Hydroconsolidation and subsidence of loess: Studies from China, Russia, North America and Europe

ENGINEERING

GEQLOGY ELSEVIER

Engineering Geology 37 ( 1994) 83-113

Hydroconsolidation and subsidence of loess: Studies from China, Russia, North America and Europe In memory of Jan Sajgalik C.D.F. Rogers a, T.A. Dijkstra a,b I.J. Smalley a a Centre for Loess Research, Civil Engineering Department, Loughborough University of Technology, Loughborough L E l l 3TU, UK b Department of Physical Geography, University Utrecht, Heidelberglaan 2, 3508 TC Utrecht, The Netherlands (Received November 30, 1992; revised version accepted January 27, 1994)

Abstract

Various approaches to the widespread problem of the hydroconsolidation and subsidence of loess have been suggested. These include considerations of rheology, thermodynamics, phase movements, particle packing, interparticle bonding, pore structure and distribution, catastrophe theory, topology, and simple structural frameworks. Chinese, North American and most European investigators tend to concentrate on mechanisms of loess collapse. The Russian literature, however, retains an extra dimension. Two approaches, the 'syngenetic' and the 'epigenetic' approach, to the formation of subsiding loess have been defined in the literature. Most investigators follow a syngenetic approach which appears to be a consequence of the aeolian idea of loess deposition. Some Russian writers, in contrast, promote an epigenetic approach in which collapsibility can develop in an originally noncollapsible material, which can then suffer from hydroconsolidation and subsidence. The basis of the phenomenon is a change in the packing structure of the major loess particles, and this can be modelled using simple Monte Carlo methods to develop appropriate structures. This paper aims to review the work done on this important subject. Serious investigation of hydroconsolidation and subsidence of loess began in the early nineteen-forties, fifty years ago, and this has been reported in a piecemeal manner. A detailed, critical review of this diverse work is now overdue and this is presented herein in the light of recent work in the United Kingdom. An attempt is made to describe the process in a phenomenological and a structural sense. Inherent in this, the role of N. Ya Denisov as 'subsidence pioneer' is considered.

1. Introduction

Berg (1964, p. 138) writing on subsidence (i.e., hydroconsolidation) states: " I n Central Asia, when new canals were constructed on the loessic grounds of the Tashkent district, the G o l o d n a y a Steppe, the Surkhan D a r y a district, along the Vakhsh canal (in Tadzhikistan), and elsewhere, and also in Northern Caucasia ( K a b a r d a canal) and in Transcaucasia, it was found that soon after the oo13-7952/94/$7.00 © 1994Elsevier Science B.V. All rights reserved SSDI 0013-7952(94)00003-K

water started flowing, the bottom of canals and the adjacent banks began subsiding. Along the canal, the loessic ground becomes fissured with vertical cracks, along which the loess subsequently collapses into terraces running parallel to the canal, the number of such terraces varying from 5 to 12 on each side. The subsidence of the canal's bottom reaches 1.5-2 m, or even 2.5 m..." T h i s is a classic example of the problem that may arise when loess is loaded and wetted. M a n y

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loesses undergo a structural collapse which leads to a phenomenon which has variously been called hydroconsolidation, hydrocompaction, sagging, subsidence, settlement, collapsibility, and soil structure collapse. It is the classical loess property in the dynamic sense. The classic loess property in the geomorphological sense is that it mantles the landscape, and the classic property in the mineralogical/petrological sense is that loess consists largely of primary mineral (quartz and feldspars) particles in the size range 10-60 ~tm. The simple view of loess sedimentology is that classic loess is deposited by the wind, which gives it the open structure that can subsequently collapse. Some workers in Europe and Russia, however, still incline to a view that allows loess properties, including collapsibility, to develop over time: a post-depositional tendency towards an inclination to subsidence. Recent Russian works are still discussing the 'genesis of collapsibility' (Kriger, 1986, 1987), and indeed Kriger has devoted an entire book to the topic. In this paper the authors have chosen to use the term 'hydroconsolidation', since it seems to serve rather better than the alternatives. The area of interest is the phenomenon of hydroconsolidation (the process) rather than, say, the observation of settlement (the consequence). Hydroconsolidation seems better than hydrocompaction, which appears to imply compaction by water rather than a consolidation allowed by the presence of water. Sagging, as used by Sajgalik (1990), is assumed by the authors to be a term used in translation, in preference to subsiding. It is actually a very adequate word to describe the phenomenon: the Oxford Concise Dictionary of 1990 defines sag as 'sink or subside under weight or pressure, especially unevenly'. There is no indication, however, of the cause of sagging if this term is adopted. Soil structure collapse occurs as hydroconsolidation proceeds. It is thus a result of the hydroconsolidation process and, as above, provides no information on the cause, rendering the term inadequate also. Although this discussion could be dismissed as mere semantics, it has served to define the scope of the paper while addressing an important point of terminology. The aims of this paper thus are to bring together some widely scattered studies on

hydroconsolidation, to produce a description of the process in the phenomenological sense, to describe the process in a structural sense, and to suggest how the original open soil structure is produced, and preserved until the moment of collapse. There is a widespread literature on hydroconsolidation. This paper aims to do more than just list it, but rather to examine how studies have developed and explain how certain approaches have arisen. There is a lack of connectivity in the literature. For example Sajgalik (1990) is concerned with the world literature on 'sagging' and he cites the Kriger (1986) book, but he obviously has no access to recent North American literature: there is no citation of Lutenegger and Hallberg (1988), or of the North American classics like Holtz and Hilf (1961) or Gibbs and Bara (1962). He obviously has no way of gaining access to the voluminous Chinese studies on subsidence and related problems. These problems arise in the other direction, although perhaps not to such a great extent. The Lutenegger and Hallberg (1988) paper deals with exactly the same topic as the Sajgalik (1990) paper, but carries no references to Kriger. This is assumed to be essentially because the Kriger works are not available in English. A world system becomes visible where a set of widely dispersed groups are studying loess collapse and hydroconsolidation quite vigorously but they are separated by linguistic barriers. While the groups are undoubtedly attempting to cross these barriers, it remains a difficult process and one which this paper hopes to assist. The obvious geographical areas of research are China (much material published in Chinese, and some latterly in English, e.g., Gao, 1988; Tan, 1988); Russia and Russian-speaking regions (much material published in Russian with some translations available, and some recent publications in English, such as Kriger and Pecsi, 1987); North America (which really means the USA, although there are one or two Canadian examples, such as Hardy (1950); all publications are in English); and Europe (important works in all relevant languages with considerable work in Eastern Europe, and in Eastern European languages, some classic studies in English such as

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Audric and Bouquier (1976) and Fookes and Best (1969), and some very significant recent works in English, such as Sajgalik (1990) and Feda (1988)). The New Zealand loess, in general, does not suffer from hydroconsolidation, and there is very little literature on the South American loess. Kriger (1987) reviewed the collapse properties of loess, concentrating on the years 1982-1987, basically under two headings: genesis of collapsibility, and prognosis of possible collapse. Although some of the language is difficult to interpret, Kriger lists three points of view on the development of the property of collapsibility that serve as a useful introduction to detailed discussions: (1) Collapse properties may be explained by the 'undercompactibility' of loess, formed as a result of the cementation of loosely deposited sediments and by water-soluble structural bonds, and the emergence of internal stresses caused by the pressure of overburden. Kriger calls this N.Ya Denisov's principle. Thus, the collapse properties of loess in semiarid climates appear after the loess is loaded by superimposed deposits. The authors believe that this is the view held by Kriger and his co-workers. (2) Collapse properties can be explained by the reduction of density brought about by hypergenetic processes. There seem to be implications here of an in s i t u loessification. This is the view supported by E.M. Sergeev, the general idea apparently being that periglacial cold conditions produce an open structure in the ground material that can subsequently collapse. (3) The third explanation of collapse properties is by way of the nature of the sedimentation process of various deposits. Kriger says that this means they belong to a definite genetic type, and that this point of view is accepted in Central Asia, largely due to G.A. Mavlyanov (see 1958). However, the argument remains unclear. Mavlyanov believes loess to be an alluvial or proluvial formation (not aeolian or soil-eluvial), so presumably he envisages some terrestrial sedimentation process producing an open-structured sediment.

2. Subsidence criteria for engineering design In loess with a low bulk density, Gibbs and Bara (1962) and Handy (1973) have shown that

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when the saturation moisture content exceeds the liquid limit, collapse of the soil structure from an increase in load is possible. They express this instability criterion in terms of the liquidity index, defined as: W--PL L I ~ - - L L -- P L

(1)

where: LI=Liquidity index; W=Field moisture content; L L = Liquid limit; and P L = Plastic limit. Thus, when the liquidity index approaches or exceeds a value of unity, experience has shown that collapse or subsidence of loess may be about to occur. Lutenegger and Hallberg (1988) state that the most common method used to identify a collapsible soil material, and the one with the most meaning to geotechnical engineers, has been the one-dimensional compression test using an oedometer. The test procedure involves applying a vertical stress to a laterally confined sample of loess at a natural moisture content, and then introducing water tO the specimen. A decrease in volume at constant vertical stress is an indication of the collapse potential of the material. The coefficient of collapse, i, initially defined by Abelev (1948), is often determined for a stress of 300 kN/m 2 (3 kg/cm 2) and is given by: Ae i= 1 + e 1

(2)

where: Ae=decrease in void ratio caused by wetting; and e l = void ratio before wetting. Generally, values of i greater than 0.02 are indicative of soils considered dangerous with respect to collapse. A simple, but widely used, criterion is that proposed by Denisov (1951), who states that subsidence is probable when: e~
(3)

eo

where: eL=void ratio at the liquid limit; and eo = natural void ratio. This criterion was successfully used in predicting subsidence in a wide range of partially saturated soils, including loess, by Holtz and Hilf (1961).

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The same criterion was adapted by Gibbs and Holland (1960) for presentation in a graphical form in which natural dry density is plotted against liquid limit. The chart is divided into two regions by lines drawn at 100% degree of saturation for soils with specific densities (sometimes termed grain densities) of 2.60 and 2.70. Soils which plot above the line of saturation are potentially metastable, that is they will undergo subsidence on wetting, while those which plot below the line behave as stable or heaving soils on wetting. This graphical criterion was successfully applied in predicting subsidence at a number of dam and canal sites on loess foundations in the United States (Gibbs and Holland, 1960) and at sites underlain by low density alluvial fan deposits along the line of the San Luis Canal in the San Joaquin Valley, California (Gibbs and Bara, 1962). Knodel ( 1981 ) used the Holtz and Hilf (1961) criteria in his study of the San Luis Canal and Clemence and Finbarr (1981) incorporated it into their review of design considerations for collapsible soils. Fookes and Best (1969) considered the criteria proposed for the recognition of partially saturated soils exhibiting subsidence on wetting at low overburden pressures and under low applied foundation stresses. These criteria require only the representative values of natural densities, Atterberg limits and natural water contents to be determined. In their discussion on the subject they state that the Denisov criterion and the derived graphical criterion are only strictly applicable to cases where wetting of the soils occurs at low overburden stresses or under low static surface stresses. Thus they cannot be satisfactorily employed for predicting the behaviour of soils when wetted under high loads. Another criterion has been proposed by Feda (1966) who states that subsidence is probable when the subsidence index KL>0.85, KL being defined as:

W°/S°-PL KL-- I_,L--PL

(4)

where: Wo=natural water content; So =the natural degree of saturation. For comparison with the Denisov criterion, the

subsidence index KL can be rewritten in the form: /~L -

eo - ep

(5)

e L -- ep

where: eo = the natural void ratio; eL = void ratio at liquid limit; and ep = void ratio at plastic limit. When eo=eL, KL=I; and, therefore, Feda's criteria is more conservative. Clemence and Finbarr (1981) describe a very simple field test: the 'sausage' test. A hand-size sample of the soil to be tested is broken into two pieces, and each is trimmed until they are approximately equal in volume. One of them is wetted and moulded in the hands to form a damp ball. The two volumes are then compared again. If the wetted ball is obviously smaller, then soil structure collapse may be suspected. This has the merit of simplicity and immediacy of the result, but is in no way adequate as an engineering tool other than in alerting the engineer to a potential problem worthy of analysis. Samples that fail this test might prove to be perfectly stable under stress acting in the field, regardless of hydrogeological condition.

3. Pioneer research

It is apparent that the problem of hydroconsolidation was not addressed in the literature before 1940. Although some significant earlier studies (e.g., Abelev, 1931) had been carried out, we see the hydroconsolidation as it is understood in this paper was first introduced around 1940, and the true pioneer is N.Ya Denisov (1940). Scheidig (1934) in his seminal work 'L6ss und seine geotechnischen Eigenschaften' mentioned loess collapse, but indicated little systematic investigation. Kriger (1986) in his book on hydroconsolidation gives three Denisov references (1946, 1953, 1972) and these probably encapsulate his ideas and set his subsidence criterion in perspective. Kriger (1965) gives a fuller list (1944, 1946, 1948, 1949, 1950, 1953, 1956). Yang (1989) starts his study with a reference to Denisov (1956), Sajgalik (1990) cites Denisov (1953), and Feda (1966) cites Denisov ( 1951, 1963). Berg (1964) gives two references to Denisov (1940, 1944), and we have located an earlier one (1934),

C.D.F. Rogers et al./Engineering Geology 37 (1994) 83-113

which is a study of the use of liquid limit determinations to indicate soil properties. Berg's treatment of Denisov is interesting because of its very critical approach, in that Denisov's early subsidence studies did not tend to support Berg's hypothesis of loess formation. Berg (1964, p. 141) writes "Therefore Denisov (1940) is wrong when he claims that the subsidence of loess-like loam in the eastern parts of Ciscaucasia attests to its aeolian origin." In the same passage he goes on to state "Relying upon his own theoretical notions of the origin of subsidence, Denisov (1944, p. 17) denies altogether that any alluvial deposits may acquire a loess-like aspect. But as the 'loessification' of alluvium is an actual fact, established by tens of researchers, easy to observe in nature, and fully described in our present paper [Berg 1964], we must conclude that the very premises of Denisov are quite erroneous. They cannot strengthen the aeolian hypothesis or refute my own views". This is a critical passage. It shows Berg at odds with Denisov because Denisov has chosen an obvious explanation of loess subsidence: the collapse of an open structure formed by aeolian deposition. Berg (1964) is a translation of a work published in Russia in 1947. We see the situation in the late nineteen forties where the Berg theory of loess formation in situ was very influential. The Berg theory still affects Russian and Eastern European studies on subsidence and this causes continuing reference to 'loessification' - - the development of loessic qualities in non-loessical ground. Kriger (1986) sees the subsiding quality of loess as a significant natural phenomenon in its own right, rather than as a simple consequence of aeolian deposition. The current authors feel that it would be reasonable to argue that the observed subsidence caused by hydroconsolidation of loess is one of the strongest arguments for an aeolianmode of deposition.

4. Digests: Different views of hydroconsolidation

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to find that there is a large literature on the topic. In this section an attempt is made to illustrate the diversity of type that is found in this literature (the literature as a whole will be considered in a later section). American authors write in English and tend to cite papers written in English; Russian authors write in Russian and tend to cite papers written in Russian. This truism hides another fundamental difference: there are differences in approach to loess itself which make an American paper typically very different from a Russian one. The Russian literature is vast. The spread of loess in Russia, the Ukraine and neighbouring countries is very large and great urban centres are affected by the loess in ways that they are not in North America. (We know of no major towns in the USA that are badly affected by loess subsidence.) In addition, Russian authors tend to have regional connections, such as Lysenko in St. Petersburg, Kriger in Moscow, Anan'ev in Rostov, and Mavlyanov in Tashkent. The history of loess investigation in Russia is complex and turbulent, with a range of views in contention, whereas in North America it is smooth and straightforward. The overall complexity of the loess scene in Russia has lent complexity to the Russian literature on collapse and subsidence. It is hard, for example, to conceive of an American book on subsidence, but Kriger (1986) filled 174 pages in Russian and the journals 'Osnovaniya, Fundamenty i Mekhaniky Gruntov' and 'Inzhenernaya Geologiya' contain a steady stream of subsidence papers. The digest section illustrates both recent and relatively recent investigation in the four demarcated regions. While it would be impossible to present a comprehensive description of work reported in the literature, it is important to clearly demonstrate the worldwide nature of research into collapsing loess and the collapse phenomenon, and to attempt to give an indication of the 'type' or 'style' of investigations that are being undertaken in various parts of the world. All of this work is considered by the current authors to derive from the pioneering work of Denisov, fifty years ago.

4.1. Introduction

4.2. Bibliographies

The structural collapse and consequent subsidence of loess soils presents an enormous engineering problem in many countries, so it is no surprise

The DLT classification The literature on loess presents many problems. The initial task herein is to give an approximate

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structure, or classification, and then to examine access to the literature and the value of various available compilations and data-bases. To demonstrate the overall structure of the loess literature we have devised the DLT classification, in which each unit of literature is classified with respect to relevant deposit, language used, and topic of investigation. It has proven convenient to give each major category six sub-divisions, as shown in Table 1. Various points need explanation, and some subjective judgements need to be justified. In the deposit section we wanted to identify the major deposit regions. In this respect the boundaries to countries are arbitrary and, for example, the Ukraine, Belarus and western Russia are included in Europe (E). The entries are in alphabetical order with the initial directly related to the deposit as a memory aid. We recognise five 'loess languages' i.e., the languages in which the bulk of the world literature is published. Although Russian is probably the leading loess language in terms of material published to date, English is possibly now more popular. The INQUA Loess Commission now works almost exclusively in English (switching from German in the mid-1970s). There are some major languages in the O group, such as Polish and Hungarian that perhaps deserve recognition, but the table is constrained to six sub-groups in the Language package for ease of use. The Topic (T) group is intended to cover all the areas of investigation in which loess has featured. Some have been more popular than others and the main thrust of investigation moves from sub-group to sub-group. During the 1980s Stratigraphy S has

become very popular and yielded very significant results. This paper is concerned almost exclusively with category E, Engineering, within which there has certainly been steady but unspectacular work since the beginning of the century. Scheidig (1934) listed a vast amount of endeavour, and there has been a steady stream of publication since then. Within topic E, the major sub-topic is that of subsidence and hydroconsolidation. While all languages are relevant to this work, there is a slight emphasis on Russian, and there is a similar emphasis on deposits A, C, E and K.

Russian geotechnology An emphasis on Russian is justifiable for several reasons. Firstly, there exists a very large Russian literature that has not been appreciated in the nonRussian speaking world. Access is improving. The two major journals in Russian in which loess subsidence is discussed are 'Osnovaniya, Fundamenty i Mekhaniky Gruntov' and 'Inzhenernaya Geologiya' and they are available in cover-tocover English translation as 'Soil Mechanics and Foundation Engineering' and 'Soviet Engineering Geology'. Nevertheless much remains beyond the reach of western researchers. The Ukraine, whose loess is largely described and discussed in Russian, is probably a country containing the most loess in the world and the one in which there exists the most pressing construction problems. Russian investigators have an extra dimension to work with because there exists, as Trofimov (1990a,b) has described, the two approaches, syngenetic and epigenetic, to the formation of the collapsing loess.

Table 1 The DLT classification for loess literature DLT Classification D Deposit

L Language

T Topic

A North America C China E Europe K Kazakhstan, Central Asia N New Zealand and others S South America

C Chinese E English F French G German O Others R Russian

A Archaeology/History/General E Engineering G Geomorphology/Hydrology/Sedimentology M Materials: Mineralogy/Petrology P Pedology/Agriculture S Stratigraphy/Dating/Palaeontology

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The philosophical aspects of loess subsidence and collapse are discussed in the Russian literature to a much greater degree than elsewhere, and there is always lurking in the background the additional problem of accounting for collapsing loess which, it is claimed, has been formed by 'epigenetic' processes. There are two significant recent books, one on collapsing loess in Central Asia and southern Kazakhstan (Kriger et al., 1981) and one on the origin of hydrocompaction properties (Kriger, 1986). As mentioned earlier, the fact that a whole book can be written on the 'origin' of hydrocompaction properties demonstrates the extra dimension in the Russian discussion.

4.3. Feda (1966) Prague, Czechoslovakia This is one of the key papers in the subsidence literature. It has the advantage of being published in an accessible journal (unlike many subsidence papers) and containing a perceptive discussion of criteria of collapsing loess and the idea of loess as a sensitive soil. There are two main conditions for the subsidence: (1) An 'internal' condition that the soil has to be sufficiently porous. The critical porosity is approximately no > 40%. (2) An 'external' condition that the stress must be sufficiently high to cause the structural collapse at the wetting of the soil. The second condition is valid only for partially saturated soils, and even for them with higher stresses the collapse will take place without wetting. The first condition is more general and common for all metastable soil structures. Jennings and Knight (1957) describe some wind blown subsident sands. The phenomenon of subsidence was shown to occur in soils of alluvial origin as well as in colluvial soils, i.e., the D and E soils, respectively, of Holtz and Hilf (1961), and even by soils of residual origin. The subsidence of granite residuum in South Africa has been described by Brink and Kantey (1961). This subsidence has been created by heavy rainfall and good drainage, causing, according to the authors, the leaching of the colloidal components of the decomposed rock. Feda recorded the subsidence of resid-

89

ual soils originating from decomposed gneiss in North India, in which the coefficient of subsidence at a normal stress of 300 kN/m 2 was 6%. The instability of the soil structure after wetting in all these cases has been revealed and it is similar to the case of loess. The natural degree of saturation So has always been considerably smaller than 1.0 (typically 0.5-0.6). The same phenomenon of subsidence should doubtless be observed for cases of increased normal stress conditions only, that is where no wetting occurs. The structural strength of soils, whose break-up causes the subsidence, may be defined in soil mechanics in one respect, by the sensitivity of soils, i.e., the ratio of the strengths of undisturbed and remoulded samples of the same porosity and water content as in the undisturbed state. Highly sensitive soils are, therefore, potentially structurally unstable soils. It is notable that overconsolidated clays are non-sensitive, while normally consolidated clays usually exhibit sensitivities in the range of 2-4. The most sensitive, and in practice the most structurally unstable soils are the quickclays. Feda describes these as low plasticity (PI= 10-15%) illitic clays whose typical feature is a natural water content that is higher than their liquid limit. Cabrera and Smalley (1973) have suggested that true quickclays are characterised by very low PIs(3-5%) and very low clay mineral contents. Although the subsidence of loess soils differs in other ways from the liquefaction of quickclays, high porosity and low plasticity index values are typically common in both types of structurally unstable soils. These soil types differ in their degrees of saturation however; quickclays are saturated, while subsident soils are unsaturated. The different genesis of subsident soils indicates that the process of the origin of metastable structures can be different, yet the soils must retain the principal condition that the final product is highly porous. The limiting porosity, necessary for the development of a subsident structure, is very close to the maximum possible of no = 47.6% for a cubic sphere packing, with each sphere having six contacts with its neighbouring spheres (the 600 pack of Smalley, 1971).

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4.4. Kraev (1971) Kiev, Ukraine About 14% of the territory of the USSR and nearly 65% of the Ukraine are covered with loess. The latter possesses several specific compositional features (predominance of dust fractions, presence of carbonates, poor weathering of the clastogene material), as well as specific properties (fast loosening, rather high filtration rate, anisotropy of the mechanical properties in vertical and horizontal directions, subsidence). Subsidence is among the most adverse features of loess soils. Experience in the construction and operation of different buildings on loess soils furnishes numerous examples of deformation suffered by buildings, industrial installations, and irrigation canals. In several areas of the Ukraine subsidence may be as great as 1.0 to 1.5 m, with thickness of the loess averaging 20 to 40 m. With approximately equal values of active porosity a loess soil is found more subsiding if it possesses lower natural humidity, contains a small percentage of clayey particles, is characterised by a small absorption capacity, and has lower indices of hydrophylic properties. All this is abundantly clear and does not require any additional explanation. The following remain to be discussed: (1) Why are clayey minerals of subsiding soils often rich in montmorillonite? (2) Why may the subsidence take a stepwise course? (3) Why may the subsidence recur with a repeated cycle of soaking? Kraev makes the following observations on his key questions: ( 1) In the mechanism of subsidence the physicochemical phenomenon of filter water disjoining action is essential. The most subsiding loess soils are formed in steppe and semi-desert conditions of arid and semi-arid zones where, due to predominance of alkaline weathering conditions, montmorillonitic clay minerals are formed. This small content of montmorillonite, present in a soil in contact clusters, swells when soaked and loosens the primarily loose soil structure, thus contributing to a more vigorous manifestation of the water wedge effect.

(2) Studies of structural bonds in loess soils have shown that they may be represented by clayey materials and various salts. The stepwise character of subsidence results from the non-simultaneous disintegration of different groups of structural bonds. (3) Recommencement of subsidence following a water-level decline or after a repeated soaking is by no means a result of recovery of the subsidence properties. The phenomenon should rather be explained by the presence in loess soils of a developed system of tubular macropores, owing to which vertical filtering is twice or thrice as great as horizontal. Thus temporarily 'closed' hydraulic systems may originate because of an insufficient horizontal discharge of water and so suspension of subsidence occurs.

4.5. Egri (1972) Budapest, Hungary Loess soils are composed of 40 to 60 various types of minerals. The most important role is, however, being played only by some 10 to 12 of them. These can be divided into two groups: (1) Active minerals, i.e., those being changed due to the process of collapse, and including: carbonates (chiefly calcite) 0.2-30% sulphates (gypsum) 0.0-3% salt 0.0-2% various clay minerals (montmorillonite, kaolinite, etc.) (2) Passive minerals, i.e., those not being changed due to the process of collapse, and including mainly: quartz 20-85% feldspar 4-40% mica 1-30% Strictly speaking, it is the presence of macropores which enables the collapse of loess soils to take place. Therefore it is quite evident that the void ratio expressing the quantity of macropores has a determining effect on the degree of collapse.

4.6. Barden, McGown and Sides (1973) Glasgow, Scotland A study made on loess material supplied by Fookes and Mellors; for further studies on this

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material see Mellors (1977). The collapse phenomenon is apparently a contradiction of the principle of effective stress which underlies all soil mechanics theory, since wetting increases pore pressures, decreases effective stress and, hence, is expected to cause heave rather than settlement. However, a more detailed consideration of the mechanism suggested that collapse is due to local shear failure between grains or peds, and hence is compatible with the principle of effective stress. The open, meta-stable structure, which is a basic requirement, was shown by Barden et al. (1973) to be almost invariably of a bulky granular nature. This is true whether a soil is basically a sand, a silt or a clay. The nature of the intergranular bonding is a much more complex problem. In all the soils studied by Barden et al. (1973) clay appeared to provide the dominant bonding agent, although in the case of loess soils calciumcarbonate cementation can be important. The clay bonds were essentially of two types. Where the clay clothes the surface of the silt- or sand-size quartz grains, the bond is simply a quartz-clayquartz adhesion. Where the clay bond tends to be concentrated in local areas it often appears to act as a buttress, although it may act together with fine silt particles, and not necessarily possess a flocculated micro-structure.

4. 7. Zur and Wiseman (1973) Tel Aviv and Haifa, Israel A definition of collapse should be general enough to include the whole variety of collapse manifestations. It is proposed that the term collapse should be applied to any rapid decrease in volume brought about by the increase in any one or a combination of the following: water content/degree of saturation, mean normal total stress, shear stress, or pore pressure. This definition recognises that collapse of the soil structure may be triggered by a variety of soil processes other than wetting. It does not follow that different causes will produce a unique collapse performance. However, if the mechanism of the collapse phenomenon is similar, the different collapse patterns should be comparable, at least qualitatively. The main findings of the Zur and Wiseman

91

study are that loess maintains a quasi-rigid structure even after saturation and that collapse behaviour depends not only on the increase of the degree of saturation and the mean stress level but on the shear stress level as well. The decrease in collapse potential at low shear stress levels as compared to hydrostatic stress conditions is particularly worthy of note. This latter phenomenon was found to exist for all the widely different methods of testing employed.

4.8. Handy (1973) Iowa, USA Handy defined collapsibility (also referred to as slumping soils, subsiding soils, or hydrocompacting soils) as a state of underconsolidation related to apparent cohesive strength of perennially unsaturated soils. If such soils ever become wetted so that their apparent cohesion is no longer sufficient to hold grains in support of the overburden pressure, the soil structure collapses and the soil rapidly consolidates and becomes 'normally consolidated' for the existing pressure. Collapsible soils may safely support loads considerably in excess of the existing overburden pressure, but only so long as their relative dryness is preserved. Handy pointed out that Iowa is ideal for the study of variations in loess properties because it lies at the heart of the USA loess area, having the thickest loess deposits in North America, and because the vertical and areal property variations in Iowa loess have been the subject of intensive study. Loess from different depths at three locations in western Iowa was tested by Olson (1958) for consolidation at different moisture contents and under stresses which bracketed the overburden pressure. Although the purpose of Olson's tests was not to ascertain collapsibility, in that the moisture content usually was not increased while the samples were under load, essentially the same compression was found to occur whether the loess was wetted before or after loading. Similar results have been reported by Holtz and Gibbs (1951) and Kane (1969), and indicate that collapsibility can be determined from a comparison of the consolidation curves obtained at natural moisture content and after saturation. An advantage of this procedure is that it allows a determination of

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collapsibility under any pressure within the range of the tests, instead of under a single pressure; a disadvantage is the introduction of sample variability, since tests are necessarily performed on different samples. Olson's data (see Handy 1973, p. 282) indicated highly collapsible loess at two western Iowa locations where a decrease in volume of more than 5% occurred on saturation. Moderate collapsibility was shown at a third location which was further from the loess source area. Kane (1969) performed consolidation tests on eastern Iowa loess under varying moisture conditions, and found only slight collapsibility. Clevenger (1956) suggested a density criterion for collapsibility of Missouri Basin loess, a dry density 7a< 1.28 (80 pcf) being conducive to large settlements. If yd>l.44 (90 pcf) the settlement should be small, while at intermediate densities the properties are transitional. Lehr (1967) indicated that compaction by piles to reduce the porosity below 40% causes loess to become insensitive to wetting; if the specific gravity is 2.70, this corresponds to 7d>1.62 (101 pcf). Gibbs and Bara (1962) developed a simple but logical criterion following an observation by Denisov (cf. Gibbs and Bara, 1962): if on saturation the soil moisture content exceeds the liquid limit, the soil will most likely collapse. Since the specific gravity is practically constant in loess in any given area, the potentiality for collapse may be calculated from the field density and the liquid limit. For example, if the specific gravity is 2.70, the saturation moisture content Ws may be calculated from: Ws= 100( 1/ya- 1/2.70)

(6)

Where 7d is the dry bulk density in Mg/m 3. Potential collapse is indicated if Ws exceeds the liquid limit, which is easily and quickly determined for any specific deposit. Handy (1973) calculated the ratio LL/Ws, where LL is liquid limit. Collapsible soils are indicated by LL/Ws < 1.0 and safe soils by LL/W~>I.O. The ratio is thus analogous to a factor of safety, even containing the characteristic element of uncertainty, except that the 'collapsibility safety factor' refers to a moisture content ratio rather than a stress ratio. Handy (1973) reached two major conclusions:

( 1) The extent of collapsible loess may be defined and mapped in Iowa (and presumably in other places) using the criterion that the saturation moisture content must equal or exceed the liquid limit, because of the systematic increase in the liquid limit and decrease in saturation moisture content with distance away from a source (see Fig. 1). (2) When density and liquid limit data are not available, a preliminary estimation of collapsibility may be made on the basis of clay content. Handy gave a fairly detailed indication of clay content effects but the essential conclusion was that increase of clay mineral content decreases the likelihood of collapse, or hydroconsolidation. This is an observation of major importance, and one not made by other investigators.

4.9. Uriel and Serrano (1973) Madrid and Santander, Spa& Uriel and Serrano (1973) were concerned mostly with collapsible volcanic soils of low bulk density. However, they did consider collapsible soils in general, and in so doing produced an interesting, diagrammatic soil structure (Fig. 2). The cemented soil is schematically represented, each rectangle symbolizing a particle or group of particles of the matrix soil, and the hatched strips representing the cementing material. They proposed seven basic hypotheses for an axisymmetrical state of stress. ( 1) The soil collapses when the cementing bonds are destroyed. (2) The soil assembly may be considered elastic up to its point of failure, possessing a modulus of elasticity Em and a Poisson's ratio v. (3) The bonds of the cementing material have the following average dimensions: width 1, length L. (4) If the soil assembly is subjected to a principal stresses of ax and a3 and the principal strains are el and e3, the displacements of the ends of each bond correspond to a unit deformation Kel and Ke3 of the space occupied by such bonds: K being a constant which will thus represent the relative stiffness of the soil particles and bond material. (5) The displacements of the ends of the bonds will set up certain stresses in the whole bond. The displacements and stresses will depend on the

C.D.F. Rogers et aL/Engineering Geology 37 (1994) 83-113

Collapse probability

U I

Kilometres

93

i UO I

1oo:%

~"20f

Clay content Flood plain

i° Fig. 1. Criteria for collapsible loess in Iowa (after Handy, 1973). Squares indicate collapsible loess from consolidation tests, circles from field density and liquid limit data. Triangles indicate analogous non-collapsible data. The collapsible loess tends to have a low clay mineral material content, 'contours' show clay content.

orientation of the bond with respect to that of the principal stresses. (6) It is assumed that no relative rotation takes place between the ends of the bonds. (7) The bonds are assumed to consist of a rocky material, whose brittle fracture corresponds to certain parameters c and ~b' or to a compressive strength crc and a tensile strength ~t. Uriel and Serrano (1973) conclude that for a cemented soil, the collapse of its structure can be produced by three causes: by tensile stress at the bond due to bending, by compressive stress, and by shear stresses. This work, therefore, adopts the classical structural engineering approach to the problem. 4.10. Koff, Komissarova and Shibakova (1978) Moscow, Russia The problem of loess soil subsidence is one of the most elaborate problems in the state-of-the-art

soil science. It is gaining particular importance for the Soviet Union, where 14% of the continential territory is made up of loess formation (Kraev (1971) reported 65% of the Ukraine loesscovered). This problem can be separated into two interrelated aspects: (1) Development of subsidence as a specific property of loess soils; and (2) Changes in structure and basic properties of loess soils in the course of subsidence. By the term 'structure' the authors imply a host of structural elements and their relationships forming a multi-component system; a 'soil'. A special (temporal) interpretation of structure is classified as texture. Quantitative methods are gaining paramount importance in structural investigations of soils. A quantitative assessment of the structured dynamism of loess soils comprises estimates of: ( 1) Structural changes of pore space at the point of additional compaction;

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Fig. 2. Structure of a collapsing soil, as proposed by Uriel and Serrano (1973). Each rectangle symbolizes a major structural particle, and the hatched stripes the bonding-cementing material.

(2) Redistribution and reorientation of the structured elements; and (3) Changes in the nature of the structural bonds. By the present time a number of works giving an estimation of structural dynamism have appeared. Several Russian researchers, including N.Ya Denisov, G.A. Mavlyanov, S.D. Vorenkevich et al. have shown that, for example, such an important structural element of loess soils as macro-pores do not always influence their subsidence. (This appears to agree with Yang's (1989) observation.) 4.11. Knodel (1981) Denver, USA

Deposits that subside because of hydrocompaction are generally one of two types: (1) Loose, moisture-deficient alluvial deposits; or

(2) Moisture-deficient loess and related aeolian deposits. These deposits, laid down with an open structure and high porosity and buried with a moisture deficiency and sufficient intergranular strength to support an increasing overburden pressure, are readily susceptible to compaction when wetting reduces their dry strength. They are generally found in regions where rainfall seldom penetrates below the root zone, and they invariably occur above the water table. Unlike land subsidence resulting from compaction at depth-of-waterbearing deposits due to a reduction of fluid pressure, hydrocompaction is a near-surface phenomenon. Since wetting progresses from the land surface downward, hydrocompaction begins near the surface and progresses downward with the advancing waterfront. This near-surface subsidence is a serious problem and has damaged canals, roads, pipelines and transmission towers in local areas. It presents particularly serious problems in the construction and maintenance of large canals. To explain and evaluate the critical condition for collapse, it was determined that the soil density was of highest importance because, for this type of subsidence to occur, the soils must be sufficiently loose so that they are capable of collapsing when their particle-to-particle bond is weakened by wetting. Soils of different types require different quantities of water to reduce them to a weak plastic condition. This suggests that a simple criterion can be used to indicate the limiting density at which any given soil will collapse upon wetting. Then, by the determination of density in situ, the soil can be evaluated as being loose and susceptible to subsidence, or sufficiently dense and unlikely to subside. One of the criteria for analysis, making use of the natural in situ density and the standard liquid limit test, is that due to Gibbs and Bara (1962). Soils of different plasticity or water-holding capacity will collapse at different densities. The liquid limit is a moisture content, determined by standard laboratory tests, which represents the weakest plastic condition of the soil, point of transition from a plastic solid to a liquid state. When the soil has a low density such that its void space is just

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sufficiently large to hold the liquid limit moisture content, saturation can easily cause a liquid limit consistency, at which state the soil offers little resistance to deformation. If the void content is less than this amount, the soil at saturation will remain in the plastic state and have greater resistance against particle shifting. It will then only settle as a normal result of loading. If the void content is greater than this amount, saturation would result in a moisture content in excess of the liquid limit and the potential for collapse would be high. If collapse did not occur, the soils would surely be in a very sensitive condition and would in effect be waiting for a load event (dynamic or shock wave, local failure causing progressive failure) that would result in catastrophic collapse.

4.12. Lutenegger (1981) Iowa, USA Lutenegger, in a short but significant item, explored the connection between loess collapse and soil sensitivity. It could be a useful connection to make in that it introduces a unifying element into a very diverse subject, and it revives a suggestion made many years ago by Denisov (see Smalley, 1981; see also Lutenegger and Saber, 1987, 1988). In the classical Terzaghi sense, loess soils would probably not be considered sensitive soils because the ratio of undisturbed to remoulded strength, at constant moisture content, is usually around 3, depending on clay content, and, therefore, would generally fall into the category of medium sensitivity. If, however, sensitivity is taken as the ratio of undisturbed to saturated strength (in unconfined compression), as indirectly suggested by Feda (1966), then some loess soils would appear to have very high sensitivities. In investigations throughout the midwestern USA, in situ stability of loess has been related to liquidity index, that is when the in situ moisture content reaches the liquid limit, usually on saturation, stability is all but lost and can be readily identified by isolated flow in boreholes. Because liquid limit is related to clay content, and saturation moisture is a function of density, this instabil-

95

ity only occurs in special circumstances, typically in low density and low clay content soils (see Handy, 1973). Denisov stated that a subsident loess is extremely sensitive, particularly when saturated. The water content in such a state is above the liquid limit. He pointed out that loess in this state preserves some strength and can form steep sides to canals and pits. When disturbed, the shear strength drops to zero.

4.13. Savvateev, Kriger, Lavrusevich and Petrov (1987) Moscow, Russia The speed of wetting of collapsible soils is known to influence the measure of deformation. It was found that after the quick phase of subsidence some secondary reinforcement of the soil and temporary ceasing (or slowing down) of the deformation takes place. It is a characteristic of the different types of loess soils in that it occurs to a different degree, and it is sometimes unnoticed (for example, during the flooding of soils in foundation pits or in oedometer tests). This fact, observed during the layer-by-layer analysis of deformations, is important for the prognosis of the development of soil deformations in the course of time. After the time of outflow, 6pw, from the beginning of flooding, some slow-suffusion-plastic deformations appear again, and may go on for 8-12 years. The dimensions and form of the source of saturation essentially influence the quantity of deformation of the collapsing soil system. The calculated deformation S is given by: S=Sp+Sw+S,

(7)

where: Sp = settlement up to saturation under the influence of mass construction: the calculation of settlement is made as for the usual non-collapsing soils; and Sw = deformation in connection with the wetting of the soil.

Sw = YhZpwMaMpMwt

(8)

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C.D.E Rogers et aL/Engineering Geology 37 (1994) 83-113

h = thickness of the calculated soil layer. 6pw= collapsibility of soil in the calculated layer; this is determined by the results of compression tests according to the formula: 6pw-

Apw+(1 - M r ) A d s hk

(9)

Ads = quantity of sample deformation under preliminary reduction of the pressure equal to the natural one. Apw= quantity of sample deformation after saturation under the calculated pressure. hk = height of sample before reduction, equal to the height of the unit ring. Mr = coefficient related to the hysteresis of soil deformation. Ma= coefficient related to the form-changing (horizontal) deformation of soil within the limits of the compressed zone under the foundation. Mp= coefficient related to the dimensions and forms of flooding objects.

Mp= M.ML MB = f l (B,H) ME = f2(L,H) B,L = width and length of flooding object. H = thickness of collapsing loess. Mwt = coefficient related to the duration of soil wetting. Sn = deformation through compaction of the underlying collapse pack. Deformation Sn must be taken under H > 1 0 m, Mp>0.8, En<15MPa (En-modulus of deformation of the underlying rock). For the calculations the deformation that should be predicted is the subsidence of conventional foundations with their base at the top level of these soils, and with dimensions in the plane, corresponding to the dimensions of the prism of subsiding soil under the influence of the weight of the buildings being erected and under the influence of soil wetting. On the basis of generalizations and mathematical analysis of large quantities of test data, some formulae have been produced, together with some nomograms for the definition of Mr, Ma, Mp and Mwt. The mere fact that so many indirectly determined coefficients are required for the calculation,

however, makes the calculation questionable for general use. 4.14. Derbyshire and Mellors (1988)

Derbyshire and Mellors (1988) conclude that it is evident from their analysis that the loess deposits of central China and southwest England are very similar in terms of their characteristic particle size ranges, mineralogy, fabric and, as they refer to potential collapsibility, 'the tendency to metastability'. Some important differences between these two loess types exist, however, and these are predominantly expressed in a higher plasticity, lower void ratio and a lower collapsibility coefficient for the English loess. This is the result of the greater degree of sorting of the English loess which induces the formation of more compact fabrics. Additionally, these loess deposits show a 'greater degree of reworking and clay particle translocation' (Derbyshire and Mellors 1988, p.172: see also Mellors 1977). Derbyshire and Mellors indicate that the assessment of the collapsibility of a soil depends on the availability of 'sufficient void spaces in its natural state to hold its liquid limit moisture at saturation'. They conclude that 'such criteria should be used with caution. Whilst they may be applicable to soils which will undergo collapse on wetting under their natural overburden pressure, they do not necessarily cater for those soils which will collapse only under additional pressure, as for example applied by the weight of a structure and are considered therefore to be of limited applicability. Greater reliance should be placed on direct measurements, such as the testing of specimens in oedometers, with material at moisture contents and loading conditions at flooding representative of field conditions' (see also Mellors 1977). 4.15. Gao Guorui (1988) Nanjing, China

Gao proposed that the study of the regional variation of loess microstructures contributes to understanding the formation and development of the structure of collapsing loess. He suggested that the formation and development of the loess structure in China may be considered to have undergone

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the following stages: the loessization stage, the formation stage, the development stage, the degeneration stage and the clayization stage. The five stages are closely related to the local climatic conditions. As the climate in the regions of the Loess Plateau in China varies from arid in the northwest to humid in the southeast, the five stages of collapsing loess are exemplified in the different regions. It follows, claims Gao, that under the present climatic conditions, collapsing loess is still developing in the northwest part of the Loess Plateau. However, the collapsibility of loess is gradually disappearing in the southeast region of the Loess Plateau in China. Gao (1988), in referring to 'loessization' cites five of his own papers and two other Chinese papers. Thus we have no indication of how he relates his 'loessization' concept to the 'loessification' process of Berg (1964) and subsequent Russians, or to Russell (1944). Gao describes loess formation as follows. The dust transported by wind from desert areas is deposited in a pocket-like region surrounded by mountains on three sides. The dust that has just been deposited is easily moved. In the continental arid climatic conditions and in the periodic rainfall season, little calcium is eluvated (Gao's word which, in the absence of clarification, is interpreted as dissolved?) from calcite and silicate. When calcium ions come into the pore liquid, they first cement the fine clay-colloidal particles (less than 5 ~tm) with a negative charge in the dust, forming clay-colloidal particle groups. This renders the dust particles firmly fixed.

4.16. Tan (1988) Beijing, China The loess from Lanzhou (capital of Gansu Province) has an average void ratio e = 1.05. The loosest theoretical packing of spheres of equal diameter has a void ratio e = 0.89. The mixed sand and silt particles in loess, which are subangular and subrounded, must be mutually separated and they can only be interconnected by cementing bonds. From the analysis of the loess structure it is obvious that its mechanical properties must be complex. In the natural state there will be a

97

distribution of moisture due to rainfall, air humidity, infiltration of water from engineering works and motion of water vapour under thermo-osmotic potentials. As the water will mainly influence the bond material, which comprises only 7.5-12.5% of the soil skeleton, minute changes in water contents may have a major influence on the mechanical properties. This must be ascribed to the presence of hydrophillic soluble salts and clay particles. A further important factor is time: loess masses show time effects as creep deformation and relaxation. Under compression the internal structure of loess may collapse and grains will be squeezed into the pore spaces resulting in mutual interlocking. In this case the loess is transformed into a soil type which is similar to silty soils and poor clays. Typical loess has a collapsible internal structure with a high initial void ratio of more than 1.0, a mechanical sensitivity to the influence of slight variations in moisture content and the big change in mechanical properties before and after structural collapse. Tan states that, before collapse the loess is a complex material sensitive to creep and relaxation which vary with the water content, and active stress and deformation field. After structural collapse Tan states that the loess behaves as poor silty clays with an angle of internal friction of about 30 ° . The current authors, however, question the statement that loess is a material susceptible to creep, because the cementation bonds (predominantly consisting of calcium-carbonates) are relatively rigid and static. Due to small-scale discrete shearing of these bonds which leads to localized collapse of the loess fabric (often associated with progressive failure), loess slopes may exhibit a creep-like morphology. Additionally, recent tests on remoulded loess samples from Lanzhou (China) indicate that they behave more like poor clayey silts with internal friction values between 35 ° and 37 °.

4.17. Grabowska- Olszewska (1988) Warsaw, Poland Collapse is one of the most interesting phenomena observed in some lithological types of loesses which are saturated. Existing literature on that subject is considerable but its ordering appears

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rather difficult. Of all the problems in the applied geosciences, the sorting-out and ordering of the literature on loess collapse is probably the most challenging, Grabowska-Olszewska offers three main observations on collapse: (1) Collapse is most intense in the stratigraphically youngest loesses in a given section. In all regions of Poland, horizons displaying structural instability most often include those of the younger Pleistocene subaerial loesses, defined as younger upper loesses and correlated with the Vistulan glaciation. (2) Collapse is almost exclusively restricted to loesses of dust size particles (with particles below 2 lain comprising slightly over 10% of the whole). (3) The collapse loesses are connected with primary aeolian accumulation and they are usually characterized by a random texture (in macro scale), pale yellow colour, and a carbonate content over 5%. They are relatively unweathered and have pronounced vertical jointing. Such loesses occur in plateau areas.

tion of information to the system its ordering increases, whereas entropy diminishes with ordering, then the information is negative entropy (I=N= -S). Of all the papers in the digest section this one by Volyanik is perhaps the most difficult to condense, or to extract the essential ideas from. What is presented above might appear confusing as a result, but it has been included to allow the essence of Volyanik's presentation to be announced, if not studied in depth. It was assumed that the reserve of mechanical potential energy less than 500 kJ/m 2 corresponds to loess series of sag type Ib according to the system of Anan'ev and Cherkasov (1983) for the northern Caucasus. This category includes sagging soils whose thickness does not exceed 10 m, and subsidence from its own weight is practically non-existent. Areas with an energy reserve of 500-700 kJ/m 2 correspond to type Ic in the same system (thickness of sagging series to 20 m, subsidence from its own weight to 50 mm).

4.19. Yang (1989) Lanzhou Region, China 4.18. Volyanik (1989) Rostov-on-Don Region, Russia ¥olyanik has attempted an evaluation of loess subsidence from its thermodynamic characteristics. Homogeneous geological bodies, he stated, do not exist in nature; therefore, we can evaluate only the statistical homogeneity and speak of quasihomogeneity. Rats (1973) has identified five levels of inhomogeneity of geological objects. In evaluating the variability of loess in the process of the rise in groundwater it is most important to evaluate the inhomogeneities of level II, engineering geological element, and level I, loess rock massif. The massifs, engineering geological elements, and individual samples of loess rocks, which have a certain volume and consist of a multitude of particles, can be viewed as thermodynamic systems. The degree of disordering of the system can be conveniently evaluated according to the volume of information it contains, and there is a relationship between the information (I) and entropy (S). These characteristics can be used to evaluate the organisation of the system. Since upon introduc-

Yang considers that the internal requirements for loess collapse are that the loess must possess both a certain amount of collapsible space and a certain amount of interparticle bonding, the strength of which decreases when in contact with water. He cites Denisov (1956) as proposing that because it has surplus porosity and the cohesion decreases on contact with water, loess collapses. This is his earliest reference. Yang studied pore-size distribution in intact and collapsed samples. He observed that pores with a statistical radius of less than 54 gm comprise over 97% of the pores, and that loess collapse is basically caused by these pores, the contribution of this pore fraction to the total collapsible space being over 80%. He divided the loess particles into three size regions: > 10 gm, 10-2 lam and < 2 gin. The > 10 gm grains comprise the 'frame element'; the < 2 lam grains form the 'clay cementing agent', and the 10-2 lam grains are called the 'filling element'. In the Lanzhou loess in general the frame element content is between 70 and 75%, the filling element between 10 and 16% and the clay

C.D.F. Rogers et al./Engineering Geology 37 (1994) 83-113

cementing agent between 10 and 15%. The cementing agent mineralogy was determined as calcite 20-30%, montmorillonite <1%, illite 40-60%, chlorite 9-16%, and kaolinite 6-10%, with vermiculite absent in most samples, and some traces of quartz and feldspar. The cement appears to be a mixture of calcite and low activity clay minerals, with clay predominating. Yang divided the bonding between grains and on pore boundaries into two types: hydro-stable and hydro-labile cementations. (NB: for the purpose of this paper some of Yang's terms have been changed. It is hoped that the meaning is preserved.) Hydro-stable cementation is a kind of bonding, the strength of which does not obviously decrease when it comes into contact with water. Hydrolabile bonding, on the other hand, suffers a decrease in strength when in contact with water. Yang lists the various bonding types as shown in Table 2. When a flocculent cementing agent comes into contact with water, the thickness of the electric double layer of its colloidal particles increases because of the diffusion of the flocculent ions. Similarly the thickness of the 'hydrous membrane' also increases, so that the strength of the bonding between the colloidal particles decreases or vanishes. Thus the bonding of the flocculent cementing agent is hydro-labile. The cementation of strongly soluble salts scarcely plays a role in the process of loess collapse. Yang gave the following reasons: (1) The content of strongly soluble salts is small, generally below 0.1%; (2) The strongly soluble salts in loess probably

99

are in a state of dissolution so that they do not have a bonding function. Yang's conclusions are fairly straightforward. Loess possesses a primary loose structure and a primary special grain size distribution - - a certain amount of the frame element, a certain amount of the clay cementing agent, and a certain amount of the filling element - - immediately after it has been deposited (Yang uses the term 'settled down', although it is not clear whether he means after settling or after settlement? The former is assumed herein). After loess is deposited the clay cementing agent would cement the grains together because of the reaction of water in loess (i.e., the clay is clayey enough to glue the particles together). With the passage of time the secondary hydro-stable cementation would form in loess, for example the carbonates and sulphates described above. All the cementations maintain the loess structure of the loess. Collapsible loess possesses collapsibility because the cementation of the clay cementing agent and the hydro-labile secondary mica are hydro-labile (our emphasis). It is valuable, points out Yang, to note that the strength, whether of collapsible or non-collapsible loess or of some other clays, will decrease when the soil comes into contact with water, so the loose structure is the special condition for loess collapse. "When we treat collapsible loess, one method is to turn its loose structure into a tight structure, another is to change the quality of the clay cementing agent or enlarge the proportion of the hydro-stable cementation". This statement provides the essential philosophy for engineering in loess. 4.20. Trofimov (1990a) Moscow, Russia

Table 2 Bonding types in loess according to Yang (1989) Hydro-stable cementation

Hydro-labile cementation

Calcium carbonate Hydrous calcium sulphate Ferrous oxide and ferrous hydroxide Hydro-stable secondary mica Secondary zeolite

Strongly soluble salts Clay cementing agent --Flocculent --Non-flocculent Hydro-labile secondary mica

Collapsibility, states Trofimov, is a specific property of loess of different geneses (aeolian, slopewash, alluvial-fan, alluvial, lacustrine-alluvial, etc.). It is expressed in the ability of loess strata to decrease their volume under a constant acting stress after wetting, as a result of which collapse of the mass's ground surface the deformation of engineering structures occur. This property makes loess soils fundamentally different from many other types of disperse soils. Trofimov gives a brief review. The genesis of

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collapsibility is an interesting and complex question, with which many Soviet researchers have been engaged e.g. Yu.M. Abelev, V.P. Anan'ev, I.M. Gor'kova, N.Ya Denisov, N.I. Kriger, A.K. Larionov, G.A. Mavlyanov, A.V. Minervin, and E.M. Sergeev. In the last decade alone it has been specially discussed in reviews by Kriger (1986), Minervin (1982), Minervin and Komissarova (1983), Sergeev and Komissarova (1986), Trofimov (1980, 1986a,b, 1987a,b) and Shaevich (1987). In spite of this, many prepositions are unclear and debatable, and a theory of the formation of collapsibility, in the full sense of the word, has not been created. According to Trofimov, collapsibility can be formed as both a syngenetic and an epigenetic property of loess. These two terms need some explanation. Collapse properties are formed either directly in the course of accumulation and early subaerial diagenesis of silty sediments in arid conditions (this is syngenetic collapsibility) or by progressive lithogenesis of young alluvial and lacustrine-alluvial deposits in subaerial conditions, or in the course of hypergene (including cryoeluvial) decompaction of soils of different ages and geneses (epigenetic collapsibility). Trofimov divides the Russian workers according to their approach, although some have looked at both aspects. It is interesting that there is just the one reference to Kriger (1986); no mention of the Kriger et al. (1981) book on collapsing loess in Central Asia. Besides the division into syngenetic and epigenetic approaches one might speculate, from the balance of such literature, that there are two distinct schools of Moscow subsidence investigators, one group related to E.M. Sergeev at Moscow State University, and one group related to Kriger at the Research Institute for Foundation and Bases (PNIIIS). Presumably all 'western' investigators are involved in studying syngenetic collapsibility, since the philosophy of epigenetic loess formation is not supported in the west.

4.21. Sajgalik (1990) Bratislava, Czechoslovakia Sajgalik divides the factors which cause sagging/subsidence into internal factors and exter-

nal factors. The internal factors correspond to the facies-genetic conditions of development and with the total internal loess structure (genesis, geomorphologic position, climate, structure, moisture, etc.). External factors include those whose tendency is not directly connected to loesses (stress intensity from loading, chemistry of infiltrated water, direction of flow and groundwater dynamics, etc.). In principle the process of sagging can be divided into three time phases, which develop gradually. In the first phase deformation of the original microstructure occurs, caused by moisture increase due to external pressure. The deformation is manifested by the disintegration of clay coatings, bridges and supports, the disintegration of aggregates and micro-aggregates, the passage of clay minerals into suspension, and the increased solution of carbonates. The solutions receive oxides of Fe and Mn, and soluble salts. In the second phase the collapse of the microstructure occurs, as the clay minerals are leached from the soil. The volume of carbonates decreases. Soluble salts and some metal oxides are washed out from the soil. A considerable compaction of the soil takes place, and its volume decreases, with the porosity decreased by about 8%. In the third phase the soil acquires a heterogeneous structure, as opposed to the original homogeneous structure. The contacts between the clay minerals are changed into physically more stable-phase ones. The clay particles are more aggregated. The silt and sand grains are also reorientated and have frequently direct contacts as opposed to the original covered ones. Even the relations change between the individual structural elements, and the percentages of the individual size fractions of pores and structural elements also change.

4.22. Miao and Wang (1991) Lanzhou, China Miao and Wang cite Feng and Zheng (1982) to establish that there are two types of loess, which they call collapsible and non-collapsible. They consider only collapsible loess. They describe loess in general as a special kind of clay soil with a three-phase system consisting of solid grains, water

C.D.F Rogers et al./Engineering Geology 37 (1994) 83-113

and air. They mention a phase approach similar to that of Alexiev (1969) and Egri (1972) but do not cite these authors. Miao and Wang list ten references, seven of these are to Chinese work, one is to Poston and Stewart (1978) on catastrophe theory, and one is to Mustafaev (1978) on foundations and bases. This is thus essentially a Chinabased paper, a view from Lanzhou. The description of loess is similar to that of Yang (1989) but with interesting terminological differences. The most solid component is the 'powder' particles. The particles with a diameter greater than 10 gm are called 'skeleton framing' (Yang's framework elements) and account for 70-75% of the particles. These are cemented by an additional 10-15% with diameters less than 2 gm. The rest, with diameters 10-2 gm are called 'stuffing' and fill the gaps. Miao and Wang develop a model based on four hypotheses. As a three-phase system, the material composition and the structural assemblage of the loess are so complex that one cannot exactly describe the grain behaviour and the changes of the microstructure in the collapse process. On the other hand, it is not necessary to do so because the main interest of civil engineers concentrates on the macroscopic deformation (i.e., the criterion approach is more important than the structural approach in practical terms). The four hypotheses are: (a) The soil grains are rigid in the deformation process; (b) The process of collapsing deformation is purely mechanical; other physical and chemical reactions are excluded; (c) The water in the system is involved in the deformation process only by way of changing the mechanical property constants of the soil; and (d) The collapsing deformation is due to the stability damage of the 'framed-void' microstructure. In order to study the response of loess under stress a representative sample is considered which should be large enough to contain the detail of the microstructure and small enough so that the average response is representative of the local response of the soil, being an ideal isotropic continuum.

101

Let V, Vs and Vp denote the total volume, solid volume and pore volume of the sample, then:

V=Vp-~--gs

(10)

The volumetric porosity and the solid density are defined as:

~p,,= Vp/ V ~ v = Vs/ V

(11)

Similarly we can define the area porosity and the area density as:

~s=Sp/S

tP~=Ss/S

(12)

Where S, S~ and Sp denote the total area of the area-element, the solid area and the pore area. It is thus easy to prove that: ~bv= ~bs Ov=O~

(13)

Hence no distinction need be made between ~bv and ~b~;~band O will be used, and obviously: ~b+O=l

(14)

The saturation of the soil is:

W= Vw/Vp

(15)

Where Vw denotes the volume of water contained in the voids. The average stress and the average strain can be introduced by: 1

alj = V(R) !alj dv

(16)

1

eq = V(R) !~iJdv

(17)

Where V(R) is the volume of an element body occupying a region R. The formulae:

g,j=~b~+O~j

(19)

are easy to derive, where the superior letters 'p' and 's' mark the relevant quantities belonging to the pore part and the solid part, respectively. Miao and Wang used a simple structure to model the framed-void microstructural element, where the solid grains (the 8:5:2 particles) are modelled by rigid rods [hypothesis a], joined to each other with an angle-stiffness k, which is a function of the moisture content [hypothesis c].

C.D.F. Rogers et aL/Engineering Geology 37 (1994) 83-113

102

The initial state of the model structure before collapsing is determined by the angle a. After this state has had an infinitesimal change described by 0, the total potential energy is: U = ~k[O 2 + ( V2n - 2c0 0] + 4L[a cos 0 - z sin 0 - a]

(20) where: a = V2(al sin ~ + az cos a), r=

V2(O"1 COS ~X--O"2

sin ~)

(21)

and (al, a2) are the principal stresses of a planestress state applied on the structure as in Fig. 3.

5. Particle shape and packing The particles comprising detrital sediments may be roughly divided into blocks, grains and flakes, and the properties of these sediments depend in some part on the properties of the individual particles. Griffiths (1952) suggested a simple relationship: P = f ( s , sh, p, o, etc.)

(22)

Any property of a sediment is a function of particle size (s), shape (sh), packing (p), orientation ctl

a2 (

a2

Fig. 3. The Miao and Wang (1989) microstructural model. Essential a 4-point framework which represents a 'framed void' in the loess structure. The angle stiffness K is a function of the moisture content. (For an explanation of the symbols see text.)

(o) and various other factors. Here we are concerned with the inter-connected properties of shape and packing. Krinsley and Smalley (1973) suggested that small quartz particles in sediments will tend to be blade shapes, more or less as predicted some years earlier on the basis of simple probability theory by Smalley (1966). We now propose that, as a first approximation, the structural unit in an airfall loess deposit be considered as a primary mineral particle (usually quartz) with an axial ratio of 8 : 5 : 2. This really is an ideal particle, in which the ratio is worked out by generating 100 particles (which only requires 300 random numbers) and then assigning them to Zingg classes (the modified Zingg classes as used by Smalley, 1966). In Zingg class IIIm, the blade shape is enormously predominant and the mean particle shape (in terms of fitting into a Zingg box) has an axial ratio of 8 : 5:2 (Rogers and Smalley, 1993). This is a shape generated in an isotropic solid material, and we argue that quartz, with many inactive cleavages, is in fact effectively isotropic (see Scott and Smalley, 1991 ). The structure that collapses in loess hydroconsolidation is an open packing of blade shaped particles with a mode size of around 30 Ixm. The mean axial ratio of the particles is around 8 : 5 : 2 but there is considerable variability about this ratio. The open packing is due to airfall sedimentation, i.e., relatively slow particle arrival, and slow build up of overburden. Contacts may be effectively fixed before effective overburden pressure develops. The disposition of the particles means that the coordination number is low, so there are relatively few particle contacts which consequently have a greater individual importance in maintaining the structure. The interparticle contacts are surely the most important part of the whole loess system, from the hydroconsolidation point of view. It would be useful if we could represent the initial packing, and the final packing, and understand the spatial reconstructions which connect the two boundary conditions. It would be useful if we could begin to model the structures, but this is an elusive and difficult task. Of all the variables in the Griffith (1952) functional equation, packing has perhaps proved the most difficult to handle. The basic reference to particle packings in sedi-

C.D.F. Rogers et al./Engineering Geology 37 (1994) 83-113

ments is the chapter by Allen (1982) and although it provides a fascinating and sophisticated look at ideal packings it exposes the enormous gap between real and ideal packings. Allen has developed the Smalley (1971) system of regular packings, particularly by introducing non-spherical particles, but there seems to be little future for the regular, unit-cell approach to the packing problem. Allen (1982) also considers the random approach to packing. The problem is how to represent the randomness that we believe typifies an in situ clastic sediment. This approach was first attempted by Smalley (1964a) via two types of radial distribution functions - - a density function and a probability function. The density function can be applied, with recognized severe constraints, to real clastic sediments (Smalley, 1964b). This method has been applied to a pebble beach (Smalley, 1964b), to a post-glacial marine quickclay (Smalley, 1978), and in this paper to a very ideal loess. It allows particle environments to be demarcated, and possibly allows a compaction process to be followed. The status of random packing studies is even more unsatisfactory than the status of regular packing studies. The early SEM studies (e.g., Smalley and Cabrera, 1969) hinted at the possibility of relating structure to properties for engineering soils (every material scientist's dream) but the promise was not fulfilled. Clay soil structure proved more complex than expected, and it could not be satisfactorily described. If packing studies are to progress it should initially be concerned with relatively simple structures. We think that airfaU loess might be the best material on which to start a really serious study of particle packing in sediments. An insight into packing parameters would certainly help the studies on collapsibility and subsidence.

6. Bonding and structures One of the most important problems in the scientific study of the hydroconsolidation phenomenon is the description of the particle packing in the loess soil system. The hydroconsolidation event is essentially a change of packing, from an open

103

packing to a much closer packing: it involves a wholesale rearrangement of the modal loess particles. This rearrangement in turn depends to a large extent on the nature of the bonding between particles. Two questions need to be answered: what is the nature of the interparticle bonding, and how does it affect the hydroconsolidation event? The response of the structural bonds to loading and wetting is at the centre of the whole hydroconsolidation phenomenon. Yang (1989) made this point, and discussed the chemistry of the bonding systems. There is a simple physico-chemical model that can be applied to the loess situation, and it is essentially the same as that developed for quickclays, highly sensitive post-glacial clay soils, by Cabrera and Smalley (1973). The quickclay is in many ways very similar to collapsible loess since it is largely composed of blade shaped primary mineral particles, formed by slow-fall sedimentation of single particles, developing a cementation bond at particle contacts, given to sudden failure, and directly connected with the cold phases of the Quaternary period. Denisov (1963), Feda (1966) and Lutenegger (1981) have made the connection and discussed loess as a sensitive soil (see also Smalley 1981 ). Cabrera and Smalley (1973) proposed two simple bond classes: long-range bonds and shortrange bonds. The essential difference between these two categories was that the long-range bond allowed action at a distance and the formation of mobile bonds, while the short-range bond functioned as a contact bond, which lost strength when it was broken. The ideal long-range bond is that found in a high plasticity (PI) clay mineral system, because, for the long-range bond to function in a soil system, electrically charged soil particles are required. The short range bond can operate between two primary mineral particles. It operates between two quartz grains but the weight of the particles masks its effectiveness. If the particle size is reduced the short-range bonds become more effective. They appear to be very similar in concept to the 'atomic' or 'point contact' bonds described by Osipov (1975). His equivalent to the long-range bond is the 'coagulation' bond. We can postulate an ideal loess deposit which consists of a random open packing of blade-shaped

104

C.D.F. Rogers et al./Engineering Geology 37 (1994) 83-113

quartz and feldspar particles, with no added clay or carbonate or cementing minerals. The interparticle contacts would be of the atomic or the short-range type. This structure would be rigid and probably dilatant. There is no reason why it should collapse when loaded and wetted. The quartz-quartz point contact is not particularly weakened by water. The system would probably fail at the bond points, under shear stress, by tensile failure. These hard contacts do occur in quickclays, which require an initial stress to cause failure. The characteristic collapse in loess hydroconsolidation occurs because the bond contacts are modified. The simple quartz-quartz bond tends not to exist in collapsing loess. There are two obvious modifiers, clays and carbonates. These co-exist in many loesses but initially they will be considered separately. Four simple possibilities are considered in Fig. 4: (1) Small clay; a small amount of clay mineral material in the system, concentrated at the bond points of the main structure (introducing factor k in the Miao and Wang (1991) treatment). The main structure is based on that postulated by Uriel and Serrano (1973; Fig. 2) and generated by Smalley (1978: Fig. 6); (2) Small carbonate; here the bond points are augmented by small amounts of carbonate cement to produce a rigid structure, the bond strength is increased but it retains its short range nature; (3) Large clay; the clay begins to fill the large spaces in the basic structure and may form a continuous clay 'phase'; and (4) Large carbonate; the carbonate fills spaces and produces large scale cementation. The response to shear stress of these four ideal systems is shown in Fig. 5, where strength s is plotted against deformation d. Curve 5b shows a classic short range response in which the strength of the original structure is quite high, but it is rigid and when it fails the strength drops rapidly. Curve 5c shows a typical long-range bond response in which the clay minerals have a powerful influence on the system (which, therefore, has a high PI) and deformation does not significantly reduce the strength. The small clay response (curve 5a) is perhaps the most interesting. If the amount of clay

I1

b

e

el

Fig. 4. Four ideal bond systems: basic structure of blade shaped primary mineral particles, plus: (a) a small amount of clay; (b) a small amount of carbonate; (c) a large amount o f clay; and (d) a large amount of carbonates. Initial structures based on Uriel and Serrano (1973; fig. 2).

is small there is still a considerable short-range nature in the system. The clay at the contacts modifies, rather than fundamentally changes, the short range behaviour and the strength falls on disturbance, but not as precipitately as in the small carbonate case. The question then arises as to how these ideal bond systems respond in the hydroconsolidation situation. The small clay model appears to fit in with Handy's (1973) observations on the Iowa loess. He found that collapsing loess had a low clay mineral content, and that when the clay mineral content was greater the loess did not collapse. The small clay system is relatively rigid

C.D.F. Rogers et al./Engineering Geology 37 (1994) 83-113

m'~d Large carbonate

105

cemented with substantial amounts of carbonate will be what Yang (1989) calls 'hydrostable' and will not be expected to subside.

7. Fabric

7.1. A speculative random structural collapse model

i Z' Z ........... % I

~, a Small clay

A soil structure model can be constructed using a very simple Monte Carlo method. This method was devised to produce a two-dimensional picture of the structure of a post-glacial clay soil (Smalley, 1978) and can be adapted to produce the structure of airfall loess. Figure 6 shows a completed structure; each particle is assigned a position in the 100 × 100 field by the selection of random coordinates, and a random positional angle is allocated; overlapping particles are rejected. Increasing frequency of overlaps means that it is difficult to

'd' deformation

Fig. 5. Strength-deformation relationships for four ideal bond systems. Carbonate bonded systems are dominated by shortrange bonds, strength drops rapidly after small deformation. Large clay system has long-range bonds which allow deformation without major loss of strength.

when dry but when wetted bond mobility occurs. The major bond points have to move in such a way that structural collapse occurs. In the large clay system plastic deformation might occur but structural collapse does not (the situation in many New Zealand loesses). A compromise is thus required. The system overall has to behave initially as a short range bonded open structure, in which the initial bonds have to be fixed and rigid but latterly local long range behaviour is required as the clay modified bond points move under the influence of water and stress. The initial shortrange nature may be emphasized by some small carbonate in the system giving additional rigidity. The carbonate responses, 5b and 5d, on the s - d diagram are not significantly different. The larger amount gives a higher initial strength and rigidity and can cope with a little deformation, but once failure occurs strength drops very rapidly. Loess

Fig. 6. Random placement model of an open, airfall, loess structure in its original state. Blade shaped primary mineral particles with interparticle bonding. Particle 10 is shown in association with test frame used for determining radial distributions. Mode coordination number is 3 (i.e., most particles have three bonds). Overlaps are marked 'eo' on reintroduction at opposite side of frame (after Smalley, 1978).

106

C.D.F. Rogers et al./Engineering Geology 37 (1994) 83-113

produce a dense packing, but the method is suitable for loosely packed materials such as loess, or quickclays. In Fig. 6 the particles are represented as flatplates with axial ratios of 8 : 5 : 2. A particle of this basic shape is believed to be a typical loess particle, a rather flatter shape than might be expected. The mode shape was deduced by generating 100 random particles, and then finding the modal axial ratio within the group of blade shapes. The compaction of the system is achieved by shrinking one axis and redraining, allowing for contacts to realign particles. This was done for contractions of 15, 30 and 45%. Figure 7 shows the system at the effective limit of compaction, at 45% linear reduction. It is possible to show how the particle environments change as the compaction process is proceeding, but only in a relatively imprecise manner. In Fig. 6 particle 10 has its immediate environment divided into three zones. The density of packing in each zone is determined by counting the number of particle centres in the zone, and then dividing by a factor related to zone size. A simple radial distribution can be produced (Fig. 8) which shows how the environment around particles varies with compaction. A loess collapse is about 15% linear compaction and at this stage there is no large scale rearrangement of the particles. The radial distributions change somewhat, for particles 8 and 10 the immediate neighbourhood becomes more crowded, and the inner ( I ) peak in the distribution curve becomes more marked. The low density cases (see

PF 0 0.8] 0.6 8

15

30

45

IMO

IMO

o

0.4

0.2 d

I0

, 10

13

, 13

18

, 18

.J,,, 2O

0

15 Compactlon

[]~._

,

20

,~,,

,

21

3O %

Fig. 8. Radial distribution (density) results for particles 8, 10, 13, 18, 20 and 21. Compactions of 0, 30 and 45%. In all cases the ordinate scale is the packing factor and the abscissa represents the distance from the reference particle. The three histogram columns refer to the inner (I), middle (M) and outer (O) zones of the test frame shown in Fig. 6.

Fig. 7. Random placement structural model after 45% linear collapse; the basic structure becomes more or less completely ordered. The level of compaction is very unrealistic, but provides a limit for radial distribution (see Fig. 8).

particle 18) may stay as low density zones at 15% compaction, but by 30% compaction the inner ( I ) values are increasing rapidly. At the effective limit of compaction (45%) all the radial distributions

C. D. F. Rogers et al. /Engineering Geology 37 (1994) 83-113

show very high inner values, and the structure is tightly packed, as shown in Fig. 7. The process shown in Fig. 8 must only be seen as the most diagrammatic of representations but it does suggest that as loess consolidation proceeds what is actually happening is the fairly modest readjustment of the packing of a set of very blade shaped particles: no really large scale structural arrangements occurs. At 45% a wholly new structure is formed, with wholly new constraints. Figure 6 looks remarkably like the Uriel and Serrano (1973) structure, and this is possibly a reasonable two-dimensional representation of a collapsing soil. We assume that the Uriel and Serrano structure is basically an arbitrary construction. The structure shown in Fig. 6 was produced in a totally random manner, and represents a low density random packing. The radial distribution concept is discussed by Smalley (1964a,b, 1978).

7.2. Change of state and sensitivity The above arguments can raise the question of how like a sensitive soil is a collapsing loess? A very sensitive soil is essentially a brittle system with a very low plasticity (PI) and which changes state fairly rapidly: short range bonds predominate in a very sensitive soil. The basic concept of sensitivity needs to be discussed to see if it can be stretched to cover both the quickclays and the collapsing loesses. Cabrera and Smalley (1973) discussed the concept of sensitivity with respect to the very sensitive soils (St > 50; the so-called quickclays). The first investigations of very sensitive clays were carried out in the Scandinavian countries (see Flodin and Broms, 1981) and in Russia (see Ter-Stephanian, 1965). They established the basic concept of a soil whose strength properties are sensitive to disturbance. The question then arises as to whether loading and wetting count as disturbance? It clearly does and thus there are similarities at this level. Perhaps a more satisfactory comparison might be in terms of structural collapse. In a quickclay the soil structure collapses, from a metastable position to a more stable configuration, and the same phenomenon occurs in a collapsing loess. Both quickclays and collapsing loesses are asso-

107

ciated with the cold phases of the Quaternary period; particle production is subject to glacial or cold-condition control. In both situations there is slow-fall single particle sedimentation which results in an open structure. The inter-particle bonds are essentially short range, but in the case of collapsing loess the short range bond system is subtly modified to offer a short-term and localised long-range nature. The open structure is characteristic of short-range bonded systems; the rigid bonds maintain the open structure, which allows the collapse. Bond augmentation in quickclays is by cementation, which makes the structure more rigid and the sensitivity more pronounced; bond augmentation in collapsing loess is by concentration of clay mineral material at the contact points. In dry conditions the clay augmented bond is rigid, but on wetting the clay softens and a controlled bond mobility is possible. The loess collapse can be rapid, but does not reach the catastrophic speed sometimes seen in quickclay failure. Too much clay would prevent the efficient structural strength, too little clay would not allow the necessary bond mobility, the right, relatively small amount of clay allows the (relatively rapid) collapse observed in collapsing loess, but also allows long term settlements.

8. Concluding discussion 8.1. Approaches to hydroconsolidation There are various ways to approach the problem of hydroconsolidation. There is an obvious distinction into the engineering approach, the detection of potentially dangerous subsiding grounds and their treatment, and the scientific approach, an attempt to understand how collapsible loess is formed and what is happening during the structural collapse process. But within these two major categories more specialised approaches can be detected and defined, and this is attempted here. Bonding: The nature and behaviour of the interparticle bond is basic to hydroconsolidation, see Yang (1989), and see Osipov (1975) for general considerations.

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C D . F Rogers et al./Engineering Geology 37 (1994) 83-113

Catastrophe theory: Only one application that we can find, in outline, by Miao and Wang (1991). Chemistry: The chemistry of bond material has been considered by Yang (1989). Some of the most interesting collapse-related chemistry deals with stabilisation. Aniline-furfural was used by Sheeler et al. (1957), and various sodium silicates by Rzhanitsyn (1978) and Sokolovich (1976). Criteria for subsidence: The practical approach to subsidence problems with an emphasis on equations and rules for designers. See Feda (1966) for a good discussion, also Malinowski (1971), Lobdell ( 1981 ) and Northey's (1969) contribution to the 6th ISSMFE Conference. Cures and correctives: There are many approaches to the problem of construction on collapsing loess; Abelev (1975) favours dynamic compaction; see Smalley (1980) for a very general list, see also Evstatiev (1988). Formation of subsiding soils: An airfall loess with an open structure is an obvious subsiding system. The question arises of whether subsiding loesses can be formed by any other means has been adresssed by many Russian workers who think it can. See Kriger (1986), Trofimov (1990a,b) and Minervin (1982). Global distribution: Collapsing loess is well known in the Ukraine, in Iowa, and in parts of China, but as loess investigation proceeds examples are turning up in unexpected places: Afghanistan (Waheed Hussani and Saboor Rahim, 1983), Thailand (Phien-wej et al., 1992), etc. Global distribution studies need more attention, and in particluar a detailed world loess distribution map is required although there is now available a revised Kriger map (Kriger, 1990). Mineralogy: Bond mineralogy is listed by Tan (1988) and we have a fairly clear view of clastic mineralogy (e.g., see Scheidig 1934). We need definitive knowledge of the mineralogy of the actual bond material. Packing of major particles: The change in packing is at the heart of the loess collapse problem - but packing is a parameter which is still impossible to define. See Allen (1982, p. 137) for a good recent review of packing. Phase movements: Egri ( 1971 ) related subsidence to phase movements at Donaujvaros.

Pore structure and distribution: Various authors comment on this in relation to space for the structure to collapse into. See GrabowskaOslzewska (1988) in particular. Prevention and Improvement: Evstatiev (1988) has surveyed and reviewed methods of improving collapsing loess. This is a useful and comprehensive review, and certainly one of the best recent review in the field of loess geotechnology. The current paper can be seen as a complement to the Evstatiev paper. Rheology: Browzin (1981) took a specifically rheological approach, see also Nuyens and Huergo (1973). As a Sensitive soft: The classic paper is by Feda (1966), but see also Lutenegger (1981) and Denisov (1963). Thermodynamics: The only specifically thermodynamical approach that we know of is due to Volyanik (1989) - - see digest section. Topology: The collapse of the loess soil structure may change the connectivity of the soil system and thus bring about topological changes. The idea has been applied to sand (Smalley 1967), but not yet to loess. Types of structural collapse: Structural collapse in a straight engineering sense has been studied by Miao and Wang (1991). 8.2. Words

Lobdell (1981) called his study of the loess in Washington and Idaho the 'hydroconsolidation potential of Palouse loess', there was no discussion of terms, the implication being that 'hydroconsolidation' is a commonly used word. It may be useful and perhaps advisable to coin a brand new term, relating to nothing else, to describe loess collapse. Assalay (pers. commun., 1993) has suggested the term 'hydrocolsolidation' as relating to already used terms but offering something new. Clemence and Finbarr (1981) wrote on 'Design considerations for collapsible soils'. They defined metastable or collapsible soils as any unsaturated soil that goes through a radical rearrangement of particles and great loss of volume upon wetting with or without additional loading. They did not use the terms hydroconsolidation or hydrocompaction.

CD.F. Rogers et al./Engineering Geology 37 (1994) 83-113

A metastable loess soil undergoes structural collapse by way of a hydroconsolidation mechanism and this leads to subsidence and compaction. Is it valid to say this? Are the terms uniquely definable? Fookes and Best (1969) set a good example by providing precise definitions: Consolidation: is used in the generally accepted soils engineering sense to define a volume change with loss of pore fluid and decrease in void ratio. (cf. the general definition, from the Oxford English Dictionary 2nd ed. - - the action of making solid, or of forming into a solid or compact mass). Settlement: is defined as the decrease in height in an oedometer test specimen or sinking through ground failure of a building or piece of land. There are two subsections for settlement: Collapse settlement occurs on flooding very rapidly with 95% of the settlement completed within 10 min; and Subsidence settlement on flooding occurs more gradually over a period in excess of 10 min, and usually hours. In this paper the term 'hydroconsolidation' has been favoured, even though it is a rather long and clumsy word. The current authors wish is to describe the phenomenon under discussion, the process that occurs when a collapsible, metastable loess is loaded and wetted. A study of hydroconsolidation includes methods of detecting it, of measuring it, of describing the process mechanism at several levels of precision, of coping with it, of preventing it, and of accounting for the presence of the soils in which it occurs.

8.3. Commentary Grabowska-Olszewska (1988) commented, with respect to subsidence and hydroconsolidation, that 'existing literature on that subject is considerable but its ordering appears rather difficult'. She rather understates the problem, since the ordering is very difficult to define. For example, Trofimov (1990a) writes of 'cryo-eluvial decompaction' which leads to 'epigenetic collapsibility', which workers in other research groups may find difficult to relate to. One aspect of the problem might focus on Trofimov, or one could use Trofimov as an example of linguistic and conceptual confusion,

109

Trofimov (1990a) writes that 'collapsibility is a specific property of loess deposits of different geneses (aeolian, slopewash, alluvial-fan, alluvial, lacustrine-alluvial, etc.)'. How many other workers, in other parts of the world will accept the idea of a loess origin other than aeolian? Are the Russians following some totally false trail, or have the rest of us missed something important, and is there a dimension to the problem that is evading us? Trofimov has been publishing vigorously since at least 1980, but his work does not seem to be cited outside Russia. How to order the literature is the most important task of the review writer. In the DLT table one variable is fixed: the E entry in the T column. The topic is engineering, since hydroconsolidation is a topic primarily for engineering geologists and geotechnical engineers. But all topics could be relevant, and are certainly worthy of study, and there is certainly no restriction on language. Northey's (1969) review was rather short of Russian material, whereas the present review is intended to offer some emphasis on the Russian language literature and to indicate the important events and advances since 1969. What are the real ground problems? They need to be separated, and separately considered, and very clearly stated. If loess is loaded and wetted, subsidence may occur due to the collapse of the soil structure, a phenomenon termed herein hydroconsolidation. The mechanism of this process has been widely investigated, and it is one of the problems that has been tackled all over the world, by Sajgalik (1990), Yang (1989), Miao and Wang (1991) and Handy (1973). The basic event in the hydroconsolidation process is a rearrangement of particles in the loess, the original packing is transformed into a new closer packing. There is a need for clearer definitions of the typical loess particle, of the initial packing, of the final packing, and of the transition process. These are all what might be called post-depositional problems, occurring in time well after the loess has been formed. There is another class of problems, considered by Gao (1988), but largely the province of Russian workers, and that is the formation of a material that has the capacity to subside. There is room here for much linguistic confusion: for example

110

CD.E Rogers et al./Engineering Geology 37 (1994) 83 113

does 'the formation of subsidence' mean the formation of a material that can subside or the actual subsidence operation? Interpretation here can be very difficult. It is suggested herein that the history of Russian theories of loess formation is such that many alternatives to aeolian deposition are embedded in the intellectual environment, and that these emerge in the subsidence literature. Trofimov (1990a,b) in particular appears to suggest that there are many ways for subsidence potentiality to arise. A way forward into the region of 'epigenetic' formation of collapsible soil structures may be via the study of systems other than loess. There is a modest literature on the collapsing sands of southern Africa, and these also appear to be residual soils. Clemence and Finbarr (1981) certainly offered three categories of collapsing soil, and while loess may be the most widespread and important, the others should not be neglected, particularly if their study can throw some light on the mechanism of structural collapse and subsidence in loess. The major task facing reviewers of the hydroconsolidation literature is to refine and analyse it and to summarise the real findings, to reach a consensus. This paper represents a step towards that aim; we want to identify major groups of investigators and indicate the style and nature of their investigation; also it would be useful, if, in the light of what they have written, we could identify problems, and research strategies.

(6) More study of the packing parameter in general; (7) A better general appreciation of the many strangenesses of Quaternary geology that affect engineers; put the Pleistocene in perspective; and (8) Find other collapse sites; e.g., the 'malos' soils in North Libya may provide a simple system, and very simple natural systems would be very useful. We also want to suggest that a way forward for students of hydroconsolidation might be to concentrate on the actions at the single particle level. We list and comment on some studies of particle nature, packing and interparticle bonding - - this surely needs to be thoroughly investigated before the fundamental mechanisms of loess collapse can be understood. Establishing criteria for collapse has not proved too difficult, but discovering the mechnisms is more demanding. Also there are still problems relating to the formation of the open structure; many investigators accept that the aeolian deposition of loess material is sufficient to form the open packing structure, but a few hydroconsolidation specialists still feel that other mechanisms operate. Most of this latter group have been influenced by the Berg in situ approach to loess formation and tend to be concentrated in Russia and neighbouring regions. Some extremely basic problems remain.

Identify problems:

Acknowledgements

(1) Rules for avoiding subsiding loess; (2) Explanations of mechanisms of collapse: (a) key factors, (b) actual collapse dynamics; (3) Discover the origin of the open structures; and (4) Treatment to prevent collapse problems.

Identify research strategies: (1) Devise a model for the open soil structure; (2) Devise a model for the open soil properties; (3) Decide on packing units, and set an accepted standard; (4) Find ways to simulate the structure: (a) by direct physical means, and (b) by computer simulations; (5) Invent new mechanisms to study the material and the phenomenon - - the standard soil mechanics tests no longer give adequate research results;

The authors wish to thank Barbara Hughes, Ruth Pollington and Stephanie Pilkington for their help in the preparation of this review. This work is part of a joint Leicester-Loughborough LAMBERT project (Loess And Modern Bibliography: Engineering Research Topics) and has been carried out in association with the Geotechnical Working Group of the INQUA Loess Commission.

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