8/12/2019 Construction of Foundations on Thawing Permafrost - Kolesov

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by A A Kolesov

CONSTRUCTION OF FOUNDATIONS ON THAWING PERMAFROSTThe f r o z e n f o o t i n g s un de r h e a te d b u i l d i n g s an d o t h e r s t r u c t u r e s i np e r m af r o st r e g i o n s th aw when t h e s e s t r u c t u r e s a r e i n u se , a nd t h e t h aw in g i s

accompanied by changes i n physicomechanical pr op er t i e s of so i l s . Thawing ofp e rm a fr o st r e s u l t s i n s e t t l e m e n t s i n e x c e s s of t h e a mounts p e r m i s s ib l e f o rc o n v e n t i o n a l b u i l d i n g s a n d s t r u c t u r e s .

A t t h e i n i t i a l s t a g e i n t h e d ev el op me nt o f m ec ha ni cs o f f r o z e n an dthawing s o i l s and founda t ion eng ineer in g 1950-1960), t h e uneven se t t l em en t sof t h e t ha wi ng f o o t i n g s were preven ted by bu i l d ing founda t ions of a sp ec ia ldes ign , such s c a se d -i n r e i n f o r c ed c o n c r e t e p l a t e s , c o nt i nu o us f o o t i n g s i nt h e form of g i r d e r w a l l s , c r o s s s t r i p f o o t i n g s , e t c . A t t h a t t i m e t h e r ew er e no s t a n d a r d r u l e s c o n c er n i ng t h e su i t a b i l i t y of c o n s t r u c t i o n m et hod sf o r t ha wi ng f o o t i n g s . T h is l e d t o a s i t u a t i o n , wh ere s u p e r s t r u c t u r e s we ree r e c t e d on s o i l s w i t h s e t t le m e n t s s e v e r a l times i n e x c e s s o f p e r mi s s ib l eamounts. S p e c i a l f o u n d a t i o n d e s i g n s w er e a l s o u se d t o p r e ve n t d i f f e r e n t i a ls e t t l i n g of t h e f o ot i ng s .

The ne x t t r e n d i n f ou n d a ti o n e n g in e e r in g was t o b u i l d a r t i f i c i a lfo ot in gs f rom thawed-out e a r t h ma te r i a l s . The method of pre-cons t ruc t ionthawing makes i t pos s ib le t o bu i l d even on i ce - r i ch pe rmaf rost .

The thawing i s done w i t h e l e c t r i c i t y , s t ea m o r w a t e r.The foo t i ng s and founda t ions des igned fo r a r e in fo rce d co ncre temanufac tur ing p la n t a r e shown i n Fig . 1 and may s er ve a s an example of t h e

above. The bu il di ng i s l o c a t e d i n a t e r r a i n composed o f i c e - r i c h c o a r seg r a v e l b ed s w it h a s an dy loam f i l l e r , w hich s e t t l e i n t h e p r o c e s s oft h a w i ~ g . I n t h i s p ro j e ct i t was d e c i d e d t o t ha w o u t t h e g ro un d p r i o r t ocons t ruc t ion . The thawing was done wi t h wa te r i n j ec te d th rough need lesa r r anged a t m i n t e r v a l s and s un k t o a d e p t h of 8 m Water w a s s u p p l i e df rom th e r i ve r , where pumping s t a t i o n w a s b u i l t , a n d w a s d r a i n e d o f fth rough a d ra inage sys t em i n th e fo rm of a t r enc h i n the thawed-out pebb lebed t h a t was g ra d ed t o w ar ds t h e r i v e r a nd ha d a l o w e r e l e v a t i o n t h a n t h efo ot of t h e thawed-out s o i l mass.

Thawing was sc he du led f o r th e t h re e summer months. Hydrothawing makest p o ss i b l e n o t o n l y t o th aw o u t t h e gr ou nd , b u t a l s o t o wash away t h e f i n e

f r a c t i o n . Th i s c o mp l et e ly e l i m i n a t e s h e av i ng of t h e thaw ed-out s o i l e ve n i ft h e g r a v e l p ad i s invaded by ground water. To red uc e d i f f e r e n t i a lse t t l e me n t s du r ing t he thawing of th e ground , t he whole bu i l d ing w a s d i v i d e di n t o s e p a r a t e b lo ck s by s e t t l e m e n t j o i n t s a t t h e b a se of t h e thaw ed-out s o i lmass.

I n r e c e n t y e a r s t h e u s e of c o n s t r u c t i o n p r i n c i p l e s on th aw in g f o o t i n g shas been s t r i c t l y r egu la t ed SNIP 11-18-76). Wherever th e se t t l em en t s o fth e thawing fo o t i ng exceed th e a dmiss ib le amoun ts , t i s impe rat iv e t o worko u t a program f o r p r e p a r i n g t h e s o i l of t h e f o o t i n g , i n c l u d i n g t h e me th od sof pre-co nst r uct i on thawing and re inforceme nt of th e ground.

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I n some c a se s a r oc ky l e d g e o r s t o n e y s o i l s t h a t d o n o t s t t l d u r i n gthawing may be enc ount ered a t some depth. t i s a d v is a b l e t o u s e t h a t k i ndof g round as a fo o t i ng fo r deep supp or t s o r p i l e s c u t t i n g t hr ou gh t h e up pe rs t r a t a of f r oz e n s o i l s t h a t a r e s u b j e c t t o s e t t l i n g . F i gu r e 2 shows a nexample of such a so lu t ion . In des igns of t h a t k ind , t h e suppor t s a r e madet o bea r l a r ge loads t o t ak e maximum advan tage of th e bea r ing c apa c i t y of th erocky ground.

For thos e cas es , where l oc a l t hawing zones form undernea th th esu p e r s t r u c t u r e s r e l e a s i n g s i g n i f i c a n t a mo un ts of h e a t , t h e F un da me nt pr oe ktp roposed i n 1968 , a va r i a n t o f p i l e founda t ions sunk t o a dep th below th ees t ima ted zone of thawing. In tha t ca se th e bea r ing ca pac i ty i s determinedby t he amount of adf ree ze of th e p i l e s i n th e zone of f ro ze n ground belowthe thawing zone, and wi th a l lowances made f o r t h e warming up of th e f roz enf o o t i n g .

The cu r r en t method of pre- cons t ruc t ion thawing of t h e ground i s t o thawo u t t h e s o i l un d er n ea t h t h e w hole a r e a of t h e b u i l d i n g o r o t h e r e n g i ne e r in gs t r u c t u r e s , w i t h i n c r e a s e i n t h e d im en si on s i n t h e p l a n t o t ha win g d ep th .

The Fundamentproekt i n s t i t u t e developed a method f o r thawing ou tp e rm a fr o st o n l y a t t h e p l a c e s , w here t h e p i l e s a r e su nk ( p a t e n t o 218741 ofJune 11, 1966, A method of i n s t a l l i n g p i l e f o u n d a t i o n s u n d e rn e a t h h e a t edbu i l d ing s on thawing pe rmaf rost ) . Th i s method co ns i s t of th e fo l lowing(Fig . 3):

a ) The ground i s thawed-out t o a de pt h of 9-10 m a t t h e p la c e s , w he reg r ou p s of p i l e s a nd p ad s f o r t h e e qu ip me nt a r e t o b e i n s t a l l e d . Thed im e ns io ns of t h e z on e t o b e t ha we d- ou t a r e c a l c u l a t e d i n t h e p l a n a s e q u a lt o a t l e a s t s i x d ia me te rs of a s i n g l e p i l e .b) The p i l e s a r e sun k t o t h e s p e c i f i e d d e p th al lo w in g f o r a t l e a s t a4-5 m e tr e -t h ic k l a y e r of s o i l b etw ee n t h e t i p of t h e p i l e s an d t h e f o o t oft h e thawed-out zone.

Two type s of fou ndat ions have been developed fo r th e ca se s , where t h es o i l s a r e us ed a s f o o ti n g s ( p r i n c i p l e 1 1) : p i l e f o u nd a t io n s ( t h e main t y p e )and f o u n d a ti o n s on n a t u r a l f o o t i n g s f o r b u i ld i n g s p a r t l y s u nk i n t o t h eground.

The le ng th of t h e p i l e s i s l i m i t e d (5-6 m) t o e n su r e t h a t t h e b e a r i n gs o i l l a ye r a t t h e ba se of t h e p i l e s i s a t l e a s t 4-5 t h i c k . The b e a r i n gs t r e n g t h of t h e p i l e s i s d et er mi ne d from t h e r e s u l t s of s t a t i c t e s t s i nthawed-out zones.

P i l e foun da t ions a r e recommended f o r t he fo l lowing ca ses :a ) i n the p resence o f sandy s o i l s w i t h a g r e a t b e a r i n g c a p a c i t y b elowt h e d e p t h of 4m ( t h e s e a s o n a l f r e e z i n g l a y e r ab ov e t h a t d e p t h i s composed ofunconso l ida ted sand) ;b ) when e x c av a t i o n of a f o u n d a t io n p i t c a n n ot b e c a r r i e d o u t

( p a r t i c u l a r l y i n t h e w i nt e r) .In th e proc ess of e l ab or at in g the method proposed by t he

Fundamentproekt, a t e ch n iq u e h a s b ee n d ev el op ed , i n w hic h t h e s e t t l i n g s o i l sa r e bo re d t h ro u gh t o a g r e a t d e p t h an d t h e u n d e r l y i n g g r a v e l d e p o s i t s a r e

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subsequen t ly thawed-out fo r some ea r t h s t ru c tu re s of t he Baikal-AmurR ai lw ay T ru nk L i n e ) . A f t e r t h a t t h e p i l e s a r e su nk i n t o t h e thawed-outpebble de pos i ts . This reduces the amount of thawing, speeds up p i l es i n k i n g , and en ab l e s u s t o u t i l i z e t h e b e ar in g l a y e r t o a c o ns id e ra b ledepth .

The method of p re -c ons t ruc t ion thawing of fo o t in g s o i l s deve loped i nt h e USSR makes t p o s s i b l e t o b ui l d on ne a r ly a l l t h e t y p e s of f r o ze ns o i l s . However, t h e m ost a ccep t ab l e t y p es of s o i l f o r co n s t r u c t i o na c co r di n g t o p r i n c i p l e I a r e san d and gr ave l ly - peb bl y o r co a r s e l yf r ag m en t a l s o i l s .

Depending on th e geocryo log ic a l cond i t ion s and so i l s , th e type s off o un d at io n s u se d i n c o n s t r u c t io n a c c o rd i ng t o p r i n c i p l e I a r e a s f o l l o w s :

1 I n the p resence of a rock l edge a t a d e pt h of 10 20 o r i n t h eca se o f thawing t o a dep th o f 1 0 i n l ow -t em pe ra tu re s o i l s , p i l ef o u n da t i on s o r deep s u p p o r t s a r e su nk below t h e l a y e r of s e t t l i n g andt h aw i n g s o i l s .

2 . P i l e founda t ion i n thawed-out zones o f l i mi te d dimens ions.3. F l e x i b l e f o un d at io n s i n t h e f orm of s t r i p s , p l a t e s o r mats l a i d ont haw in g f o o t i n g s and ab s o r bi n g d i f f e r e n t i a l s e t t l em en t s o f t h e g ro un d.

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Figure 1 Diagrammatic representation of hydrothawing of frozen grounda Plan of the site; b Cross section I I; c Plan of the area ofthawing operations; 1 The area undergoing thawing; 2 Water main;3 Pumping station; 4 Waste water drain; 5 Watercollectingtrench; 6 Distribution hoses; 7 Building under construction;8 Distribution pipes; 9 Thawed out soil.

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Figure 2 . In sta l l at io n of p i l foundations on rocky s o i l s a Zones ofthawing of th e ground; b Construction of th e p i l foundation;1 Outlines the zone t o be thawed out over th e s p e c if i e d time;2 Sand bank; 3 Peat; 4 Gravel bed; 5 Rock; 6 Pi l e s ; 7 Cement s andso lu tio n; 8 Pilework.

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gure 3. Methods of installing pile foundations on thawing soils a Thecurrent scheme of preparing the ground for construction withallowances made for thawing when the building is in use; b Theproposed method of installing pile foundations in thawed outzones of limited dimensions; 1 Permafrost thawed out prior toconstruction; 2 Estimated zone of thawing over the life of thebuilding; 3 Permafrost; 4 Local zones of thawed out soil.