PROBLEMS

OF

FOUNDATION

S. S. Vyalov

ENGINEERING

ON P E R M A F R O S T

SOILS UDC:624.139.2

During the fifty y e a r s of Soviet power, e n o r m o u s a r e a s of the north and n o r t h e a s t of our Motherland, which w e r e p r e v i o u s l y b a r r e n a r e a s , have been developed. New industrial regions have a r i s e n in the basins of the Kolyma and P e c h o r a r i v e r s , on T a i m y r Peninsula, in Yakutia, and in the F a r East. Shafts and m i n e s , f a c t o r i e s and plants, l a r g e c i t i e s , e l e c t r i c power plants, and highways have sprung up where the foot of man had s c a r c e l y trod. But despite these achievements only a s m a l l p a r t of the truly inexhaustible natural r e s o u r c e s of Siberia and the F a r E a s t is p r e s e n t l y being used. T h e r e f o r e , the P a r t y cons i d e r s the f u r t h e r development of the e a s t e r n and n o r t h e a s t e r n regions of the Soviet Union to be a p r i ority assignment. T h e d i r e c t i v e s of the 23rd C o n g r e s s of the CPSU concerning the plan of d e v e l o p m e n t of the national e c o n o m y during 1966-1970 s t a t e : "It is n e c e s s a r y to i n c r e a s e industrial and a g r i c u l t u r a l productivity in r e g i o n s e a s t of the U r a l s , w h e r e e n o r m o u s r e s o u r c e s of industrial raw m a t e r i a l s , fuel, and e l e c t r i c a l energy are concentrated, w

An expansion of the old and the c o n s t r u c t i o n of new e n t e r p r i s e s and cities is envisaged on the basis of the existing and newly d i s c o v e r e d deposits of coal, oil, gas, gold, d i a m o n d s , a n d o t h e r natural r e s o u r c e s . T h e r e is to be an e n o r m o u s volume of industrial, civil, hydraulic, and t r a n s p o r t c o n s t r u c t i o n in the oilb e a r i n g regions of W e s t e r n Siberia, at the Udokan c o p p e r deposit, at the diamond deposits of Yakutia, at N o r i l ' s k and Talnakh, in the Magadan region, and in many other p l a c e s . C h a r a c t e r i s t i c f o r m o s t of these constructions is the fact that they m u s t be c a r r i e d out on p e r m a f r o s t soils, which occupy about 47~ of the t e r r i t o r y of our country. Construction on p e r m a f r o s t soils has a n u m b e r of p e c u l i a r i t i e s , without c o n s i d e r a t i o n of which the s t r u c b a r e quickly undergoes d e f o r m a t i o n and c o l l a p s e s . T h e r e f o r e , the p r o b l e m of developing r e l i a b l e and economic methods of construction on p e r m a f r o s t soils ~s of exceptional impor'mnce. Although our nation has had to deal with p e r m a f r o s t for hundreds of y e a r s , a s y s t e m a t i c and c o m p r e h e n s i v e study of p e r m a f r o s t soils began o n l y d u r i n g the time of Soviet power. In p r e r e v o l u t i o n a r y time builders did not know how to c o m b a t the insidious p r o p e r t i e s of f r o z e n ground and a l m o s t all l a r g e s t r u c tures e r e c t e d on them w e r e s e v e r e l y d e f o r m e d within s e v e r a l y e a r s and b e c a m e u n s ~ t a b l e . T h e r e f o r e , the f i r s t attempts of developing n o r t h e r n regions r e q u i r e d thorough investigations of p e r m a f r o s t soils. The f i r s t r e s u l t s of these investigations which opened a new page in the study of p e r m a f r o s t soils was the publication in 1S27 of the book by M. I. Sumgin " P e r m a f r o s t Soils Within the USSR." L a t e r the monographs of N . A. T s y t o v i c h and M. I. Sumgin " P r i n c i p l e s of the Mechanics of F r o z e n Soils, w etc. w e r e published. The wide scope of construction during the f i r s t F i v e - Y e a r Plan led to the need f o r m o r e extensive investigations of p e r m a f r o s t , for which purpose a commission, chaired b y A c a d e m i c i a n V. A. Obruchev, was c r e a t e d under the Academy of Sciences of the USSR. In 1936, the c o m m i s s i o n was changed to the c o m m i t t e e on p e r m a f r o s t , and in 1939 the Institute of Geocryology of the Academy of, Sciences of the USSR was c r e a t e d . L a t e r , i m p o r t a n t investigations in the a r e a of c o n s t r u c t i o n on p e r m a f r o s t soils w e r e c a r r i e d out at this institute, at the R e s e a r c h Institute of B a s e s , and at i n d u s t r i a l organizations of the north (Dai'stroi, V o r k u t s ~ o i , N o r i l ' s k Combine, G l a v s e v m o r put', and others}. At p r e s e n t a n~v stage in the development o f n o r t h e r n a n d ~ a s t e r n r e g i o n s is a r i s i n g , and, accordingly, r e s e a r c h e r s a r e faced with new p r o b l e m s . Now, not only is the volume of c o n s t r u c t i o n in p e r m a f r o s t region~ m a r k e d l y increasing, b u t a l s o the c h a r a c t e r of the s t r u c t u r e s being built is substantially changing. N o r t h e r n cities a r e beginning to be c o v e r e d with h i g h - r i s e and l a r g e - p a n e l buildings and with t a r g e b r o a d span i n d u s t r i a l s t r u c t u r e s . Hence the c h a r a c t e r of the t h e r m a l and mechanical i n t e r a c t i o n of s t r u c t u r e s T r a n s i t e d f r o m Osnovaniya, Fundamenty i Mekhanika Gruntov, No. 5, pp. 1 7 - i 8 , S e p t e m b e r O c t o b e r , 1967.

and bases is becoming complicated, and the loads on the soils are increasing, and the requirements imposed on the reliability and durability of structures are being elevated. All this requires an improvement of the calculation methods and an increase of the theoretical level of this area of construction science. The problem of ensuring the stability of structures on permafrost soils first of all involves creating a reliable base which does not complicate the superstructures and which prevents the occurrence of excessire settlements. Consequently, it is necessary to avoid excessive settlements and not adapt the structures of the buildings to them (such methods are possible only in the case of specially developed designsL To r e duce settlements we must know how to improve and strengthen the bases. Such an idea was formulated in the new chapter SNiP II-B.6-66 nBases and Foundations of Buildings and Structures on Permafrost Soils. Design Stand~rds,"and is expressed as follows: a) Construct bases of structures on rock and other little-compressible soils (ff they are present in the construction region); b) take measures to preserve the frozen state of the soils of the base (bynatural or artifical means) and to lower taeir temperature if necessary (for example, in the presence of high-temperature frozen soils); c) use prethawing and compaction of the frozen soils of bases; d) replace the slumping soils of the bases; e) strengthen the soils by physicochemieal means. In the problem of developing these designs of deep supports, spread-foot supports, etc. arise when constructing major structures on deep rocky and little-compressible soils. In this respect the experience of the Noril'sk builders, which should be studied and developed, is of interest. I~ many cases ice-saturated slumping permafrost soils occur within the upper 10-15 m, and soils comparatively little-compressible upon thawing occur lower. It is expedient to construct foundations on these soils, permitting their thawing, and to use deep supports with a spread-foot, for example clubfoot piles. It is necessary to examine the problem of carrying deep supports below the anticipated thawing zone in permafrost soils. However, such a solution is still associated with some risk, since in this case a prescribed position of the thawing boundary should be guaranteed.

The retention of the frozen state of the soils of b a s e s - o n e of the most developed and utilized methods of construction on p e r m a f r o s t - i s being successfully used in practice. The problem involves a f u r t h e r i m provement of this method, in particular as related to s t r u c t u r e s with large sp~us, for which it is n e c e s s a r y to develop special methods of cooling the base, f o r example by means of pipes placed trader the floor o r along foundations, the construction of a i r spaces, etc. We particularly distinguish the problem of construction on plastic-frozen, high-temperature soils. It is n e c e s s a r y to develop methods of cooling such soils, both generally under the entire building and locally n e a r the foundation. Of promise is the use of self-cooling foundations designed by Gapeev with the use of kerosene as a coolant or by Long (USA), where propane is used f o r such purposes, which p e r m i t s constant maintenance of a p r e s c r i b e d low t e m p e r a t u r e in soils around foundations. These m e a s u r e s a r e r a t h e r effective also for construction of certain dams on p e r m a f r o s t soils. If preservation of the permanently f r o z e n state is technically o r economically inexpedient, the method of prethawing of p e r m a f r o s t soils in the bases of s t r u c t u r e s should be introduced m o r e widely since it is one of the effective ways of improving the construction properties of the base and f o r ensuring stability of s t r u c t u r e s . The method of thawing bases during s e r v i c e of s t r u c t u r e s is used only when the soils slump v e r y little. For a wider intz~duction of the method of preconstruction thaw2ng it is n e c e s s a r y to devote m o r e attention to problems of improving the technology of this p r o c e s s and to ways of making it c h e a p e r . In particular, this pertains to the development of effective methods of sinking boreholes in p e r m a f r o s t s o i l s , since this stage of the works is one of the most laborious and expensive. Another urgent problem is the selection, with consideration of local conditions, of the method of thawing soils (electro- and hydro-methods) and an i n c r e a s e of the efficiency of this p r o c e s s . It is p a r t l e u l a r l y important to solve the problem of compacting soils followin~ thawing by mechanical and physico-

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mechanical means. Also of importance is the problem of replacing slumping soils by nonslumping soils in the bases of structures; this method is rather widely used in the USA. The use of physicochemical methods is promising for solving the problem of improving the properties of permafrost and thawing soils of bases. As the latest investigations have shown, by means of these methods it is also possible to control t~e strength properties of frozen ground, for example, by reducing the soil freezing and heaving forces within the active layer or by increasing the freezing forces within the p e r m a f r o s t stratum° Piles a r e one of the most p r o g r e s s i v e types of foundations f o r construction on p e r m a f r o s t soils with re%ention of their f r o z e n state. Pile foundations have become widely used. However, ~ e i r use r e q u i r e s a f u r t h e r improvement of foundation practice, selection of more efficient types and designs of piles, study of the actual conditions of the behavior of pile foundations and an increase of their bearing capacity, and also an improvement of calculation methods, et~. An improvement of the technology of placing f r o z e n - i n piles should be directed toward the development of m o r e effective methods of driving boreholes, especially for soils with c o a r s e fragmental inclusions. Quite promising f o r this purpose is the use of v i b r a t o r y impact mechanisms, combinations of v i b r a t o r y and t h e r m a l methods, the thermal (gas-jet) method, and the development of new highly efficient drilling mechanisms. In plastic-frozen soils the driving of piles into guide holes or directly into the p e r m a f r o s t stratum by means of v i b r a t o r y mechanisms and diesel h a m m e r s , especially pipe piles, is quite effective. T h e efforts of investigators in the a r e a of improving the designs of pile foundations should be directed p r i m a r i l y to increasing the bearing capacity of piles, devoting attention to problems of using shaped piles, long piles with lengthwise variable reinforcement, hollow piles, shell piles, etc.

It is possible to increase the bearing capacity of pile foundations by rational methods of driving them. F o r frozen-in p i l e s - b y selecting the appropriate composition of the s l u r r y and of its v i b r a t o r y compaction when filling; for piles in plastic-frozen s o i l s - b y improving the method of driving and by artificial lowering of the soil t e m p e r a t u r e , In the latter case the use of hollow piles with air cooling inside the cavity is especially effective. Finally, there is a large untapped r e s e r v e of bearing capacity of piles in the existing calculation methods. In particular, at present piles a r e calculated with r e s p e c t to long-time streng'~h as related to the highest mean monthly temperakLre. It is expedient to take into account a drop of soil tern~ p e r a t u r e during the winter season, t. e., to introduce into the calculation a change of t e m p e r a t u r e in time and accordingly, to take into account the change in time of strength and deformational properties of frozen soils. This will p e r m i t a m o r e complete utilization of their mechanical properties. It is n e c e s s a r y to unify the method of field tests of piles under various frozen-ground conditions and f o r different driving methods in o r d e r to accumulate experimental data on the bearing capacity of piles, having differentiated the strength c h a r a c t e r i s t i c s as a function of different factors. An important problem related with a d e c r e a s e of the cost of substructure works is the improvement of methods of excavating frozen soils when digging foundation pits, trenches, underground excavations, etc. Unforkmately, this problem is presently not receiving due attention. T h e r e f o r e , it is n e c e s s a r y to conc e n t r a t e maximum efforts to investigate in this a r e a , mainly on the development of highly efficient mechauisms for excavating frozen soils. When constructing lightly loadedfoundations (transmission lines, trestles, light bridges, low-story bulldings, etc.) on heaving soils, control of frost heave gains importance. First of all,it is necessary to ohemln factual data on the magnitude of heaving forces in relation to frozen-ground conditions and to differentiate standard values of these forces, for which purpose all interested departments should carry out field experiments and observations according to a unified program. At the same time,it is necessary to concentrate efforts on the development of measures to combat heaving forces (physicochemicalstabilization of soils, thermoregulation of freezing depth, etc.). It is necessary to point out the particular but important problem of construction on frozen coarse fragmental soils and weathered rocks, which, for example, for such areas as Magadan or the region of the diamond deposits in Yakutia,is a paramount problem. The indicated soils, on one hand, are difficult to excavate in a frozen state and, on the other, can s e t t l e appreciably upon thawing, slide down the slopes of excavations, etc. It is necessary to organize a comprehensive study of the properties of these soils and to work out methods of using them as bases of structures.

In construction practice v e r y little attention is devoted to the proper engineering preparation of an a r e a , although under conditions of permanently frozen ground the observance of the calculated temperature r e g i m e of the soils of the construetionsite depends upon this. T h e r e f o r e , it is beneficial to use the experience of Canada and Alaska, where roads and approaches a r e laid out before the s t a r t of construction, drainage is accomplished, drainage blankets a r e widely used, and all measures a r e taken to p r e s e r v e the natural plant c o v e r . One of the main conditions f o r ensuring stability of s t r u c ~ r e s on p e r m a f r o s t soils is the high quality of site explorations. A comprehensive elucidation of the frozen-ground conditions of the construction site and a prediction of the changes which will occur as a r e s u l t of development of the site should be the purpose of *,he explorations. It is n e c e s s a r y to develop detailed methods of conducting explorations with the use of the l a t e s t geophysical methods, use of a e r i a l photography, etc. P a r t i c u l a r attention should be devoted to problems of field determinations of the c h a r a c t e r i s t i c s of frozen soils. T h e solution of problems of construction on p e r m a f r o s t soils should be based on a further development and ~mprovememt of theoretical principles of geocryology. In o r d e r to be prepared to answer new, m o r e complex practical problems, it is n e c e s s a r y to have a large theoretical background and the theory should s t a y ahead of practice. This r e q u i r e s a f u r t h e r intense development of theoretical investigations, among which f i r s t and foremost, in our opinion, are: t h e o r y of strength and theology of f r o z e n soils: t h e o r y of deformation (consolidation) of thawed soils; t h e o r y of heat and mass t r a n s f e r of f r o z e n , freezing, and thawing soils; naimre of the strength of frozen soils and physicochemical methods of acting on the strength p r o perties of these soils; r~ities

of the process of f r e e z i n g of soils, migration of water, and f r o s t heave;

p r o c e d u r e and equipment f o r investigating and determining the physicomechanica! and thermophysical p r o p e r t i e s of frozen, thawing, and f r e e z i n g soils. AIo~g with theoretical investigations it is n e c e s s a r y to improve engineering methods of calculations, in p a r t i c u l a r , with r e s p e c t to the limit states with consideration of the rheologica! properties of soils, variability of soil t e m p e r a t u r e in depth and in time, variability and nonuniformity of strength,and deformarive c h a r a c ~ r i s t i c s . An indispensable condition of the successful solution of problems of construction on p e r m a f r o s t soils is experimental design and construction, which, however, is used in extremely limited volumes. It is nece s s a r y to take all measures for the wide development of experimental construction. We have presented the basic problem~ of foundation engineering on p e r m a f r o s t soils. It will be quite valuable to l e a r n the views of engineers and scientific workers concerning the general trends of study of these problems and specific examples of their solution.