ., Tk:r%n&rll, %he inta~nat ionnb authori%y on ao il mechanics, ha8

in~sratigs.%ed p ~ o b l s m a~hd propouad a p r ~ o a u t l o n a g rcaaawe ta g t i a d

agaimt f a i % u ? a . , A.6 m@at%oned eaxlisr in t h i a amor or an dew,. t h e r e l e a oxit-

i c a l b i g h t t u :%%luh a g i w n slope OW 'be cut

i n

a ccahoafv@ ma-harial, D L ,

Tsrzaghi has 6k~m ~ i ; h ~ ~ r @ % i e a l l y . that if the maximuria haigb% 0% olope in a.

V S S W I P ~ 02ay i n kept dsvm %o the

rnmimwm

height fo ~ i c s h a vertiml. @u% o m

be m ~ G e !.n t b . ~ naterial, %hen a ~ a l e g w r d o m bs grovided againat fa91me by 8pread. Rna :?'Bowo Stna'la a liml.tatiaa, applied in a o n j u u t i o n wlth EWA

analysla f o r Lho @ircrll..a.r a r c type of fail-~ase el$ould giv@ protaol"cfcala waJ~et failure, 32 7:l.o~ caf p o m d Hater tcpwwds %ha aPay can hrs readil-J Wiv@rtad th3a 8huh~Ld bs done arn such water can ,enstrate %be water bewing layers of srmd ox sA::.t and b u i l d up a prasewe In She p4aros,

1n a ~.oaationm In the United S t a t s a vshichcas v a ~ v e d clay dopoaittl

,

$Pow failurea have basn edb~csswed at ixntar-aabs of abcr* 20 yeam, corraep-

ond:lng ta ~ r . : o d a Q P at~xdmarm ralnfaill. The frequenuy of such failures,

o f couree, deq~snds on conditfoaa of' arxisti~g water table lavelbs and the

faotor of safety exiotfng ,,si*o-p increased pore water ~ ~ B B ~ U T B , , and the

(12)

Ee: S t s i p p i a . ~

---

of Clqy from tlas South %b of the

"B1*

_{Orebody [ p q e 2.0)}

&p%yilag

Dr,

TerzaghiFe method, the critical, height t o which a

vertical o u t

can

be made i n tihe varved o l a y at Steep Rook is approximately

22 feet. This value ie based on the relatively small number of

uncoaf

imd oompreesion t e s t s .that have been performed, and may be aubjeot to revi~fon

as further teats m e performed,

morn

this data, I suggeet that the

f i n a l

bench intervtxL8 b e 20 feet apart v e r t i c a l l y , This should give protection a$alnst the periodic buS.lding up of pore water grsasure, ExamLnatisn o f

Section8

B

t o 9 shorn t b t i n ths more recent ~ t r i p p f - n g operation8 -the beneb b t e r v a l s ere approximately 20 feet, thus conforming to the

s e o o ~ r P 8 a t io.no

Tae method of ealcuAa%ioa of the maxianurn ~ e : ~ % i c a l c u t i n a o l a ~

bank f a ahm8 at -bhe end of %heat3 no%eo, "srd a

c o w

of

Professor TayAorP

e

(13)

ge:

St-i Caa3f fron the South End of t h e ItB" Orebody (Page

XI)

cor~cz,usI

ON

After

an

examilnation of Mr. Samuel's proposed scheme, I should l i k e

t o make t h e following comerltiat

I

am of t h e opinion t h a t the clay need not be removed t o such an extent

as

suggested, l e a , to a l e v e l varying from l l 4 7 , 5 at Section 10 t o 1UO at Section 15, S t a b i l i t y calculations on Section 10 t o 15 show t h a t benches can be formed at higher levels while s t i l l retaining t h e atabili'ty

of: the slopes. Obss~vatioas of the condj.tions oxis'ting aL Secti.ona b to 9 show 42ia-k rngr propsc3d amendnents do no% incur any mre severe conditions

t h ~ i e x i s t i n k h e s e Y ~ C ' G ~ O ~ S , SecLions 13 t o 15 do not lie on the line of grsn%est. slope o f t h e clay, Therefore s.Lability calculatTons on these

anci;icmz c m bs zisleading, asd now sectLol~o should be drawn on the line of

,cgea.-test slope so . t h a t a plan of t h e propxed benches can be properly &:am,

However, I't is difficult; t.o draw a pl.ar? 7:i.e~ clt' -the 'benches p l ~ t t ~ i n g from seciions, as: tbs sand and gravel. levels v 3 ~ t o a great extent, P l o t t i n g a plan from t h e seetians gives an extrercely e r r a t i c arrar!gement o f benches due

.f;o t h e varinticns,

H

have dram up a sketch plan, which is attached t o t h i s

nermranclwn, in which t h e o f bench l o c a t i o n i s ahowno T h i s planp r&th

the approximate locat,ion of the benches shown thereon, w i l l serve as a guide

to t h e se'tting o.ut of the benches,

Pt

i s ~ * e a l i s e d that l o c a l conditions and

exigencies m k a ii; d i f f i c u l t ta follow a set plan, and t h e sketch plan sub-

mitted i s in.ter:ded t o be, a general guide t o t h e scheme of t h e slopes rather

than a rigid specification,

Denna1::ds f o r production of ore may a l t e r the planning and c a l l for an accelsa:atior?. of the stripping programme, The following are gwneral rec- ommsndatlons tktat should be followed i n order t o afford s a f e t y against

(14)

Re: S t r i p Clay fi-om_the South k d o f the

"B"

Orebodg (page 12) the crand and g r ~ ~ e l l e v e l l i e 8 throughout a t approximately the same l e v e l ax

t h e t o e of t h e slope. Where the sand and gravel surface

parallels

the varved clay surface, these values are too severe and can be modified,

In

such ta case individual slopes should be checked as shown i n Figure

5 ,

Check

skability

from

X t o C; Check s t a b i l i t y from

F

t&

A

Fissure

5

.,%

With slopev witr! 20 feat rises bsttmen benches,and w i t h berms 50 f e e t wfde,

8, l i m i t i n g t o t a l rise of

54

f e e t above t h e toe is recommended, Greater rises

a r e p e d s ~ i b l s by widening the berrns and so f l a t t e n i w the overall slope, The

following table gives tho refationehip between rise and overall s l o p ,

Dfffersnce

in elmration

betmg3 -LOB of slope and

c r e s t of s l o p , Horizonbal distance between t o e of shops and c r e s t of slope Feet Feet Eqluivalent overall slops measured from crest t o t o e o f slope

-

B y t h e use of these figures as a general guide, slopes can be l a i d out on section8 on the l i n e a of t h e greateat elope of t h e clay, Protection againat circular

arc

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Bet S t X 0 i - i -. of C l a ~ f r o r n

-

the

S o u t h s o f - Orebody-

O'JW

&3

1

f ~ i l u r a s can t h u s be given, By limiting difference

in

level between benches

t,o 20 feet, an attempt can be m d e to etabjlise the slope against failure by

flow and swead, Using a 20 feet rise between benches at a slope of 1

in 3

'I

and knowing the total rise,the overall equivalent slope can be found from t h e above t a b l e and the necessary widths of benches determined,

!!'hes~_values have been baaed

on

a rela$Jxly small nunher of t e s t a

considering t h e _extent of" t h o area i n v o P ~ e d ~ C h o c k t e s t s for tile unes,niined LB

coiupresaior.

sP;~&~~&h

of t h e s 9 o _ i l 3 $ ~ d be carrigd QU~, ?t every o p h u n i t y ,

-

The I.nformation obtained from the foregoing analysis is inLended as a g1.13.de and mei; he combi-ned w i t n the observations and experience of t h e operators on t h c J D ~ , shod-d not be taken a s a rigid s p e c i f i c a t i o n but

shoulci bs ~ s s d ~ d ' t h good judgnen-b, as f l e x i b i l i t y of thought ~mst be retained

(16)

Be:

S t , - . -

from the South End

--.

of the "Bn &@body (page

14)

Specimen Calculations used j,n Slow Anal-mis

]L3

To

f irld the height to which a bark can be saf s l y c u t to obaexlre t h e

following conditions:

2 '

Average valueof shear s t r e n g t h of clay S

.

4

B . / b

2

576

i b / f t 2

-

0 ! 9 p 3 d used

3

by Ttwlor)

Density of S l a y

-

j

-

= 105 lb/rtt

.

Bank rises at 1 vartic.il to

3

horieontal for a vert-ical distance of 20 feet

has a 50 feet h e m , rises a ~ a i n at 1 i n

3

for 20 feet e t c . T h i s gives

an

o

nveraga o v e r a l l slope value of ngproxhatsly

13

Fsactcr o f safety against circular arc failure 1,25

Ratio of distance f r j m slope crest Lo hard underlying aurface to vertical

F r o m Taylor '2s curvet3 :

' 0

Stab-Uity Nurnber for De 1 and D(

=

13

and _@ o l a

O,m

_,C

.0,81 o " , H p C

-

₅

₇₆

=

54

_Feet.

FkH

E

_k.

0,81

1,25x105X0081

' Height to which bank can be

carried

-

54

Feet.

(17)

ge;: Stpi- ,,-. o n t h e South Ehd of t h e T3n

-

O m b o m (page 15)

2 ,

To

find the height to which a bank can be cut vertically in a clay having the s m e properties as

in

example 1,

Crit:ical h e i p ? t t o which a clay can be cut vertically

3.85

2

Where S

=

shear strength of s o i l lb/%t 2

8

1

density of sooil ib/ft3

.",

B 3.85 x

4

approximately 22 feet

y05

A

%able is a t t a c h e d b e l o w showing the variation in t h i s critixal height

to w h l c l l a v e r t i c a l c u t can be nasda as the shear strength and density vaslbea,

Values of C e t i c t d l Height

(18)

L. a Q~IJ.'.:'~ o r ' :\lqt' C.

r,,,

2i* ,.-, , . -. ..-* -"".-L--.- , . -.

.''a7,;!ll 9;; f ~ r a?;?; bj...!ii;y ~ ) f ' c'.o:lc~ 3:; {: :A,: :I;. . i...kinixh S r:i.lc 1. ;is '~:3:~!:;o~?~i;:';.

u?

"ii!.ii:i5 j"1c'l.: ;?e

cmi 9cr.re aB a m@:>ms >I' jud,?ilag ~~1.lt%i*l_os :f ~ j . X : i . ;

O L ~ P C

w l l 2 . b ~ ? ~~?ilc)U'bt~d:I',,y s E ; B b 3 , ~ ,

ua.d.cu'f3'~ l:dl.;r m a t c~i-.~.c:. :, o.? 3f douZ:~i;i'ui, s L : i h l l l t y c If t )LC: 8% ~ibli.lt y appears doubtful

,

tho

f a c t o r of a u S b z ; ~ v h i l respect t o f a . i l u u s h o u l d he computed uccordinc to morc: dota1;lci; and oxpt:t dletfioda of co.lcu.k& ion,

~t ? c ecddorn p o ~ s i b l e is1 o t a b i l l t y problms 01' indeecl any enrthpaork p r o b 3 . m ~ to makc use

,i' ,3 lai6ge f a c t o r of s a f e t y , Val1r9s as h i g h as 1,s my 5 e w e d in d t 3 ~ i g n , b u t u s u a l l y

from eccmomic couejideratlons R valu.rt. this 1me;e Xs 30% aliowa31cio Soverd. la]-gr, earth

dam:: r c c e r ~ t l y constructed were ~ C S I : ~ I C ~ I tn ~ I V C a FacLor as s a f e t y in thc r e ~ i o n o r

r.e

t o 1 ~ 3 ~

nhelte B n.> unit shct~r:ln(; s.tcarii;t;::~ p "=.' u:,p.l %eta ef.fL;zt ire st rlesc on t b.:, 813r7-T.c.26: 0 ;

~~:~;p%ur:: i a,v.ch nn due t :: -24c ovcx.bu.~~don j

r.: e u n i t a o h ~ i s i o n oP $'lo soil

4

t b cn$e oi' i.nicm~:.l. fr.%c.bisxi. of t h c soi:i.

It must b e noted theit t h o v a l u e s for. cohesion arid img.2~ of :b-rt.:~nah f"xbfuet;bou. e r o not

necessari1.y constants f o r a given s o i l , but are iatondsd t o r q r a ~ o n t c_umtit ies whioh m:

be depe:nded upon to hold for a duf i n . i t e set of conditions,

The shear strength f o r a eoft clay often taken as one hn3.f the zomprossive strcngch

(19)

Eff e c t 3 of .3aturation.

A e l o w be eubjeotod to the followlne; condldions.

Case

I.

Caaplote Submergen~o~Tho free water rsurfaoo i a at tho t o p of tho sl.ope,

--

a

tho s o i l i n f u l l y aaturat ad. U s e t h o su'hmcrceu u n i t wei,zht for

o u l o u l t i t i c n ~ . Thi u aqualu

a.

and varies from 56 to 70

Ib,

por oubu ft, for tho goncrccL n m of soils, T U B is t h o moot favourablc caoe,

!:ern 2* S;idd~il D D - N Q W ~ O ~ : ~ ~ Th:io 10 t h e : 3 ~ 1 l e as 5330 I , but with the .~VOC mtor

.-

--__1--...

su:lilonly rcnovuct , ToyJ.ar ou;l,;.;csce ?illat a r?.od.~r'lcii cu~;;,.I..c 61: ~r.ti;rn:'.l.. f r i c ? ; i c : .

bc: used i n t h i s c a w . K t is sn approrimat e oolut ion

,

but t h e error i~ on thi!

cafe s l d e . The t a t a l u n i t w9ight shouJ,d bc usad i n the calcr3at-ions,

Case 3, Capillary Saturation, The c Is no fLow. no aup~:ly, no evaporation, The

-

gores of the soil, are fflleci w i t h Tr.laf;orvhol.d by capillarity, a condition

camon i n typical cohcsivc e o i l a . The total u n i t mei*t should be used in

calculat ions, a value givon by f a s

+

e

]

m(l varying from IIt; to I30

o a l c u l a t ion

1 -

a+ 0

Ib.

ger cub. ft.

.

13 the unit weigh; of water,

-

8, i s the PPac~..'ic ~ r $ . v & y of t h o soil ~rains ~ % ~ a l l y fallin,: in tho r a g e 2,60 to 2=80

Jew

8

.+ i e t h ~ v o i d F ~ ' . ~ ~ ~f t h e e soil, and i s t h e r a t i o of t h e vol,un~s of t h c s a i d a tombhe

1 volmo of the s s Z L d ~ , r is the innit w o i ~ h f ; of L:'? 9

IU

mi _is_{t h e natural mator}_{c o n t ~ . - ;}_sf_{t h c}_{soil andic}_{the ratio}_of_tho_{volume of}_{t h e voids}

LA

-to tho volw~o of tho 301idae )?DF 8 f u l l y saturated soil e equals VJ < :.ss

m

-

note: w, w e i g h t of water i n coil

_-

_.4 ₁₀₀

_p

weight of so1ida

(20)

(21)