Loess improvement methods

Engineering Geology, 25 (1988) 341--366

341

Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands

LOESS IMPROVEMENT METHODS

DIMCHO EVSTATIEV

Soil Improvement Section, Geotechnical Laboratory, Bulgarian Academy of Sciences, Sofia (Bulgaria) (Received January 16, 1986; accepted in revised form September 10, 1986) ABSTRACT Evstatiev, D., 1988. Loess improvement methods. Eng. Geol., 25: 341--366. The paper deals with the present-day state of the methods of improvement of loess soils as a base and material for engineering constructions. The experience of the Soviet Union, Bulgaria, the United States, China and other countries where loess is widespread has been taken into account. The classification of loess bases into 2 types and 4 sub-types has been suggesteddepending on the thickness of the loess and total overburden collapse. The improvement methods have been classified into 8 groups depending on their nature and the results achieved: (1) compaction by rollers, heavy tampers, soil piles, moistening and by explosions or vibrations and other similar methods; (2) modification of the granulometric composition by the addition of coarser material; (3) stabilization on the surface or in depth by means of injection or mixing with various binders and chemical reagents; (4) loess improvement by replacement with various cushions or jet grouting; (5) reinforcement comprising methods in which the strength of loess is improved by putting bodies of tensile resistance into it; (6) geomembranes; (7) desiccation by different draining systems, electro-osmosis or hygroscopic materials; and (8) correction, terracing, grassing and afforestation of slopes. The applicability of the methods, their recent development as well as typical case histories have been discussed. INTRODUCTION

Loess and loess-like soils are widespread in the Soviet Union, China, the United States, Bulgaria, Romania, Poland, Germany, Brazil, Australia and a number o f other countries in all parts of the globe. They have varied origin, but what unites them is their specific granulometric and mineral composition (mainly silty soils containing mostly quartz and varied quantities of feldspars, mica, carbonates and clay minerals), their characteristic non-consolidated structure (soft soils with great porosity n = 40--50%, the existence of a great volume of macropores), their low degree o f saturation Sr and the binding o f the sand and silt particles with the clay and carbonate mass, which is not resistant to water, or with the capillary forces. These features of the composition and structure predetermine the existence o f the two most important building properties of loess: collapsibility at saturation caused by overburden or by the additional load of the installations and considerable permeability. In building codes and textbooks on engin-

342

eering geology and soil mechanics, loess is usually discussed in the same group with muds, peat, expansive soil and artificial non-consolidated soils under the c o m m o n heading of structurally unstable soils. The unfavourable properties of loess have long been known to builders. For instance, in the first Bulgarian capital Pliska built in the 7th--8th centuries A.D. on loess up to 8 m thick, a big water supply reservoir and the foundations of some heavy buildings had subsided (Evstatiev et al., 1983). The loess base underlying the foundations of the palaces, cult buildings and parts of the fortress wall had been compacted with the use of short w o o d e n piles. Interest in loess has intensified especially during the current century, and since the 1930's in connection with the growing urbanization, industrialization and modern farming. This is especially relevant for the Soviet Union where considerable civil, power industry and agricultural construction had to be accomplished in regions of widespread collapsible soils. That is w h y loess research related to construction has developed most extensively in that country, and the works of Soviet researchers have laid the foundations of almost all main aspects of the loess problem (e.g., Abelev, 1948; Abelev and Abelev, 1979; Denisov, 1946, 1953; Larionov et al., 1959; Anan'ev, 1964; Mavlianov et al., 1978; Mustafaev, 1979; Kraev and Costianoi, 1980; Krutov, 1982). Since World War II, loess research related to construction has rapidly developed in Bulgaria (Stefanoff and Kremakova, 1961; Minkov, 1968), China (Chang Tsung-Hu, 1960), Romania (Gr~ciun and Popescu, 1963), the U.S.A. (Krinitzsky and Turnbull, 1967; Sheeler, 1969; Handy, 1973) and in many other countries. As a result of all these investigations, the specificities of loess as a soil base for various constructions have been elucidated b y and large, and a number of methods of secure foundations have been developed. The existing methods of counteracting the collapsibility of constructions and water leakage can be divided into three big groups: constructive measures, water isolation measures, and loess improvement methods. The first group includes the various methods of reinforcing structures and pile foundations by means of which the loess is essentially traversed and strong soil reached. The second group includes the methods of water isolation preventing the access of water to the loess base. The third group of methods subject to consideration in this work comprises all methods whereby collapsibility and filtration are counteracted by a modification of the loess properties by w a y of compaction, stabilization, replacement or some other ways. It is difficult to determine to which group some methods belong. For instance, the short pyramidal piles are referred to as a m e t h o d of loess compaction, b u t they are at the same time part of the foundation of a building. There are similar difficulties in defining some of the methods of reDlacement, of g eomembranes, and so forth. Soviet experience has proved that constructive and water isolation measures alone cannot solve the problem of stability of the overwhelming part of

343 constructions erected on collapsible loess. Numerous data indicate that the rising level of ground water is an inevitable process in all built or irrigated areas, notwithstanding the water isolation measures that had been taken (Smirnov and Bogdanov, 1974; Litvinov, 1974). Best results have been achieved in the optimum combination of the advantages of the three groups of methods. This is especially relevant to seismic regions. The effectiveness of a decision taken regarding foundation works in loess depends to an extremely high degree on the quality of the geological engineering investigations and of soil mechanics tests, which should provide information about the thickness of the loess layer and of the collapsible zone, the extent of collapse under overburden or additional load, permeability, etc. Of special importance is the defining of the type of loess base, which largely determines the choice of the best suited method. Loess is probably the most frequently compacted and stabilized soil. An evidence of this is the existence of scores of methods of modifying its building properties. The author of this paper has set to the task of analyzing these methods and indicating to what degree they have been worked out. In this connection, a specification has become necessary regarding the existing type patterns of loess bases and the elaboration of a new classification of the methods of loess improvement, which would help elucidate their nature and inter-relatedness. LOESS BASE TYPE PATTERNS According to Soviet norms (Rukovodstvo..., 1977), depending on the extent of collapsibilityunder geological load, 5 n, loess bases are divided into Type I (5 n ~ 5 cm) and Type II (5 n P 5 cm). This classificationhas been accepted in Bulgaria (Pravilnik..., 1983) and in some other countries. Loess base Type I is usually ~ 8 m thick, but there is also loess of a thicker collapsible zone, which does not collapse under overburden. The loess base of Type II is ~8--10 m thick, sometimes reaching a thickness in dozens of metres (in China, Central Asia, Bulgaria and elsewhere). The value of 5 n is most frequently obtained by laboratory methods, but experience has shown that sometimes there are considerable disparities between laboratory calculated and actual collapses. It has been recommended, therefore, that the type of loess base be determined by experimental wetting in situ. In some countries this is rendered more difficult by the great horizontal non-homogeneity of loess (Minkov and Evstatiev, 1977). Suggestions have been made to introduce sub-types of loess bases in connection with the application of the methods of their improvement (Minkov and Evstatiev, 1975, 1977). The author considers that from that point of view it would be sufficient if the two main types are divided into two subtypes each depending on the thickness of the loess (Table 1).

344 TABLE 1 Loess base t y p e p a t t e r n s

Type

Sub-type

I, Sn < 5 c m

Ia , h s < 5 m I b, h s > 5 m

II, S n > 5 c m

II a , h s < 1 5 m II b, h s > 15 m

CLASSIFICATION OF LOESS IMPROVEMENT METHODS

Anan'ev (1976) divided loess improvement methods into two groups: methods of mechanical compaction and methods of physico--chemical stabilization. In these two groups, he distinguishes between methods of surface stabilization and methods of deep improvement, taking into consideration 20 methods with 17 varieties. His classification does n o t include the methods of surface stabilization employed in road and water irrigation construction, the m e t h o d of mechanical stabilization, the methods of reinforcement, of drainage of water-saturated loess, and some new methods of deep stabilization of loess. In this paper, another classification has been presented taking into consideration all used or experimented methods of improvement of the building properties of loess. The methods have been classified into 8 groups (Table 2). The first group comprises the methods w h e r e b y the properties of the loess are improved through its compaction; the second group - - t h r o u g h modification of its granulometric composition; the third -- through the creation of qualitatively new cohesive contacts; the fourth -- through replacement of some of the collapsible loess b y non-collapsible materials; the fifth -- through incorporating elements into the loess, which have tensile strength; the sixth -- through the use of polymer membranes; the seventh -- through drainage; and the eighth -- through slope cutting, terracing, planting of grass and afforestation of loess slopes. The drawing up of this classification has been accompanied b y difficulties due to the non-homogeneity of the classified objects. Besides, the nature of the methods as well as the function they perform had to be taken into consideration. Some of them are t o o complex and involve elements or entire c o m p o n e n t parts of other methods. For instance, during surface stabilization using non-organic binders, a change of the granulometric composition and compaction take place. In some of the methods of reinforcement and replacement, bodies of stabilized soil are used, b u t in this case it is n o t the soil stabilization that is the main element determining the soil's mechanical behaviour of the improved base or slope. Consequently, the classification presented has the ambition of taking into consideration all kinds of interventions in loess soils aimed at the improvement of their building behaviour, i.e. all methods w h e r e b y loess itself is used

345 as a base or material, while some of its physical or mechanical properties are corrected. Moreover, the main principle of classification is both the way in which the improvement has been accomplished and its actual results. STATE OF LOESS IMPROVEMENTMETHODS Table 2 shows that building practice now has at its disposal scores of methods and their varieties in countering collapsibility and filtration leakage in loess soils. Most of them have been worked out or adapted to loess in the Soviet Union. In that country, every other year since 1960 all-Union conferences have been held on stabilization and compaction of soils, and the proceedings of these conferences reflect the continuous advance in this field. The proceedings of the last conferences have mostly been taken into consideration here, since they contain the latest achievements of Soviet researchers. Besides publications in various journals, a number of normative documents, which regulate the application of different methods have been used, as well as the following more important monographs: Bezruk, 1965, 1971; Litvinov, 1969, 1977; Goncharova, 1973; Abelev, 1975: Bannik, 1976: Anan'ev, 1976; Voronkevich, 1981; Tokin, 1984; Grigorian, 1984; Krutov et al., 1985.

1. Loess compaction This group comprises the methods whereby an increase of the density of loess is achieved with the ensuing elimination of collapsibility, reduction of permeability and greater bearing capacity. This is realized under the influence of static or dynamic forces, through injection of clay suspensions or under the influence of the adsorptive action of water. In some of the methods a combination of the effects described has been used.

1.1. Compaction of the surface using rollers and tampers This is the best known and most widely applied method in construction. Huge masses of compacted loess are used for road embankments, earth dar~, various water irrigation facilities, levellers and back embankments in civil engineering. Compaction proceeds in optimal moisture content Wopt until the attainment of the standard density Pd,s. Depending on tl~e clay content, Woot ranges between 12% and 19%, while p d.s ranges between 1.62--1.78 g/cm 3. The shape of Proctor's curves depends very much on the granulometric composition and in some loess varieties Pd.s is attained in a narrow range of water content. When compacted, typical silty loess in Bulgaria has the following soil mechanics properties: Wopt= 159'a--16%; Pd.s = 1.68--1.70 g/cm3; modulus of total deformation E0 -- 20--30 MPa; modulus of elasticity E = 25--40 MPa; cohesion C = 0.025--0.05 MPa; angle of internal friction ~ -26--30 ° ; coefficient of filtration 10-7--10 -s cm/sec.

1.2. Compaction by heavy tamping The dynamic energy of a concrete tamper is used, the tamper being dropped by the outrigger jib of a scraper. This method was first applied in

3.2.4. 3.2.5.

3.2.2. 3.2.3.

Surface stabilization by : Cement, lime and waste materials B i t u m e n and b i t u m i n o u s emulsions Macromolecular c o m p o u n d s Salts, acids and alkali Stabilization in depth by: Injection of silicate grouts; Electroand gas silicatization Injection of gases Injection of cement, lime and other grouts Injection of large molecular c o m p o u n d s Mechanical mixing with Portland c e m e n t and lime

Improvement of the granulometric composition Stabilization

2.

3. 3.1. 3.1.1. 3.1.2. 3.1.3. 3.1.4. 3.2. 3.2.1.

Compaction by:

Rollers, light tampers, vibration plates Heavy tampers S h o r t pyramidal piles R i b b e d foundations Soil piles Gas explosions C o m p a c t i o n injections Injection of clay suspension Moistening Moistening and deep vibration Moistening and surface explosions Moistening and deep explosions Injection of vapour Water stream

1.

Methods

1.1. 1.2. 1.3. 1.4. 1.5. 1.6. 1.7. 1.8. 1.9. 1.10. 1.11. 1.12. 1.13. 1.14.

No.

Classification of loess i m p r o v e m e n t m e t h o d s

TABLE 2

I b and II a I b and II a

civil civil

civil civil

civil

I b and II a I b and II a I b and II a

road, road, road, road,

water water water water

irrigation irrigation irrigation irrigation

all kinds civil, water irrigation civil civil civil civil, water irrigation civil civil water irrigation, civil civil civil, water irrigation civil, water irrigation civil civil road, water irrigation

Kind of c o n s t r u c t i o n

types types types types

all all all all

all types Ia Ia Ia I b and II a I b and II a I b and II a I b and II a II a and IIb I b and II a I b and II a I b, II a and II b II a II a and IIb all types

T y p e of loess base

experimentation good, literature

medium, literature medium, literature

high, regulations

high, instructions good, literature medium, literature experimentation

high, instructions high, instructions high, instructions m e d i u m , publications high, instructions m e d i u m , publications experimentation good, publications high, instructions good, publications high, publications high, instructions experimentation experimentation good, literature

Degree of elaboration

6. 7. 7.1. 7.2. 7.3. 7.4. 7.5. 8.

5,

3.2.6. 3.2.7. 4. 4.1. 4.2. 4.3. 4.4.

Surface draining Drainage boreholes and wellpoints Horizontal boreholes Electro-osmosis Hygroscopic substances Correction, terracing, grassing and afforestation of slopes

Reinforcement Geomembranes Desiccation by:

Soil cushion Sand cushion Soil-cement cushion Cement-bentonite grout introduced by jet-grouting

Replacement by:

Mixing with cement by jet-grouting Burning of liquid and gas fuels

all all all all all all

types types types types types types

all types all types

~ a n d Ib I b and II a

i b, ]]a grJ_dIIb I b and II a

civil civil civil road and civil civil all kinds

civil and road water irrigation and civil

civil and water irrigation civil civil civil

civil civil

high, regulations high, instructions good, literature good, literature medium, literature high, literature

good, literature good, literature

good, instructions good, literature high, instructions good, literature

good, literature high, regulations

--3

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the 1930's in the Soviet Union (Abelev, 1975). Since World War II, it has been used in almost all countries building on loess soils. Initially the weight of the tamper was 2.5--3 tons, b u t n o w it has been increased to 15--20 tons and the compaction effect has grown to 4 m (Minkov et al., 1980; Minkov and Donchev, 1983). This makes it possible to use the method besides in sub-type I a, in sub-type I b, too, and in combination with other methods even in sub-type II a (Minkov et al., 1980, 1981). The best results have been obtained in degree of saturation Sr between 0.35 and 0.60. In the Soviet Union depth in which Pd > 1.60 g/cm 3 has been adopted as the border of the compacted zone (Rukovodstvo, 1977). According to the Bulgarian regulations, this border is at Pd > 1.55 g/cm 3 (Pravilnik..., 1983). A new design of collapsible tampers is used in Bulgaria whose lower surface is tapered off in the shape of a cone, and compaction is more effective using this particular shape. If heavy tampers of a conical or pyramidal shape are fastened to a leading column, compacted pits are obtained serving as beds for isolated or strip foundations. In the Soviet Union, scores of buildings have foundations made in this w a y (Krutov et al., 1985}. There are cases where heavy tamping has not produced the expected result; this is due to the higher than Wopt water content of the soil or the existence of fossil soil in the loess, whose carbonate horizon is very difficult to compact (Minkov et al., 1980). As in other structurally unstable soils, the weight of the tampers has to be further increased in order to obtain a better compaction effect.

1.3. Compaction by short pyramidal piles This m e t h o d has been in use in the Soviet Union since 1965. It consists of driving into the loess of a concrete pile 3--4 m long with a cross section in the upper part 60 × 60 up to 70 × 70 cm and in the lower part most frequently 10 × 10 cm. A compacted zone is formed along the length of the pile which bears most of the stresses. In this w a y compaction with pyramidal pries is more effective than compaction with prismatic piles. The m e t h o d produces the best results in loess base of I a sub-type, b u t there are numerous examples of its application in I b bases. An important advantage of this method is the complete mechanization of all technological operations. It has been used in the Soviet Union and in Bulgaria in foundation works of hundreds o f pre-fabricated buildings up to 9 storeys high (Lubenets, 1974; RSN 224-75; Golubkov, 1976; Toshkov, 1982; Grigorian, 1984).

1.4. Compaction by ribbed foundations Various kinds of foundations have been designed with the aim of compacting loess during construction of buildings under the action of their weight. Their base instead of being fiat contains elongations in the shape of ribs, cotters, pyramids, etc. Experiments started in 1962 in the Ukraine, and this method, have since been used in the foundation works of scores of housing and farm buildings (Anan'ev, 1976; Gohibkov, 1976).

349

1.5. Compaction by soil piles This m e t h o d has been applied in the Soviet Union since 1934. Currently it is performed with special percussion drilling machines using rods weighing up to 2700 kg. The percussion drill with a diameter of 50--60 cm ends in the shape of a cone with a 30 ° angle at the top. Boreholes up to 15 m long are drilled around whose wails a compacted zone with a diameter of up to 1.50 m is formed. The boreholes are filled with loess which is compacted in portions of 100--200 kg by the percussion drill. The compaction can be achieved by driving in of various shapes of reinforced concrete piles, which are subsequently taken out and the holes made are filled with concrete, compacted soft or soil--cement (Mitchell, 1970; Anan'ev, 1976; Krutov, 1982; Grigorian, 1984). Successful attempts have been made in the Soviet Union of compacting the walls of a borehole through the blast action of an electric spark (Lomize et al., 1974; Anan'ev, 1976). During the past two years, tests have been carned out in Bulgaria trying to improve one of the well-known variants of compaction by soil piles, in which after a borehole with a diameter o f 8--10 cm has been drilled, a linear blasting charge is set off (Anan'ev, 1976). During these tests, after the charge has been placed, the borehole is filled with silicate grout. After the explosion, an opening with a diameter of up to 0.80 is obtained, whose walls are impregnated with silicate grout. The opening is then filled with plastic soil--cement mixture. Usually the uppermost 2--3 m of loess remain u n c o m p a c t e d and that is w h y compaction by soil piles is combined with compaction by heavy tamping. This m e t h o d has been successfully used in the foundation works of scores of industrial, housing and administrative buildings, and m a n y writers consider it one of the most important methods in loess base of sub-type II a (Litvinov, 1974; Abelev, 1975; Bannik, 1976; Voronkevich, 1981; Krutov, 1982).

1.6. Compaction by gas explosions The m e t h o d is Soviet invention No. 480357 (Kirillov, 1983). The compaction is done with a heavy device 1.5--2.0 kN linked with pipes along which gas and water are fed from the surface. The device is fastened to a crane and sinks into the loess due to its own weight, facilitated by the water fed under pressure from its lower part. After reaching the desired depth, the device is withdrawn and at the same time a series of explosions of the gas are set off. A column of loess compacted up to Pd = 1.50--1.65 g/cm 3 with a diameter of up to 1.20 m is obtained. The equipment used is comparatively light and easily movable. The m e t h o d has been applied in urban construction, but is also considered promising in water irrigation projects (Frolov and Kirillova, 1983).

1.7. Compaction injections This m e t h o d has been successfully used in other uncompacted soils (Brown and Warner, 1973) and there are reasons to expect t h a t i t would be effective in loess, too. The small diameter borehole is widened by the injection of

350 cement mortar and the surrounding soil is compacted. For this purpose, reinforced rubber baloons are also used in the Soviet Union, which are dropped in the borehole and then expanded through the feeding of water under pressure 1.5--2.0 MPa (Litvinov, 1969). The widened openings are filled with concrete or soil cement.

1.8. Compaction by injection of clay suspension The compaction is done through the injection of clay grout under pressure 150--350 kPa into the loess. In the zone of penetration of the suspension around the borehole, the porosity of the loess drops down to 37--38%, the collapsibility is entirely eliminated and the modulus of total deformation increases by 3.0--3.5 times. This method has been successfully applied in the United States (Holtz and Hilf, 1961) and in the Soviet Union (Shehovtsev, 1962).

1.9. Compaction by preliminary moistening The natural susceptibility of loess to collapse after moistening due to overburden has been used. This method was first applied in 1914--1915 in the Gladnata Steppe in Russia and ever since it has been used in that country in the building of hundreds of facilities chiefly in water irrigation construction (Anan'ev, 1976; Bannik, 1976; Litvinov, 1977; Ukazaniya..., 1979; Voronkevich, 1981). After World War II it has been applied in a number of projects in the United States (Gibbs and Bara, 1967; Lofgren, 1969; Dudley, 1970), in Romania (Beles et al., 1969), in Bulgaria (Minkov and Evstatiev, 1975), in Brazil (Wolle et al., 1981) and elsewhere. Usually moistening is applied to a shallow excavation where a constant water level is maintained in the course of several months until the collapse deformations fade out. Compaction can be accelerated by vertical sand drains especially if water is fed into them under pressure. There are suggestions to accelerate compaction also by superimposed embankments (Anagnosti, 1973). In the Soviet Union, successful attempts have been made to stabilize loess by moistening during the construction of buildings, as well as to the straightening of buildings tilted as a result of collapse (Anan'ev, 1976). This is the most cost efficient of all existing methods of compaction of loess with great thickness, but its application gives rise to some difficulties. The length of moistening causes some ecological problems, the existence of slow post~ollapse deformations delays construction works, the spread of fissures beyond the compacted areas threatens the existing installations, etc. The topmost 5--6 m of loess remain uncompacted and therefore additional heavy tamping may become necessary. The effectiveness of the method is significantly improved by combining moistening with the application of dynamic energy.

1.10. Compaction by preliminary moistening and deep vibration Like other non-consolidated soils, water-saturated loess is well compacted using heavy torpedo-like vibrations attached to the outrigger's jib (Kanatov,

351 1974; Auan'ev, 1976). So far, this method (known as vibroflotation in the Western countries) has not been widely applied to loess, but this is anticipated in the foreseeable future. Litvinov (1977) has put forward a method of compaction of watersaturated loess with the help of directed vibration flow produced by a flat vibrator sunk into a borehole.

1.11. Compaction by preliminary moistening and underwater explosions This method is used in water irrigation and hydro-power construction in the Soviet Union and in Bulgaria (Yadgarov et al., 1974; Minkov and Evstatiev, 1975; Anan'ev, 1976; Askarov etal., 1981). After preliminary moistening of the base, as described in section 1.9, linear charges are placed on the bottom of the water basin. The explosions are set off with a water column usually more than 1.0 m high serving as a weight. The result is a faster and greater effect of compaction. The possibilities of using the method for bases of type I are also expanded.

1.12. Compaction by preliminary moistening and deep explosions This method was developed in the Soviet Union in the early 1960's (Litvinov, 1977). Its application to construction started in 1967. Loess is moistened with drain boreholes situated at a distance of 3--5 m from each other. A metal pipe with a widening in the lower part is sunk in additional boreholes or in the drain boreholes, with 5--7 kg of explosive put in it. After the moistening of the loess, which does not continue until complete saturation, the explosives placed in several boreholes are simultaneously set off. The powerful blast corresponding to a 12 degree MSK earthquake causes the loess texture to break and there is a quick collapse reaching up to 2 m. The enormous quantity of gas set free during the blast also contributes to the rapid consolidation. It dissolves in the water and affects its viscosity. In built areas, the compacted site is surrounded by deep narrow trenches in order to avoid the propagation of fissures beyond it. Using this method, loess is compacted within a few weeks only, and moreover, considerably less water is used. The density achieved is much greater than during common moistening. The method has been used on a number of industrial and urban sites in the Soviet Union and the soil compacted in this way exceeds I million m 3. In Bulgaria, it has been applied in high-rise housing construction (Donchev, 1980). The method is very cost-effective and suitable in eliminating collapse. In earthquake conditions, there are certain risks of the liquefaction of loess, since it has been long overmoistened. Moreover, considerable settlement during construction has been measured in some high-rise buildings in Bulgaria, construction having started directly after the completion of compaction. The method requires a well trained team of borehole mechanics, a geologist, a geophysicist and an explosion works specialist, and this impedes the work of the builders. It is most effective in work of great volume, particularly in water irrigation and hydro-power construction.

352

1.13. Compaction by water vapour Preliminary collapse of loess is caused by superheated water vapour injected into the loess. The m e t h o d has been used in Dnepropetrovsk, U.S.S.R. in straightening buildings tilted as a result of the collapse of part of the foundations (Varinichenko, 1974). It could also be used for preliminary compaction of loess foundations.

1.14. Compaction by water stream Jet-grouting technology is used (Kirillov et al., 1983). Loess is cut by a water stream fed by a monitor under great pressure in a borehole from the b o t t o m upwards. A column with a diameter of up to 2--3 m or a strip with the same width is obtained of loess precipitated under water which has lost its collapsibility. In the course of time, the physico--mechanical properties of the soil are improved due to the dispersion of the water and the decrease of the water content.

2. Improvement o f the granulometric composition The best or optimal granulometric mixture consists of gravel, sand, silt and clay, which under the concrete natural conditions and way of applying, has the best strength and resistivity in the compacted state. Such a mixture is mostly used in road, less often in hydro-power and rarely in civil engineering. In the Soviet Union, a great number of investigations of this problem, including loess were carried out in the 1930's and continued after World War II (Bannik, 1976). The optimal granulometric mixture for loess is obtained by adding sand or river gravel. Scores of kilometres of roads have been built in Bulgaria on a base of such a mixture with Ip = 3.5 and Pd = 2.25--2.30 g/cm 3 (Nachev, 1964). Laboratory studies have shown that the strength parameters of the mixture are substantially improved after the addition of small quantities of binders.

3. Stabilization o f loess Included here are methods in which the improvement of the binding properties o f loess is achieved chiefly through the creation of new structural bonds, as a result of which cohesion increases. In some of the methods this is realized by breaking the natural texture, and in others, that texture is fixed and strengthened by injection of various binders, chemical reagents or fuel mixtures. There are two groups of methods depending on whether stabilization is performed on the surface or in depth.

3.1. Methods o f surface stabilization In these methods stabilization is achieved by mixing the soil with binding materials or chemical reagents in special mixers or on the site using rotary fresno, while compaction is obtained using road rollers, small tampers, vibrat o r y plates and various types of vibrators. The n u m b e r of stabilized layers and their thickness depend on the concrete application.

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3.1.1. Stabilization using cement, lime and some waste materials. This method is most widely applied in road construction (Bezruk, 1965, 1971; Ganse, 1973) as well as in streets and farm grounds construction (Minkov and Evstatiev, 1975). The greatest number of studies on this problem have been made in Bulgaria (Evstatiev, 1969, 1984; Evstatiev and Minkov, 1975), in the Soviet Union (Bezruk, 1965, 1971) and in the United States (Handy, 1956). These studies have established the optimal proportions of the binding materials, the mechanism of formation of strength and the strength and technological performance of loess--cement mixtures. It has been found that when the quantity of the clay fraction ~17%, lime produces a better effect than cement. Investigations have also been carried out to replace cement by residual ash from cement plants and by activated fly ash from thermo-electric plants. Its application in water irrigation construction has been of a comparatively recent date. In Bulgaria, 15 balancing reservoirs have been built whose bottoms with a total area of 130,000 m 2 have an impermeable screen consisting of one or two soil--cement layers, each one 0.15 m thick and covered by a protective soil layer 0.15--0.20 m thick (Minkov and Evstatiev, 1975). In loess base of type II, the soil is first moistened until deformations under overburden stop. Compaction by heavy tampers is applied under the dykes of the balancing reservoirs. Attempts have been made to apply soil--cement revetments of canals, but this method, however, has not been widely used (Minkov and Evstatiev, 1975; Tokin, 1984). In the Soviet Union loess-cement has long been used for making bricks, foundations and floors of animal farms, etc. (Vilenkina, 1961; Tokin, 1984). 3.1.2. Stabilization by bitumen and bituminous emulsion. Loess varieties containing little clay are well stabilized using hot bitumen and especially bituminous emulsion. The Soviet Union has the greatest experience in this area as great quantities of bitumen-stabilized soil have been used in road construction (Bezruk, 1965, 1971). In other countries with loess soils, this method is not competitive to the stabilization using inorganic binders. 3.1.3. Stabilization by macromolecular compounds. Numerous laboratory experiments have been made with loess and similar soils (Kardoush et al., 1957; Demirel and Davidson, 1960; Bezruk, 1965, 1971), but so far, none of the macromolecular compounds have found application in road construction on a loess base, because of their high cost. Some compounds with surface active properties soluble in water are more promising as they can be used for building impermeable screens (Koval'chuk et al., 1971) or as an additive to loess--cement mixtures improving their compactability, impermeability, freeze and water resistivity (Evstatiev, 1969) I 3.1.4. Stabilization by salts, acids and alkali. Successful attempts have been made in the Soviet Union and in Bulgaria to reduce the filtration leakage of irrigation canals into loess soils by superficial treatment with a solution of

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NaC1 or by building a screen of salted loess covered b y compacted soil (Sokolovskiy, 1952; Ermolaeva and Rel'tov, 1971; Minkov and Evstatiev, 1975). The increase of the content of Na ÷ in the adsorbed complex causes a thickening of the double water layers on clay particles, peptization of the colloids and reduction of the effective porosity. Besides NaC1, Na, CO3, Na3PO4 and NaF have been suggested for use in improving the properties of loess (Afanas'ev, 1962). Of greatest interest are salts like sodium h e x a m e t o p h o s p h a t e (NaPO3)6 which form stable c o m p o u n d s in the soil. The collapsibility of loess is eliminated after it has been soaked with diluted acid solutions (Anan'ev, 1976). In New Zealand, loess has been successfully stabilized using phosphoric acid (Evans and Bell, 1981). Attempts have been made in water irrigation construction to build impermeable screens using loess processed with a diluted solution of NaOH or alkaline suspensions of peat and brown coal treated with NaOH (Dumanskiy, 1954). Some salts or alkaline and alkaline earth elements in small quantities improve the strength of loess cement mixtures (Evstatiev, 1969).

3.2. Stabilization in depth This group of methods has developed with particular intensiveness during the past two decades in the Soviet Union (Goncharova, 1973; Bannik, 1976; Litvinov, 1977; Voronkevich, 1981). While stabilization in depth was formerly used chiefly to stabilize the foundations of subsided buildings, now, as a result of a n u m b e r of technological improvements, it has become possible to do it as a preliminary treatment of the loess base. It is accomplished through the injection of various chemical reagents or fuel mixtures, or through the mechanical or jet-grouting mixing with binders. As a result, new cementing bonds are created and cohesion is considerably increased; collapsibility is eliminated and the bearing capacity grows. In this work, loess stabilization in depth means an improvement of the building properties of the soil all over the area of the foundations and of the entire collapsible zone. When only part of the collapsible layer under the foundation is replaced or is cut in depth by strips or columns of stablized soil between which collapsible loess remains, the terms replacement and reinforcement of the soil base have been respectively used. 3.2.1. Injection o f silicate grouts. In 1944, Askalonov (1959) suggested the monosolution method based on the chemical interaction between the adsorbed ion complex, water-soluble salts and water glass. The mechanism of that interaction has been studied b y a number of writers (Voronkevich, 1981). The m e t h o d has a great deal of variants depending on the composition of the silicate grout and the technological differences. Up until 1982, the m e t h o d had been applied to more than 800 projects in the Soviet Union (Beketov, 1983), which enabled its continuous improvement, the reduction of its prime cost and the drawing up of normative documents regulating its use. The effectiveness of silicatization is enhanced b y adding ammonia or formamide solution to water glass, by the successive

355 injection of increasingly less concentrated solutions, while in certain conditions the free absorption of water glass by the surface is possible through the drain boreholes (Sokolovich, 1980; Rzhanitsin, 1974; Abramova and Voronkevich, 1983; Beketov, 1983). The effectiveness of silicatization can be substantially increased by alternating water glass injections with CO2 injections (Sokolovich, 1980; NIIOPS, 1973). A deeper penetration of the water glass is achieved as well as a greater strength of the stabilized loess. In clay or moistened loess, as well as in loess of destroyed texture, the penetration of the solution can be increased in an electro-osmotic way (Akimov, 1962; Venkov, 1966; Bronstein, 1968). So far, silicatization has been applied using grouts with density most often ranging between 1.10 and 1.15 g/cm 3, injected under pressure 0.2--0.4 MPa. It is the writer's belief that it is more promising to inject water glass under great pressure, i.e. to apply squeeze grouting widely used in other dispersive soils, which would result in a greater penetration of the grouts and compaction of the loess. This method could likewise be used in straightening some installations tilted as a result of subsidence of part of the foundations.

3.2.2. Injection of gases. Successful attempts have been made to stabilize loess by the injection of ammonia (Sokolovich, 1980; Voronkevich, 1981). Ammonia enters into physico--chemical interaction with Ca~* of the soil as a result of which highly dispersed Ca(OH)2 is obtained. Some 5--8 kg of ammonia are needed per 1 m 3 of loess. The effect is enhanced if the soil is additionally injected with CO2. Attempts have also been made to inject silicotetrafluoride SiF4, which is a waste product of the manufacture of superphosphates (Afanas'ev, 1962). 3.2.3. Injection of cement, lime and other grouts. In the state of Arizona (U.S.A.), the foundations of buildings affected by collapse have been stabilized with a solution of lime with a lime--water ratio 1:3 (Sultan, 1971). Similarly lime--slag injections have been used in the Soviet Union (Dolgih, 1962). In Bulgaria preparatory works are going on for tests with a cement solution by squeeze grouting. 3.2. 4. Injection of large molecular compounds. Successful tests have been made in the Soviet Union with carbamide resin (Kuleev, 1961; Voronkevich, 1981) which needs a solidifier if the loess has a high carbonate content (Sokolovich, 1980). The stabilizing effect of epoxide--phenol mixtures has also been tested (Zgadzai, 1968) and of some other large molecular compounds (Goncharova, 1971; Voronkevich, 1981). 3.2.5. Stabilization by mechanical mixing with Portland cement. A number of methods are known of stablizing collapsible loess in depth by mechanically mixing it with Portland cement or other binding materials which have the same effect (Minkov and Evstatiev, 1975; Anan'ev, 1976; Tokin, 1984).

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In some of t h e m boreholes are first made by percussion drilling w i t h o u t taking out the soil o n t o the surface, i.e. according to the technology of soil piles, and the hole obtained is filled with a soil--cement mixture prepared on the surface. Stabilization with micro piles is similar; several reinforced concrete piles with a diameter of 0.1 m and length of 1.4--2.5 m are simultaneously driven into the loess. While t h e y are being taken out, the openings are filled with plastic soil cement mixture (VRSN-74; Anan'ev, 1976; Tokin, 1984). This m e t h o d has been widely used in agricultural construction in the Soviet Union. According to another method, the mixing of the soil and cement is done in the process of drilling itself: first a borehole of a smaller diameter is drilled, in the course of whose widening the soil is mixed with cement. Mixing from the b o t t o m upwards has also been applied using a special auger.

3.2. 6. Stabilization through mixing by a jet-grouting monitor. Jet-grouting technologies have become recently widespread in f o u n d a t i o n works in different soils (Aschieri et al., 1983). In one variant, the soil is not taken out onto the surface, but is mixed in the borehole itself with a cement solution fed by a m o n i t o r in the form of a cutting stream under great pressure up to 40 MPa. A column of plastic soil cement with a diameter of up to 2 m or with thin walls of various configuration is obtained. Loess is very suitable for stabilization according to this technology, because it is easily washed by the stream of water and the resulting plastic cement loess mixture has great strength. The first tests in the Soviet Union (Hasin et al., 1984) and in Bulgaria have produced very good results and currently this is probably one of the most promising technologies of deep stabilization of loess. 3.2. 7. Stabilization by burning o f liquid or gas fuels. In its present form, this m e t h o d has been developed for loess soil by Litvinov (1977) and has been successfully applied in f o u n d a t i o n works of hundreds of buildings in the Soviet Union and a number of other countries. The application of this m e t h o d is technically and economically expedient in the following cases: (a) in stabilizing the foundations of tall existing buildings and installations (high chimneys, blast furnaces, water towers, multistoreyed buildings, etc.); (b) in arresting the deformations of buildings and installations t h a t have already subsided. The fuel mixtures are burnt in closed boreholes under pressure. The expenditure of air per hour in the case of liquid fuel is 25 m 3 per 1 kg of fuel on the average, and in the case of gas fuel it is 10 m 3 per 1 m 3 of gas. In a borehole with a diameter of 0.15--0.20 m, a stabilized soil column with a diameter of 1.5--2.0 m and depth of 8--10 m can be built in the course of 8--10 days. Usually the stabilization is done in groups of 12--15 boreholes. Loess is burnt at a temperature of 300--1000° C whereby its collapsibility is entirely eliminated and its bearing capacity greatly increases.

357 4. Replacement with other soils

This group includes several methods in which part of the collapsible surface layer directly under the foundations or in depth is excavated and replaced by compacted or stabilized loess or some other suitable soils or materials. The ground becomes a non-homogeneous medium and the interaction between the foundation, the improved and unmodified loess is of particular importance for its bearing capacity. Using modern computer methods such as the method of finite elements, considerable progress has recently been made in forecasting the mechanical behaviour of that complex ground. Replacement on the surface is used in loess of lesser depth and includes several well elaborated methods. The building excavation is made a little deeper and then the distance between its bottom and the foundation is filled with a cushion of compacted or stabilized loess, clay or coarser material. In this way the danger of collapse of the loess layer situated directly under the foundation where the stresses are the greatest is eliminated. Replacement in depth is accomplished by excavating the entire collapsible layer with a scraper or in some other method and substituting with suitable material. 4.1. Soil cushion In the Soviet Union this method was used in loess soils even before World War II. There are several instructions regarding the design, carrying out and control of construction works. Usually, local loess soil is used, but there have been cases when soil cushions have been made out of clay or sandy clay. The thickness of the cushion is usually 1--2 m, and it is compacted in layers in the way described in 1.i. When the loess layer is deeper, the soil cushion can be combined with compaction by a heavy tamper. Considerable experience has been accumulated in the use of these cushions in civil engineering (MinkoV and Evstatiev, 1975; Anan'ev, 1976) in all countries building on loess soils. In the Soviet Union, China and Romania, they are also used in the construction or reconstruction of irrigation canals in collapsible loess (Vremeunie ukazania, 1975; Bally, 1982; Wang Ching-Tu, 1982). There are also cases when the soil cushion has not accomplished its purpose. This is due to the use of unsuitable types of loess base or to poor-quality construction. Soil compaction is difficult in a small building site and during the moist seasons. 4.2. Sand cushion Sand cushions have been successfully used in a number of projects in the Soviet Union (Golubkov, 1976). Sand is easily compacted and when the depth of the loess is not great, it is possible to increase the load of the foundations. It has been applied in combination with compaction by heavy tamping (Krutov et al., 1985). 4.3. Soil--cement cushion Soil--cement cushions have been widely used in foundation works on collapsible loess soils in Bulgaria only, to date (Minkov and Evstatiev, 1975;

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Minkov et al., 1981). More than 90 buildings and other installations have been constructed on soil--cement cushions including a large nuclear p o w e r plant, industrial and p o w e r installations, high TV towers, and residential and administrative buildings. Instructions have been worked o u t on the design and construction of soil--cement cushions ( U k a z a n i a . . . , 1976). The soil--cement cushion is built using loess from the building site itself, mixed with 3--7% Portland cement and compacting in layers of 15--20 cm at Wopt until the attainment of p d.m~. The thickness of the cushion is usually 1--1.5 m and only in rare cases reaches or exceeds 3 m. It has a modulus of total deformation of 80--120 MPa and unlike the other cushions has a capacity of redistributing the stresses transmitted to it by the foundations on a larger area. In the calculation of the thickness of the soil--cement cushion, the methods of mechanics of layered media are used, the m e t h o d of finite elements offering great conveniences (Karachorov and Gechev, 1984). The results obtained b y this method match field observations and measurements very well (Minkov et al., 1981; Evstatiev et al., 1985). In foundation works of isolated or strip foundations, the admissible load of the cushion is usually 0.25--0.30 MPa, b u t there are cases of greater loading. The soil--cement cushion can be used in loess bases of Type I, but in combination with heavy tamping it has also been used in Sub-type II a (Minkov et al., 1980). After the big Vrancea earthquake of 1977, it was found that buildings and installations erected on a soil--cement cushion had experienced considerably less damage than those built on natural loess (Minkov and Evstatiev, 1979).

4.4. Replacement in depth by jet-grouting The practice of excavating the entire collapsible layer and its replacement b y another soil has long been c o m m o n in engineering. This is accompanied b y great difficulties when the thickness of the loess is considerable and especially when the construction is in the limited space of urban sites. In this case, the replacement can be comparatively quickly accomplished b y jetgrouting methods taking away the loess and replacing it by a cement-bentonite mixture. The possibility of mixing loess and a cement mixture on the site (cf. 3.2.6.) should be taken into consideration in making technical and economic estimates. 5. Loess reinforcement According to current ideas (Shlosser et al., 1985), reinforcement comprises all methods in which the strength or bearing capacity of loess is improved b y putting in it bodies of tensile strength. Reinforcement is horizontal or vertical. The methods of horizontal reinforcement include the various methods of building retaining walls of soil reinforced with metal or polymer bands, grids and nets, as well as the methods of "nailing" slopes. Some loess-like sands alone are suitable for the construction of retaining walls of reinforced soil. The experimentation of some of the ways of "nailing" steep loess slopes is of considerable interest, especially from the point of view of their seismic resistance.

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The methods o f vertical reinforcement include the building of columns in the loess base, or of deep narrow underground walls of loess, stabilized with Portland cement or water glass by injection or using jet-grouting techniques, as well as by burning of fuel mixtures whereby a considerable part of the loess remains unchanged. Significant successes have been obtained in the Soviet Union in the reinforcement of collapsible loess bases with columns or underground walls of silicated loess (Beketov, 1983; Beketov et al., 1983). For instance, during the construction of the Energomach Works in Volgodonsk, 2142 boreholes and about 16,000 tons of water glass were used to build underground walls of silicated loess.

6. Geomembranes In loess soils geomembranes are chiefly used in water irrigation construction, in building water reservoirs and enhancing the impermeability of the concrete revetments of the canals (Minkov and Evstatiev, 1975). For instance, in greatly collapsible loess in Bulgaria, a screen at the bottom of a balancing reservoir has been used consisting of a lower loess-cement layer covered by a polyethylene membrane on which 15 cm of compacted loess was placed. In the Soviet Union a screen of polymer membrane covered by a layer of soil is used in the construction of irrigation canals in loess. Observations have been made for many years on the durability of the screen depending on the kind of polymer, the thickness of the membrane and the soil and climatic conditions. In that country, too, polymer membranes have been used for securing the isolation of water in the foundations of buildings. Geomembranes are expected to be still more widely used in the future in combating collapsibility and filtration leakage of loess soils.

7. Desiccation o f loess Many years of observations in the Soviet Union (Litvinov, 1974), Bulgaria and elsewhere have shown that the rising of ground water levels in built areas, and especially in irrigated lands, is an inevitable process. In some cases, the ground water level is only a few metres below the surface. During foundation and excavation works, this very often calls for using various methods of drying the loess.

7.1. Surface draining There are well known and widely applied methods of draining surface waters to prevent the moistening of the foundations of buildings, to protect deep excavations from inundation during construction, etc. Although the importance of these measures is well known, they are often underestimated in practice. For instance, the collapse of hundreds of small houses in Bulgaria was caused only because the water coming from the rainwater pipe had been left to soak freely in the loess foundation.

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7.2. Drainage boreholes and wellpoints There are cases of successful ground water lowering in loess using vertical drainage boreholes, b u t in the comparatively small Dardy's coefficient of loess between 10 -3 and 10 -s cm/sec, weUpoint drainage has proved most effective (Bannik, 1976; Voronkevich, 1981). The vacuum in the boreholes is b e t w e e n 26 and 78 kN/m 2. The water collected in the borehole is periodically p u m p e d out and in this way the soil mass is dried. 7. 3. Horizontal drainage boreholes Horizontal drainage boreholes have successfully been used in Bulgaria during the past few years in draining water-saturated loess in slopes (Tsvetkov, 1985). They have been made according to a m e t h o d developed in Czechoslovakia of a drill resting in the soil, whereby great productivity is achieved because the metal drainage pipe with a diameter of 80--100 mm is inserted into the loess without casing of the borehole. The m e t h o d has been particularly effective in landslides in water-saturated loess soils. 7.4. Electro-osmosis The principle of desiccation b y electro-osmosis of soils is well known and has been described in the literature (Bannik, 1976; Mitchell, 1976; Voronkevich, 1981). One interesting example is the case of a deep railway excavation in the F R G traversing water-saturated loess. The high level of ground water caused loess liquefaction when the excavation exceeded 2 m in depth. As a result of electro-osmotic desiccation, a 7 m deep excavation could be made with sloping 1:0.75. Before the switching on of the electric current, the water was p u m p e d o u t at a discharge of 0.02 m3/24 h, after which the discharge increased to 3 m3/24 h. 7. 5. Hygroscopic substances Loess under isolated foundations can be desiccated also by driving through it perforated pipes filled with CaC12 (Litvinov, 1969). Water-saturated loess in the Soviet Union has been successfully desiccated with quick-lime compacted in boreholes of 200 to 500 mm (Abelev, 1983). Beside drying the soil, the lime also compacts it as a result of the great voluminous expansion during its hydration. 8. Correction, terracing, grassing and afforestation of slopes These methods will n o t be considered because they are very well known and have long been applied. The problem of the stability of loess slopes has specially been investigated by Lohnes and Handy (1968). Some specific peculiarities of loess should be kept in mind while determining the most suitable slope inclination. This will be illustrated by one example in Bulgaria. In the construction of an industrial enterprise, on the basis of laboratory data, it was calculated that the inclination of the 10 m high slope should be 1:1. The builders first made an excavation with an inclination of

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the slope close to the vertical and then began the correction of the slope according to the design. Autumn rains began at that time greatly moistening the part of the slope with 1:1 inclination and causing its sliding. At the same time, the slope close to the vertical remained dry and was not affected by any deformation whatever. Trees and plants like accacia, alfalfa and some other grasses have the ability of greatly desiccating loess by their roots, as a result of which its cohesion considerably grows. This is used in improving the stability of natural and artificial slopes. At the same time it should be kept in mind that the collapsibility of the loess desiccated in this way may increase considerably. CONCLUSION

It becomes evident from this analysis that when compared with other soils, probably the greatest number of methods have been elaborated for loess in the endeavour to improve its building properties. In the degree to which they have been elaborated, and in applicability, these methods can be classified into three groups: (1) methods of a high degree of elaboration and widespread use in practice (1.1, 1.2, 1.3, 1.5, 1.9, 1.12, 3.1.1, 3.2.1, 3.2.7, 4.1, 4.3, 5, 6, 7.1, and 8 from Table 2); (2) methods of a fairly good degree of elaboration which are comparatively more rarely used (1.4, 1.8, 1.10, 1.11, 2, 3.1.2, 3.2.5, 3.2.6, 4.2, 4.4, 7.2, 7.3 and 7.4) and (3) methods of a fairly poor degree of elaboration or at the stage of experimentation. Almost all methods of the first two groups have been included in national or departmental normative documents in the Soviet Union, Bulgaria, Romania and elsewhere. Now investigations are being conducted for the further improvement both of the widespread methods and of those which have not yet been well developed. It is important also that the methods applied have such a prime cost that their use would not significantly raise the cost of construction. According to statistics provided by Soviet sources, and on the basis of Bulgarian experience, the prime cost of 1 m ~ of loess soil improved using some of the methods, changes approximately as follows: heavy tamping -1.5--2.5 rubles; soil piles -- 1.6--6.2 rubles, injection of clay suspensions 1.5 rubles; moistening and deep vibrations -- 0.8--1.5 rubles; moistening and surface explosions -- 0.30--0.50 rubles; moistening and deep explosions -0.5--1.2 rubles; compaction with water vapour -- 1 ruble; silicatization -3--10 rubles; injection of large molecular compounds -- 12--30 rubles; burning of liquid and gas fuels -- 2--10 rubles; soil cushion -- 1--2 rubles; sand cushion -- 3--4 rubles; soil--cement cushion -- 5--8 rubles. With the exception of group 1.9., the other methods do not require a time delay prior to beginning construction. All methods used have convenient tests that need to be conducted to verify that the improvement has been successful. During the past few years, some new methods have been developed like, for instance, jet, grouting technologies, improved methods of deep vibration, gas explosions, etc., which are expected to further develop in the foreseeable future and to improve the possibilities of foundation works in loess.

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At present the question of improving loess bases of sub-types I a, I b and partially II a may be considered comparatively well tackled. There are few methods applicable to sub-type II b, i.e. to a loess base of great thickness. Foundation works in such a base are so far an unresolved problem worldwide. Of major importance for the future advance in this field, apart from the technological improvements, will also be the investigations concerning the nature of the processes which modify the building properties of loess, as well as the methods of a more precise forecasting of the building behaviour of the improved loess base. REFERENCES Abelev, M.Yu., 1975. Metody ustroistva iskusstvennykh osnovaniy. MISI Publ., Moscow, 77 pp. (in Russian). Abelev, M.Yu., 1983. Stroitel'stvo promyshlennykh i grazhdanskikh sooruzheniy na slabykh vodonasyshchennykh gruntakh. Stroyizdat, Moscow, 248 pp. (in Russian). Abelev, Yu.M., 1948. Osnovy proektirovania i stroitel'stva na makroporistykh gruntakh. Stroivoenmorizdat, Moscow, 204 pp. (in Russian). Abelev, Yu.M. and Abelev, M.Yu., 1979. Osnovy proektirovania i stroitel'stva na prosadochnykh makroporistykh gruntakh. Stroyizdat, Moscow, 271 pp. (in Russian). Abramova, T.T. and Voronkevich, S.D., 1983. Primenenie formamidsilikatnogo rastvora dlia zakrepleniy lessovykh gruntov. Zakreplenye i uplotnenie gruntov v stroitel'stve. Tezisy dokladov na X Vsesoyuznom nauchno-tekhnicheskom soveshchaniy. Rostov on the Don. Stroyidat, Moscow, pp.17--18 (in Russian). Afanas'ev, N.V., 1962. O himicheskoi stabilizatsiy lessovykh porod. Materialy soveshchania po zakrepleniyu i uplotneiyu gruntov, Kiev, pp.78--80 (in Russian). Akimov, A.A., 1962. Elektrosilikatizatsia lessovykh gruntov. Materialy soveshchania po zakrepleniyu o uplotneniyu gruntov, Kiev, pp.382--385 (in Russian). Anan'ev, V.P., 1964. Mineralogicheskiy sostav i svoistva lessovykh gruntov. RGU, Rostov on the Don, 218 pp. (in Russian). Anan'ev, V.P., 1976. Tekhnicheskaya melioratsia lessovykh gruntov. Publ. Rostov University, Rostov on the Don, 118 pp. (in Russian). Anagnosti, P. 1973. Storage dams founded on collapsing loess soils. Proc. 8th ICSMFE, Moscow, 2(2) (Paper 4/2): 7--11. Aschieri, F., Jamiolkowski, M. and Tornaghi, R., 1983. Case history of a cut-off wall executed by jet-grouting. Proc. 8th ECSMFE, Improvement of ground, Helskinki, 1 : 121--126. Askalonov, V.V., i959. Silikatizatsia lessovykh gruntov. Gos. izd. Literatury po stroitel' stvu, arkhitekture i stroitel'nym materialam, Moscow, 78 pp. (in Russian). Askarov, H.A., Bobylev, L.M. and Margotiev, A.N., 1981. Improving construction properties of loamy soils. Proc. 10th ICSMFE, Stockholm, 3: 585--586. Bally, R.J., 1982. Engineering geological problems for specific planning of land with loessic soils. Proc. IV Int. Cong. IAEG, New Delhi, 1 (Theme 1): 425--435. Bannik, G.I., 1976. Tekhnichestaya melioratsia gruntov. Vishcha Shkola, Kiev, 303 pp. (in Russian). Beketov, A.K., 1983. Silikatizatsia prosadochnykh lessovykh gruntov. Zakreplenie i uplotnenie gruntov v stroitel'stve. Tezisy dokladov na X Bsesoyuznom nauchnotekhnicheskom soveshchaniy, Rostov on the Don, Stroyizdat, Moscow, pp.10--13 (in Russian). Beketov, A.K., Seleznov, A.F. and Kachan, Yu,I., 1983. Effektivnost' primeneniya silikatizatsiy gruntov pri stroitel'stve zavoda 'Energomash' vo Volgodonske. Zakreplenie i uplotnenie gruntov v stroitel'stve. Tezisy dokladov na X Vsesoyuznom nauchnotekhnicheskom soveshchaniy, Rostov on the Don. Stroyizdat, Moscow, pp.35--37 (in Russian).

363 Beles, A.A., Stanculescu, I.I. and Schally, V.R., 1969. Pre-wetting of loess soil foundation for hydraulic structures. Proc. 7th ICSMFE, Mexico, 2: 17--25. Bezruk, V.M., 1965. Ukreplenie gruntov. Transport, Moscow, 340 pp. (in Russian). Bezruk, V.M., 1971. Ukreplenie gruntov v dorozhnom i aerodromnom stroitel'stve. Transport, Moscow, 246 pp. (in Russian). Bronstein, B.E., 1968. Zakreplenie tyazhelykh lessovidnykh suglinkov s narushenoi strukturoi metodom elektrosilikatizatsiei. Materialy k 6-omu Vsesoyuznomy soveshchaniyu po zakrepleniyu i uplotneniyu gruntov. Publ. Moscow University, Moscow, pp.318-322 (in Russian). Brown, R.D. and Warner, J., 1973. Compaction grouting. J. Soil Mech. Found. Div., Proc. ASCE, SM 8: 589--601. Chang Tsung-Hu, 1980. Novyye danyye of lessovykh porodakh Kitaya. Sovetskaya Geologiya, 7:72--81 (in Russian). Cr~ciun, F. and Popescu, D., 1963. Paminturi macroporice in Republica Populara Romina. Raionare si proprietati geotehnice. Bucuresti, Comitetul de stat al apelor, Institutul de studii prospeciuni, Bucuresti, 52 pp. Demirel, T. and Davidson, D.T., 1960. Stabilization of a calcareous loess with calcium lignosulfonate and aluminium sulphate. Proc. Iowa Acad. Sci., 67:290--313. Denisov, N.Ya., 1946. O prirode prosadochnykh yavleniy v lessovidnykh suglinkakh. Izd. Soy. nauka, Moscow, 176 pp. (in Russian). Denisov, N.Ya., 1953. Stroitel'nye svoistva lessa i lessovidnykh suglinkov. Gosgeolitzdat, Moscow, 154 pp. (in Russian). Dolgikh, P.D., 1962. Zakreplenie slabykh gruntov molotymi shlakami i gorelymi porodami. Materialy soveshchaniya po zakrepleniyu i uplotneniyu gruntov, Kiev, pp.407--409 (in Russian). Donchev, P., 1980. Compaction of loess by saturation and explosion. International Conference on Compaction, Paris, France, pp.313--317. Dudley, J.K., 1970. Review of collapsing soils. J. Soil Mech. Found. Div., Proc. ASCE, SM 3 (Proc. paper 7278): 925--247. Dumanskiy, A.V. (Ed.), 1954. Bor'ba s fil'tratsiei vody v lessovykh gmntakh. Pubh Acad. Sci. Ukrainian SR, Kiev, 148 pp. (in Russian). Evans, G.L. and Bell, D.H., 1981. Chemical stabilization of loess, New Zealand, Proc. 10th ICSMFE, Stockholm, 3: 649--658. Ermolaeva, A.N. and Rel'tov, B.F., 1971. Polevye issledovaniya po iskustvennomy zasoleniyu lessovidnogo suglinka i uplotneniyu ego v opytnykh nasypyakh. Materialy VII Vsesoyuznogo soveshchaniya po zakrepleniyu i uplatneniyu gruntov. Energiya, Leningrad, pp.500--503 (in Russian). Evstatiev, D., 1969. Zazdravyavane na lyossovi i glinesti gruntove s portlandtsiment i var. Dissertation. Bull. Acad. Sci., Inst. Geology, Sofia, 209 pp. (in Bulgarian). Evstatiev, D., 1984. Formirane na yakosta na tsimentopochvite. Publ. Bulg. Acad. Sci., Sofia, 94 pp. (in Bulgarian). Evstatiev, D., Rashev, R. and Georgiev, P., 1983. Zemnata osnova na Pliska. Muzei i pametnitsi na kulturata, XXIII, pp.18--23 (in Bulgarian). Evstatiev, D., Miler, G. and Karachorov, P., 1985. Settlements of TV tower built on stabilized loess. Proc. 11th ICSMFE, San Francisco, 2: 1127--1128. Frolov, N.N. and Kirillova, T.N., 1983. Nekotorye osobennosti uplotneniya gazovzryvnym sposobom lessovykh gruntov. Zakreplenie i uplotneniem gruntov v stroitel'stve. Tezisy dokladov X Vsesoyuznom nauchno-tekhnicheskom soveshchanii, Rostov on the Don, Stroyizdat, Moscow, pp.170--171 (in Russian). Ganse, R. Van., 1973. Immediate stabilization of wet soils with lime. Proc. 8th ICSMFE Moscow, 2(2): 233--237. Gibbs, H.J. and Bara, J.P., 1967. Stability problems of collapsing soil. J. Soil Mech. Found. Div., Proc. ASCE, 93 SM 4, (July): 572--594. Golubkov, V.N. (Ed.), 1976. Novye fundamenty na stroikakh Odessy. Izd. Mayak, Odessa, 108 pp. (in Russian).

364 Goncharova, L.V., 1973. Osnovy iskustvennogo uluehsheniya gruntov. Publ. Moscow University. Moscow, 376 pp. (in Russian). Grigoryan, A.A., 1984. Svainye fundamenty na prosadochnykh gruntakh. Stroyizdat, Moscow, 162 pp. (in Russian). Handy, R.L., 1956. Stabilization of Iowa loess with Portland cement. Iowa State University Library, Ames, Iowa, Ph.D. thesis. Handy, R.L., 1973. Collapsible loess in Iowa. Proc. Soil Sci. Soc. Amer. 37(2): 281--284. Hasin, M.F., Malishev, L.I. and Broid, I.I., 1984. Struinaya tekhnologiya ukrepleniya gruntov. Osnovaniya, fundamenty i mekhanika gruntov, 5 : 1 0 - - 1 2 (in Russian). Holtz, W.G. and Hilf, J.W., 1961. Settlement of soil foundations due to saturation. Proc. 5th ICSMFE, Paris, 1: 673--679. Kanatov, V.O., 1974. Uplotnenie lessovogo prosadochnogo grunta m e t o d o m gidrovibratsiy. Materialy VIII Vsesoyuznogo soveshchaniya. Zakreplenie i uplotnenie gruntov v stroitel'stve. Izd. Budivel'nik, Kiev, pp.292--295 (in Russian). Karachorov, P. and Gechev, P., 1984. Izsledvane na napregnato deformiranoto sustoyanie na dvusloini osnovi po metoda na krainite elementi. Inzhenerna geologia i hydrogeologia, 1 4 : 2 0 - - 2 7 (in Bulgarian). Kardoush, F.B., Hoover, J.M. and Davidson, D.T., 1957. Stabilization of loess with a promising quaternary ammonium chloride. Highway Research Board, pp.736--754. Kirillov, Yu.A., 1983. Gazovzryvnoi m e t o d glubinnogo uplotneniya prosadochnykh lessovykh gruntov. Zakreplenie i uplotnenie gruntov v stroitel'stve. Tezisi dokladov X Vsesoyuznom nauchno-tekhnicheskom soveshchaniy, Rostov on the Don, 1983. Stroyizdat, Moscow, pp.144--146 (in Russian). Kirillov, A.A., Kirillov, Yu.A. and Skurlyagin, A.A., 1983. Issledovanie gidrodinamicheskogo sposoba glubinnogo uplotneniya prosadochnykh lessovykh gruntov. Zakreplenie i uplotnenie gruntov v stroitel'stve. Tezisy dokladov na X Vsesoyuznom nauchno-tekhnicheskom soveshchaniy, Rostov on the Don, Stroyizdat, Moscow, pp.143--144 (in Russian). Koval'chuk, Yu.F., Molodykh, I.I. and Batyuk, V.P., 1971. Rezul'taty primeneniya PAV v protivofil'tratsionnoi zashchite gruntov na orositel'nykh kanalakh i bodoemakh yuga Ukrainy. Materialy k VII Vsesoyuznomy soveshchaniyu po zakrepleniyu i uplatneniyu gruntov. Izd. Energiya, Leningrandskoe otdelenie, pp.326--328 (in Russian). Kraev, V.F. and Kostyanoi, M.G., 1980. Stroitel'nye svoistva glinistykh gruntov Ukrainy. Izd. Naukova dumka, Kiev, 154 pp. (in Russian). Krinitzsky, E.L. and Turnbull, W.J., 1967. Loess deposts of Mississippi. Geol. Soc. Amer. (Spec. Paper 94) 64 pp. Krutov, V.I., 1982. Osnovaniya i fundamenty na prosadochnykh gruntakh. Budivel'nik, Kiev, 222 pp. (in Russian). Krutov, V.I., Bagdasarov, Yu.A. and Rabinovich, I.G., 1985. F u n d a m e n t y v vytrambovannykh kotlovanakh. Stroyizdat, Moscow, 163 pp. (in Russian). Kuleev, M.T., 1961. Nekotorye osobenosti protsessa zakrepleniya lessovykh gruntov rastvorami karbamidnoi smoloi. Osnovaniya, fundamenty i mekhanika gruntov, No.6 (in Russian). Larionov, A.K., Priklonskiy, V.A. and Anan'ev, V.P., 1959. Lessovye p o r o d y SSSR i ikh stroitel'nye svoistva. Gosgeoltekhizdat, Moscow, 367 pp. (in Russian). Litvinov, I.M., 1969. Glubinnoe ukreplenie i uplotnenie prosadochnykh gruntov. Budivel'nik, Kiev, 184 pp. (in Russian). Litvinov, I.M., 1974. O stroitel'stve na slabykh i prosadochnykh gruntakh, ikh uplotneniy i zasrepleniy. Obobshchayushchiy doklad. Materialy VIII Vsesoyuznogo soveshchaniya. Zakreplenie i uplotnenie gruntov v stroitel'stve. Izd. Budivel'nik, Kiev, 288 pp. (in Russian). Litvinov, I.M., 1977. Ukreplenie i uplotnenie prosadochnykh gruntov v zhilishchnom i promyshlennom stroitel'stve. Budivel'nik, Kiev, 288 pp. (in Russian). Lofgren, B.E., 1969. Land subsidence due to the application of water. Reviews in engineering geology, Vol.2, Geol. Soc. Amer., Boulder, CO.

365 Lohnes, R.A. and Handy, R.L., 1968. Slope angles in friable loess. J. Geology, 76(3): 247--258. Lomize, G.M., Semushkina, L.A., Kirillov, Yu.A. and Abramkina, A.V., 1974. Uplotnenie lessovykh prosadochnykh gruntov elektroiskrovym metodom. Materialy VHI Vsesoyuznogo soveshchaniya. Zakreplenie i uplotnenie gruntov v sizoitel'stve. Izd. Budvel'nik, Kiev, pp.311--317 (in Russian). Lubenets, Ch.K., 1974. Opyt zakrepleniya i uplotneniya gruntov na stroikakh Ukrainskoi SSR. Materialy VIII Vsesoyuznogo soveshchaniya. Zakreplenie i uplotnenie gruntov v stroitel'stve. Izd. Budivel'nik, Kiev, pp.27--46 (in Russian). Mavlyanov, G.A., Kasymov, G.A., Smolina, L.B., Vaiman, E.N., Stupakova, L.F. and Yunuskhodjieva, M.T., 1978. Fiziko-khimicheskie, inzhenerno-geologicheskie i seismicheskie svoistva lessovykh porod Uzbekistana. Tashkent, FAN Publ., 255 pp. (in Russian). Minkov, M., 1968. Lyossut v Severna Bulgaria. Kompleksno izsledvane. Sofia, Bull. Acad. Sci., 202 pp. (in Bulgarian). Minkov, M. and Evstatiev, D., 1975. Osnovi, oblitsovki i ekrani ot zazdraveni lyossovi pochvi. Sofia. Tehnika, 220 pp. (in Bulgarian). Minkov, M. and Evstatiev, D., 1977. The loess as a structurally complex formation. The Geotechnics of Structurally Complex Formation Int. Symp., Capri, Italy, VohII: 122--371. Minkov, M. and Evstatiev, D., 1979. On the seismic behaviour of loess soil foundations. Proc. 2nd Nat. Conf. on Earthquake Eng., Stanford University, pp.988--996. Minkov, M., Evstatiev, D. and Donchev, P., 1980. Dynamic compaction of loess. Int. Conf. on Compaction, Paris, France, pp.345--349. Minkov, M., Evstatiev, D., Donchev, P. and Stefanoff, G., 1981. Compaction and stabilization of loess in Bulgaria. Proc. 10th ICSMFE, Stockholm, 3: 745--748. Minkov, M., Evstatiev, D., Karachorov, P., Slavov, P., Stefanoff, G. and Jelev, J., 1981. Stresses and deformations in stabilized loess. Proc. 10th ICSMFE, Stockholm, 2: 193-197. Minkov, M. and Donchev, P., 1983. Development of heavy tamping of loess bases. Proc. 8th ECSMFE, Helsinki, 1983, Session 7: 797--800. Mitchell, J.K., 1970. In place treatment of foundation soils. J. Soil Mech. Found. Div., Proc. ASCE. 96(1): 73--110. Mitchell, J.K., 1976. Fundamentals of Soil Behaviour, John Wiley, New York, 422 pp. Mustafaev, A.A,, 1979. Raschet osnovaniy i fundamentov na prosadochnykh gruntakh. Vyshaya Shkola Publ., 367 pp. (in Russian). Nachev, P., 1964. Mehanichna stabilizatsiya na putishtata. Godishnik na NIIP, 1:33--52 (in Bulgarian). NIIOPS, 1973. Rekomendatsiy po gazovoi silikatizatsiy peschanykh i lessovykh gruntov. Stroyizdat, Moscow, 33 pp. (in Russian). Pravilnik za proektirane. Plosko fundirane (2.03.01), 1983. Byuletin za stroitel'stvo i arhitektura, XXVII (January--February) 1(2): 1--91 (in Bulgarian). Rozhkova, T.N. and Volkov, F.E., 1983. O zakrepleniy lessovykh gruntov rastvorami NaOH. Svainye fundamenti, Ufa, pp.94--100 (in Russian). RSN 224--75, Ukazaniya po proektirovaniyu i ustroistvu fundamentov iz piramidal'nykh svai. 1975. Gosstroy USSR, Moscow, 90 pp. (in Russian). Rukovodstvo po proektirovaniy osnovaniy zdaniy i sooruzheniy, 1977. Stroyizdat, Moscow, 377 pp. (in Russian). Rzhanitsyn, B.A., 1974. Nekotorye itogi rabot v oblasti khimicheskogo zakrepleniya gruntov. Obobshchayushchiy doklad. Matcrialy VIII Vsesoyuznogo soveshchaniya. Zakreplenie i uplotnenie gruntov v stroitel'stve. Izd. Budivel'nik, Kiev, pp.99--112 (in Russian). Sheeler, J.B., 1969. Summarization and comparison of engineering properties of loess in the United States. Highway Res. Rec., 212: 1--9. Shekhovtscv, V.S., 1962. Primenenie kol'matatsiy dlya zakrepleniya lessovykh prosa-

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