DISCUSSIONS

PRINCIPLES ON

COLLAPSIBLE

FOR

THE

DESIGN

OF

STRUCTURES

SOILS UDC

624.041:624.131o23

From the Editorial Board. In connection with the revision of the current SNiP II-B.2-62 norms for design of bases and foundations on collapsible soils, the Editorial Board of this journal has published M. N. Gol'dshtein's article "Principles of building design on soils prone to slump-type settlement owing to wetting" (No. 6, 1969) and V. I. Krutov's article "Basic theses on the design of buildings and structures on setting soils (discussion)" (Noo 3, 1970). These articles were discussed by I. A. Rozenfel'd (No. 5, 1970), N. I. Kriger, F. S. Tofanyuk, and B. P. Markin. In view of the impossibility of publishing all these discussions in their entirety, the Editorial Board requested ¥. I. Krutov to prepare a selection of the basic recommendations made therein, and to discuss them (V. I. Krutov's comments are printed in small type).

N. I. Kriger proposes introducing the following changes in the SNiP norms: I. Introduce the terms

"loess" and "loesslike soils," adopted by the geologists.

2. Divide into the following different types the addition deformations which take place when soils are wetted: collapse under the dead weight of the soil ("true collapse"), collapse under additional loads from the foundations ("additional or conditional, false collapse"), collapse occurring as a result of dissolution of salts after prolonged infiltration ("posteollapse deformation"),* and deformation under squeezing, according to N. Ya. Denisov, related to the sharp decrease of the strength of loess soils when they are wetted. 3. For the selection of the anticollapse measures, together with the type of soil conditions and the type of collapse, to take into consideration the possible increase of the water content of the soils during the operation of the buildings and structures, as well as the classification of the structure according to the economic category, importance, and constructional features. In the last part of his discussion, No I. Kriger states that he considers it necessary to carry out a technical-economic analysis of the different construction principles applied in recent years. I. In the SNiP norms being prepared (Section "Nomenclature for foundation soils") it is stated that collapsible soils include mainly loess and loesslike soils (sandy loams, foams, clays). It would not be convenient to change the generally adopted name of "collapsible soils," which characterizes certain specific properties, by the terms "loess" and "loesslike soils," since, in the first place, the name "collapsible soil" is more general and comprises many types of soils, and, in the second place, not all types of loesses and loesslike soils have collapsibility properties. 2. In the draft for the SNiP norms

it is stated that the collapse of soils is divided into:

collapse caused by the loads from the foundations, within the limits of the deformable

zone;

collapse caused by the dead weight of the soil, within the limits of the zone of collapse under the dead weight of the soil; *No I. Kriger's terminology

is given ia parentheses.

Translated from Osnovaniya,

Fundamenty

i Mekhanika

Gruntov, No. 2, pp. 37-39, March-April,

© 1972 Consultants Bureau, a division of Plenum Publishing Corporation, 227 West 17th Street, New York, N. Y. 10011. All rights reserved. This article cannot be reproduced for any purpose whatsoever withou.t permission of the publisher. A copy of this article is available from the publisher for $15.00.

136

1972o

p o s t c o l l a p s e d e f o r m a t i o n s , o c c u r r i n g as a r e s u l t of dissolution and washing out of salts a f t e r p r o longed infiltration of w a t e r into the soil. Thus, No I. K r i g e r ' s r e c o m m e n d a t i o n s in this r e s p e c t a r e a l m o s t fully satisfied. As r e g a r d s the fourth type of s e t t l e m e n t , namely, d e f o r m a t i o n s caused by squeezing, a s s o c i a t e d , a c c o r d i n g to N. Yao Denisov, with a s h a r p d e c r e a s e of the strength of a loess soil when wetted, there a r e not sufficient grounds for including it in the SNiP n o r m s . As shown by r e c e n t investigations c a r r i e d out at the S c i e n t i f i ~ R e s e a r c h Institute for Foundations (see V. I. Krutov and A. Go B o z h k o ' s a r t i c l e " P h a s e s of d e f o r m a t i o n of s l u m p prone soils under foundations," Osnovaniya, Fundamenty i Mekhan. Gruntov, No. 2, 1972), N. Ya. Denisov appIies the t e r m "squeezing d e f o r m a t i o n s " to the collapse caused by the loads t r a n s m i t t e d by the foundstions, within the limits of the d e f o r m a b l e zone. 3. N. Io K r i g e r ' s r e c o m m e n d a t i o n to take into consideration the possible d e g r e e of i n c r e a s e of the w a t e r content during the p r o c e s s of operation of buildings and s t r u c t u r e s is r e f l e c t e d in the SNiP n o r m s now being p r e p a r e d , in connection not only with the selection of anticoltapse m e a s u r e s , but also with the collapse a n a l y s i s f o r nonuniform d e f o r m a t i o n s of the foundation beds of buildings. A basic principle in the d r a f t for the SNiP n o r m s and in the guides for t h e i r application is the a n a l y s i s of the foundation beds and of the buiIdings on the b a s i s of the d e f o r m a t i o n s . In o r d e r to r e a l i z e this principle in the designs, p r e c i s e methods a r e r e c o m m e n d e d for d e t e r m i n i n g the absolute and the differential collapse, for designing buildings on the b a s i s of nonuniform d e f o r m a t i o n s , for d e t e r m i n i n g the n o r m a t i v e p r e s s u r e s , etc. In c o n f o r m i t y with these methods, which take into c o n s i d e r a t i o n not only the c h a r a c t e r i s t i c f e a t u r e s of the buildings but also the soil conditions, the n o r m s e s t a b l i s h different types of foundations, construction m e a s u r e s , and methods of p r e p a r i n g the foundation beds. With this a p p r o a c h , it is not n e c e s s a r y to u s e a c l a s s i f i c a t i o n of buildings and s t r u c t u r e s a c c o r d i n g to t h e i r economic c a t e g o r y . In the d r a f t for the SNiP n o r m s it is r e s o l v e d not to p r e s c r i b e d e f o r m a t i o n a n a l y s e s for buildings and s t r u c t u r e s on Type-I soils, when the following conditions apply: the mean pressure on the foundations does not exceed the value of the initial collapse pressure;

few s t o r i e s , in the c a s e of a b s e n c e of a wetting p r o c e s s , with loads of up to 8 t o n s / m on s t r i p footings, and of up to 40 tons on isolated footings. 4. N. I. K r i g e r ' s r e c o m m e n d a t i o n to c a r r y out a t e c h n i c a l - e c o n o m i c a n a l y s i s of the different cons t r u e t i o n p r i n c i p l e s d e s e r v e s attention, and it would be d e s i r a b l e if the leading design organization would c a r r y out this w o r k for the r e g i o n s pertaining to their s p h e r e of action. Fo S. Tofanyuk, b a s e d on the r e s u l t s of e x p e r i m e n t a l wetting of a trench in the city of I s k i t i m , at which a f t e r full wetting of the entire collapsible l a y e r , the actual m e a n collapse amounted to 1.7 cm, while the calculated collapse was t h r e e t i m e s as high, d r a w s the conclusion of " u s e l e s s n e s s " and "conditionality" of collapse a n a l y s e s of soils. The i n a c c u r a c y of the a n a I y s e s is explained by the g r e a t v a r i a b i l i t y of the values of the r e l a t i v e collapsibility, which r e a c h e s 50% in the region of the Ob River, n e a r N o v o s i b i r s k . In this connection, it is p r o p o s e d to introduce in the SNiP n o r m s the concept of a c c u r a c y and r e l i a b i l i t y of collapse p r e d i c t i o n s . F. S. Tofanyuk c o n s i d e r s it n e c e s s a r y to p r e p a r e , for collapsible soils, tables giving the n o r m a t i v e and design values of the strength c h a r a c t e r i s t i c s c and ~o, and of the d e f o r m a t i o n c h a r a c t e r i s t i c E, for the c a s e s of n a t u r a l w a t e r content and of s a t u r a t e d state. The c o m p a r i s o n s made by many organizations and s p e c i a l i s t s between the actual and calculated values of the collapse both under the foundation loads and under the dead weight of the soil in a r e a s with T y p e - I and T y p e - I I soils, taking into c o n s i d e r a t i o n the coefficients of working conditions r e c o m m e n d e d in the draft for the SNiP n o r m s , indicate that, in g e n e r a l , the d i f f e r e n c e s between those values do not exceed -* 10-30%, which is e n t i r e l y a c c e p t a b l e for p r a c t i c a l calculations. On the whole, it m a y be stated that a t the p r e s e n t time the reliability of collapse analyses lies within the same limits as the reliability of settlement analyses for foundations on ordinary soils. The largest deviations are observed in small-collapsibility soils in which the settlement caused by the foundation loads is 2-3 cm, and the settlement under the dead weight of the soil is 8-15 cm. In this connection, for such cases, when large-scale construction is undertaken at a given site, the draft for the SNiP norms emphasizes the convenience of carrying out field tests. Thus, F. S. Tofanyuk's conclusion, which is evidently based on isolated tests, is not convincing. However, his observations about the great variability of the values of the relative collapsibility obtained from

137

compression tests is correct for all the regions where collapsible soils prevail; they are partially considered in the draft for the SNiP norms in connection with the determination of the values of the soil characteristics. Tables containing the normative and design values of the strength and deformation characteristics of collapsible soils with a natural water content and in the saturated state are unquestionably needed. For their preparation, it is necessary to take into consideration the fact that the strength and deformation characteristics of collapsible soils depend not only upon the density and the water content but also-to a lesser degree -upon many other factors. These factors should be considered in the preparation of those tables° Vo P. Markin considers it inconvenient to classify collapsible soils according bility for a standard pressure of 2-3 kg/cm 2, since: maximum

values of the relative collapsibility may be observed

the adoption of standard pressures

to the relative collapsi-

for smaller pressures;

may be interpreted as a return to the NTU

137-56 norms.

Vo P. Markin proposes taking into account, in the new chapter of the SNiP norms, the inevitability of the increase of the water content of collapsible loess soils after the buildings are completed, as well as indicating also in the norms that not only loess soils possess collapsibility properties. The experience with the application of the SNiP II-B.2-62 norms indicates that, along with the division of loess soils into two types, which is advocated by N. I. Kriger, F. S. Tofanyuk, and B. P. Markin, there is a need for a classification of collapsible soils according to the relative collapsibility. This classification should be used for a general evaluation of the collapsibility properties of the soils at the construction sites, the determination of the depth of the layers to be compacted, the stabilization of the soils, the depth to which piles should be driven, etc. As Bo Po Markin correctly states, the classification of collapsible soils on the basis of the relative collapsibility, as determined from the standard pressure, is not wholly successfill. For this reason, the classification used for relative collapsibility in the draft for the SNiP norms is based on soil pressure of i, 2, and 3 kg/cm2o The inevitability of the increase of the water content of collapsible soils in the low-moisture state, after the completion of the buildings, is considered in the "draft for the SNiP norms in connection with the analyses for normative pressure, settlement, and collapse, and with the selection of the construction measures. In particular, it is especially emphasized that the normative pressures on undisturbed and compacted collapsible soils, even when it is not possible to wet them from above, are determined by the strength characteristics corresponding to the water content of the plastic limit. B. P. Markin's observation to the effect that, in addition to loess and loesslike soils, other types of soils may exhibit collapsibility properties, is correct and has been covered in the draft for the SNiP norms. In the observations submitted by Io Ao Rozenfel'd (No. 5, 1970) it is stated that the division of soils into Types I and II, made in the SNiP II-B.2-62 norms, based on a value of 5 cm of the collapse under the dead weight of the soil, is a formal one° Moreover, he points out the weakness of the current classification of collapsible soils according to the typos of conditions and proposes discarding the arbitrary limit of -> 5 cm for the division of soils into the above types° The division of soil conditions into types, at construction sites, is not made on the basis of a formal particularization, but on the basis of the possibility of collapse of the soil under its dead weight, that is, it essentially reflects the nature and laws of the deformation of foundation beds under the applied loads, and, consequently, predetermines not only the selection of the measures for ensuring structural safety of the buildings but also the possible methods of construction. The experience with the application of the SNiP II-B.2-62 has confirmed the rationality of such a classification of Ioesssoils, and, consequently, given the lack of new recommendations, it is not correct to attribute weakness to it. Type -I so il conditions are undoubtedly lighter for con struction than Type-II conditions, since with greater simplicity, smaller funds, and lower labor costs, the strength and normal operation of the building are ensured by completely eliminating the collapsibility properties of the soils. This does not mean, however, that the possible value of the settlement of foundations built on undisturbed soils in construction sites with Type-I soil conditions is in all cases smaller than for Type-If conditions, or that buildings and structures may be constructed without any special measures on Type-I soils.

138

Unfortunately, as in other classifications, it is not possible to discard the arbitrary limit for division of collapsible soils into two types° In the determination of the possible value of the collapse under the dead weight of the soil, fromthe results of laboratory tests, and in the experimental wetting of trenches in the field, a certain, sometimes small value of the possible collapse is inevitably found. Since it practically does not affect the strength of the building, it may be recommended. As the arbitrary limit, a value of the possible collapse of 5 cm under the dead weight has been adopted in the SNiP norms. I.A. Rozenfel'd objects to this value, but he does not propose a justifiable alternative. I. Ao Rozenfel'd's proposal for separate consideration of the collapse of soils under the loads transmitted by the foundations and under the dead weight of the soils is correct and has been covered in the draft for the SNiP norms. Also worthy of attention are his recommendations to take into consideration ha the SNiP norms the possible degree of wetting of collapsible soils, according to the techniques used for constructing the building. However, the solution to this problem is complex and has been considered only partially- in the SNiP norms.

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