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STABILITY AND SWELL PRESSURE CHARACTER-ISTICS OF COMPACTED CLAYS

H . B . SEEDU n i v e r s i t y o f C a l i fo r n ia

A B S T R A C TT h i s p a p e r d i s cu s s es s o m e o f t h e f a c t o r s a f f e c ti n g t h e d e n s i t y v s . st a b i li t y r d a t i o n -s h ip s o f c o m p a c t e d c l a y s a n d s o m e o f t h e l im i t a t i o n s o f th e c o n c e p t t h a t t h e s t a b i l i ty

o f a c o m p a c t e d s o i l i n c r e a s e s w i th a n i n c r e a s e i n d e n s i t y .A s e r i e s o f t e s t s p e r f o r m e d o n s i l t y c l a y , in wh ic h s t a b i l it y wa s m e a s u r e d b y t r i a x i a lc o m p r e s s io n te s t s , a r e d e s c r ib e d . I t i s s h o w n th a t t h e r e la t i o n s h ip b e tw e e n d e n s i t ya n d s t a b i l i t y o f a s o i l d e p e n d s o n t h e c r i t e r i o n u s e d t o d e f in e s t a b i l i t y ; t h e g r e a t e r t h ep e r m i s s ib l e s t r a in b e f o r e a s a m p le i s c o n s id e r e d u n s t a b l e , t h e g r e a t e r i s t h e p o s s ib i l i t yth a t a n i n c r e a s e i n d e n s i t y w i l l c a u s e a n i n c r e a s e i n s t a b i li t y . F o r t h e s i l t y c l a y , wh e nth e s t a b i l i t y i s d e f i n e d a s t h e l o a d r e q u i r e d t o c a u s e 1 0 p e r c e n t s t r a in o r l e s s , a n i n c r e a s ein d e n s i ty a t a g iv e n wa te r c o n t e n t c a u s e d a n i n c r e a s e o r d e c r e a s e i n s t a b i l i t y d e -p e n d i n g u p o n t h e w a t e r c o n t e n t a n d t h e r a n g e o f d e n s i t i e s i n v o l v e d ; a t c o n s t a n td e g r e e s o f s a tu r a t i o n , h o we v e r , a n i n c r e a s e i n d e n s i t y a lwa y s c a u s e d a n i n c r e a s e i ns tab i l i ty .T h e e f f e c t o f c o m p a c t i o n m e th o d o n t h e d e n s i t y v s . s t a b i l i t y r e l a t i o n s h ip i s i l l u s t r a t e d .T e s t d a t a a r e p r e s e n t e d f o r tw o s o i ls , a s i l t y c l a y a n d a s a n d y c l a y , c o m p a r in g t h es t a b i l it i e s , a t v a r i o u s wa t e r c o n t e n t s a n d d e n s i t i es , o f p a r t i a l l y s a tu r a t e d s a m p le s c o m -p a c t e d b y im p a c t m e th o d s , s t a t i c p r e s s u r e a n d k n e a d in g a c ti o n . I t i s s h o w n th a t , a te q u a l wa t e r c o n t e n t s a n d d e n s i t i e s , s a m p le s c o m p a c t e d b y s t a t i c p r e s s u r e s e x h ib i t h ig h e rs t a b i l i t i e s t h a n s a m p l e s c o m p a c t e d b y i m p a c t o r k n e a d i n g m e t h o d s ; a l s o , t h a t t h es t a b i l i t i e s o f s a m p le s a t e q u a l d e n s i t i e s a n d wa te r c o n t e n t s , c o m p a c t e d b y im p a c t a n dk n e a d i n g m e t h o d s , a r e v e r y s i m i l a r , w i t h k n e a d i n g c o m p a c t i o n p r o d u c i n g s o m e w h a th ig h e r s t a b i l i t i e s a t l o we r d e g r e e s o f s a tu r a t i o n , a n d im p a c t c o m p a c t i o n p r o d u c in gs l i g h t l y h ig h e r s t a b i li t i e s a t h ig h e r d e g r e e s o f s a tu r a t i o n . F o r s t a t i c c o m p a c t i o n , b o ths o i l s , s h o we d a n i n c r e a s e o f s t a b i l i t y w i th i n c r e a s e i n d e n s i t y a t a l l wa t e r c o n t e n t s ,wh i l e f o r s a m p le s c o m p a c t e d b y im p a c t a n d k n e a d in g m e th o d s , a n i n c r e a s e i n d e n s i t ya t a g iv e n wa te r c o n t e n t d id n o t n e c e s s a r i l y l e a d t o a n i n c r e a s e i n s t a b i l i ty .

T h e e f f e c t o f s a tu r a t i o n o n t h e s t a b i l i t y o f c o m p a c t e d c l a y s i s s h o w n a n d t h e i n f lu e n c eO f w a t e r c o n t e n t a t c o m p a c t i o n o n th e s t a b i l i ty o f s a t u r a t e d s a m p l e s h a v i n g t h e s a m ed e n s i t y a n d wa te r c o n t e n t i s i l l u s t r a t e d .F in a l l y , a c o m p a r i s o n i s m a d e o f t h e s t a b i l i t y a n d s we l l p r e s s u r e s d e v e lo p e d a tv a r io u s d e n s i t i e s o f s a m p le s o f t h e s a n d y c l a y c o m p a c t e d b y k n e a d in g a c t i o n a n d s t a t i cp r e s s u r e a n d s u b s e q u e n t l y s a t u r a t e d b y e x u d a t i o n o f m o i s t u r e f r o m t h e s a m p l e s u n d e rs t a t i c l o a d s . I t i s s h o w n th a t v e r y m u c h h ig h e r s we l l p r e s s u r e s a n d s t a b i l i t ie s a r ee x h ib i t e d b y t h e s t a t i c a l l y c o m p a c t e d s a m p le s t h a n b y t h e s a m p le s p r e p a r e d b yk n e a d in g a c t i o n .INTRODUCTION

T h e c o m p a c t i o n o f s o il s t o i m p r o v e t h e i r e n g i n e e r i n g p r o p e r t i e s i s c o m -m o n p r a c t i c e i n e a r t h w o r k c o n s t r u c t i o n . I n o r d e r t o e n s u r e t h a t t h e s o i l i sa d e q u a t e l y c o m p a c t e d , i t i s c u s t o m a r y t o m a k e d e n s i t y m e a s u r e m e n t s o n t h ec o m p a c t e d s o il a n d t o r e q u i r e t h a t t h e d r y d e n s i t y e q u a l o r e x c e e d a c e r t a in

483

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484 CHARACTERISTICS OF CO MP AC TE D CLAY S

m i n i m u m v a l u e ; t h is m i n i m u m v a l u e is u s u a ll y a h i g h p e rc e n t a g e ( 9 0 o r9 5 p e r c e n t ) o f t h e h i g h e s t m a x i m u m d e n s i t y w h i c h c o u l d r e a s o n a b l y b ee x p e c t e d f r o m f ie l d c o m p a c t i o n e q u i p m e n t . A s l o n g a s t h e d e n s i t y e x c e e d st h e d e s i r e d m i n i m u m v a l u e , c o m p a c t i o n i s c o n s i d e r e d t o b e s a t i s f a c t o r y ;a n d i t i s u s u a l l y c o n s i d e r e d t h a t t h e h i g h e r t h e d e n s i t y t o w h i c h t h e s o i l i sc o m p a c t e d , t h e g r e a t e r w i ll b e th e i m p r o v e m e n t i n i ts e n g i n e e r i n g p r o p e r t i e s .I n a d o p t i n g t h e d r y d e n s i t y a s a c r i t e r i o n o f s o i l c o m p a c t i o n i t i s i m -p l ic i tl y a s s u m e d t h a t d e n s i t y is a m e a s u r e o f t h e d e s i ra b l e c h a r a c t e r i s ti c s o fa so il such as s t r en g th o r s t ab i l i ty , com pres s ib i l i t y o r pe rm eab i l i t y . Re cen ti n v e s t i g a t i o n s ( U . S . W a t e r w a y s E x p e r i m e n t S t a t i o n , 1 9 4 9 - 1 9 5 0 , 1 9 5 1 ;F o s t e r , 1 9 5 3 ; S e e d a n d M o n i s m i t h , 19 5 3 ) h a v e s h o w n , h o w e v e r , t h a t th i s

i s n o t n e c e s s a r i l y s o . T h e p u r p o s e o f t h i s p a p e r i s t o d i s c u s s s o m e o f t h el i m i t a ti o n s o f t h e c o n c e p t t h a t a n i n c r e a s e in d e n s i t y l ea d s to a n i n c r e a s e i ns t a b i li ty a n d t o d e s c r ib e s o m e o f t h e f a c t o r s w h i c h a f f e c t t h e d e n s i t y v s .s t a b i li ty r e la t i o n s h i p o f a c o m p a c t e d c la y . T h e e f f e c t o f c o m p a c t i o n o n t h es w e l l p r e s s u r e e x e r t e d b y a c o m p a c t e d c l a y is a ls o b r i e f l y d i s c u ss e d .L A B O R A T O R Y C O M P A C T I O N T E S T S

A v a r i e t y o f l a b o r a t o r y c o m p a c t io n p r o c e d u r e s a r e i n u se a t th e p r e s e n tt im e . T h e m o s t w i d e l y u s e d m e t h o d i s t h a t o f d r o p p i n g a w e i g h t o n t o th es u r f a c e o f t h e s o i l - - a p r o c es s r e f e r r e d t o a s i m p a c t co m p a c t io n . I n s o m ecases t he so i l i s compac t ed by sub j ec t i ng i t t o a s t a t i c l oad wh ich i s bu i l tu p s l o w ly t o s o m e p re d e t e rm i n e d v a l u e a n d t h e n r e l e a s e d - a p ro c e ss r e -f e r r e d t o a s s ta t i c c o m p a c t i o n . I n o t h e r m e t h o d s t h e s o il i s c o m p a c t e d b yr e p e a t e d l y a p p l y i n g a p r e d e t e r m i n e d p r e s s u r e t o s m a l l a r e a s o f t h e s o i l ,m a i n t a i n i n g t h e p r e s s u r e f o r a s m a l l e l e m e n t o f t i m e , a n d t h e n g r a d u a l l yr e d u c in g th e p r e s s u r e - a p ro c e ss t e rm e d k n e a d i n g c om p a c tio n .

S a m p l e s p r e p a r e d a t t h e s a m e d e n s i t y a n d w a t e r c o n t e n t b y t h e v a r i o u sc o m p a c t i o n m e t h o d s h a v e d i f f e r e n t s t r e s s - s t r a in c h a r a c t e r is t ic s . H o w e v e r ,i n v e s t i g a t i o n s i n d i c a t e t h a t s a m p l e s p r e p a r e d b y k n e a d i n g a c t i o n b e a r t h ec l o s e s t r e s e m b l a n c e t o s a m p l e s c o m p a c t e d b y e q u i p m e n t i n t h e f i e l d ( S e e de t a l . , 1 9 5 4 ) . T h i s m e t h o d o f c o m p a c t io n h a s t h e r e f o r e b e e n u s e d f o r p r e -p a r i n g s a m p l e s f o r t h e m a j o r i t y o f t e s t s d e s c r i b e d i n t h e f o l l o w i n g p a g e s .

D E T E R M I N A T I O N S O F S T A B I L I T YT h e s t a b i l i t y o f a s o i l m a y b e b r o a d l y d e f i n e d a s t h e l o a d o r p r e s s u r ew h i c h i t c a n s u p p o r t w i t h o u t e x c e ss iv e d e f o r m a t i o n . E x a c t l y w h a t c o n -s t i t u t e s e x c e s s i v e d e f o r m a t i o n i s a m a t t e r o f o p i n i o n , a l t h o u g h i t i s m o r eo r l e ss g e n e r a l l y a g r e e d t h a t a s t r a i n o f a b o u t 1 0 p e r c e n t i s c e r t a i n l y i n t h i s

c a t e g o r y . B e c a u s e o f t h e d if f ic u l ty o f o b t a i n i n g a d i r e c t m e a s u r e o f t h es t a b i li t y o f a s o i l, a v a r i e t y o f l a b o r a t o r y t e s t m e t h o d s f o r m e a s u r i n g i n d i c e so f s t a b i l i t y h a v e b e e n d e v e l o p e d , a n d t h e s e i n d i c e s h a v e b e e n c o r r e l a t e dw i t h t h e p e r f o r m a n c e o f s o ils u n d e r l y i n g p a v e m e n t s .I n C a l i f o r n i a , t h e i n d e x o f s t a b i l i t y u s e d f o r d e s i g n i s t h e R e s i s t a n c e

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H . B . S EE D 4 8 5V a l u e a s m e a s u r e d b y a H v e e m S t a b il o m e t e r t e st ( H v e e m a n d C a r m a n y ,1 9 4 8 ) . T h i s i s a c lo s e d - s y s te m , t ri a x i a l - c o m p r e s s i o n t es t , a s s h o w n i nF i g u r e 1 , u s i n g a s p e c i m e n 4 i n . i n d i a m e t e r a n d a b o u t 2 89 in . h igh . T h ev e r t i c a l l o a d o n t h e s p e c i m e n i s a p p l i e d a t a c o n s t a n t r a t e o f s t r a i n ( 0 . 0 5i n c h e s p e r m i n u t e ) w h i l e t h e p r e s s u r e i s a l lo w e d t o b u i l d u p i n t h e l iq u i dc e ll w h i c h e n c i r c le s a n d c o n f i n e s t h e s p e c i m e n l a te r a l ly . T h e l a t e r a l p r e s -s u r e , P m t r a n s m i t t e d t h r o u g h t h e s p e c im e n w h e n t h e v e r t i ca l p r e s s u r e , P v ,i s I 6 0 p s i is r e c o r d e d . A n i n d i c a t io n o f t h e s u r f a c e r o u g h n e s s o f t h e sp e c i -m e n i s t h e n o b t a in e d b y d e t e r m i n i n g t h e d i s p l a ce m e n t , D , o f t h e s p e c im e n ,a n d t h e R e s i s t a n c e V a l u e o r S t a b i l o m e t e r R v a l u e is c o m p u t e d f r o m t h ef o r m u l a :

100R = 1 0 0 2 . 5 / P v " \ 1T h e R v a l u e s d e t e r m i n e d i n t h i s w a y h a v e b e e n c o r r e l a t e d w i t h t h e s e r v i c eb e h a v i o r o f s o il s u n d e r p a v e m e n t s a n d h a v e b e e n f o u n d t o b e a s a t i s f a c t o r ym e a s u r e o f r e l a t i v e s t a b i l i t i e s .

A n o t h e r t e s t w h i c h m a y b e u s e d t o m e a s u r e t h e r e l a t i v e s t a b i l i t i e s o fs o i l s i s t h e t r i a x i a l c o m p r e s s i o n t e s t p e r f o r m a n c e u n d e r c o n s t a n t l a t e r a lp r e s s u r e . E i t h e r t h e u l ti m a t e s t r e n g t h o f a te s t s p e ci m e n o r t h e l o a d r e-q u i r e d t o c a u s e a p a r t i c u l a r s t r a in m a y b e u s e d a s a n i n d e x o f s t ab i li ty . T h i st y p e o f t e s t h a s t h e a d v a n t a g e t h a t c o m p a r i s o n s o f t h e r e l a t i v e s t a b il i ti e so f s a m p l e s m a y b e m a d e f o r d i f f e r e n t m a g n i t u d e s o f a l lo w a b l e s t r a i n ; h o w -e v e r i t i s n o t s o r e a d i l y p e r f o r m e d a s t h e s t a b i l o m e t e r t e s t .

\ Head f e s t i n g M a c h i n e /

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: ' ~ " ~ ' / ~ " ~ ' ) legibleOloohragmL quld n d e r s m a l l' M ; '. . . . . . . . . .

N Nlg:O%~163N o f e :S p J c l m e n g i v e n l a t e r a l s u p b o r f b y f lexible $ ;de wa i lw h i c ht ronsm~ls h o r l z o n f o l p r e s s u r e tO iquid.MagnitudB o fpressure m a y b e r e a d o n gauge.

Fig . L - - Hv eem S tab i lo m e te r .

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486 CHARACTERISTICS OF COMPACTED CLAYS

Fig. 2 ~- Expan sion p r e s s u r e d e v i c e : (~) mold~(b) provin g bar, (e~ adj usta ble b~se~

E F F E C T O F A L L O W A B L E S T R A I N O N T H E D E N S I T Y V S.S T A B I L I T Y R E L A T I O N S H I P O F C L A Y S O I L C O M P A C T E DB Y K N E A D I N G A C T I O NThe form of the densi ty vs . s tab i l i ty re la t ionship for a c lay so i l com-pacted by knead ing ac t ion and the effec t of the def in i tion of s tab il i ty onth is re la t ionship may be shown by the resu l t s of tes t s performed on a s i l tyc lay so il f ro m Vicksburg , Miss iss ippi (Se ed and Monism i th , 1953) . Ser iesof tes t s were made to es tab l i sh the re la t ionship be tween densi ty and watercontent and between s tab i l i ty and water content for the so i l a t each of s ixdi fferent comp act ive effor t s. A ft er com pact ion , samples were subjec ted tot r i a x i a l c o mp re s s i o n t e s t s o f t h e u n c o n s o l i d a t e d - -u n d ra i n e d t y p e a n dindices of s tab i li ty a t 1 percen t and 10 perc ent s t ra in were ob ta ined by

de te rmina t ion o f t he modu l i o f de fo rmat ion a t 1 pe rcen t s t ra in and thes t re sses requ i red to cause 10 pe rcen t s t rain . Mo du lus o f de fo rm at ion a t1 percent s t ra in i s def ined as the ra t io of s t ress to s t ra in a t the poin t on thes t ress vs . s t ra in curve where the s t ra in i s 1 percent ; i t i s numerica l ly equalto one hundred t imes the s t ress requi red to cause 1 percent s t ra in .

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H . B . SEED 4 8 7

T h e r e s u l ts o f t h e s e te s t s a r e s h o w n in F i g u r e s 3 a n d 4 . F i g u r e 3 s h o w st h e d e n s i t y v s . w a t e r c o n t e n t c u r v e s , a n d t h e m o d u l u s o f d e f o r m a t i o n a t1 p e r c e n t S t r a in Vs. w a t e r c o n t e n t c u r v e s , f o r e a c h c o m p a e t i v e e f ft ,r t. I tw i ll b e s e en t h a t f o r e a c h c o m p a c t i v e e f fo r t t h e m o d u l u s o f d e f o r m a t i o n i sl o w o n t h e w e t s i d e o f o p t i n m m b u t t h a t i t b e g i n s t o i n c r e a s e a s t h e o p t i -m u m w a t e r c o n t e n t i s a p r o a c h e d a n d , f o r t h e r a n g e o f w a t e r c o n t e n t s u s e di n t h e s e t e s t s , c o n t i n u e s t o i n c r e a s e w i t h d e c r e a s i n g w a t e r c o n t e n t o n t h ed r y s id e o f o p t i m u m e v e n t h o u g h t h e d e n s i ty o f t h e s a m p l e s is d e c r e a s in g .T h e s ig n i fi c an c e o f t h e s e r e s u lt s is b e s t s e e n f r o m t h e m o d u l u s o f d e -f o r m a t i o n - d e n s i t y - w a t e r c o n t e n t re l at io n s h ip s h o w n i n F i g u r e 5. T h e s ec u r v e s , s h o w i n g m o d u l u s o f d e f o r m a t i o n v s . d r y d e n s i t y a t a s e r i e s o f c o n -s t a n t w a t e r c o n t e n t s, w e r e i n t e r p o l a t e d f r o m t h e re s u l t s i n F i g u r e 3 . I t w i llb e s e e n t h a t t h e e ff e ct o f i n c r e a s e d d e n s i ty o n t h e m o d u l u s o f d e f o r m a t i o no f t h e s o i l d e p e n d s o n b o t h t h e w a t e r c o n t e n t a n d t h e r a n g e o f d e n s i t i e sc o n s i d e r e d . A t a w a t e r c o n t e n t o f 1 3 p e r c e n t , a n i n c r e a s e i n d r y d e n s i t yf r o m 1 0 0 t o 1 1 0 lb . p e r c u . f t . c a u s e d a n i n c r e a se i n m o d u l u s o f d e f o r m a t i o nf r o m 1 9 0 t o 4 7 0 k g . p e r sq . c m . ; a t a w a t e r c o n t e n t o f 1 7 p e r c e n t , h o w e v e r ,t h e s a m e i n c r e a s e i n d e n s i t y c a u s e d a r e d u c t i o n i n m o d u l u s o f d e f o r m a t i o nf r o m 1 2 0 t o 5 0 k g . p e r sq . c m . T h u s a g i v e n i n c r e as e i n d e n s i ty m a y i n -c r e a s e o r d e c r e a s e t h e m o d u l u s o f d e f o r m a t i o n d e p e n d i n g o n t h e w a t e rc o n t e n t o f t h e s o i l .

A g a i n , a t a w a t e r c o n t e n t o f 1 3 p e r c e n t , a n i n c r e a s e i n d r y d e n s i t y f r o m1 0 0 t o 1 1 4 l b . p e r c u . f t . c a u s e d a n i n c r e a s e i n m o d u l u s o f d e f o r m a t i o nf r o m 1 9 0 t o 5 6 0 k g . p e r s q . c m . , b u t a f u r t h e r i n c r e a s e f r o m 1 1 4 t o 1 2 0l b . p e r c u . f t . c a u s e d a r e d u c t i o n i n m o d u l u s o f d e f o r m a t i o n f r o m 5 6 0 t o1 75 k g . p e r s q. c m . T h e e f f e c t o f i n c r e a s e d d e n s i t y m a y t h u s b e t o i n c r e a s eo r r e d u c e t h e m o d u l u s o f d e f o r m a t i o n d e p e n d i n g o n t h e r a n g e o f d e n s i t i e sc o n c e r n e d .T h e e f fe c t o f d r y d e n s i t y a n d w a t e r c o n t e n t o n t h e s t r e s s r e q u i r e d t oc a u s e 1 0 p e r c e n t s t r a i n is s h o w n i n F i g u r e s 4 a n d 6 . A s f o r t h e m o d u l u so f d e f o r m a t i o n d a t a , th e r e l a t io n s h i p b e t w e e n d e n s i t y a n d s t a b il it y a t c o n -s t a n t w a t e r c o n t e n t s s h o w n i n F i g u r e 6 h a v e b e e n i n t e r p o l a t e d f r o m t h et e s t d a t a in F i g u r e 4 . I t is r e a d i l y s e e n t h a t w i t h i n t h e r a n g e o f d e n s i t i e si n v e s t i g a t e d , t h e r e i s c o n s i d e r a b l e d i f f e r e n c e b e t w e e n t h e e f f e c t o f i n c r e a s i n gd e n s i t y o n t h e s t re s s r e q u i r e d t o c a u s e 1 0 p e r c e n t s t r a i n a n d o n t h e m o d u l u so f d e f o r m a t i o n a t 1 p e r c e n t s t r a in . A t w a t e r c o n t e n t s b e l o w 1 3 p e r c e n tt h e s t r e s s r e q u i r e d t o c a u s e 1 0 p e r c e n t s t r a i n i n c r e a s e s c o n s i s t e n t l y w i t hd e n s i t y b u t f o r w a t e r c o n t e n t s a b o v e 1 4 p e r c e n t , th e s t r e s s r e q u i r e d t oc a u s e 1 0 p e r c e n t s t r a i n m a y i n c r e a s e o r d e c r e a s e w i t h i n c r e a s e i n d e n s i t yd e p e n d i n g o n th e r a n g e o f d e n s it ie s c o n s i d e r e d : h o w e v e r t h e ef fe c t o fd e n s i t y c h a n g e s i s n o t a s g r e a t a s f o r m o d u l u s o f d e f o r m a t i o n .I t w i ll b e s e e n f r o m t h e s e r e s u l t s th a t t h e e f fe c t o f d e n s i t y a n d w a t e rc o n t e n t o n t h e s t r e n g t h o f a p a r t i a l l y s a t u r a t e d s o i l d e p e n d s o n t h e c r i t e r i o nu s e d t o d e f i n e st a b i li t y . T e s t r e s u l t s f o r t h e s il ty c l a y s h o w t h a t i f t h e u l t i -m a t e s t r e n g t h o r t h e s t r e s s r e q u i r e d t o c a u s e 2 0 p e r c e n t s t r a i n i s u s e d a st h e i n d e x o f s t a b il it y , t h e n w i t h i n t h e r a n g e o f w o r k i n g c o n d i t io n s , s t a b i l it y

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H . B . S ~E D 4 9 1a l w a y s i n c re a s e s w i t h d e n s i t y . H o w e v e r , t h e s t r a i n a t w h i c h s t a b i l it y i sd e t e r m i n e d i n m o s t d e s i g n p r o c e d u r e s i s c o n s i d e ra b l y l es s t h a n 1 0 p e r c e n t ,a n d o n t h e b a s i s o f t h i s t e s t d a t a i t w o u l d t h e r e f o r e b e e x p e c t e d t h a t f o rp a r t i a l l y s a t u r a t e d s o il s c o m p a c t e d b y k n e a d i n g a c t i o n , a n i n c r e a s e i n d e n s i t yw o u l d n o t n e c e s s a r i l y l e a d t o a n i n c r e a s e i n s t a b i l i t y .I n o r d e r t o o b t a i n s o m e i d e a o f t h e c o n d i t i o n t o w h i c h t h i s s o i l m i g h t b ec o m p a c t e d in th e f ie ld , s t a n d a r d P r o c t o r a n d m o d i f ie d A _ A S H O c o m p a c t i o nt e s ts w e r e c o n d u c t e d t o d e t e r m i n e th e o p t i m u m w a t e r c o n t e n t s a n d m a x i -m u m d r y d e n s it ie s g i v e n b y t h e s e c o m p a c t io n p r o c e d u r e s . T h e t e s ts g a v et h e f o l l o w i n g r e s u l t s :

Compaction Optimum Maxim umtest wa ter content dr y densityStandard Pr oc to r 18% 105 lb./cu, ft.Modified A A SH O 14% 117 Ib./eu. f t.

O n t h e b a si s o f t h e s t a n d a r d P r o c t o r c o m p a c t i o n t e st , th i s s o il m i g h t b ec o m p a c t e d a t a w a t e r c o n t e n t o f 1 8 p e r c e n t t o a r e l a t i v e c o m p a c t i o n o fl O 0 p e r c e n t , t h a t i s t o a d r y d e n s i t y o f 1 0 5 lb . p e r c u . f t . A t t h i s w a t e rc o n t e n t , b o t h t h e m o d u l u s o f d e f o r m a t i o n a t 1 p e r c e n t s t r a in a n d t h es t r e n g t h o f t h e c o m p a c t e d s p e c i m e n s s h o w a s l i g h t r e d u c t i o n w i t h i n c r e a s ei n d e n s i t y ; a t a r e l a t i v e c o m p a c t i o n o f 1 0 0 p e r c e n t b a s e d o n t h e s t a n d a r dP r o c t o r t e s t , t h e m o d u l u s o f d e f o r m a t i o n a n d t h e s t r e n g t h a r e h i g h e r t h a nc a n b e a t t a i n e d a t h i g h e r v a l u e s o f r e l a t i v e c o m p a c t i o n .

I f c o m p a c t io n is c o n t ro l le d b y t h e m o d i fi ed A A S H O c o m p a c t io n t e st ,t h e s o i l m i g h t b e c o m p a c t e d a t a w a t e r c o n t e n t o f 1 4 p e r c e n t t o a r e l a t i v ec o m p a c t i o n o f 9 0 , 9 5 o r 1 0 0 p e r c e n t d e p e n d i n g o n t h e s p e c i f i c a t i o n s , t h a ti s t o a de n s i ty be twe e n 105 a n d 117 lb . pe r c u . f t . I t w i l l be s e e n f r omF i g u r e 9 t h a t a t a w a t e r c o n t e n t o f 1 4 p e r c e n t , t h e m a x i m u m v a l u e o f t h em o d u l u s o f d e f o r m a t i o n i s a t t a i n e d a t a d e n s i t y o f 1 1 3 l b . p e r c u . f t . , t h a ti s a r e l a ti v e c o m p a c t io n o f 9 7 p e rc e n t . H o w e v e r , ~ f u r t h e r i n c r e as e i n re l a -t i v e c o m p a c t i o n t o 1 0 0 p e r c c r ~ t c a u s e s t h e m o d u l u s o f d e f o r m a t i o n t o b er e d u c e d to o n l y 3 3 p e r c e n t o f i ts m a x i m u m v a l ue . I n t h e c a s e o f s t r e n g t h( f o r 1 0 p e r c e n t s t r a i n ) , t h e s t r e n g t h i n c re a s es w i t h d e n s i t y u p t o a r el a t iv ec o m p a c t i o n o f 99 p e r c e n t w i t h o n l y a s l i gh t r e d u c t i o n o c c u r r i n g i f t h e r e l a -t i v e c o m p a c t i o n is f u r t h e r i n c r e a s e d t o 1 0 0 p e r c e n t . T h e s e r e s u l t s i n d i c a t e,t h e r e f o r e , t h a t f o r c o m p a c t i o n a t t h e o p t i m u m w a t e r c o n t e n t d e t e r m i n e db y t h e m o d if ie d A A S H O c o m p a c t i o n t es t, t h e d e n si ty m a y b e in c r e as e dt o a r e l a ti v e d e n si t y o f a l m o s t 1 0 0 p e r c e n t ( b a s e d o n t h e m o d i f ie d A A S H Ot e s t ) w i t h b e n e fi c ia l e f f ec t s o n s t a b i l i t y ; t h is h a s a l s o b e e n s h o w n b y e x -t e n s i v e f i e l d e x p e r i e n c e .T h e s e r e s u l t s i l lu s t r a t e , h o w e v e r , t h e d e l e t e r io u s e ff e c ts o f o v e r - c o m p a c t -

i n g a s o il a s w e l l a s t h o s e o f u s i n g t o o h i g h a w a t e r c o n t e n t . A t t h e o p t i -m u m w a t e r c o n t e n t o f t h e s t a n d a r d P r o c t o r c o m p a c ti o n t e s t ( 1 8 p e r c e n t ) ,t h e m a x i m u m m o d u l u s o f d e f o r m a t i o n a t 1 p e r c e n t s t ra i n w a s o n l y 9 5 k g.p e r s q . c m . w h i l e a t t h e o p t i m u m w a t e r c o n t e n t o f t h e m o d i f i e d A A S H Oc o m p a c t io n t e st ( 1 4 p e r c e n t ) , t h e m a x i m u m m o d u l u s o f d e fo r m a t i o n w a s

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4 9 2 CHARACTERISTICS OF COMPACTED CLAYS3 8 5 k g . p e r s q . c m . A g a i n , a t a w a t e r c o n t e n t 1 p e r c e n t a b o v e th e m o d i f i e dA A S H O o p t im u m , th e m a x i m u m s ta b il it y f o r a re la t iv e c o m p a c t io n o f9 5 p e r c e n t w a s o n l y a b o u t 6 5 p e r c e n t o f t h a t a t t a i n e d a t t h e s a m e r e l a ti v ec o m p a c t i o n a t th e o p t i m u m w a t e r c o n t en t . T h e i m p o r t a n c e o f c a re f u l w a t e rc o n t e n t c o n t r o l in o b t a i n i n g t h e m a x i m u m s t a b il i ty o f p a r t i al l y s a t u r a t e ds o i l s c a n n o t b e o v e r - e m p h a s i z e d .E x a m i n a t i o n o f t h e d a t a i n F i g u r e s 3 a n d 4 s h o w s t h a t f o r e a c h c o m -p a c t i o n c u r v e t h e r e i s a m a r k e d c h a n g e i n s t a b i l i t y a t a b o u t t h e o p t i m u mw a t e r c o n t e n t . A t w a t e r c o n t e n t s a b o v e t h e o p t i m u m t h e s ta b i li ty is l o w ,w h i l e a t w a t e r c o n t e n t s b e l o w t h e o p t i m u m t h e s t a b i l i t y i s r e l a t i v e l y h i g h .T h e s e r e s u l t s w o u l d s e e m t o i n d i c a t e t h a t t h e l o w e r v a l u e s o f s t a b i l i t y a r ea s s o c i a t e d w i t h h i g h e r d e g r e e s o f s a t u r a t i o n r a t h e r t h a n p a r t i c u l a r w a t e rc o n t e n t s a n d t h a t t h e d e g r e e o f s a t u r a t i o n m a y b e a m o r e i m p o r t a n t f a c t o ri n d e t e r m i n i n g s t a b il i ty t h a n t h e w a t e r c o n t e n t. C u r v e s o f m o d u l u s o f d e -f o r m a t i o n a t 1 p e r c e n t s t r a in v s . d e n s i t y f o r v a r i o u s c o n s t a n t v a lu e s o ft h e d e g r e e o f s a t u r a t i o n a r e s h o w n i n F i g u r e 7 a ; t h e s e c u r v e s h a v e b e e no b t a i n e d b y i n t e r p o l a ti o n f r o m t h e d a t a i n F i g u r e 3 . F o r a g i v e n d e g r e e o fs a t u r a t i o n , i n a l l c a s e s t h e m o d u l u s o f d e f o r m a t i o n i n c r e a s e s w i t h d e n s i t y .T h e s a m e is t r u e o f s t r e n g t h a s m a y b e s e en f r o m F i g u r e s 7 b a n d 7 c . I ti s i n t e r e s t i n g t o c o m p a r e t h e c u r v e s i n F i g u r e 7 a w i t h t h o s e s h o w i n g t h er e la t i o n s h ip b e t w e e n d e n s i t y a n d m o d u l u s o f d e f o r m a t i o n a t c o n s t a n t v a lu e so f w a t e r c o n t e n t in F i g u r e 5 . W h i l e t h e s t a b i li t y o f t h e s o il a t a g i v e n w a t e rc o n t e n t m a y i n c r e a s e o r d e c r e a s e w i t h a n i n c r e a s e i n d e n s i t y , i t m a y b ec o n c l u d e d f r o m F i g u r e 7 t h a t , w i t h i n t h e r a n g e o f d e n s i t i e s a n d w a t e r c o n -t e n t s i n v e s t i g a t e d , f o r a g i v e n d e g r e e o f s a t u r a t i o n , t h e s t a b i li t y w il l i n c r e a s ew i t h a n i n c r e a s e i n d e n s i t y a n d , f o r a g i v e n d e n s i t y , t h e l o w e r t h e d e g r e eo f s a t u r a t i o n t h e h i g h e r w i l l b e t h e s t a b i l i t y .E F F E C T O F C O M P A C T I O N M E T H O D O N T H E D E N S I T Y V S .S T A B I L I T Y R E L A T I O N S H I P O F P A R T I A L L YS A T U R A T E D C L A Y S

I n r e c e n t y e a r s , i n v e s t i g a t i o n s h a v e b e e n c o n d u c t e d t o d e t e r m i n e t h ee f fe c t o f d i f f e r e n t m e t h o d s o f c o m p a c t i o n o n t h e d e n s i t y v s . s t a b i l it y r e l a -t i o n sh i p o f c l ay s oi ls a n d t o p r o v i d e c o m p a r a t i v e d a t a f r o m w h i c h t e n t a t i v ec o n c l u si o n s r e g a r d i n g t h e q u a l i t a ti v e n a t u r e o f t h e s e e ff e ct s m a y b e d r a w n( U . S . W a t e r w a y s E x p e r i m e n t S t a t i o n , 1 9 4 9 - 1 9 5 0 ; S e e d e t a l . , 1 9 5 4 ) .T h e d e n s i t y v s . s t a b i l i t y r e l a t i o n s h i p s f o r s a m p l e s o f a s i l t y c l a y s o i lp r e p a r e d b y k n e a d in g , i m p a c t a n d s t a t ic c o m p a c t i o n a re s h o w n i n F i g u r e 8 .I n t h e s e t e s t s t h e s t a b i l i t i e s w e r e d e t e r m i n e d b y a H v e e m S t a b i l o m e t e ra n d t h e s a m p l e s w e r e t e s te d as c o m p a c t e d . I t w i ll b e s e e n t h a t t h e m e t h o do f c o m p a c t i o n h a s a s i g n i fi c a n t e ff e c t o n t h e t e s t r e s u l ts .F o r p u r p o s e s o f c o m p a r i s o n , t h e d e n s i t y vs . s t ab i l it y r e la t i o n s h ip s s h o w ni n F i g u r e 8 a r e r e p r o d u c e d in F i g u r e 1 0. F i g u r e 1 0 a c o m p a r e s th e s ta b i li -t i e s a t v a r i o u s w a t e r c o n t e n t s a n d d e n s i t i e s , f o r s p e c i m e n s o f t h e s i l t y c l a yc o m p a c t e d b y k n e a d i n g a n d i m p a c t m e t h o d s . I t w i l l b e se e n t h a t i n g e n e r a l

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9 ~ 1 l I 0 0 I 0 ~ 1 0 4 1 0 6 1 0 8 l i d I1 "~ 1 14 1 1 6 1 1 8 / R O l B fD r y O e n s i f y - - I b p e t c u f fFig. 7b-- Relationship between dry densit y andst re ss at 20% strain f or constant degree s ofsaturation.

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0 9 e / 9 0 1 0 2 i o 4 I O e / o e / / o / M / / 4 l i e l i e 1 2 o I ND r y D e n J i f y - - I t r p e r c u f fFig. 7c-- Relationship between dry density andst re ss at 10% strain f or constant degre es ofsaturation. 493

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F i g . 9 b - - E f f e c t of k n e a d i n g a n d s t a t i c c o m p a c t i o n o nd e n s i t y v s . s t a b i l i t y r e l a t i o n s h i p a t c o n s t a n t w a t e rc o n t e n t s f o r s a n d y c l a y .

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F i g . 1 0 b - - E f f e c t o f k n e a d i n g a n d s t a t i c c o m p a c t i o no n d e n s i t y v s . s t a b i l i t y r e l a t i o n s h i p a t c o n s t a n t w a t e rc o n t e n t s f o r s i l t y c l a y .

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H. B. SEED 497the density vs. stability curves for these two methods of compaction arevery similar in form but that kneading compaction produces slightly higherstabilities at the lower densities and impact compaction results in somewhathigher stabilities at the higher densities. The higher densities on the curvesin this type of plot are associated with the higher degrees of saturation;thus at higher degrees of saturation impact compaction produces the higherstabilities while at lower degrees of saturation, kneading compaction pro-duces higher stabilities. Comparison of the densities at which impact com-paction begins to give higher stabilties with the position of the line of opti-mums for the compaction curves shows that it is at water contents justbelow and above the optimum water content for the particular compactiveeffort being used that impact compaction causes higher stability than knead-ing compaction; at water contents well below optimum, kneading compac-tion results in the higher stabilities. However, at no stage is there any verygreat difference between the results obtained by these two methods.It is interesting to note that both kneading and impact compaction resultin samples which, at lower degrees of saturation, show an increase instability with increase in density at a given water content but at the higherdegrees of saturation show a reduction in stability with increase in densityat constant water content. This reduction in stability with increase in den-sity is more pronounced for specimens prepared by kneading compactionthan for those prepared by impact compaction.The stabilities, at various water contents and densities, of samples of siltyclay prepared by kneading and static compaction are compared in Figure10b. The most important difference in the results for these compactionmethods is that samples prepared by static compaction always show an in-crease in stability with an increase in density at any water content while forsamples prepared by kneading compaction the stability is reduced if, atwater contents greater than about 12 percent, the density is increased atthe higher degrees of saturation. This difference results in a considerablediscrepancy between the stabilities of samples prepared by static and knead-ing methods. For example, at a water content of 15 percent and a density of116 lb. per cu. ft., the Resistance Value of a sample prepared by kneadingcompaction is only 22, while that for a sample prepared by static compac-tion is 56, an increase of approximately 150 percent. In terms of pavementthickness for a typical flexible pavement designed by the State of Californiadesign procedure for a highway with heavy traffic, a Resistance Value of22 would indicate a required thickness of pavement and base of about 18inches, while a Resistance Value of 56 would indicate a required thicknessof only about 9 inches. If kneading compaction most satisfactorily repro-duces the effects of field compaction, the dangers of designing a pavementfor a partially saturated subgrade condition on the basis of tests on samplesprepared by static compaction are immediately evident.At lower degrees of saturation, the stabilities of samples of the silty clayprepared by static and kneading compaction appear to be almost identical.Thus, at equal water contents and densities, the stabilities of samples pre-

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498 CHARACTERISTICS OF COMPACTED CLAYSp a r e d b y s t a t i c c o m p a c t i o n w e r e a l w a y s e q u a l t o o r g r e a t e r t h a n t h o s e o fs a m p le s p r e p a r e d b y k n e a d i n g c o m p a c ti o n . C o m p a r i s o n o f t h e c u r v es i n F i g -u r e s 1 0a a n d 1 0b s h o w t h i s to b e t r u e a ls o f o r s t a t ic a n d i m p a c t c o m p a c t i o n .T h e d e n s i t y v s . s t a b i l i t y r e l a t i o n s h i p s f o r s a m p l e s o f a s a n d y c l a y p r e -p a r e d b y k n e a d i n g , im p a c t a n d s ta t ic c o m p a c t i o n p r o c e d u re s a r e c o m p a r e di n F i g u r e s 9 a a n d 9 b . I t w i ll b e s e e n t h a t in g e n e r a l t h e q u a l i ta t i v e e f f e c tso f t h e d i f f e r e n t c o m p a c t i o n m e t h o d s a r e s i m i l a r t o t h o s e d i s c u s s e d f o r t h es i l ty c lay .

E F F E C T O F S A T U R A T I O N O N T H E S T A B I L I T Y O F AC O M P A C T E D C L A YI n t h e l a r g e m a j o r i t y o f c a s e s , p a v e m e n t d e s i g n s a r e b a s e d o n t h e a s -s u m p t i o n t h a t t h e c o m p a c t e d S u b g r a d e w i l l b e c o m e s a t u r a t e d a t s o m e t i m ei n t h e l i f e o f t h e p a v e m e n t w h i c h i t s u p p o r t s . I t i s o f i n t e r e s t t h e r e f o r e t oc o n s i d e r t h e e f f e c t o f s a t u r a t i o n o n t h e s t a b i li ty o f a c o m p a c t e d c l a y .T h e e f f e c t o f s a t u r a t i o n o n t h e s t a b il it ie s o f s a m p l e s o f s a n d y c l a y p r e -p a r e d a t v a r i o u s w a t e r c o n t e n t s u s i n g a c o n s t a n t k n e a d i n g c o m p a c t i v e e f f o r ti s s h o w n in F i g u r e 1 1. S o m e o f t h e s a m p l e s w e r e t e s t e d a s c o m p a c t e d w h i l eo t h e r s w e r e t e s t e d a f t e r s a t u r a t i o n a t c o n s t a n t d r y d e n s i t y . F o r s a m p l e sp r e p a r e d a t w a t e r c o n t e n t s b e l o w t h e o p t i m u m , t h e r e i s a l a r g e c h a n g e i n

s t ab i li ty a f t e r s a t u r a t io n b u t f o r s a m p le s p r e p a r e d a t w a t e r c o n t e n ts a b o v et h e o p t i m u m , a n d t h e r e f o r e w i t h i n i t ia l ly h i g h d e g r e e s o f s a t u ra t io n , t h e r ei s o n l y a s l i g h t c h a n g e i n s t a b i li t y a f t e r s a t u r a t i o n .T h e l a r g e e f f e c ts o f s a t u r a t i o n o n t h e s t ab i li ti e s o f s a m p l e s h a v i n g i n i ti a l lyl o w d e g r e e s o f s a t u r a t i o n c a u s e s th e f o r m o f t h e d e n s i t y v s . s t a b i li ty a f t e rs a t u r a t i o n r e l a t i o n s h i p to b e q u i t e d i f f e r e n t f r o m t h o s e d i s c u s s e d p r e v i o u s l y .F o r s a m p l e s p r e p a r e d u s i n g a c o n s t a n t c o m p a c t i v e e f f o r t, t h e s t a b i l i ty a f t e rs a t u r a t i o n v s . in i ti a l w a t e r c o n t e n t r e la t i o n s h i p is s i m i l a r in f o r m t o t h e d r yd e n s i t y vs . i n i ti a l w a t e r c o n t e n t r e la t i o n s h ip . H o w e v e r t h e s t a b i li ty d o e sn o t d e p e n d s i m p l y o n t h e d e n s i t y t o w h i c h t h e s o i l i s c o m p a c t e d , b u t a l s o

o n t h e w a t e r c o n t e n t a t w h i c h i t i s c o m p a c t e d ; e x a m i n a t i o n o f t e s t d a t as h o w s t h a t t h i s f a c t o r m a y h a v e a n a p p r e c i a b l e e f f e c t o n t h e s t a b i li ty o f as o il a f t e r s a t u r a t i o n .E F F E C T O F W A T E R C O N T E N T A T C O M P A C T I O N O N T H ES T A B I L I T Y O F S A T U R A T E D C L A Y

T h e e f f e c t o f w a t e r c o n t e n t a t c o m p a c t i o n o n t h e s t a b il it ie s o f s a t u r a t e ds a m p l e s o f a s a n d y c l a y i s s h o w n b y t h e t e s t re s u l t s in F i g u r e 1 2. S a m p l e sw e r e p r e p a r e d b y k n e a d i n g c o m p a c t i o n , u s i n g a c o n s t a n t c o m p a c t i v e e f f o r t ,a t w a t e r c o n t e n t s a b o v e a n d b e l o w t h e o p t i m u m . A l l s a m p l e s w e r e t h e ns a t u r a t e d w i t h o u t a n y a p p r e c i a b le c h a n g e i n d e n s i t y a n d t h e i r s t ab i li ti e sw e r e d e t e r m i n e d b y H v e e m S t a b i lo m e t e r te s ts . T h e c o n d i ti o n s o f t h e s a m -p l e s a f t e r c o m p a c t i o n a n d a f t e r s a t u r a t i o n a r e s h o w n i n F i g u r e 1 2 ; i t w i l lb e s e e n t h a t a l l s a m p l e s w e r e e s s e n t i a l l y f u l l y s a t u r a t e d a t t h e t i m e o f t e s t -

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H . B . SE ED 499

q, 8 0 - ~ . .

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a /b 1 14 / o L E G E N D9 5om l~les tested a s c o m p a c t e do 5ornp les tested a f te r sa tura tion112I0 I I 12 13 14 15 16 17W a t e r C o n t e n t - p e r c e n t

Fig. 11-- Effect of saturation or, the stability of compacted samples of sandy clay.

i n g. A l s o s h o w n i n F i g u r e 1 2 a r e p l o t s o f th e s t a b il it y a f t e r s a t u r a t i o n v s .f i n a l w a t e r c o n t e n t a n d v s , f i n a l d e n s i t y .

I t i s a p p a r e n t t h a t f o r t h i s s o i l , t h e s t a b i l i t i e s o f s a m p l e s c o m p a c t e d a tw a t e r c o n t e n t s b e l o w o p t i m u m a r e c o n s i d e r a b l y h i g h e r t h a n t h o s e o f s a m -p l es c o m p a c t e d a t w a t e r c o n t e n t s a b o v e o p t i m u m . T h e s i g n i fi c a n ce o f t h isr e s u l t m a y b e se e n a s f o ll o w s . T h e m a x i m u m d e n s i t y f o r t h is so il i n a m o d i -f ie d A A S H O c o m p a c t i o n t e s t i s a b o u t 1 23 lb . p e r c u . f t. A t a r e la t i v e d e n -s i ty o f 9 5 p e r c e n t t h e s o il w o u l d t h e r e f o r e h a v e a d r y d e n s i t y o f 1 17 lb .p e r c u . f t. U s i n g t h e d a t a in F i g u r e 1 2, i f t h e s o il w e r e c o m p a c t e d to th i sd e n s i t y a t a w a t e r c o n t e n t o f 1 5 . 5 p e r c e n t , a n d t h e n s a t u r a t e d w i t h o u tc h a n g e i n d e n s it y , t h e R e s i s t a n c e v a l u e w o u l d b e a b o u t 2 7. I f t h e s o il w e r ec o m p a c t e d a t a w a t e r c o n t e n t o f 1 1.5 p e r c e n t , u s i n g th e s a m e c o m p a c t i v ee f f o r t , t h e s a m e d e n s i t y w o u l d b e a c h i e v e d b u t a f t e r s a t u r a t i o n w i t h o u tc h a n g e i n d e n s i t y t h e s o i l w o u l d h a v e a R e s i s t a n c e v a l u e o f a b o u t 4 1 , a ni n c r e a s e o f a b o u t 5 0 p e r c e n t o v e r t h e p r e v i o u s v a l u e .

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T h e m a g n i t u d e o f t h i s e f f e ct w i ll n o t b e t h e s a m e f o r a ll s o il s b u t i t m a yw e l l b e o f s i g n i f i c a n c e f o r e c o n o m i c a l p a v e m e n t d e s i g n .

E F F E C T O F C O M P A C T I O N M E T H O D O N T H E S T A B I L I T YA N D S W E L L P R E S S U R E S O F S A M P L E S S A T U R A T E DB Y E X U D A T I O NI n t h e p a v e m e n t d e s i g n p r o c e d u r e u s e d b y t h e S t a t e o f C a l i f o r n i a D i v i -s io n o f H i g h w a y s , s a m p l e s a r e p r e p a r e d b y k n e a d i n g c o m p a c t i o n , a t w a t e rc o n t e n t s u s u a l l y o n t h e w e t s id e o f o p t i m u m , a n d a r e t h e n s a t u r a t e d b ya p p l y i n g s t a t i c p r e s s u r e u n t i l m o i s t u r e i s e x u d e d . T h e s e s a m p l e s a r e t h e nu s e d t o d e t e r m i n e t h e s t a b i l i t y o f t h e s o i l a n d a l s o t h e s w e l l p r e s s u r e e x e r t e db y t h e s o i l w h e n i t is c o n f i n e d i n a m o l d a n d i m m e r s e d i n w a t e r .S w e l l p re s s u r e s a r e m e a s u r e d i n t h e s t a n d a r d d e v i c e s h o w n i n F i g u r e 2 .A f t e r m o i s t u r e h a s b e e n e x u d e d , t h e s a m p l e i s a l l o w e d t o s t a n d f o r h a l fa n h o u r w i t h t h e e n d s o f th e m o l d c o v e r e d . A p e r f o r a t e d m e t a l d i s c w i t ha v e r t i c a l s t e m i s t h e n p l a c e d o n t o p o f t h e s a m p l e a n d t h e m o l d i s f i t t e d i nt h e s w e l l p r e s s u r e d e v i c e . I n t h i s p o s i t io n t h e l o w e r e n d o f t h e s a m p l e r e s t so n t h e a d j u s t a b l e b a s e o f t h e d e v i c e a n d t h e b a s e is a d j u s t e d u n t i l t h e s t e mo f t h e p e r f o r a t e d p l a t e o n to p o f t h e s a m p l e is j u s t t i g h t a g a i n s t t h e p r o v i n gb a r . W a t e r i s t h e n p o u r e d o n t o p o f t h e s am p l e a n d th e s u b s e q u e n t d e f le c -t i o n o f t h e p r o v i n g b a r o v e r a p e r i o d o f s e v e r a l d a y s i s m e a s u r e d b y a d i a lg a u g e . T h e p r o v i n g b a r s a r e r e l a t i v e l y s ti ff , a p r e s s u r e o f 1 p s i e x e r t e d b yt h e s a m p l e c a u s i n g a d e f l e ct i o n o f o n l y 0 .0 0 3 i n c h ; t h u s o n l y a v e r y s l i g h te x p a n s i o n o f t h e s o i l i s p e r m i t t e d d u r i n g t h e s w e l l p r e s s u r e m e a s u r e m e n t s .A f t e r t h e s w e l l p r e s s u r e h a s b e e n m e a s u r e d , t h e s t a b i l i t y o f t h e s a m p l e i sd e t e r m i n e d b y a H v e e m S t a b i lo m e t e r t es t.T h e s ig n i fi c an t ef fe c ts o f c o m p a c t i o n m e t h o d o n t h e s w e ll p r e s s u r e s a n ds t a b i l i t i e s d e t e r m i n e d i n t h i s w a y f o r c l a y s a m p l e s s a t u r a t e d b y e x u d a t i o na f t e r p r e p a r a t i o n b y k n e a d i n g a n d s t at i c c o m p a c t i o n a r e s h o w n b y t h e t e str e s u l ts i n F i g u r e s 1 3 a n d 1 4.T h e r e s u l t s o f t e s t s o n a s a n d y . cl ay a r e s h o w n i n F i g u r e 1 4. I t w i ll b es e e n t h a t f o r e q u a l d e n s i t i e s , s a m p l e s p r e p a r e d b y s t a t i c c o m p a c t i o n h a v ec o n s i d e r a b l y h i g h e r R e s i s t a n c e v a l u e s a n d s w e l l p r e s s u r e s t h a n s a m p l e sp r e p a r e d b y k n e a d i n g c o m p a c t i o n . A t a d r y d e n s i t y o f 1 1 0 l b s. p e r c u . f t .,w h i c h f o r t h i s s o i l i s t h e m a x i m u m d e n s i t y d e t e r m i n e d b y t h e s t a n d a r dA A S H O c o m p a c t io n t es t, t h e R e s i s ta n c e v a lu e o f a s a m p l e p r e p a r e d " bys t at ic c o m p a c t io n i s a b o u t 3 0 p e r c e n t g r e a t e r t h a n t h a t o f a s a m p l e p r e p a r e db y k n e a d i n g c o m p a c t i o n a n d t h e s w e ll p r e s s u r e is a b o u t 4 0 0 p e r c e n t g r e a t e r .A t a d e n s i t y o f 1 23 lb s . p e r c u . f t ., w h i c h i s t h e m a x i m u m d e n s i t y a s d e -t e r m i n e d b y t h e m o d if ie d A A S H O c o m p a c t io n te s t f o r t h is s o il, th e R e -

s i s t a n c e v a l u e o f a s a m p l e p r e p a r e d b y s t a t i c c o m p a c t i o n i s a l s o a b o u t3 0 p e r c e n t g r e a t e r t h a n t h a t o f a s a m p l e p r e p a r e d b y k n e a d i n g c o m p a c t i o n ,a n d t h e s w e l l p r e s s u r e i s a b o u t 1 0 0 p e r c e n t g r e a t e r .T h e r e s u l t s o f t e s t s o n a s i l t y c l a y a r e s h o w n i n F i g u r e 1 3 ; t h e q u a l i t a -t i v e e ff e c ts o f t h e c o m p a c t i o n m e t h o d s a r e s i m i l ar t o t h o s e d e s c r i b e d a b o v e .

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502 CHARACTERISTICS OF COMPACTED CLAYS

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Fig. 14-- Density vs. stability and density vs. sw el l- pr es su rerelationships for samples of sandy clay prepared by static andkneading compaction and saturated by exudation.

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5 04 C H A R A C T E R I S T I C SF COMPACTED CLAYS

F o r t h e si lt y c la y , t h e m a x i m u m d r y d e n s it y i n t h e s ta n d a r d A A S H O c o m -p a c t io n t e s t w a s 1 0 7 l bs . p e r c u. f t . a n d t h e m a x i m u m d r y d e n s i t y i n t h em o d i f i e d A A S H O c o m p a c t i o n t e s t w a s 1 17 l bs . p e r c u . f t . A t a d r y d e n s i t yo f 1 0 7 I b s . p e r c u . f t . , t h e R e s i s t a n c e v a l u e s a n d s w e l l p r e s s u r e s f o r s a m -p l e s p r e p a r e d b y k n e a d i n g a n d s t a t i c c o m p a c t i o n a r e a b o u t t h e s a m e ; b u ta t a d r y d e n s i t y o f 1 1 7 I b s . p e r c u . f t . t h e R e s i s t a n c e v a l u e o f a s a m p l ep r e p a r e d b y s t a t i c c o m p a c t i o n i s a b o u t 2 0 0 p e r c e n t g r e a t e r t h a n t h a t o f as a m p l e p r e p a r e d b y k n e a d i n g c o m p a c t i o n w h i l e t h e s w e l l p r e s s u r e i s a b o u t8 0 0 p e r c e n t g r e a t e r .I f , a s a v a i la b l e e v i d e n c e w o u l d i n d i c a t e , k n e a d i n g c o m p a c t i o n m o r e c l o s el yd u p l i c a t e s t h e e f fe c t s o f f ie ld c o m p a c t i o n e q u i p m e n t t h a n d o e s s t a t ic c o m -p a c t i o n , t h e e r r o n e o u s v a l u e s f o r d e s i r a b l e p a v e m e n t t h i c k n e s s w h i c h m i g h tb e o b t a i n e d i f d e s i g n s w e r e b a s e d o n t h e r e s u l t s o f t e s t s o n s a m p l e s p r e -p a r e d b y s t a t ic c o m p a c t i o n a r e i m m e d i a t e ly a p p a r e n t .

C O N C L U S I O NT h e p r i m a r y p u r p o s e o f t h i s p a p e r h a s b e e n t o d e s c r i b e s o m e o f t h ef a c t o r s a f f e c t i n g t h e s t a b i li t y a n d s w e l l p r e s s u r e c h a r a c t e r i s t i c s o f c o m -p a c t e d cl ay s . I n m e t h o d s o f p a v e m e n t d e s ig n b a s e d o n th e r e s u l ts o f t e s t sp e r f o r m e d o n s a m p l e s p r e p a r e d i n t h e l a b o r a t o r y , i t i s d e s i r a b l e t h a t t h et e s t s a m p l e s s h o u l d h a v e t h e s a m e p r o p e r t i e s a s t h o s e o f t h e s o i l c o m p a c t e di n t h e f ie ld . A k n o w l e d g e o f t h e d e n s i t y v s . s t a b i l i t y re l a t i o n s h i p s f o r p a r -t i a l l y s a t u r a t e d s o i l s i s r e q u i r e d i f t h e m o s t e c o n o m i c a l p a v e m e n t s a r e t ob e d e s i g n e d f o r a r e a s i n w h i c h i t i s r e l a t i v e l y c e r t a i n t h a t t h e s u p p o r t i n gs o il w i ll n o t b e c o m e s a t u r a t e d ( U . S . E n g i n e e r D e p t ., 1 9 5 1 ) . A n u n d e r -s t a n d i n g o f t h e i n f lu e n c e o f d e n s i t y a n d w a t e r c o n t e n t a t c o m p a c t i o n is r e -q u i r e d f o r t h e m o s t e f fi c ie n t p l a c e m e n t o f s o i l s i n t h e f ie ld . I t i s o n l yt h r o u g h a b e t t e r u n d e r s t a n d i n g o f t h e f a c t o r s a ff e c ti n g t h e p r o p e r t i e s o fc o m p a c t e d s o i l s t h a t t h e s e r e s u l t s c a n b e a c h i e v e d a n d s o i l s m a y b e u s e dt o b e t t e r a d v a n t a g e i n e a r t h w o r k s t r u c t u r e s .

R E F E R E N C E SFoster, C. R. (July 1953) Reduction in soil strength with increase in density: P r o -ceedings Separate No. 228, Am erican Society of Civ il Engineers, N ew York.Hveem, F. N., and Carmany, R. M. (1948) The factors underlying the rational designof pavements: Proceedings, Highw ay Research Board, v ol. 28, pp. 101-136.Seed, H.. B., Lund gren, R., and C han, C. K . (1 95 4) The effect of compaction methodon the stability and swell pressure characteristics of soils: Highway ResearchBoard, Bulletin 93, 1954.Seed, H. B., and Monismith, C. L. (1953) Some relationships between density andstability of subgrade soils: Highway Research Board, Bulletin 93, 1954.U.S. Engineer D ep t . (July 1951) Airfield pavement design: Flexible pavements:Engineering Manual for Military Construction, Part XII, Chapter 2, Appendix B.U.S. Waterways Experim ent Station (A pril 1949 -June 19.50) Soil compaction investi-gation: Technical Mem orandum 3-27 1: 1-5, Vicksburg, Mississippi.U.S. Waterways Experiment Station (May 1951) Airplane landing mat investigation,engineering tests on steel, pierced type, M8 and aluminum~ pierced type M9: Tech-nical M emo randum 3-324, Vicksburg, M ississippi.