Studies of Shear Strength and Creep of Frozen Soils With the Use of Various Apparatus - Dokuchaev

8/12/2019 Studies of Shear Strength and Creep of Frozen Soils With the Use of Various Apparatus - Dokuchaev

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by V V DokuchaevA.S. Gerasimov

STUDIES OF SHEAR STRENGTH ND CREEP OF FROZEN SOILS WITH THE USE OF VARIOUSAPPARATUS

S he ar s t r e n g t h of f r o z e n s o i l s t o be u s ed f o r t h e d e s i g n of f o u n d a t i o n san d f o o t i n g s , can b e de t er m in ed w i t h t h e h e l p o f d i f f e r en t t y p es o fappa ratu s . The same equipment can a l so be used f o r determining t h e cre epc h a r a c t e r i s t i c s , i n p a r t i c u la r t h e c o e ff i c i e n ts of v i s c o s i t y , f o rc a l c u l a t i n g t h e d e fo r ma t io n s of f o o t i n g s i n i c e and i c e - r i c h s o i l s .

T es t s c a r r i e d o u t u n de r co n d i t i o n s of combined s t a t e o f s t r e s si n c l u d i n g t r i a x i a l c om pr es si on , a r e t h e m ost e f f e c t i v e o n es f o r de t e rm i n in gt h e s t r en g t h and c r eep ch a r ac t e r i s t i c s . However, t h e equ ip ment r eq u i r ed f o rs u ch t e s t s i s s t i l l under development . The apa ra t us used i n cu r re n tp r a c t i c e a r e of t h e s i m p l e s t k i n d e n a b l in g on e t o c on du ct t h e t e s t s byp en e t r a t i o n of a c i r c u a r l o ad i n g p l a t e , by u n i ax i a l com p ress i on , by s h ea r i n gt h e s o i l w i t h i n t h e s pec im en o r a c r o s s t h e a d f r e e z i ng s u r f a c e of t h efoun dat i on model [1 ,2] .

P en e t r a t i o n t e s t s w i t h t h e h e l p of a c i r c u l a r l o a d i n g p l a t e p r op os ed b yTsy tov ich , have been ex t ens ive ly used by Vyalov. They y i e l d r e l i a b l er e s u l t s f o r s o i l s w i t h ma ssiv e, f i n e l y b edded, o r r e t i c u l a t e c ry og en ics t r u c t u r e s , and h av i ng a small a n g l e of i n t e r n a l f r i c t i o n . Clay s o i l s andf i n e o r s i l t y s an ds w it h a n i c e con t en t c lo se t o un i t y commonly be long t ot h a t t y p e of s o i l s . The l a r g e - s ca l e t e s t s c a r r i e d o u t a c co rd i ng t o t h a tmethod h av e s e r v ed a s b a s e s f o r o b t a i n i n g d a t a o n t h e r e s i s t an ce t o n or ma lp r e s s u re f o r f r o z e n s o i l s va r yi ng i n c om po si ti on a nd i c e c o n t e n t and a td i f f e re n t t empera tu res . Tabu la t ion of such da ta was o r ig in a l ly p roposed byVyalov, and became p a r t of s ta nd ar d documentation.

U n i ax i a l com pr es si on i n b en ch a r b o r p r e s s e s u n d e r co n s t an t l o ad o ru n de r l o ad a p p l i e d i n s t a g e s , o r i n V ya l ov s d yn am om et ric ap p a r a t u s u n de r acon t inuous ly vary ing load , i s a un iv er sa l and dependable means of t e s t in gs o i l s w i t h d i v e r s e c r yo g en i c s t r u c t u r e s i n c l u d i n g i c e -r i c h s o i l s [ l-31. I nt h e s e t e s t s however , th e normal p res s ure on th e shear p lane in cre ase s wi thinc rea s in g loa d on t h e specimen. There fore r e l i a b l e va lu es of long- te rms t r e n g t h c an be o b ta i n ed o n ly f o r l i m i t e d v a l u e s of t h e a n g l e of i n t e r n a lf r i c t i o n , a s i s a l s o t h e c a se i n t h e t e s t s w i t h a s p h e r i c a l l o a di n g p l a t e .The u n i v e r s a l n a t u r e of t h i s ki n d of t e s t i n g l i e s i n t h e f a c t t h a t te n a b l e s on e t o o b t a i n by r e l a t i v e l y s i m pl e means, n o t o n ly t h e s t r e n g t hc h a r a c t e r i s t i c s , b u t a l o s t h e c h a r a c t e r i s t i c s of d e f or m a b il i ty of s o i l s ,w hich c a n be u s ed t o c a l c u l a t e ( d e si g n ) f o o t i n g s a c c o r d in g t o t h e t h e o r y o fin he r i t ed c ree p recommended by Vyalov fo r f ro zen s o i l s , a s w e l l a s acco rd i n gt o t h e t h e or y of v i s co u s f lo w a p p l i e d f o r i ce a n d i c e - r i c h s o i l s .

Shear apparatus make t p o s si b l e t o s t ud y c re ep a t d i f f e r e n t r a t i o s o fn or ma l t o s h ea r s t resses bu t t he y have t h e shortcomin g of non-uniformd i s t r i b u t i o n of s t r e s s w i t h i n t h e s pe cim en w i t h a t t e n d a n t l o c a l re mo uld in g.The e f f e c t of t h i s f a c t o r i s p a r t i c u la r l y n o t ic e ab l e i n f r oz en s o i l s a ts h e a r stresses e x ce e di n g t h e t r a n s c i e n t c r e e p l m t z ( y ie l d s t r e s s ) .


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D ur in g t e s t i n g of i c e and i c e -r i c h s o i l s w i t h a n i c e c on t e nt o f n b = 0.6 i nSado vski i ' s appa ratu s PRS*-1 designed i n co-operat ion wi t h Goro dets ki i , i twas d i sc o v e re d , f o r ex am pl e, t h a t t h e c l e a r a n c e b etw ee n t h e en d p l a t e n s d i dno t a lways co in c ide wi th th e th i ck ness of th e shea red zone . Th i s wasc l e a r l y v i s i b l e i n c r os s -s e ct io n s of t h e s pe cim ens a f t e r t h e t e s t s . I n t h et e s t s c a r r i e d o u t i n i c e a t a t e m p er a t ur e of -3OC, t h e se me asure me ntsc o in c id e d o n ly a t a c l e ar a n ce of 10 ( t = 0.4 MPa); a t a sma l l e r o r l a rg e rc le ara nce t h e d i f f er en ce between th es e measurements was inc rea se d by af a c t o r o f 1.5 t o 3. For i c e -r i c h s o i l s ( t = 0.6 MPa) t h e th ic kn es s of t h esh e a re d z on e was p e r c e p t i b l y g r e a t e r t h a n t h e c l e a r a n c e , when t h e l a t t e r wassm a l l e r t h a n 3 0 mm

I n th es e t e s t s a s p e c i a l c a s e i s t h e s h e a ri n g of f r o ze n s o i l a c r o s s t h ea d f r e e z i n g s u r f a c e of t h e m a t e r i a l of t h e f o un d at io n . I f t h e e f f e c t i v es h ea r s t r e s s e s a r e o n ly s l i g h t l y g r e a t e r t h an t h e t r a n s i e n t c r e e p s t r e n g t ho f t h e s o i l , t h e s t r e n g t h an d s t r a i n p a ra m et e rs d et er mi ne d d u ri n g t h ed is pl ac em en t a c r o s s t h a t s u r f a c e a r e s u f f i c i e n t l y r e l i a b l e . A pp ar at us , i nw hi ch c y l i n d r i c a l ( Tsy t o v i c h ' s ) o r f l a t (E rm ak ov 's ) p r o be s a r e f o r c e d( p r e s s e d ) t h ro ug h t h e s o i l un de r t e s t h av e a l s o fo un d p r a c t i c a lap pl i ca t ion . The presence of th e normal stresses o c c u rr i ng w h il e t h e s o i lf r e e z e s i n t h e c a s i ng s , and t h e no n-u ni fo rm d i s t r i b u t i o n of s t r e s s e s i nf r o z e n s o i l a t t h e bo tto m o f t h e c a s i n g s , a r e among t h e main d i s t i n c t i v ec h a r a c t e r i s t i c s of t h e s e a p p a ra t u s.

The above r ev iew shows t ha t t he r e su l t s of th e t e s t s depend on th ec h a r a c t e r i s t i c s o f t h e t e s t i n g a p p a r a t u s u se d. To d e t e rm i n e t h e d e g r e e oft h a t e f f e c t , t h e Amderma u nd er gr ou nd l a b o r a t o r y c a r r i e d o u t s p e c i a l p a r a l l e lt e s t s* * i n t h e po p u l ar a p p a r a t u s d e si g n e d by S a d o v sk i i , Tsy t ov i c h an dErmakov (Fig. l ) , as w e l l a s w i t h t h e h e l p of a c i r c u l a r l o a d i n g p l a t e . tt h e s u g ge s t io n o f t h e a u t h o r s p e n e t r a t i o n t e s t s were a l s o c a r r i e d o u t , i nwhich c y l in d r i c a l model s were p r e s se d t h ro ug h t h e s o i l f r o z e n on t o t h ec a s i n g s . I t was a na lo go us t o e x p er im e nt s c a r r i e d o u t i n a n a p p a r a t u ss i m i l a r t o t h a t of Tsy t o vi c h , b u t w i t ho u t t h e bottom . The r e su l t s of t h e s et e s t s w ere combined w i t h t h e r e s u l t s o b t ai n e d p r e v i o u s l y i n s i n g l e- s h e arappara tus des igned a t t h e Lenmorn iip roekt [4 ] .

A r t i f i c i a l l y p r e p ar e d sp ec im en s we re f r o z e n un d er i d e n t i c a l t e m p er a t ur econd i tons , and wi t h rough ly the same vo lume o f s o i l pe r cm2 o f th e she arsu r f a c e . The t e s t s were conducted a t a tem per atu re of -3OC w it h Amdermac l a y loam ( su g l in o k ) ( p l a s t i c i t y i n d e x 0.10) h a v in g t h e f o l lo w i n gp a r t i c l e -s i z e d i s t r i b u t i o n : 10 mm f r a c t io n 0 .1%, 10-5 mm 0.6%,5-2 0.7%, 2-1 0.7%, 1-0.5 mm 0.5%, 0.5-0.25 0.5%,0.35-0.1 7.1%, 0.1-0.05 34%, 0.05-0 -0 1 mm 26.4%,0.01-0.002 mm 11.3%, and 0.002 mm 18.1%. The t o t a l moi s tu re con t en tof t h e s o i l w a s 0.30-0.32, s l i g h t l y g r e a t e r t h a n t h e l i q u i d l i m i t which wasequ al t o 0 .28.

In th e app arat us of Tsytovi ch and Ermakov t h e s o i l was packed andf r oz e n i n l a y e r s 10 ( i n t h e f or me r a p pa r at u s) o r 5 mm ( i n t h e l a t t e r ) i nt h i c k n e s s , a s w e l l a s t h ro u gh o ut t h e c a s i n g s .

* T r a n s l i t e r a t i o n**The t e s t s were performed by Artemov, Kurkov and Yarkin


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In t he app arat us of Tsytovich , Sadovs ki i and Ermakov th e lo adi ng wasdone i n i n c re m e n ta l s t a g e s , a s w e l l a s w i t h c o n s t a n t l oa d s. The s i z e o f t h es t e p s i n e a ch t e s t s was 0.05 MPa up t o a v a l u e c l o se t o t h e a n t i c i p a t e dsh ea r s t re ng th and 0 .025 MPa the re af te r . Each loadi ng s ta g e was mainta inedu n t i l e i t h e r t h e p re de te rm in ed d e fo r ma t io n was a t t a i n e d , o r t h e i n i t i a t i o nof t h e s t e a d y - s t a t e f l o w s t a g e . I t was assumed th a t defo rmat ion wass t a b i l i z e d i f t h e i n cr e me n t o v e r 24 h o u r s d i d n o t e xc ee d 0.01 i n t h ef i r s t two appara tu s and 0.002 mm i n t he l a s t a p p a r a t u s . T e st s i n r i n gc a s i n g s w ere c a r r i e d o u t by u s i n g t h e p r oc e du r es s p e c i f i e d f o r T s y t o v i c h l sa p p a ra t u s , and s p h e r i c a l p l a t e t e s t s we re pe rfo rm ed i n a s t r a n d a r d a p p a ra t u sac co rd in g t o GOST* 21-48-75 B ui ld in g Code).

S he ar s t r e n g t h v a lu e s o f f r o z e n s o i l s de te rm in ed i n d i f f e r e n t a p p a r a t u sS he ar s t r e n g t h MPa) across a

No of t h e Specimensa p p a ra t u s f r o z e n smooth s t e e l c o r ru g a te d s t e e l c o n c r e t es u r f a c e s u r f a c e s u r f a c e

I n l a y e r sThroughout2 I n l a y e r sThroughout3 I n l a y e r sI n l a y e r sThroughoutThroughout4 Throughout

5 ThroughoutFOOTNOTE. Acc ordi ng t o t h e d a t a fro m t h e t es t s i n A V S a d o v s k i i l sa p pa r at u s, t h e s h e a r s t r e n g t h a c r os s t h e s o i l s u r f a c e i s 0.30 MPa.

The t e s t s w er e r e p e a t e d t h r e e o r more t i m es i n t h e a p p a r a t u s d e s i gn e dby Tsy tovi ch 1, Fig. l ) , by t h e au th or 2 ) , by Ermakov 3 ) , by Sadovskii4 ) , a nd a t t h e Le nm o rn ii pr oe kt 5 ) . The d a t a f ro m t h e t e s t s w ere u se d t oc a l c u l a t e t h e a v e ra ge v a l ue s o f s h e a r s t r e n g t h a c r o s s t h e a d f r e e z i ngs u r f a c e s Rc s e e t h e t a b l e ) . I n t h e se t e s t s s h e a r s t r e n g t h was t h e v a l u eof s t r e s s a which th e s tea dy- st a te f low ra t e was 0.01 mmlday, andc or re sp on de d t o t h e damping c r i t e r i o n of s t r a i n s , o r t h e s t r e s s v al u e a twhich t he damping o f s t r a i n s sp ec i f i e d a s t o t a l was obse rved and which wasd e te r mi n ed f ro m t h e i n t i a l a b sc i s s a of t h e r h e o l o g i c a l c u r v e m ark ed i n t h et a b l e w i t h a n a s t e r i s k ) . I n a d d i t i o n t o t h e v a lu e s of s t r e n g t h f o r c o n r et esu r f a c e s of t h e 5 t h c l a s s of r o.ug hn es s, s t r e n g t h v a l u e s f o r su r f a c e of t h e3rd class o f r o ug h ne s s a r e g i v e n i n p a re n t h e se s . A s was d e te r mi n ed e a r l i e r ,t h e c o n v er s io n f a c t o r h e r e i s 1.15 [ 5 ] The equivalent cohesion determinedw i t h t h e h e l p o f a c i r c u l a r l oa di ng p l a t e w a s 0.28 MPa.

The d a t a o b t ai n e d s ug g e s t t h a t t h e t y p e of t e s t i n g a p p ar a t us a s t h o s ed e s c ri b e d ab ov e h a s v e ry l i t t l e e f f e c t o n t h e mea sured s h e a r s t r e n g t h . Themethod o f f r e e z i n g t h e s o i l i s of some impor tance i f t h e mat e r i a l t o whicht h e s o i l a d fr e e ze s h a s a c o rr u ga t ed s u r f a ce . I t i s t h e r e f o r e p r e f e r a b l e i ns uc h t e s t s t o f r e e z e t h e s o i l i n l a y e rs . Removing t h e bo tt om of t h ea p p a r a t u s i n t o w hi ch p i l e s a r e d r i v e n , a l s o i mp ro ve s t h e a gr ee me nt b et we ent h e r e s u l t s o b t a in e d i n t h e s e t e s t s a nd t ho s e of p l a ne s h e a r t e s t s . Removal


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of t h e bo tt om makes t h e s t a t e of s t r ess i n t h e s o i l more d e f i n i t e , w hich n o to n l y im pr ov es t h e a c c ur a c y of s t r e n g t h d e t e r m i n a t i o n s , b u t a l s o i n c r e a s e st h e p o t e n t i a l o f t h e a p p a ra t u s, a s i t c a n a l s o b e u se d f o r d e t e rm i ni n g t h ec o e f f i c i e n t of v i s c o s i t y . A p p l ic a t io n o f t h e l o a d i n s t a g e s d oe s n o tp r e c lu d e d e t e r m i n a t i o n of t h e s h e a r i n g s t r e n g t h . This may be due t o t h en e g l i g i b l y s m a l l c o n s o l i d a t i o n of s o l i d l y f r o z e n s o i l s , w h i ch becomed eform ed a t t h e s t a g e of t r a n s i e n t c r ee p .

The a d f re e z e s t r e n g t h of t h e s o i l t o t h e p i l e i s dete rmined f rom t e s t sw i t h m od el s, a s w e l l a s f ro m t h e t e s t s i n t h e a p p a r a t u s d e s c r i b e d ab ov e.Many rese ar ch er s , inc lud ing Vyalov [ 6 1 h av e n o te d t h a t t h e r e s u l t s o b ta in e dd ep en d o n t h e d ia m e t e r of t h e p i l e s . It i s t h e re f o r e of i n t e r e s t t oe v a l u a t e t h e s t r e s s e d s t a t e and t h e development of s t r a i n s i n t h e s o i ls u rr o un d in g t h e model p i l e . L et u s f i r s t co n s i de r t h e c a s e o f a f l u t e d( g ro o v ed ) m od el p i l e w i t h a r a d i u s ro. The model i s p r e s se d i n t o ab o t t o m l e s s c a s i n g a nd i t s d is pl ac em en t a t t a i n s t h e s t a g e of t r a n s i e n t c r e ep .The r a d i u s o f t h e zo ne of t h e s o i l , i n w hich t h a t s t a g e of c r e e p d ev e lo p s,i s rh= ro (zo / zz ) , whe re z0 and 7 a r e t h e s h ea r s t r esses on t h e c y l i n d r i c a ls u r f a c e of t h e p i l e w i t h a r a d i u s to c or re sp o nd in g t o t h e t r a n s i e n t c r e eps t r e n g t h .

I n t h e c a s e o f a n a x i a l l y s y m m e tr i ca l m odel a nd i n t h e a b se n c e o fr a d i a l d i sp l a ce m e n ts , t h e a n g l e of s h e a r i s I = I-= here w i s t h eZTL ~di sp l acement a lon g t he z -ax i s o f t he model , and p = r / r h . For av i s c o - p l a s t i c body w i t h v i s c o s i t y 1, t h e r a t e of c hange n t h e r g l e ofa ws h e a r i s given by = (7->)/q, it c an be w r i t t e n a s (z-z3),

where aw l a t = v i s t h e d is pl ac em en t r a t e of t h e p i l e spat rl

Equa t ion (1 ) i s v a l i d o n l y i f t h e d i a m et e r of t h e c a s i n g 2 r0 z0 /z z; i fn o t , t h e d i sp la ce me nt vh of t h e f r o z e n s o i l r e l a t i v e t o t h e c a s in g must a l s ob e t a k e n i n t o a cc ou nt . I n o r d e r t o re d uc e vh i t i s a d v is a b le t o i n c r e a s et h e r o u gh n es s of t h e i n n e r s u r f a c e of t h e c a si n g . F or i c e w i t h z z 0 w ec a n o b t a i n i n a s i m i l a r manner

V i s c o s i t y c a n b e c a l c u l a t e d by s u b s t i t u t i n g t h e e x p e r i m e n t a ll y d e te r mi n edv a l u es o f t h e o t h e r p a ra m et er s i n t o t h e ex p r e s s i o n s ( 1 ) and ( 2 ) .F o r e x p e r i m e n t a l v e r i f i c a t i o n of t h e s e e q u a t i o n s , Artemov c a r r i e d o u t

p e n e t r a t i o n t e s t s w i t h f l u t e d m odels f r o ze n i n t o t h e i c e ( r o = 1.8 cm) i n ac y l i n d r i c a l c a s in g ( r h = 10.35 cm, 8 cm i n h e i g h t ) a t -3OC and unde rc o n s t a n t l o a d s . Co ar se -g r ai ne d s a n d was g l u e d t o t h e i n n e r s u r f a c e o f t h ec a s i n g , and i c e was f r o z e n i n l a y e r s 2 mm i n t h i ck ness . For compar ison o ft h e d a t a o b t a i n ed , u n i a x i a l c om pr es si on t e s t s w er e c a r r i e d o u t w i t h i c e


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spe ci me ns p r e p a r e d f ro m t h e i c e f r o z e n i n t h e se c a s i n g s . The specimensmeasured 50 mm i n d i amete r and 100 mm i n h ei gh t. The v a l u e s of v i s c o s i t yc o e f f i c i e n t o b t a i n e d f ro m t h e se two t es t s were c l os e t o each o the r : 16500i n t h e f o rm e r i n s t a n c e a nd 1 80 00 MPa i n t h e l a t t e r case .The problem examined above can be extended t o p i l e s , f o r which as o l u t i o n i n t h e form of 1 ) w a s proposed e a r l i e r by Vyalov [61. For two

p i l e s a t i d e n t i c a l v al ue s of z0 and zz, e q u a t i o n 1 ) g i v e s

his c o n d i t i o n of s i m i l i t u d e i s equ iva le n t t o th a t ob ta ined by Tyabin andqu ot ed by Vyalov [61. A t v = c o n s t w e have

C on se qu en tl y, t o o b t a i n i d e n t i c a l s t ress v a l u e s a t t h e p i l e- s o il i n t e r f a c ef o r p i l e s h a vi ng d i f f e r e n t d i am e te r s, t h e i r di sp la cm e nt r a t e s mst b ed i f f e r e n t . I f we a c c e p t v l = 0.01 mm/day a s th e cr i t e r i o n f o r th e dampingo f t h e s t r a i n s of t h e s t e a d y - s t a t e f l ow f o r a model w i t h r o = 2cm, t oo b t a i n i d e n t i c a l s h e a r s t r e n g t h v a lu e s from t es t s wi th mode ls of a l a rg e rr a d i u s t h a t v a l u e of t h e d i sp la c e m e nt r a t e m ust b e i n c r e a se d r0 , 2 / r0 , 1times, e .g . f o r r0 ,2 = 10 cm i t sho u ld be 0.05 mm/day.

I f , however , w e assum e on e s i n g l e c r i t e r i o n of damping f o r a l l modelp i l e s h av in g d i f f e r e n t d i am e te r s, t h e v a l u e of s h e a r s t r e n g t h o b ta i ne d f o rt h e m odel w i t h a l a r g e r r a d i u s w i l l b e sm a l l e r t h a n t h e v a l u e d e te r mi n ed f o rth e mode l wi th a sm a l l e r r a d i u s . The g raph i n F ig. 2 g ive s va lue s ofz0,2/-rO, a s a f u n c t i o n t h e r a d i u s ro of t h e m odel, f o r d i f f e r e n t= z o , l z z . model with a r a d i u s of cin w a s assumed t o be th e ana logue.For t h e s o i l t e s t ed s e e t h e t a b le ) a t v = 0.01 mm/day, 1.2. The valueof t h e c o e f f i c i e n t f o r c o nv e rs i on of t h e s h e a r s t r e n g t h o b ta i n e d fr om t h ep e n e t r a t i o n of t h e model i n t o t h e c a s i ng t o t h e s h e a r s t r e n g t h of t h e s o i lon a f u l l - s i z e p i l e i s roughly 0.9.

I f t h e s h e a r s t r e n g t h v a l u e i s d e te r mi n ed fro m t h e i n t e r s e c t i o n of t h er h e ol o g ic a l c ur ve w i t h t h e s t r e s s a x i s , i .e . i f t i s assumed t h a t v 0 ,t h e n r0 ,2 / r0 1 = 1, t h e e f f e c t of th e d iamete r of th e model i s excluded, and

= . his e f f e c t c a n a l s o be i g no r ed i n t h e d e te r m in a t io n of t h eu imate lo ng -te rm s t r e n g t h by e x t r a p o l a t i o n of t h e r e s u l t s of sh o r t - te r mt e s t s u nd er c o n s ta n t l o a d s, i f t h e e x t r a p o l a t i o n fo rm ul as u se d l e a d t o alo ng -t erm s t r e n g t h v a lu e o v er t h e l i f e of t h e s t r u c t u r e t h a t c o i n c id e sw i t h z

The ab ov e c o n s i d e r a t i o n s a r e v a l i d f o r t h e c a se , w he re t h e h o r i z o n t a ls t r e s s e s n ormal t o t h e l a t e r a l s u r f a c e of t h e model and of t h e p i l e a r euni form o r abs ent . On t h e co nt r ary , a s has a l re ady been demonst ra ted byV ya lo v [ 6 1, t h e d i a m e t e r h a s a n a p p r e c i a b l e e f f e c t on t h e e x p e r i m e n t a l l yde te rmined va lue of th e u l t im a te long- te rm sh ea r s t r en g th .

De ta i l ed s tu d i es a long the se l i ne s have been ca r r i e d ou t by Kardymon[ ] and Tikhomirov [ 8] , who have demonst ra ted th a t t h e s t resses n orm al t ot h e s h e a r s u r f a c e a r e t h e most i m p or ta nt f a c t o r . These s t r e s s e s a r i s e i n


6/9

t h e p r o ce s s of v e r t i c a l l o ad i n g of m od els an d p i l e s d u r i n g f r eez i n g of t h es o i l i n c a s i ng s o r i n b or eh ole s h a ft s . Kardymon discove red t h a t wi thi n c r e a s e i n t h e l a t e r a l di me ns io ns of t h e model p i l e s t h e d eg r e e of t h e i rl a t e r a l co mp re ssi on un de r t h e a c t i o n of a x i a l l o a ds i n c r e a se d s l i g h t l yi n s t e a d of d e c r ea si n g. But t h e f o r c e s e x e r t i n g t h e g r e a t e s t i n f l u e n c e o nth e magni tude Rc a r e t h e l a t e r a l co mp re ssi on f o r c e s a r i s i n g w h il e t h emodels o r p i l e s k r e b e in g f r oz e n i n t o t h e s o i l . With a n i n c r e a s e i n t h et h ic k n e ss of t h e f r e e z i n g s o i l and i n t h e r i g i d i t y of t h e c a s i ng ( o r of t h ef r o z e n s o i l ma ss ), and w i t h a d ec r ea s e i n t h e d i am e t e r of t h e m odel p i l e s ,t h e s p ec i f i c m agn it ud e of t h e s e f o r c e s ( p er u n i t a r e a of t h e l a t e r a ls u r f a ce ) i n c r e a s e s p e r cep t i b l y . M or eo ve r, t h e i n c r ea s e i s p a r t i c u l a r l yp ro mi nent when t h e s o i l f r e e ze s i n a c l o s ed s p ace. A s t he l a t e r a ld imensions of th e model s (p i l es ) inc r ea se , th e in te n s i ty of compres sionf o r c e s r a p i d l y d e c l i n e s .

It s h o ul d be n o t ed , h ow ev er , t h a t t h e no rm al s t r e s s e s o ccu r r i n g d u r i n gt h e f r e ez i n g of t h e s o i l become r e l ax ed w i t h ti me . The d eg r ee of t h a tr e l a x a t i o n o v er t h e l i f e of t h e s t r u c t u r e h as n o t been de te rm in ed . Thev a l i d i t y o f t h e t e s t s w i t h models a nd p i l e s , d u r i n g w hic h l a r g e n orm als t r e s s e s a r i s e , i s t h e r e f o r e q u e st i o na b l e . F r ee z in g o f s o i l i n l a y e r smarkedly r educes t he magnitude o f th e compress ive fo r ces ac t i ng on th e ode1( s e e t h e t a b l e ) .

A s was demonstra ted by Vyalov e t a l . [ I ] , t h e r h eo l o g i ca l cu r v e can b er eg a r d ed a s a curve composed of two l i n e a r segments wi th t h e po in t ofi n f l e c t i o n c o rr es po nd in g t o t h e s t r e s s t z whe re z, i s t h e l i n e a r c r ee pl i m i t i . e . t h e s t r e s s b ou nd in g t h e i n i t H a l l T n ea r s eg ment . I n t h i s c as et h e l i n e a r d i s pl a ce m en t r a t e of t h e p i l e model i s expressed through

where q1 and q 2 a r e t h e v i s c o s i t y c o e f f i c i e n t s c or re sp on di ng t o t h e f i r s tand seco nd l i n e a r s egm ent s o f t h e r h eo l o g i ca l cu r ve ; z i i s t h e v a l u e of t h es t r e s s in t e r ce p t o f th e s econd l i n e ar s egment on th e X-axis ; and r i s t h e

rr ad i u s of t h e cy l i n d e r a t w hi ch z o 2;.r~

When models wi th a s tand ard ( smooth ) con di t ion of th e sur fa ce a r ep r es se d i n , t h e t o t a l d is pl ac em en t r a t e of t h e model, a f t e r t a k i n g i n t oa cc ou nt t h e e f f e c t o f t h e weaker c o n t a c t s o i l l a y e r , i s

where v i s t h e r a t e d e t er m in ed by t h e f l o w of t h e f ro ze n s o i l mass andca l c u l a t ed f ro m f o r r m l a s ( 1 ) o r 5); and v t i s t h e r a t e de t er m in ed by t h ef l o w of t h e co n t ac t l ay e r .


7/9

Assuming, on th e bas i s o f da ta i n [ 9 and of th os e from our own s tu di e sw i t h t h e u s e o f c o n c r e t e m od el s, t h a t t h e v a l ue s of v i s c o s i t y c o e f f i c i e n t sof t h e c on t a ct l a y e r and s o i l mass are t h e same, w e f i n d t h a t

f o r t h e c a se wh ere t h e f l o w ra tes c or re sp on d t o t h e i n i t i a l segment of t h er h e o l o g i c a l c ur ve . I n t h i s e q u a t i o n ro and r t a r e t h e r a d i i o f t h e modelwftth fnd wi thou t th e th i c knes s of th e co n ta c t l a ye r be ing accoun ted fo r ; andTO, 2 3 a r e t h e s t ress a l o n g t h e g e n e r a t r i x of t h e m odel an d t h e t r a n s i e n tc r e e p l i m i t o f t h e c on t a c t l a y e r , r e s p e c t i v e ly .

I f t h e f lo w o f t h e s o i l i n t h e c o nt a c t l a y e r o c cu r s a t r a t e sc o r re sp o nd i n g t o t h e s e co n d l i n e a r s eg me nt o f t h e rh e o l o g i c a l c u r v e, t h e nt h e e x p r e s s i o n f o r v w i l l have a somewhat different form obtained byt r a n s f o r m i n g e x p r e s s i o n ( 5 ) .

To i l l u s t r a t e t h e a bov e, w e s h a l l examine t e s t s o f two conc re te mode lsh a vi n g su r f a c e r ou g hn e ss , of t h e 5 t h c l a sq a nd w i t h r a d i i r = 1 0 c m . ~ e tu s assume vk 0.01 mmlday, T = 26 N l c m T = 20 N I C I ~ ~ , 25 / cm2 ,kand ro-ro = 0.3 cm .k k , 2 2For rk w e have 2 = ( r O , l / r O , l ) TO 6 22.6 N l c m z ~091 2.3c o n se q u e n t l y t h e r e i s no f lo w a t a c o n s ta n t r a t e i n t h e f r o ze n s o i l o u t s id et h e c o n t a c t l a y e r . Assuming i n t h i s e xa mp le , t h a t t h e c o n t a c t - l i n ed i sp l ac e me n t r a t e i s d e te r mi n ed by t h e f l o w i n t h e i n i t i s eg me nt o f t h e

rheo log ic a l cu rve , we ob ta i n v = v t , = 1.268Iq = loQ mmlday, and f o r= l O c m w e f i n d v = 1 . ~ ~ 1 0 - 2 m r n f d a y b y u s i n g f o r m l a s 6 ) n d ( 7 ) .r0.2 k,2I t i s t h us e v i de n t t h a t t o o b t a i n i d e n t i c a l s h e a r s t r e n g t h va lu es f o rmodels wi th diam ete rs of 20 c m and 4 c m r e s pe c t iv e l y, t h e t o t a l cr e epdamping c r i t e r i o n must be inc rea sed by one- th i rd . t v k , l = vk0.01 mm/day, i .e. a t a n i d e n t i c a l d amping c r i t e r i o n va l ue , t h e d i f f e r e n c eb etw ee n t h e s t r e n g t h v a l u e s h a s b ee n c a l c u l a t e d a s 6 . C o nseq ue nt ly i n t h eabsence of normal s t resses t o t h e s h e a r s u r f a c e t h e e f f e c t of t h e d ia me te ro f t h e model d e c l i n e s w i t h d e c r e a s i n g ro u gh n es s o f t h e su r f a c e .


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BIBLIOGRAPHY1 Vyalov S.S., Gorodetskii S.E., Ermakov V.F. et al. Metodika

opredeleniya kharakteristik polzuchesti, dlitel noi prochnosti iszhimaemosti merzlykh gruntov (Methodes of Determining Creep, Long-termStrength and Compressibility Characteristics of Frozen Soils). Moscow,Nauka, 1966, 132 p.

2. Rukovodstvo po opredeleniyu fizicheskikh, teplofizicheskikh imekhanicheskikh kharakteristik merzlykh gruntov (Guide forDeterminations of Physical, Thermal and Mechanical Characteristics ofFroze Soils). Moscow, Stroiizdat, 1973, 191 p.

3. Vyalov S.S., Dokuchaev V.V., Sheinkman D.R. Sil nol distye grunty kakosnovaniya sooruzhenii (Ice-rich Soils as Footings for Superstructures).Leningrad, Stroiizdat, 1976, 166 p.

4. Velli Yu.Ya., Karpov V.M., Ivanov V.N. Rezul taty polevykh ilaboratornykh issledovanii sil smerzaniya merzlykh gruntov (Results offield and laboratory studies of adfreeze forces of frozen soils). -In:Tr. IV soveshch. seminars po obmenu opytom stroitel stva surovykhklimaticheskikh usloviyakh (Proceedings of the IV Conference forExchange of Experience Pertaining to Construction under Harsh Climatic

bokovoi poverkhnosti svai na soprotivlenie sdvigu po nei merzlykhgruntov (The effect of the material and of the roughness of the lateralsurface of the pile on its resistance to shearing of frozen soils alongthat surface). -In: Inzhenernoe merzlotovedenie (EngineeringPermafrost Studies). Novosibirsk, Nauka, Sib. otdnie, 1979,pp. 74-77.

6. Vyalov S.S. Reologicheskie svoistava i nesushchaya sposobnost merzlykhgruntov (Rheological Properties and Bearing Capacity of Frozen Soils).Moscow, Izd-vo AN SSSR, 19 59, 191 p.

7 . K a p d b ~ ~ o nfl Uccnenoman~e onpormmnenmu rpynTa cnamry no 60 ~o m o ii omepxlloc-TH IIMOPO)l(eHHbIX C U ~ G 03H0fi CbOpMbl: A B TO PC C ~.HC. KaHll. TeXH. HayK. Bn aw -~ C T O K : anbneeocr. nonmrexn. HHT 1 9 6 7 . 2 6 c .8. T ux o~ up oe C.M. Mopoanoe ny9enme rpyHToa m OrpaHHreHHOM 0 6 b e ~ e . Tp.HU UO Cll , 1978, awn. 69. c. 137-142.9 . 6 0 ~ d 8 ~ ~ h ' K O.H. C J ~ O ~ C K U ~8 f l p o r ~ ~ c l bA ~ O P M H ~ Y ~ M O C T ~epJnOrO rpYHraHa KOHTaKT CO C K B ~ O ~ . O C H O ~ B H H R ,Y H ~ B M ~ H T ~ IMeXaHHKa rpyHTOm. 1975,3. c. 22-25.


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Figure 1 Diagrammatic repr esentation of the apparatus f o r t e st in g fr ozens o i l s i n shear designe d by: 1-Tsytovich 2-Dokuchaev andGarasimov 3-Ermakov 4-Sadovski i and 5-the Lenmorproekt.

Figure 2. Variation of the values of ~ ~ ~ / r ~ ~ith the radius of themodel r

Documents

Studies of Shear Strength and Creep of Frozen Soils With the Use of Various Apparatus - Dokuchaev