FOUNDATION ENGINEERING

STABILITY OF

OF

FOUNDATIONS

SOIL

HEAVING

DURING

V.

B. Shvets

and

UNDER

CONDITIONS

FREEZING

B. I. Koehengin

UDC 624.139.26+624.131.434:624.131.537

Foundation s t a b i l i t y under conditions of f r o s t heaving h a s been widely elucidated in the l i t e r a t u r e [1-9]. T h e r e a r e c o n t r a d i c t o r y opinions with r e s p e c t to this p r o b l e m . Most r e s e a r c h e r s h a v e e x p r e s s e d the n e e d for refining the r e c o m m e n d a t i o n s of Construction Specifications and Regulations (SNIP) on founding depth in o r d e r to allow for the p o s s i b i l i t y of heaving the foundation soil. However, m a n y r e s e a r c h e r s ignore the r o l e of p r e s s u r e t r a n s m i t t e d by the foundation to the heaving soil, which at c e r t a i n values can m a r k e d l y d e c r e a s e and even c o m p l e t e l y eliminate the d e v e l o p m e n t of f r o s t heave of the soil under the foundation [1, 9, 2, 6, 8]. F o r a n u m b e r of y e a r s the authors h a v e been making f u l l - s c a l e o b s e r v a t i o n s of the stability of buildings with foundations situated above the f r e e z i n g depth in clay soils under the c l i m a t i c and soil conditions of Central U r a l s , of the development of heaving with r e s p e c t to depth, and of the b e h a v i o r of lightly loaded e x p e r i m e n t a l foundations with different founding depths; and have also c a r r i e d out field and l a b o r a t o r y inv e s t i g a t i o n s to e s t a b l i s h the effect of initial w a t e r content and p r e s s u r e on the heaving p r o p e r t i e s of clay soils. In 1958-1960, m o r e than 120 o n e - s t o r y h o m e s with a 0.6-0.8 m depth of the continuous footings and contact p r e s s u r e o v e r t h e i r b a s e of 1.5-1.8 k g / c m 2 w e r e c o n s t r u c t e d Near P e r v o u r a l ' s k , at El'nichnoe village. Around the p e r i m e t e r of the buildings was c o n s t r u c t e d an asphalt blind a r e a 0.8-1 m wide. The foundation soils w e r e r e p r e s e n t e d by slightly m o i s t diluvial l o a m s and clays (to 1-1.5 m) p a s s i n g d e e p e r hheaveo MM

t

hhcavc, mrn

,

/ 'Jr lI

'~t~ 3O

° 8O

\! ,

\o

\ fSO

o Fig. 1. D i s p l a c e m e n t s hheav e of e x p e r i m e n t a l foundations as a function of f r e e z i n g depth H. 1, 2, 3) E x p e r i m e n t a l foundations with a depth o f 0.5, 1.2, and ].9 m, r e s p e c t i v e l y .

fS6O

\

fSgl

Fig. 2. D i s p l a c e m e n t s of e x p e r i m e n t a l foundations at different t h i c k n e s s of the backfill. ]) Backfill 0.6 m ; 2) 0.7 m ; 3) 0.8 m ; 4) 1.2 m ; 5) 1.7 m ; 6) 2.5 m .

S v e r d l o v s k . T r a n s l a t e d f r o m Osnovaniya, Fundamenty i Mekhanika Gruntov, No. 3, pp. 26-28, M a y June, 1966.

198

r T y p e of soil

1. R o c k y m a r l (in addition to clayey) c o a r s e - f r a g m e n t a l soils with c o a r s e - and m e d i u m - g r a i n e d sandy fill, c o a r s e - and m e d i u m - g r a i n e d sands 2. Fine and powdery sands, clayey marls, sandy loams with consistency B <0, loams and clays with B --<0.25 3. Loams and clays with consistency B --<0.5 4. Fine and powdery sands, clayey marls, sandy loams, foams and clays regardless of consistency 5. Sandy loams with consistency B > 0, foams and clays with consistency B > 0.5

D i s t a n c e f r o m grading s u r f a c e !to g r o u n d - w a t e r l e v e l during i f r e e z i n g with c o n s i d e r a t i o n of s e c t i o n 3.1 of SNiP I I - B . 1 - 6 2

Depth of foundation from grading surface

Any

Does not depend on f r e e z i n g depth E x c e e d s the design f r e e z i n g depth by 2 m and m o r e

The s a m e

Designated in a c c o r d with point 4 of c o m m e n t s

L e s s than design f r e e z i n g depth o r e x c e e d s it by l e s s than 2 m

Any

Not l e s s than design f r e e z i n g depth

into eluvial l o a m s . O b s e r v a t i o n of the s t a t e of t h e s e buildings c a r r i e d out by leveling in a r a n d o m o r d e r for 2-3 y e a r s e s t a b l i s h e d that the m a x i m u m d i f f e r e n c e of v e r t i c a l d i s p l a c e m e n t s of the c o r n e r s of the buildings as a consequence of f r o s t h e a v e of the b a s e was 1-2 c m , i.e., l e s s than that p e r m i s s i b l e a c c o r d ing to SNIP. The buildings did not have v i s i b l e d e f o r m a t i o n s . I n v e s t i g a t i o n s e s t a b l i s h e d s o m e i n c r e a s e of w a t e r content of the clay s o i l s (before the s t a r t and d u r ing m a x i m u m f r e e z i n g ) n e a r the foundations of t h e s e buildings a f t e r 3 y e a r s of s e r v i c e , e s p e c i a l l y in the u p p e r l a y e r s w h e r e the c o n s i s t e n c y b e c a m e s e m i s o l i d and p a r t i a l l y p l a s t i c in p l a c e of solid. T h i s slight change in w a t e r content was p r o m o t e d by w e l l - p e r f o r m e d v e r t i c a l planning in laying out the village on a slope, the v i r t u a l a b s e n c e of backfill n e a r the foundation s p a c e s , and the r e l i a b l e w o r k of the blind a r e a , the width of which exceeded the depth of the foundations by a f a c t o r of 1.25. Along with the f a v o r a b l e e x p e r i e n c e of c o n s t r u c t i n g low- s t o r i e d buildings on clay soils above the f r e e z ing depth, there are adverse examples also. Thus, at Pervoural'sk low-storied buildings, whose foundations were at a depth of 1.2 m (which amounts to 2/3 of the standard freezing depth) in saturated diluvial and eluvial foams and sandy loams with a ground-water level at a depth of 1.2-1.5 m, experienced appreciable deformations during the first winter as a consequence of heaving in the base soil [8]. The effect of frost heave on lightly loaded (up to 2 kg/cm 2) foundations having a different depth was studied at two experimental construction sites. Displacements of the foundations were determined by precision leveling relative to fixed wall markers. The experimental foundations (site No. 1) were made of 0.4 × 0.4-m cinder blocks with a foundingdepth of 0.5, 1,2, and 1.9 m, and were loadedto a contact pressure of 1.9-2 kg/cm2. The supporting soils to a depth of I. I 1.2 m were represented by diluvial sandyloam (W= 19%, Wp Z16%, WI =20%)passing lower into eluvial sandy loam (W= 15%, Wp = 13%, W1 = 19%). The ground-water level was at a depth of Im.. The site was regularly cleaned free of snow. The results of observing displacement of the experimental foundations andthe depth of freezing, recorded by a Danilin cryopedometer, are shown in Fig. 1. The different magnitudes of displacement of tl~e experimental foundations are due to the different action of normal and tangential heaving forces which are related, in turn, with the freezing depth. Thus, displacement of foundation I having a depth of 0.5 is mainly due to normal heaving forces of the 1-m thick soil layer that froze underneath the base. As a consequence of the relatively small size of the foundation, the effect of the pressure transmitted by it on a decrease of heaving was negligible.

199

Displacement of foundation 2 is by a f a c t o r of 2-3 less than that of foundation 1. With a 0.3 m thickn e s s of the f r o z e n l a y e r under the foundation base, which was in a state of s t r e s s owing to the t r a n s m i t t e d p r e s s u r e , the effect of n o r m a l heaving f o r c e s m a r k e d l y d e c r e a s e d . The tangential heaving f o r c e s f o r m e d along the side s u r f a c e of the foundation p r i m a r i l y affected the magnitude of heaving, Displacement of foundation 3 was insignificant owing to the absence at n o r m a l heaving f o r c e s , and also because of a dec r e a s e in the action of tangential f o r c e s as a r e s u l t of the retaining f o r c e of the anchor (lower end of the foundation) in the thawed soil. The e x p e r i m e n t a l foundations at Sverdlovsk (site No. 2) w e r e individual foundations of an industrial building under c o n s t r u c t i o n that had been left unfinished. The size of the foundation base was 2.5 × 2.5 m, the c r o s s section of the foundation columns was 1.2 × 0.8 m, the thickness of the backfill was 0.5-2.5 m. The p r e s s u r e s t r a n s m i t t e d by the foundations to the supporting soil (with consideration of the s u r c h a r g e of the backfill) w e r e 0.35-0.7 k g / c m 2. The supporting soil was composed of eluvial sandy loams a n d l o a m s with a water content close to the plastic limit. T h e ground w a t e r s w e r e at a depth of 1.3-1.5 m below the foundation b a s e . F i g u r e 2 shows that at a total f r e e z i n g depth of 1.1-0.9 m (depending on ~he backfill layer) the diff e r e n t magnitudes of d i s p l a c e m e n t s of foundations 1-5 a r e due to a different r a t i o of n o r m a l and tangential f o r c e s . No d i s p l a c e m e n t s of foundation 6 was observed, since loads of 100 kg p e r c e n t i m e t e r of the foundation column p e r i m e t e r , with consideration of the s u r c h a r g e f r o m the backfill, proved to be sufficient to a b s o r b the tangential heaving f o r c e s . The m o s t interesting data w e r e obtained in a l a b o r a t o r y study of the effect of initial water content and p r e s s u r e on the magnitude of f r o s t heave. The e x p e r i m e n t s w e r e c a r r i e d out on ten different (with r e s p e c t to type of bedrocks) types of eluvial clay soils of a natural composition in closed and open (free seepage of water) s y s t e m s on s p e c i m e n s enclosed in m e t a l r i n g s with a d i a m e t e r of 26 and height of 10 cm. In c o n f o r m i t y with the adopted p r o c e d u r e , the s p e c i m e n s w e r e f r o z e n at a t e m p e r a t u r e of - 8 to - 1 2 ° in insulated boxes whose design precluded f r e e z i n g of the specimens on the sides and bottom and permitted t r a n s m i t t i n g local s t r e s s to the soil f r o m plates with an a r e a of 15-16 cm 2 (Sj. To avoid f r e e z i n g of the soil to the ring, the inside s u r f a c e of the l a t t e r was c o v e r e d with graphite lubricant. On the b a s i s of generalizing the data of l a b o r a t o r y t e s t s set up to study the effect of initial water content and of field o b s e r v a t i o n s of heaving of the s u r f a c e of eluvial clays, loams, and sandy l o a m s , we elicited the decisive effect of initial w a t e r content and water supply conditions on f r o s t heave. It was e s t a b lished that in a closed s y s t e m c o r r e s p o n d i n g to the absence o f ground w a t e r s close to the limit of seasonal freezing, we can r e l e g a t e loams and clays with initial w a t e r content W <-Wp + 0.25 Ip to conditionally nonheaving and little-heaving soils, according to M. F. K i s e l e v ' s classification (the heaving coefficient Kheav e is, r e s p e c t i v e l y , less than 1 and 3.5%). For sandy loams, with consideration of the s m a l l r a n g e of change of the plasticity index Ip and the a c c u r a c y of d e t e r m i n i n g the w a t e r content ± 1-270, the value of W should be not m o r e than the w a t e r content at plastic limit. F o r conditions of the open s y s t e m all types of clay soils experience intense heaving r e g a r d l e s s of the initial w a t e r content and consistency. Similar r e s u l t s w e r e obtained by V. M. Sokolova [10t and V. M. Karpov [7l in l a b o r a t o r y and field investigations of f r o s t heave of clay soils having a s e d i m e n t a r y origin. At the s a m e time l a b o r a t o r y investigations r e v e a l e d the significant role of the t r a n s m i t t e d local p r e s s u r e on d e c r e a s i n g the heave of f r o z e n soil. Experiments showed that the magnitude of external local p r e s s u r e (stress) equalizing the n o r m a l f o r c e s of f r o s t heave that f o r m in the soil can be taken for clay soils of solid, semisolid, and plastic c o n s i s t e n c y in a closed s y s t e m to be, r e s p e c t i v e l y , in the r a n g e 1-2 k g / c m 2. In the open s y s t e m and closed s y s t e m with W - W p + 0.5 Ip, the values of the equalizing p r e s s u r e exceed 3 k g / e m 2. C o m m e n t s : 1. The founding depth in c o a r s e - f r a g m e n t a l soils with a fill, in addition to point 1 of the table, is designated in relation to the type and condition of the fill with r e s p e c t to water content and g r o u n d - w a t e r level. 2. The designation of the founding depth in m a r l s , r e p r e s e n t i n g s e v e r e l y weathered r o c k y soils, depends on the type and location of the g r o u n d - w a t e r level. 3. Construction of shallow foundations in soils of point 2 of the table should be c a r r i e d out with cons i d e r a t i o n of the topography ( p r i m a r i l y in w a t e r s h e d areas) with a minimum volume of backfill and its m a n d a t o r y compaction, with eareful v e r t i c a l grading and with impervious blind a r e a s around the building

200

p e r i m e t e r at l e a s t 1.2 h wide with h < 1 m and not l e s s than h when h > 1 m, w h e r e h is the height of the buried p a r t of the foundation. 4. Shallow foundations in the s o i l s of point 3 of the table a r e p e r m i t t e d when the r e q u i r e m e n t s of point 3 of the c o m m e n t s a r e o b s e r v e d and at a contact p r e s s u r e of at l e a s t 2 k g / c m 2. 5. The s u p p o r t i n g s o i l s given in points 2-5 of the table should be p r o t e c t e d against wetting by s u r f a c e w a t e r s and against f r e e z i n g . 6. The m i n i m u m depth of founding under soil conditions p e r m i t t i n g a shallow foundation is adopted for each region on the b a s i s of c o n s t r u c t i o n e x p e r i e n c e . The data of l a b o r a t o r y e x p e r i m e n t s set up to study the effect of p r e s s u r e on f r o s t h e a v e w e r e c o n f i r m e d by field investigations* c a r r i e d out during the winter of 1964-1965 at t h r e e e x p e r i m e n t a l s i t e s r e p r e s e n t e d by eluvial l o a m s , c l a y s , and sandy l o a m s in closed and open s y s t e m s . T h e s e investigations showed the dependence of the intensity of f r o s t h e a v e on the p r o p e r t i e s of the supporting soil, d e g r e e of its s t r e s s , and size of the c o m p r e s s i b l e zone of the soil under e x p e r i m e n t a l foundations having an a r e a of 0.25, 1, and 2.25 m 2. As a whole, the obtained data c o n f i r m the p r i n c i p l e f o r m u l a t e d by M. N. G o l ' d s h t e i n [1] and f i r s t e x p e r i m e n t a l l y checked by N. N. M o r a r e s k u l [9] that an i n c r e a s e of load on the soil r e d u c e s heaving and even c o m p l e t e l y stops it. G e n e r a l i z a t i o n of c o n s t r u c t i o n e x p e r i e n c e in the U r a l s , an a n a l y s i s of c a s e s of d e f o r m a t i o n s to buildings and s t r u c t u r e s c a u s e d by f r o s t h e a v e of foundation soils, and the r e s u l t s of field and l a b o r a t o r y inv e s t i g a t i o n s at the U r a l P r o m s t r o i n i i p r o e k t [8], c a r r i e d out during 1959-1965 and p r e s e n t e d in p a r t in this a r t i c l e , enable us to suggest a table for d e t e r m i n i n g foundation depths with allowance for heaving of the soils during f r e e z i n g , which is a d e v e l o p m e n t and r e f i n e m e n t of T a b l e 6 in SNiP I I - B . 1 - 6 2 . LITERATURE 1. 2. 3. 4.

5.

6.

7.

8. 9. 10.

CITED

M . N . G o l ' d s h t e i n , D e f o r m a t i o n s of E a r t h D a m s and B a s e s of S t r u c t u r e s during F r e e z i n g and T h a w ing [in Russian], T r a n s z h e l d o r i z d a t (1948). G . F . G o r y a i n o v , Foundation Depths [in Russian], Osnovaniya, Fundamenty i Mekhanika Gruntov, 6 (1962). B . I . D a I m a t o v and V. M. K a r p o v , Stability of Foundations on Clay Soils with Deep Seasonal F r e e z i n g [in Russian], Osnovaniya, F u n d a m e n t y i Mekhanika Gruntov, 4 (1963). M . F . K i s e l e v , F r o s t H e a v e and M e a s u r e s to Reduce D e f o r m a t i o n s of Foundations in Heaving Soils. Collection 52 of NII Osnovanii " M e a s u r e s Against F r o s t H e a v e of Soils and I t s H a r m f u l Influence on Foundations" [in R u s s i a n l , G o s s t r o i i z d a t (1963). V . P . Ushkalov, F o r c e s and D e f o r m a t i o n s of F r o s t Heaving of Foundations and M e a s u r e s to Reduce T h e m . Collection 52 of NII Osnovanii " M e a s u r e s Against F r o s t Heave of Soils and Its H a r m f u l I n fluence on Foundations" [in Russian], G o s s t r o i i z d a t (1963). N . A . T o l k a c h e v , E x p e r i m e n t a l I n v e s t i g a t i o n s of N o r m a l F o r c e s of F r o s t H e a v e . Collection 52 of NII Osnovanii " M e a s u r e s Against F r o s t H e a v e of Soils and I t s H a r m f u l Influence on Foundations" [in Russian], G o s s t r o i i z d a t (1963). V . M . K a r p o v , Investigation of the Conditions and the D e v e l o p m e n t of M e a s u r e s P e r m i t t i n g C o n s t r u c t i o n of Building Foundations in Heaving Soils. R e p o r t of the S e m i n a r "Efficient Methods of Founding in Regions of Deep S e a s o n a l F r e e z i n g of Soils" [in Russian], Izd. L e n i n g r a d s k o g o obl. otd. o b s h c h e s t v a " Z n a n i e , " 1 (1964). V . B . Shvets, Shallow Foundations in the U r a l s [in R u s s i a n l , S r e d n e - U r a i ' s k o e knizhnoe i z d a t e l ' s t v o , S v e r d l o v s k (1965). N . N . M o r a r e s k u l , I n v e s t i g a t i o n s of N o r m a l F o r c e s of Soil Heaving. A u t h o r ' s a b s t r a c t of C a n d i d a t e ' s D i s s e r t a t i o n , LISI (1950). V . M . Sokolova, Investigation of the Dependence of D e f o r m a t i o n of Clay Soils during F r e e z i n g on Initial W a t e r Content. Collection 52 of hriI Osnovanii " M e a s u r e s Against~ F r o s t Heave of Soils and Its H a r m f u l Influence on Foundations" [in Russian], G o s s t r o i i z d a t (1963).

* L a b o r a t o r y and field investigations of the effect of p r e s s u r e on the intensity of f r o s t h e a v e of eluvial clay s o i l s w e r e p e r f o r m e d with p a r t i c i p a t i o n of B. N. M e l ' n i k o v ; the r e s u l t s of t h e s e e x p e r i m e n t s will be published s e p a r a t e l y .

201