Quick Soils and Flow Movements in Landslides Ackermann, E

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Quick Soils and Flow Movements in Landslides Ackermann, E.

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NATIONAL RESEARCH COUNCIL OF CANADA

/ TECHNICAL TRANSLATION 839

QUICK SOILS AND FLOW MOVEMENTS IN LANDSLIDES

ERNST ACKERMANN

FROM Z. OEUT. GEOL. GES. 100: 427 - 466. 1948

TRANSLATED BY

D. A. SINCLAIR

THIS IS THE FIFTY· FOURTH OF THE SERIES OF TRANSLATIONS PREPARED FOR THE DIVISION OF BUILDING RESEARCH

OTTAWA

1959 NATIONAL RESEARCH COUNCIL OF CANADA

Techn1cal Translatlon 839

•

Tltle: Qu1ck s01ls and flow movements 1n landslides • (Qu1ck erden und Fllessbewegelngen bel Erdrutschen)

Author: Ernst Ackermann

Reference: Zeitschr1ft der deutschen geolog1schen Gese11sohaft, 100: 427-466, 1948

Translator: D.A. S1nc1a1r, 'Translat10ns Sect10n, N.R.C. Llbrary PREFACE

Landslides and mud flows are a major problem in • certain areas of Canada. The magnitude of the problem was illustrated by the Nicolet Landslide of November 1955 in which three people lost their lives. Recent investigations of the Nicolet and other landslides have shown that much is to be learned about the properties and behaviour of certain of our fine-grained sediments. This translation of a work by Dr. Ernst Ackermann of ｾｴｴｩｮｧ･ｮ cites numerous examples of Scandanavian land• slides which are similar in many'respects to the land• slides that occur in the Eastern marine clays. This translation is a companion to an earlier one by the same author "Thixotropy and Flow Properties of Fine-Grained Soils" (N.R.C. Technical Translation 150) in which Dr. Ackermann ｾ･ｦｩｮ･､ what he means by thixo• tropy and gave a method of determining the "stiffening limit" of a soil. In this work the concepts of thixo• tropy are applied occurrences of landslides which the author has studied. The Division of Building Research is grateful to Mr. D.A. Sinclair for this translation which, it is hoped, will prove useful to those engaged on studies on Canadian landslides.

Ottawa, H.F. Legget, September 1959 Director TABLE OF CONTENTS

Thixotropy and related concepts ••••..•••••••••••••••••.••••••• 3

Thixotropy. ''Ii ••••••••••••••••••••• _,_ •••••••••••••••••••••• •• 3 The qu Lck consistency••••••••••••••.•••..••••••••.•••••••• 5 Flow-prone deposits•••••••••••••.••••••••••••••••••••••••• 6 Structural collapse and strength changes in thixotropic and non-thixotropic solIs•••••••••••••.•••••••••••• 7 Flowing in the liquid consistency range••••••••••••••••••• 9 Flowing in the quick consistency range•••••••••••••••••••• 9 Flowing in the plastic consistency range•••••••••••••••••• 9 Flowing deformation under high pressure•••••••••••••••••••12 • Earth slides and earth flows ••••••••••••••••••••••••••••"••12 Motions involving thixotropic liquefaction and re• solidification of qulck so11s•••••••••••••••.••••.••••••••••••13 Sliding motions•••••••••••••••••.••••.••••.••.•••..•. lI!, ••••13 • The flow movements of qUlck clays which become thickly liqu1d••••••••••••••••••••••••.•.•.•..••••..••••••14 Flow movements of qulck clays whlch become thinly liqu1d••.••••••••••••••••••••••. ,_ ••.....••••••.•••••••••••16 Sub-aquatlc flow movements •••••••••••••••••.•••••..•••••••18 General descrlptlon of thlxotroplc flow movements •••••••••19 Combined slide and flow movements •••••••••••••••••••••••••24 The occurrence and initlation of thlxotropic flow movernents •••••••••••••••••••••••••••••••••••••••••••••••••27 Fast flow movements of non-coheslve masses ••••••••••••••••••••30 Slow flow movements of mucky 80ils•••••••••••••••••••.••.•••••31 Flow movements of plastlc solI masses•••••••••••••••••••.••••••33 Recent spread of thixotropically lnfluenced flow movements •••••••••••••••••••••••••••••••••••••••••••••••••••••38 Summary•••••••••••••••••••••••••••••••••••••••••••••••••••••••44 L1 terature•••••••••••.•••••••.••••.••.••••••.••••••••••.••••••47 Tables•••••••••••••.•••••.•...... ••.•.•.•.••••••••••••••••••••49 Figures••••••••••••••••..••••••••...••••••••••••••••••••••••••51

.. QUICK SOILS AND FLOW MOVEMENTS IN LANDSLIDES

The soil displacements referred to generally as "landslides" can be classified according to the nature of the motion into sliding, flowing and falling phenomena. With few exceptions, insufficient attention has hitherto been paid to the part played by the flow movements. Generally speaking only the slow movements of this type are well known, e.g. sdlifluc• tion (Andersson 1906) and creeping debris flows (Albert Heim 1932). Rapid motions such as "sand falls" or "subsidences" (von Terzaghi 1925) have received little attention. Furthermore, consideration of such rapid movements has been restricted to non-cohesive, sandy solIs beoause of the general belief that motions of the quick sand variety occur only in sands, not in clays (Glossop and Skempton 1945) • This belief has remained widespread, although for some decades contrary observations have been made in lakes of Switzerland, in various coastal regions of Norway and recently also in Canada. Because of ourrent vlews on landslides definite flow processes have been repeatedly explained as slides, or - where the two types of motion were combined, as is frequently the case - the flow com• ponent has been neglected and its importance in the overall process overlooked. Thus the correctly observed and described flow move• ments involved in the subsidences occurring along the shores of Lake Zug provided the basis for Arnold Heim's (1908) representation of underwater slides. The terrible flow catastrophe which devast• ated the Vaerdal, east of Trondheim, in 1893 was explained on the assumption (which was correct in other cases) that rain water run• If ning down into desiocation cracks had softened the clay and caused "sliding". In the investigations of Swedish railway disasters several flows were discovered as well as slides, but no importance was attached to them as far as the mass displacement processes were concerned (Statens JArnvAgars etc., 1922). On the other hand the full expositions by Norwegian specialists, such as G. Holmsen

...... __J -3-

(1929-1946), of more recent flow movements have not unfortunately become wldely known, Even thls rapld survey of the pertinent literature lnd1cates that our general ldeas on landslldes need revlflion in quite a number of cases. From the numerous slides and flows observed by the author in Norway, and on the basls of the discovery, resulting from a new method of investigation (Ackermann 1948), that not only

qUick sands, but also quick clays can suddenly become liquid,, an effort will be made here to determine and define the nature and importance of the flow movements relative to the known sliding move• ments. In doing so it is absolutely essent1al to consider the effect of the change-of-state phenomenon known as thixotropy. The importance of these lnvestigations, apart from their value in geologlcal research, lies in the posSlbl1ity of re-examlning our ldeas about the char3cter of certain sUbaquatlc soil dlsplacements wlth the aid of dry-land observatlons and thereby explalning the causes underlying certain petrological phenomena involving sedlments.

Thlxotropy and Related Concepts

Thlxotropy The term thixotropy refers to reverslble changes of vlscosity occurrlng ln concentrated suspensions of very small partlcles solely as a result of mechanlcal forces·. The phenomenon of thixotropy appears very strlkingly in many gels and muds made from clay or other dlspersed systems. For example, an apparently solld clay mud can be made so liquid by belng shaken ln a glass tube, that lt can be poured out of the • tube. Thls same mud, whlch is so thlckly liquid durlng the shaklng,

• After careful conslderation lt was declded not to undertake an exact colloid-chemlcal explanation of thixotropy here. Reference ls therefore made to the pertinent literature (Freundllch 1935, Eucken 1944, Jlrgensens-Straumanes 1949). The factors of lmportance from the mlneraloglcal polnt of vlew were lnvestlgated by H. Winkler, 1938• I ...... J -4- becomes sufficiently solid aga1n after be1ng allowed to stand for a certa1n length of t1me that it no longer flows even when the tube is 1nverted. Thixotropy is an interfac1al phenomenon fam1liar to colloid chem1stry. It was discovered first in suspens10ns, but also occurs in clayey pastes and l1quids. In a solid-11qu1d thixotropic system the solid particles are not in direct contact with each other but are surrounded by f1lms of water which separate them. In the state of rest (the gel state), therefore, the particles can bu1ld up only a very loose structure (a honeycomb or house-of-cards structure) in which the forces acting between the particles are very small. This framework 1s so sensit1ve that only a small amount of energy is needed to bring about its collapse. It is the collapse of the structure which initiates the liquefaction or slurry state. When ｾｨ･ structure is d1sturbed the particles break away from the loose gel state, swirl about at random 1n the d1spersion medium wh1le in the slurry state and then gradually find their way back again to an orderly interarrangement. It is characteristic of th1xotropy, therefore, that after the mechanical stress. is removed the loose structure is reformed again during a certain period of rest. The solid1ty increases. Th1s radical change of state from quast-solid to liqUid and back again, which on discovery was called thixotropy, will be our chief concern in the present work. The process inclUding its rest1ng states, can conveniently be divided up into the follow1ng stages: Und1sturbed deposit, quasi-so11d; Sudden collapse of a loose framework; Temporary l1qu1d state; Gradual reso11d1f1cat10n; Quas1-sol1d state. Needless to say, there are no distinct boundar1esbetween the d1fferent stages, but only trans1tion zones. The transition processes vary widely 1n 1ntensity and duration, depending on the kind of soil and 1ts consistency. The f1rst two -5-

processes (structural collapse and liquefaction) can also occur alone, i.e., without being followed by the structural re-arrange• ment which is characteristic of thixotropy, and by the resolidifi• cation associated with it. There are other flow processes which occur in sediment formations and which do not have the character of thixotropy. In such cases, any solidification which occurs after liquefaction can only come about through the loss of pore water. As already indicated, thixotropy is found not only in artifi• cial suspensions, but also in natural deposits, where it again • appears as a change of viscosity which is capable of being repeat• ed indefinitely. I shall therefore refer to it as an alternate strength prOcess. In nature (slopes of terraces, pit workings, hydraulic fills and excavations) thixotropic liquefaction often comes about only as a result of comparatively strong forces of a kinetic nature, e.g. earthquakes, explosions, rail-transmitted shocks from clearance and supply trains, vibrations from track-shifting machines, ex• cavators, pile drivers, etc. However, as will be shown later, hydrodynamic pressure gradients can also lead to a thixotropic structural collapse. The Quick Consistency (cf. Fig. 4) Let us consider the changes which take place in a soil with increasing water content. From the dry or solid state the solI first acquires plastic consistency, limited by the plastic limit on the one hand and the liquid limit on the other. However, pro• longed liquid consistency does not begin with the latter, as assumed by Atterberg (1911). On the contrary, between the plastic and liquid form we find the so-called "quick" consistency· • (Ackermann 1950), in which the soil is quasi-solid and becomes temporarily liquid under mechanical stress. A temporary liquefac• tion followed by resolidification to the quasi-solid state takes

• still referred to as "thlxotropic consistency" in 1948.

______ｾａＮ "'I -6-

place within a period of rest whose duration (minutes to weeks) in different soils for equal water contents depends on the degree of thixotropy. The more pronounced this is, and the less clay the soil contRins, the longer this resolidification takes and the greater is the tendency to flow. The qUick consistency 1s separated from the (permanently) liquid consistency by the stiffening limit·. The liquid consistency is not reached, therefore, when the wetting (or other changes in the soil of similar effect) has so far exceeded the water content defining the liquid limit that it is even ｧｲ･｡ｴｾｲ than the water content of the stiffening limit. Only then does the system mineral substance + water remain permanently liquid. We must distinguish between the phenomenon of thixotropy in its crudest form, namely the alternation solid-liquid-solid, as it Occurs in the qUick consistency, and thixotropy as a property which is less strikingly characteristic of the plastic and liquid con• sistencies. Flow-Prone Deposits Examination of numerOus clay samples from Norway has shown that thixotropy is not confined to extremely fine-grained soils with colloid grain sizes, such as fine and coarse clays, but also Occurs in comparatively coarse soils, e.g. sands of low clay con• tent (Ackermann 1948). Certain calcareous slime and mud deposits (Boswell 1948) such as bog limes and silty sands with admixtures of organic substances (von Moos 1945) also showed thixotropy. Since the thixotropy decreases and dies out with decreasing clay content, its presence must be demonstrated in each individual case • especially with fine sands of low clay content - by suitable methods (lim1ting value, stiffening lim1t). However, for most soils which • conta1n clay and are suff1ciently fine-grained the presence of th1xotropic properties can quite generally be presumed on the basis of Boswell's results (1948) with widely different soils from all parts of the earth•

• See N.R.C. TT-150. -7-

If these soils contain sufficient moisture, electrolyte, organic substance, etc., so that they appear quasi-solid (apparent• ly stable) in the state of rest, but become fluid under mechanical stress without changing their composition due to structural collapse, and in the course of a certain standing time pass through the cycle quasi-solid - fluid - quasi-solid, they are referred to as quick solIs or soils in the qUick consistency state. \fuile thixotropy can be present in very different, ｳ｡ｮｾｹＬ clayey and limy fine sediments, quick consistency is found principally in the weakly thixotropic soils of low clay content. , The commonest quick sOils, therefore, belong to the silt group and other soils from the region of transition between the fine sands and the coarse clays. It should be noted, however, that not all flow-prone soils, i.e., soils which I'nay suddenly become fluid under mechanical stress, belong to the quick soils with thixotropic properties. As already related in 1948, there are transitions from the genuine ｱｵｾ｣ｫ sands of low clay content which are slightly thixotropic, to the ｣ｬｾｹｬ･ｳｾＬ non-thixotropic, but nevertheless flow-prone running sands· (Fig. 2). Inasmuch as flow-proneness i s associ at.ed with loose bedding, it can also occur in cohes i.onLes s soils, e.g. fine-grained sands which are not thixotropic and thus do not exhibit thixotropic resolldlfication. The flow-prone sands which have been studied in some regions consist of non-thixotropic running sands (Freundlich and JUliusburger 1935), and in other cases of slightly thixotropic qUick sands (Ackermann 1948, Tables No.1 - 3). In every case it is necessary first to decide, by investigation, to which of the two groups a given sand belongs. Structural Collapse and Strength Changes in Thixotropic and Non-Thixotropic Soils (cf. Fig. 3)

1# The flow movements of sediments are initiated by collapse of the loose bedding structure, ao von Terzaghi had already pointed

• Translator's note: The German word is "Schwimmsand", normally translated as "quicksand". As defined here, however, what is popularly known as "quicksand" would not be classified among the "quick soils". It was therefore deemed advisable to reserve "quick sand" to translate the German "Quicksand", meaning somewhat thixotropic and somewbat clayey sand. -r

-8- I!

out as early as 1926 in his explanation of settlement flows. The changes of strength initiated by structural collapses are different for non-thixotropic soils. The particles of loosely bedded, coarsely clastic (hence non• thixotropic) sediments (e.g. running sand; debris tending towards mud flow) in the event of structural collapse give off water after liquefaction and then assume a more compact form. They become solid. However, if the pore water can be expelled only in small quantities, or not at all, as in the case of certain limey Rnd other very fine-grained, and hence more or less impermeable sOils, • and if the soil is non-thixotropic the particles cannot form a more • solid structure. After the structural collapse, therefore, the soil remains weak. The thixotropic soils, on the o'!;her hand, have acquired com• paratively high strength values during the long geological time they have lain undisturbed, despite the retention of their loose structure. This, strength value will be referred to here as the

) "deposition strength" (H 3 and is a multiple of the value after

structural collapse (H 1 ). When the deposition structure is dis• turbed the strength falls to a fraction of its previous value (from

H to H ). Immediately after termination of the disturbance a cer• 3 1 tain rearrangement of the loose structure begins, and hence there

is a small increase of strength (H -+ H in Fig. 3). In the event 1 t of repeated structural disturbances the strength falls in every

case to its minimum H1 and then rises again as before. During a comparatively long period of rest the "thixotropic end value" Ht is gradually approached (Ackermann ,1948). Thus a small part of the deposition strength 1s recovered , during solidification by thixotropic structural arrangement. How• ever, most of the loss of strength is non-reversible, i.e., is due to non-thixotropic causes. This is true even of clays with high thixotropy values. The non-reversible strength-loss component in• creases with decreasing thixotropy until finally, in the non• thixotropic soils, it becomes the sole factor. The loss of strength'also varies with the consistency. It is greater for the qUick clays, which show a very radical change of

IT -9-

strength (but generally a low degree of thixotropy), than for clays with plastic consistency. The latter have a ｨｩｴｾ･ｲ thixotropic end strength, which, however, depends not. on the deposition strength, but on the ｳｴｲ･ｮｾｴｨ in the disturbed state. Flowing in the Liquid Consistency Range If a fine-grained, thixotropic sediment is in a state of pasty• liquid consistency, the paste will become more mobile as a result of stirring. As G. Winkler (1938) has shown, the temporary decrease of viscosity occurring in permanently liquid suspensions whe'n they are vigorously stirred is also a thixotropic phenomenon. " Flowing in the Quick Consistency Range (cf. Fig. 4) The term "thixotropic flow" will be used here to denote the flow movements of qUick soils (i.e. loose rocks in the quick con• sistency state) during temporary (thixotrOPic) liquefaction into thickly 1iquid-to-viscous pastes. In this state the individual particles are separated Rnd move turbUlently. Thixotropic flow can take place as a result of the weight of the soil itself. In the case of qUick soils which become thickly liquid it can be started by even a small application 'of energy, whereas quick clays approach• ing a viscous state only become fluid under the action of somewhat stronger kinetic forces (e.g. repeated shocks and vibrations). A necessary condition of flow, of course, is that the quick 80i1s are free to move either laterally or upwards. As stated previously, the flowing takes place under constant water content and concludes with the gradual resolidification of the liquefied soil paste into a quasi-solid or apparently stable soil. Flowing in the Plastic Consistency Range The viscosity change from quasi-solid to liquid and back again • which is characteristic of qUick consistency is only a particularly strong form of thixotropy. It is also present as a property of the soil when the latter is not in the qUick consistency form but is, for example, plastic or liquid. In both these consistency regions the phenomenon of thixotropy is expressed merely in a transient decrease of internal viscosity. For example, in the case of a structural disturbance due to remoulding a plastic clay becomes -10-

softer and its strength is ､ｾ｣ｲ･｡ｳ･､Ｎ The strength increases again, hovrever-, when the remoulding action stops. This strength increase in the state of rest is a characteristic criterion of thixotropy. Only when the soil contains an amount of water beyond that corresponding to the liquid limit (quick consistency state) does the thixotropic viscosity variation reach tte marked alternation between quasi-solid and liquid behaviour. But if the soil showing thixotropic properties has a water content below the liquid limit (plastic consistency) then the ｲｾｶ･ｲｳｩ｢ｬ･ alternations of strength II' are accomplished within the limits of the plastic state. There is • no change of aggregate state. In plastic clays, therefore, the effect of thixotropy is less noticeable. In addition to this there is the fact that even in plastic clays the differences in strength of disturbed and undisturbed clays, first demonstrated in soil mechanics by A. Casagrande (1932), are due for the most part to the irreversible destruction of the deposition strength, so that only part of the strength loss due to structural disturbances can be compensated for by thixotropic resolidificatlon. In structural disturbances of quick clays the viscosity drops to about 1/50 - 1/300 of the original deposition strength. In the case of clays in the plastic consistency range the strength is reduced, from the deposition state to the disturbed state, to 1/10 - 1/50 of the original value. Here the decrease is less, but the increase of strength from the disturbed state to the thixo• tropic end strength is greater than for qUick clays (cf. Fig. 3). Important from the standpoint of soil movements is the fact that the strength reduction under mechanical action is associated with a decrease of internal friction (Hvorslev 1937). The move• • ments of plastic soil masses are thereby facilitated. The de• crease of friction during motion is a thixotropic effect. The im• portance of thixotropy for the movement of plastic soil masses lies chiefly in the way in which it facilitates the motion. With the aid of thixotropy conditions the motion itself creates condi• tions which in turn facilitate further motion. -11-

Where ground movements occur as flowing processes it is necessary to distinf,Uish between those flows which take place as a

result of gravity only - possible only in soils 1ITi th liquid or quick consistency - and those plastic flows which take place under pressure (Fig. 5). In the latter case the water envelopes surround• ing the individual particles are broken and must first be re• formed. If the particles undergo mutual displacement without dis• turbing the water envelopes the process is called, according to .. 'l.'erzaghi (1925), a "sliding flow". Since it is difficult to dis"• tinguish between these processes in nature, the two movements will be referred. to here simply as "plastiC flow". In experiments with plastic Vienna marl and with clay from the Little Belt, Hvorslev (1937) showed that under a pressure stress s omewhat below the breaking load a "plastic flow preceding fracture" takes place (Fig. 5). This flow process is associated with a re• duction of strength, which is followed by an increase of strength when the pressure is removed. Thus thixotropic phenomena are present in plastic flows preceding fracture which occur in suitable soils under a critical load. Of fundamental importance here is the fact that the thixotropic change of strength occurs without qny vibration and precedes shearing; thus it is due, not to kinetic stress, but to pressure exerted in a certain direction. Theoreti• cally this is explained by orientation of the particles perpendicu• larly to the pressure stress. In sufficiently loose ground the accompanying change of position, especially of the lamelliform particles, results in a transient reduction of strength or in• creased mobility, which can be also regarded as a preliminary stage in the collapse of the structure. If fracture does not occur the new orientation evidently results in a new arrangement which in • turn yields an increase of strength. The more softly plastic a cohesive soil, the greater will be its tendency to undergo plastic flow prior to fracture and the closer will be its water content to the liquid limit. This state, described by Scheidig (1939) as "liquid plastic", can easily be recognized by its consistency indices between +0.25 and 0 (cf. -12-

Table I). As the consistency index approaches zero and negative values appear, i.e. as the water content passes the liquid limit, the soil enters the transition state between "plastic" to "thixo• tropic" flow. No sharp line can be drawn. It thus becomes all the clearer that thixotropy plays an important part in the flow processes even of plastic solI masses that do not become liquid (cr , p. 33). Flowing Deformation Under High Pressure • Flowing deformations are possible, of course, only in loose rocks of thixotropic character whose solid pal .... icles are still • surrounded by envelopes of water, organic material, etc., so that the motions of the particles relative to each other take place outside their boundary surfaces. Their bedding must also be loose enough so that after a structural collapse the former structure can be partly re-formed again in the sense of a thixotropic re• solidification. Only that part of the structural breakdown is thixotropic which during the solidification or resting period can be compensated for by a structural build-up to the thixotropic end-strength. The possibility of re-forming a loose structure will gradually disappear in the event of progressive consolidation with decreasing water content in the vicinity of the lower ｰｾｴｩ｣ limit. Thus we can expect to find transitions to non-thixotropic, plastic flow or flowing deformations such as occur under the pressure of huge superpositions (orogenic compression) in strongly bedded clays and argillaceous schists with slight interstitial moisture, e.g. in • tunnels and galleries. Here the movement can no longer take place within the water envelopes of the smallest particles. It occurs • at the boundary surfaces or, as in the case of salt, gypsum, marble and other rocks, partly also at translation ｳｵｾｦ｡｣･ｳ within the particles themselves (cf. Fenner 1938)*. Earth Slides and Earth Flows In a slide the friction at the sliding surface is less than the internal friction between the individual particles of the * NRC TT-515 -13- sliding mass. The latter retains its internal cOhesion. However, if the internal friction between the Lndi vi duaI particles is less than the friction between the moving mass and its supporting sur• face, the mass will flow. Its individual particles will be dis• placed relative to one another so that its original cohesion vanishes. This difference in the nature of the motion affects the motion process and the configuration of soil displacements. A basic distinction should therefore be made between earth slides with predominantly sliding movements and earth flows with pre• dominantly flowing motions. The extent to which both turbulent and laminar flow occur in earth flows remains to be investigated.

Motions Involving Thixotropic Liquefaction and Resolidificatiol1 of Quick Soils

Sliding Motions In many land slides the sliding masses are in the solid to plastic consistency state and a particularly softly plastic or softened clay acts as the "lubricant" along the slide horizon. Here we shall use the example of a ground fracture in the Lerkedal railroad section near Trondheim (Fig. 6, top profile) to illustrate sliding motions in which the sliding takes place in a comparatively thin quick-soil horizon which becomes quite thickly liquid. Before and after the motion this quick soil is quasi-solid. During the motion it is liquefied. As a result of this its shearing strength is reduced to a fraction of the deposition state (Ackermann 1948), and the frictional resistance approaches zero, depending on the viscosity of the liquefied soil. Cohesive soils in the quick consistency state, constitute an ideal sliding horizon at the instant their structure is destroyed and they are liquefied. EVidence of the presence of such a flow-prone horizon can already be obtained - in view of the close relationship between thixotropy and plasticity - by the simple methods used to determine the Atterberg consistency limits. A soil is in the qUick consistency state when the water content is above the liquid limit, or if the -14-

relative consistency is negative (cf. Fig. 6 and Table I, No. 52, 54, 55, as well as Table II). As already stated (Ackermann 194B), the soil movements in the Lerlcedal cutting were started by vibrations from debris-hauling trains and excavators during the deepening of the cutting. Slides of this kind, therefore, with liquefaction of a qUick-sOil horizon, can be caused by dynamic influences alone. No additional moisture at the slide horizon is needed, as in most of the land slides hitherto described. As in the Lerkedal quick-soil horizon, similar flow-prone sit• , uations can occur just below the dry crust (load, Fig. 11) or in the lower parts of a soft-to-liquid-plastic clay series (e.g. at Bekkelaget, a bay on the Oslofjord, Table I, No. 48-52). Many sub• aquatic slides whose masses show the' contorted crumpling indicating a plastic consistency form, known from older deposits, must have taken place on such quick-soil horizons. The changing strength of the quick-soils explains their stability during the depositing of younger strata. Its sensitivity to vibrations could result in liquefaction during earthquakes. Such a liquid sliding horizon made sliding possible even on qUite flat slopes. If the qUick sOil 1s thicker then the character of the movement changes. The quick solI then participates not only passively, in the role of a lubricant, but also as an active medium which definitely determines the nature of the motion. Flow motions occur which are capable of shifting huge masses of earth in a very short time. This kind of motion and displacement will now be illustrated by a few examples from the Scandinavian coastal regions. The Flow Movements of QillDk Clays which Become Thickly Liquid Opposite the above-mentioned Lerkedal slide, on the other slope of the same cutting, there was a 5 m. thick-clay lens (Fig. 6 lower profile). Here the coarse clay had the same plasticity index (P = 8 - 12) and thixotropy index (t = 6 - 13, and at times up to 20) as the quick clay of the first example. Its flow-prone• ness, therefore, was very great. At the same time, since the water content towards the centre and base of the quick-clay lens rises -15-

to 38.7%, the soil contains up to 13.5% excess water above the liquid limit. The consistency indices can be used here for a rough estimate of the quick consistency, since there were found to be close relationships between plasticity and thixotropy in the Norwegian blue clays. In the present example the relative consist• ency has the negative value -1.75. The graphic presentation of this high index illustrates the flow-proneness of the quick Qlay.

The latter became liquefied under the rail-transmitted shocks, of the debris-hauling train as soon as a sufficient load had been removed from the slope by neepening the bottom of the cutting. The liquefied quick clay moved downwards towards the foot of • the slope, causing the surface dry crust at first to move sideways, thus producing a crack. As the pasty soil continued to move down• wards this crack opened wider and wider. Finally, the liquefied qUick clay broke through the thin dry shell at the foot of the slope and flowed down onto the bottom of the cutting. The dry crust of the natural surface, about 2 m. thick, was carried down• wards and moved towards the centre of the cutting. As it did so, it broke up into single lumps subject to antithetical rotations. After four months' rest and solidification the qUick clay was again disturbed by vigorous excavating. It became liquid once more and flowed downwards. This time, however, it did not break through the crust, but merely caused the bottom of the cutting to buckle about one metre upwards. In both cases the principal movement was a downward flow. The sinking and breaking up of the more solid top strata were merely accompanying phenomena. In Lerkedal the flow process during the motion and the changes of form were affected by the fact that the masses which were moved were only 30 metres wide and the qUick clay was only thickly liquid. However, the flow-prone clay of most Norwegian earth flows in the disturbed state is essentially thinly liquid. The nature of the flowing movements varies accordingly. -16-

Flow Movements of Quick Clays which Become Thinly Liquid An especially impressive soil movement of typical Norwegian qUick clays is the Vaerdal catastrophe of 1893 (Fig. 7). In a 125 metre high terrace the young quaternary, marine coarse clay under a covering of sand or dry crust was, according to H. Reusch (1901), so flow-prone that no drainage ditches could be dug in it because at 70 - 100 em. depth they became filled with qUick clay oozing from the side. Also, at the deep-cut mouth of the Follobach which was situated in a 30-year old soil displace• ment, the quick clay was so yielding that cattle and people could , not step on the bank. Horses which passed over the threatened ground half an hour before the event were very restless. They probably detected the first indications of the soil movements. These ｢･ｾ､ｮ at the banks of the lower Follobach with several small landslides. With receding erosion additional such slides would have disturbed the hitherto unmoved quick-clay region further up• stream and trus have started the flow process in the region of the brook course. In half an hour about 55 million cubic metres of qUick clay were involved in 'the flow movement. The masses of mud flowing downwards kept dragging with them new parts of the surface crust above them and carried them out into the valley. ｾｩｴｨ the lumps of more solid dry crust, trees, several farm houses and people were carried for kilometres down the valley. A few inhabitants of the FolIo farm were saved after being carried 6 kilometres on the roofs of their wooden houses in the soupy clay mud. At the upper end of the caldron produced by the earth flow, here and there parts of the dry crust which had not broken off stood isolated in the air with sods and trees, the underlying qUick clay having flowed out. The mud took approximately two hours to flow 7 - 8 kilometres and finally covered an area of about 2 8.5 km. • The river, dammed by the clay for half a day, formed a 2 lake which covered an area of 3.2 km. • At Varild, according to G. Holmsen (1934), an eye witness ob• served how the trees at the foot of a quarry dump swayed during the beginning of the earth flow because the soil of the valley was -17-

moving, and shortly afterwards a 3 - 4 metre wide fountain of mud burst forth. After this had flowed for 3 - 5 min. the entire bot• tom of the valley collapsed with a frightful crashing noise. A tall wave of mud flowed in the direction of the fjord, moving first to• wards one slope of the valley and then towards the other. The boulders from the dump floated like packing cases on a river. The mud filled the flat upper end of the Varild Fjord and covered an area twice as great as the outbreak region. According to G. Holmsen (1934) the level of Lake ｇｲｵｮｧｾｴ｡､ｶ｡ｴｮ was lowered two metres. After the surface of the water sank, cracks opened in the steep shores and lumps of the clay soil slin • • down. At the same time, a wood pile situated at the same place slid a short distance into the water. Wood cutters trying to rescue this wood three days later sUddenly noticed that the ground was moving and saw the trees sway• ing. By fleeing hastily they were able to get out of the way of the crashing trees and the emerging flow of quick-clay mud right behind them. The dry crust collapsed above the latter. The trees were carried down into the lake with clods of earth clinging to the roots on top of the flowing mud. In barely one hour about 720,000 cubic metres of mud flowed from a region 200 metres wide and 1200 metres long into the 70 metres deep lake. The start of the earth flow was apparently due to excess pressure from the interstitial water after the dry crust of the bank had given way under pressure from the nearby wood pile. What we have here, in fact, is the flow of a thickly liquid mud such as has been repeatedly observed and described in connec• tion with similar soil movements, e.g., the Tiller landslide of 1816 (Helland 1894, p. 134), MOrset 1893 (Reusch 1901), Koksdad 1924 (0. Ho1msen 1929, p. 5), Gretnes 1925 (ibid. page 17); Braa 1865 (ibid. p. 23), Moum 1931 (G. Holmsen 1934, p. 8), Tharnshavn 1930 (ibid. p. 5), Leirnesset 1939 (G. and P. Holmsen 1946, p. 7), Holund 1942 (ibid. p. 14), Kverne 1944 (ibid. p. 55), Aaserumvatn 1940 (ibid. p. 64) . In the descriptions of the Swedish earth movements in BohuslAn the flow of clay 1s also mentioned several times (Statens ｊｾｲｮｶｾｲｳＬ -18-

etc., 1922). On pictures of this a mud is clearly recognizable with sharp-edged lumps of the dry crust floating in it. Sub-Aquatic Flow Movements Quick clays and thixotropic liquefactions were also observed at some of the numerous shore line collapses on Norwegian lakes, rivers and fjords. Sub-aquatic flow movements of post-glacial clays were started on April 30, 1930, in Thamshavn at low water and receding tide as a result of deposits on the bank with a load of 2 only 0.4 kgm./cm • According to G. Holmsen (1934), at least 20 - 30 million cubic metres of mud 11 km. wide flowed along the • bottom of the fjord towards the sea and bedded itself in the centre of the fjord with depths in places of 50 metres. The distance of transport in this case, determined by soundings is especially noteworthy (Fig. 13). In the sub-aquatic landslide of Hommelvik (April 14, 1942, at 8.00 p.m.), also caused by deposits, a strip of tne shore approxi• mately 400 metres long, consisting of more than 150,000 cubic metres of blue clay with covering strata about 300 metres wide, moved as far as a depression in its path in the bed of the fjord which it filled to a depth of several metres (Fig. 8). From the moving masses in this case the presence of quick clay could be shown (Table II, No. 59, 60). The slide terrain is situated at the mouth of a small brock. The movement took place at low tide. The slide can therefore be attributed to the coincidence of several unfavourable conditions: loose bedding of the basal qUick clay, hydrostatic pressure gradient due to the inflowing underground water and receding tide water, and increased interstitial water pressure due to the rapid depositing of a clearance dump. Comparable with the Scandinavian flow movements is the Zug catastrophe in July 1887 in which a mud flow comprising approxi• mately 150,000 cu. m. consisting of loosely bedded silty sand and bog lime flowed out for 1,000 metres over a lake bed sloping pre• dominantly at 1 - lio (4.4%) (Heim 1932, p. 42 and v. Moos 1948). Presumably the mud, flow of an apparently solid lake sediment in -19-

OntariO described by Legget and Peckover (1948) belongs to this class of flow movement. General Description of Thixotropic Flow Movements The typical movements of quick soil maSSes can be described as follows: The material which determines the nature of the flow process consists of fine-grained soil masses which by reason of their con• tent of water, electrolytes, organic substance, etc., are in the quick consistency state (quick soils, Ackermann 1948; cf. also Table II at the end of the present work). Its composition may vary from fat clay to sand containing a small amount of clay. Cal• careous mud and soils wlth organic substance (especially mud) also belong to this class (v. Moos and Butsch 1944). In Norway the qUick clay consists predominantly of uoarse clay (silt). On dis• turbance of their structure these qUick soils are capable of ex• periencing the changes of state from quasi-sOlid to liquid and back again without changing their water content. A condition of fa&t-f1owing movement of mass is that these qUick solIs should be several metres thick and there should be a possibility of lateral shifting. A steep angle of slope or in• clined bedding is not essential to a flow; a gentle slope of the sort prevailing in river beds is sufficient. The flow movements observed in Norway occur predominantly at the edges of terraces and on the shores of bodies of water. By preference they are seen in the vicinity of the mouths of small tributary valleys or streams. At these places comparatively coarse sedimentary material was washed down during deposition of the clays by tributary streams, thereby increasing the flow-proneness of the soil. In addition to thlS, the salt content of the solI is ex• tracted by the underground water constantly seeping through and thus the electrolyte content, as well as the degree of thixotropy, ls reduced. A structural diagenesis takes place. In the course of geological development of the clay two factors combine here, which at times may increase the flow-proneness until a definlte danger of flow exists. -20-

Sometimes there are certain advance signs of future soil movements. Some months before the Lade flow (cf. p. 25) several electric and water conduits broke. In the Lerkedal cutting a crack began to appear half a day before the flow movement took place. A half hour before the earth flow in Vaerdal some horses shied in the area of the upper ｾｯｶ･ｭ･ｮｴＮ Immediately before the mud out• break at Varild alJ the trees on the floor of the valley near the point of outbreak began to sway. Thus, at least in the case,s ob• served by eye witnesses, it can be shown that the main movements are prepared by subcrustal motions of the qUick clay which Just .. .. before the beginning of the catastrophe results in vibrations of the surface and ultimately in the breaking open of the dry crust (or , pp , 12, 33 "Flow preceding fracture"). A flow process which 1s visible on the surface is frequently prepared for or initiated by a "initial slide". Th1s is the term used by G. Holmsen (1946), for many years an expert on Norwegian landslides, to denote smaller slldes which occur, for example, on slopes which have been oversteepened by erosion. Owing to the sliding of blocks the rigid and tough dry crust which covers the quick clay is worn. The vibrations resulting from the fall of dry blocks of clay can presumably produce a liquefaction of the quick clay, which now breaks out at the posi t Lon of the slide and begins to flow. At the beginning of the movement various noises were heard which were described as crashes, booms, roars "cannonball thunder", howling or Whistling. They prove the suddenness of occurrence of the principal motion. As the tide wave observed from two miles away at Thamsha.vn (Holmsen 1934) shows, SUb-aquatic flow motions of the thixotropic nature also begin with the suddenness of an avalanche. Rapidity of motion and breVity of the process are typical of the flow movements of quick soils. Only in a few caSes was it pos• sible to rescue people from the areas in which the soil movements occurred (Hommelv1k 1942). Frequently the surprised 1nhabitants of the regions have drowned in the mud (Lade 1944). Most of the flows -21-

of this type have Jasted only a few minutes. Only very large quantities of sailor viscous quick soil can remain in motion for several hours. The huge masses of mud in the Vaerdal rolled forward "like a wave of water, so rapidly that no horseman could have kept up with it" (Tischenclorf 1894). Above the flowing ｾｵＱ｣ｫ clay the dry crust loses its support. In small strips. one block after another drops down (Norwegian: lelrfall) producing further impulses to liquefaction and the pro• cess is propagated uphill. On some occasions it is also a slow process. Thus in Ilsviken (Trondheim) 1944 a sort of slow motion • sinking of the blocks was observed. This type of backwards motion might be predominant. Impulses must also be expected in the upper part of the future area of motion which cause liquefaction (e.g. Lerkedal). These would be propagated within the qUick clay stratum in the direction of the flow or hvr1 r o s t a t i c pressure gradient. This makes possible a valleyward f'Lov: of the quick clay masses thus liquefied which now, depending on their mass and energy. can either leap over or break through the foot of the slope, again ｡｣｣ｯｭｰｾｭｩ･､ by the sink• ing of the rigid dry crust in the form of blocks. The blocks of dry crust are borne along downwards by the thickly liquid cl&y mud. They float like blocks of ice in water and even carry along trees which then sink slowly in the mud (Mourn, Varild, Grungstadvatn). After-slides, by which the oversteepened slope at the upper edge of the soil movement adapts itself to the new slope conditions, may occur within a few hours or several months, or adjacent soil masses may start to move. A good example of this is afforded by the land• slide of Lade where the principal movement of the 11th April 1944 was followed on July 26 and 27 and July 30 and 31 by after-movements (Table III). Not infrequently, nearby older depressions show that a rather large part of the area in question 1s liable to such movements, which may then reoccur in the course of decades, e.g. Braalia 1831, 1848, 1858, 1860, 1928 (G. Holmsen 1929), Vaerdal (Tischendorf 1894) and Lade. -22-

If the flow occurs in a terrace at higtl alti t ude the th ickly liquid qUick clay may sometimes drop valleywards so violently as to start ｣ｬｬｭ｢ｾｮｧ up the opposite slope ("Schuss" flow).. The clay masses f'Lowfn g down from the Tiller church on ｲｾ｡ｲ｣ｨ 7, 1816, splattered the opposite slope of the Nidelv valley to a ｨ･ｩｾｬｴ of 50 ｾ･ｴｲ･ｳ (Helland 1894). At Holund (1942) the qUick-clay mud climbed on three or four occasions as much as 25 metres up the valley slopes (P. Holmsen 1946) (cf. also Varild, p .. 16) .. The lower the viscosity of the flowing clay mud the further it is able to flow down valley. In the year 1345 in the Guldalen landslide of Hagar the mud flowed valleywards more than 20 km. (Fig.. 13). The qUick-clay mud can also flow for several kilometres in sub-aquatic flow movements, e.g. 11 km .. in the case of ｔｨ｡ｭｳｨ｡ｾｬ on May 2, 1930. The water buoyancy would produce still further movements. The break-out recess is usually about 100,000 m. 2 in size and in most flows has the shape of a cauldron (Fig. 7) .. Only seldom does the break-out region attain 550,000 m.2 (Tiller) and in Vaerdal (1893) it was 2,942,000 m.2 (Holmsen 1946). Frequently 3 the clay masses (generally 1 to 3 million m.. ) flow from a narrow opening like the neck of a bottle into the valley, e.g. in Vaerdal 1893, in Moum 1931 and in Kverne 1944. The area of deposit of the flowing clay mud generally extends along the course of the second largest river. It therefore begins at directly adjacent terraces, just below the mouth (Vaerdalen, Kverne). On the other hand, if the layer of flowing clay is high above the floor of the valley the mud in the tributary will flow down as far as the main valley (Tiller, Holunn) or the coast (Brae). In sub-aquatic flows the mud moves towards the deep troughs of the watercourse ahead (Hommelvik, Thamshavn). The valley flats above the mouth of qUick-clay mud are covered by the latter for only a short distance (Kverne 1946). The thickness of mud masses deposit• ed reaches 10 metres (Tiller), 12 metres (Vaerdal) and more .. The river is often temporarily dammed up (Tiller, Vaerdalen, Lade) until the water can find another bed. -23-

The mud displaced by the flow gradually becomes quasi-solid again. However, it now has considerably less strength than in the original deposition state. A new deposit therefore tends to flow much more easily than before (Vaerdal). At Lake Aaserum the emerg• ent quick clay has not undergone any appreciable solidification after three years (G. Holmsen 1946), although it is thixotropic (Table II, No. 71). With ｲ･ｾ｡ｲ､ to the motion process and the flat slope of the terrain, there is no difference in thixotropic flows - as opposed to the slides of a different kind described by Padding (1931) and Heim (1908) - between sub-aquatic and sub-aerial processes. The only difference is the fact that in purely SUb-aquatic flows (with• out participation of parts of the shore) blocks of dry crust, slope debris, terrace blocks and other components of the sub-aerial sur• face are absent. Since thixotropic liquefaction is independent of the water content, it is of no importance whether a quick-soil horizon is above or below the water level. It is of fundamental importance, however, to note. that sub-aquatic earth flows of a thixotropic nature have the same character as the sub-aerial. In all qUick-soil movements we may expect ｦｬｯｷｩｮｾ motions, 1.e., turbulent mixing up of the material. However, no finely contorted deposits can take place such as Heim (1908) concluded from the Zug catastrophe. The contortion horizons of older deposits, which have generally been attributed to sub-aquatic movements should be as• cribed to the sliding motions of plastic strata, the main masses of which were not liquefied. In summing up, it is particularly important to emphasize that flows with thixotropic liquefaction of qUick soils: occur without change of water content and may be repeated; take place rapidly; flow on gently sloped surfaces; spread out their material for several kilometres and may transport blocks of their former surface with deposita lying thereon. -24-

Combined Slide and Flow Movements The slide Rnd flow movements thus far mentioned have been especially clear examples. Frequently, however, soil displacements are observed in which both processes together determine the result• ing pattern. This will be explained with reference to a diagram (Fig. 9). 1. A slide with a typically curved sliding surface, the posi• tion of which is determined by a thin qUick-clay bed (black). The blocks of solid dry crust have sunk but have remained coherent. At the foot of the slope their weight is counteracted by the plastic clay which has been pushed upward to form a wall. - This type of flow corresponds to the usual slides and ground fractures taking place over a sliding bed with low shear resistance. 2. (a) A terrace with a very thick, tapering quick-clay bed. (b) During the motion the quick clay has all flowed out except for a small remainder. - The dry crust has disinte• grated into individual blocks which have become fully sepa• rated and floated downwards. The deposition area is ex• tensive, often being kilometres wide. A flow process of the Vaerdal type. 3. (a) A gently-dropping slope with a dry crust and a quick• clay bed decreasing in width. (b) The qUick clay has flowed down valley hut has not succeeded in breaking through the dry crust except in a few isolated places. The latter is strewn with faults and shows many gaping cracks in the lower part, but has almost entirely retained its cohesion. In the upper part of the • zone of motion it has sunk down over the masses of mud which have flowed out. In the lower part, however, it has been raised by the masses of mud flowing together there and has been vaulted upwards. Flowing and sliding here have equal importance in the pattern of soil movement. Their extent is locally restricted. The settlement region and -25-

deposition region are side by side. -A sUbcrustal flow movement of the Lade type. As an example of such a combined 'soil movement we may here cite the slining flow of Lade (1944) a suburb of Trondheim, Fig. 10. In this region, eight months before the first catastrophe, several bomb craters had been maoe in the slope alongside the Ladeallee, but these had no immediate effect. However, during the following months electric wires broke several times. This pointed to slow, small motions of the ground. In addition to this the seepage of water under pressure from damaged water conduits into the masses of silt in the ground could not be prevented. According to eye witnesses (Haug's report, cited by P. Holmsen 1946, p. 48) on the 14th of April 1944 "the region neFlr the stream in the western part of the slide began a wavy motion which was propagated upwards so that the entire grassy slope undulated". Then a piece of the Ladeallee 175 metres long together with the bombed terrain slid down. The process took place so qUickly that along with the sinking parts of houses ann huts people were also pulled into the ground and were killed. In the lower part of the slide area the bottom of the slope along with the adjacent ground of the valley was raised and tre Ladebach stream was dammed. Fairly large after-motions followed this first one at the end of July. These extended the land s 1 ide regions in a half moon upwar-ds and increased the heaVing in the lower part of the reglon of motion. At several points lateral displacements 20 - 30 metres wide transverse to the slope gradient could be observed. These occurred particularly below the mud zone which consisted of qUick clay. Both in the eastern and western parts of the slide this zone had dis•

• tinctly qUick consistency (Table II, No. 61-63), but was comparative• ly thickly liqUid and therefore did not belong among the typical thinly liqUid clays. The quick clay came to light in a zone along the Ladeallee and moved with in the region of motion westwards to its lower end (sample 691). After the slide it was detected by means of drilled samples, together with liqUid-plastic clays of the dry crust and in -26- the boundary areas of the slide which had remained in place. In several places, instead of the natural quick clay (No. 905a) a slimy, evil-smelling "mud" full of cellulose fibres and smelling badly of faeces (sample 805) was found. Thus sewer water must have penetrated the clay and mixed thoroughly with its substance. Owing to the admixture of organic material the clay was able to absorb twice its normal water content (w = 68%). Nevertheless, with k = -0.81 it has a smaller relative consistency than the ｴＮｭｒｾｴ･ｲ･､ natural qUick clay (k = -1.39). It is not clear, therefore, whether the flow-proneness of the qUick clay had been increased by artificial changes. In any case there were one or more liquefiable horizons present under the dry crust over the entire region of move• ment and these flowed down valley underneath the dry crust at the start of the movement. As a result a' mOre gradual slope of the sur• face was obtained, as is evident in both the upper and lower pro• files. This is a special case of morphological slope flattening which is controlled solely by "subcrustal" flow phenomena. Special attention should be given to. the large bulges formed at the bottom of the mass which has moved. Here the dry crust became strewn with a network of cracks during the gradual increase of the bulging, so that there was danger of the dry crust opening up and allowing the mud to break out again. Fortunately the presence here of a rein• forced concrete storage shed offering resistance to the motion of the clay masses prevented this from happening. P. Holmsen (1956) attrihutes the first movement to the weakening of the dry crust when the foundation of this structure was being laid. Against this in• terpretation is the fact that the quick clay did not flow forth here but was held up. Nor could the present author detect any trace of the initial slide postulated by P. Holmsen. The Lade slide is important inasmuch as it could be shown that the qUick clay had absorbed an additional quantity of water [thiS was not yet known to P. Holmsen (1946)]. In the liquefaction of quick clays in nature, therefore, it 1s absolutely essential to take into account the possibility that the -27-

soils - which are alrearty in danGer of flawing - may absorb still more water. In that CRse we would be confronted not with a purely thixotropic liquefaction, but with a transition to fluid soils whose resolidification would depend on the giving up of water. In any case, modifications and superpositions of the decisive in• fluences can be expected, so that of course every landslide will have its special character. The Occurrence and Initiation of Thixotronic Flow Movements • The catastrophic flow movement in the Vaerdal in 1890 was ex• plained by Terzaghi (1925) by means of the same hypothesis employ• ed more recently by Loos (1937) to explain the flow movement during the railroad accident at Vita Slkkudden. It 1s assumed that during the dry period cracks occurred in the clay and that rain water collected in these cracks. As a result, the weight of the clay masses was increased. At the same time, it was assumed that the water turned some of the clay into mud in which the unsoftened lumps of clay floated. This picture, which is correct for certain earth movements of a different kind, cannot explain the presence of quick clay either below an undamaged dry crust, or in underwater deposits. The demonstration of quick consistency on the part of the soil masses which have flowed out in various soil movements (Table II) explains the flow phenomena of the catastrophes in question more readily than the above hypothesis. The former description, accord• ing to which the interior of these flow-prone terraces is ｡ｳｳｴｾ･､ to consist of liquid mud even in the state of rest, had already been refuted in Sweden.* The month-long observations of quick clays in the Lerkedal once more confirm the fact that in the state of rest qUick clay is solid, or rather quaSi-SOlid, and only becomes liquid when disturbed. The immediate stiffen1ng of quick clay after it has flowed has been observed, e.g. in Lerkedal (Ackermann 1948, p. 12) and Lane. The bodies of those who had drowned in the mud at Lade were dug up

Statens ｊａｲｮｶｾｧ｡ｲ Geotekniska Meddelanden 2, p. 17. -28-

the day after the first flow movement. This was only possible be• cause the clay had already become reso11dified to an extent to where each excavation made with the spade would not close up agaln lmmediate1y due to an inflow of liquid mud. The qUick-clay mud which flowed through a window of house No. 20 hardened underneath the window sill in the form of a debris cone with flow bulges, 1.e., as soon as it attained the comparatively calm interior of the house. From a sufficient number of examples, it has now been shown • that in the movements of quick-clay soils which can be described as "earth flow" the soil is • solid or quasi-solid before the movement; liquid, i.e., thlcklyliquid to viscous during the flow; increaslngly quasi-solid after the flow. Finally, by laboratory investigations, as I have shown on a former occasion (1948), the quick condition itself has been demon• strated (Table II). The movement of qUick clays is initiated, depending on the character of the thixotropy,. by disturbances to the structure, generally due to a small application of energy. The latter is pri• marily of a dynamic kind, ･Ｎｧｾ vibrations, such as may be produced by earthquakes, volcanic explosions, rail-transmitted shocks, ramming, explosive discharges and bombs. Data. from many soil move• ments also show that static forces (directional ｰｲ･ｳｳｵｲｾＩＬ due, for example, to the construction of barriers, the piling of debris, the removal of loads or the oversteepening of slopes by fluvial erosion, etc., as well as changes in the ground water and surface water level and the accompanying changes of hydrostatic pressure ln the interstitial water can bring about the liquefaction of quick so11s and hence their movement. After floods and spr1ne tides the soils along banks and coasts are not infrequently subject to interstitial water superpressures which lead to spontaneous soil movements. The following soil moveMents took place at low tide or during ebb tide: -29-

ｔｨ｡ｾｳｨ｡ｶｮ 1930 2nd May 8:00 a.m. Hommelvik 1942 14th Aug. 7:35 a.m. Ilsviken 1944 26th May 7:35 a.m. (N.W. 9, 15) Ilsviken 1944 30th May 2:30 p.m. (N.W.12, 30) As is known in hydraulic engineering practice, stability con• ditions in slopes are especially unfavourable in the presence of a rapidly falling water-level. The weight of the dry-falline solI zone is increased by the removal of the buoyant forces. As the water-level falls the buoyancy decreases more rapidly than the ex• cess interstitial water corresponding to the former pressure is able to escape from the finely porous soil. The shear strength de• pending on the internal water presdure remains low for some time longer. On the other hand, the forces acting outwards and downwards immediately become effective. Thus we must expect to find a hydrostatic superpressure in the layer of soil between the old water-level and the new one. The pressure gradient and the rapid change in the balance of forces have a disturbing effect on the sensitive fine structure of the qulck clay and on the cohesion of individual clay partlcles or the films of water adhering to them. The tendency of the flow pressure is towards a reorientation of the solid partioles. Thls produces a collapse of the structure and the entire mass of soil is brought into a state of flow. In assessing the initiating factors it must be borne in mind that there may sometimes be a certain lapse of time between the initial liquefaction and the start of the flow movement of the entire mass. For example, the Fjellvik flow near Sandefjord took place fourteen days after the bomb explosions on the 22nd April 1945 (P. Holmsen 1946 b). Hvorslev (1937), in an experiment, ob• ｳｾｲｶ･､ that plastic flow did not begln until six hours after the application of a load. It ls frequently noted that the solI is not a material whose properties always remain the same, but that these vary under dif• ferent natural and artificial influences (weatherlng, ground water, etc.). Thus a plastic clay may turn into a qUlck clay as a result -30-

of salt being leached out through beds of sand or gravel (leachin8 diagenesis, Ackermann, 1948), and as a consequence a solid slope may gradually be converted into a flow-prone one. Investigations are also needed into whether the thixotropic movements frequently occurring under moors (e.g. Tiller, Braa, Grungstadvatn) are in• fluenced by infiltration of the coarse clays by humic acids and the associated changes in consistency. Usually a decision concern1.ng the causes of a flow, as in the case of slides, will seldom be clear-cut. Generally several unfavourable factors combine to pro• duce a catastrophe.

• Fast ｆｾｯｷ Movements of Non-Cohesive Masses

Thixotropic flow movements share with Terzaghi's "settlement flows" (Terzaghi 1925) the characteristics of loose bedding, sudden collapse of the structure and a fast rate of flow.* It is not impossible, therefore, that in certain "settlement flows" - e.g. of qUick sands with a low clay content - thixotropic structural collapses have alRo been involved. However, Terzaghi emphasizes that the settlement floNS are associated with non• cohesive soils and that an abundant influx of water and a fast underground water current are further contributing causes. In this respect they approach murl flows and similar soil movements where water is the transporting agent. They differ from thixotropic flows in the behaviour of the water and the structure. After the movement the characteristic thixotropic increase of strength is absent. No loose bedding is re-formed. Movement terminates with the expulsion of water and permanent reduction of the pore volume. Of course, the conditions for a repetition of the movement can be produced by a renewed influx of water. Since in nature we find transitional types of soil between thixotropic and non-thixotropic SOils (Ackermann 1948), so we can expect to find transitions between

* It is not yet known whether the flow movements of lacustral rleposits (silts and coarse clays) which occur only once, as described by Legget (1948) should be included in this class. -31-

soil movements with and without thixotropy. As long as the thixo• tropic component of a flow movement is doubtful its initiation can be described simply as a "structural collapse".

Slow Flow Movements of Mucky Soils

Creeping flow movements occur very frequently in weakly co• hesive soils whose marked flow-proneness ｨｾｳ already been emphasized (Ackermann 1948). The "crawling-earth glaciers" described by Albert Heim (1932) depend on extreme wetness and move comparatively slowly. Their speeds vary from a fe\"i millimetres a day to a metre per minute. They often last for several days as "acute" movements, whil'9 the "chronic" ear-t.h movements are r-eaumed in wet years and may continue for decades (Ackermann 1950 b). During dry periods they often reduce their speed or stop flowi.ng altogether. Recently flow movements have been reported in France by A. Cailleux and J. Tricard (1950). Cf. also G. ;oJagner (1950, Table 58). The creeping processes of saturated flowing sOils known as solifluction (slope solifl'.Jction according to Troll 1949) are of the same kind. They differ from the above-mentioned movements (those which cease during the dry periods of summer) not only with respect to their different behaviour in depth, but especially in the fact that they come to a halt during winter frost periods. In both caSes the movement depends priMarily on the amount of water furnished in the form of precipitation, ground water or springs. In the dry state the soil absorbs tbis water greedily. However, since owing to the moderate clay content the soil cannot bind much water it soon passes through the narrow ranges of plastic and thixotropic consistency, attains a liquid state and flows down .. hill slowly as a muck. The flow is purely gravitational, without other mechanical influence. Gravity suffices to overcome the fric• tion between the particles and to prolong the flow processo For characteri7.ation of the flow process it should also be em• phasized that in the debris from ｒｾｴｨ marls, for example (Ackermann 1950 b), the separate lumps of soil have a soft, very Olly coating ｾｨｬ｣ｨ permits the individual particles to sllde against -32- one another. Thus the conditions necessary for a genuine flow ｰｲｯ｣ｾｳｳＬ namely low internal friction between the separate particles, are satisfied. The changes in friction, cohesion and other mechanical properties of a boulder clay in the event of excessive wetting have been discussed by L. Bendell and Ruckli (1937) in con• nection with the Emmenegg and Dallenwil ground movements. Like a glacier, a debris flow is more rapid in the centre than near the edges, where the shearing strength is exceeded in the zone of greater friction and increasing tensile stress. Sliding planes and tension cracks occur. Reactions of this kind on the part of materials which behave in a plastic manner under pressure have also become known from experiments of Cloos (1931). They can be regard• ed here as associated phenomena, since the preponoerant form of movement in these debris displacements is not sliding but flowing. As Heim states, "we have the impression of a stream with fluid structure. The debris piles up in front of barriers, adheres to these and is accelerated if the gradient is increased or if the lateral profile is ｣ｯｮｳｴｲｬ｣ｴｾＱＢＮ We now come to ｴｨｾ question of what part thixotropy might play in the slow flow movements. Even in a continuously fluid state thixotropic suspensions (according to ｾｎｩｮｫｬ･ｲ 1938) undergo a reduction of viscosity due to disturbances of structure, as in stirring. Even in the case of saturated fluid solls and very wet muds of thixotropic soils we may expect to find a thixotropically induced reduction of internal friction as soon as the flow process is initiated. This reduction of viscosity - which can only be slight owing to the low degree of thixotropy of such relatively non-cohesive soils - can be regarded as the thixotropic component of creeping soils movements. Consequently, undisturbed muck can remain motionless on a comparatively steep slope until the friction has first been over• come. After the movement has begun, however, it can continue flow• ing over a fairly gentle slope. Besides this slight effect of thixotropy on the motion itself, however, a quite temporary stage of genuine quick consistency in -33- the form of an apparent stability can occur even in weakly thixo• tropic mucks. This 1s just after saturation has been reached or, conversely, just before the soil dries out. In thiS temporary con• dition its liquefaction ann motion can be produced by vibrations or other impulses of a kinetic nature. On the slopes of open pit workings, for example, spontaneous movements of apparently "drained" slopes which have been "stable" for a rather lone time are some• times produced. In creeping soil movements, therefore, only a subordinate role can be ascribed to thixotropy. The character and form of movements of mucky soils are determined mainly by the degree of soaking, which is generally very great. This reveals the close relationship of such movements to cold lahar and thinly liqUid mud flows. Thixo• tropy probably plays no appreciable part in the water mud flows (lahars) occurring during sudden downpours or in volcanic eruptions, where water acts as the transport medium.

Flow Movements of Plastic Soil Masses

If plastic soil masses do not move alone the slidine planes of individual blocks (as is often observed on the surface of the ground) but as plastic flows due to the pressure of natural or arti• ficial loads, this movement is associated with a disturbance to the bedding structure of all particles. Such structural collapses are associated not only with the known, discontinuous decrease of strength preceding the collapse, but also with a subsequent increase of strength, as mentioned at the beginning and as confirmed for ｴｨｾ case of ScancHnavian clays by Hvorslev (1937) and Haug (1940). Thus the "plastic flow preceding collapse" occurring in the zone of plastic deformation is shown to be a phenomenon influenced by thixotropic collapse of structure. It proceeds slowly, in contrast to the fast flow movements of liquefied quick soils. With respect to the behaviour of plastic olays under pressure, the investigations of Hvorslev (1937) have shown that a slow plastic flowing begins even under comparatively small stresses. In the Little Belt clay which was studied these stresses were only a third -34-

of the maximum shearing stress at fracture. If the load is gradu• ally increased the rate of plastic flow prior to fracture increases proportionally with the change of stress to a multiple of its ori• ginal value. At the same time the strength decreases, a fact Hhich Hvorslev also attributes to thixotropic disturbances of the struc• ture. Thus several hours may elapse between the application of the breaking load and the actual occurrence of fracture, depending, among other things, on the degree of thixotropy. The extent to which the "null friction" accompanying displacements of water con• tent (literal translation: "squeezings out") due to sudden loadlng (Scheidig 1941, p. 185) is influenoed by thixotropy has not yet been investigated. It should be noted that plastic flow can begin at stresses below the breaking value and that the rate of flow may vary even though the pressure remains constant. If the maximum 109.d is less than the breaking load then the plastic yielding motion will at first increase up to a maximum and then decrease again. The clay will recover part of its original strength in the form of thixo• tropic sti ｦｦｾｮｩｮｬＡＬ and the extent of the recovery will depend on the

degree of thixotropy. Although II pl a s t i c flow prior to fracture" can be attributed partly to thlxotropically influenced structural changes it is not due entirely, as the following examples will show, to dynamic causes, but also to ､ｩｲｾ｣ｴｩｯｮ｡ｬ pressure, 1.e., a static influence. As is known, a softly plastic flne clay differs from coarse clay (silt) in the fact, amongst others, that a pellet of the latter .. becomes damp and pasty when shaken in the hand. Only as a result of vibration does the weakly thixotropic SOil, even in the plestic state, Buffer a structural collapse, combined with a reduction of shearing strength and of the apparent angle of internal friction. The latter no longer happens, since a shifting of the soil in the form of a yielding soil takes place. The friction values found by Casagrande and Terzaghi in experiments using shearing apparatuses, are often too high. In this case the laws of hydrodynamics must replace those of stress analysiS for ｲｩｾｬ､ materials. -35-

In tectonic experiments with plastic clay, as carried out by H. Cloos (e.g. ＱＹｾＱＩＬ the elastic character of the deformation be• fore fracture was probably due, in part at least, to thixotropic causes. Corresponding to the laboratory observations, we observe the flow of plastic soil masses in nature where, of course, static and kinetic stresses are frequently combined and tl'eir r-especti ve com• ponents may not be clearly distinguishable. Since thus far only one or two fine-grained soils undergoing plastic movement have been examined for tbixotr0pic changes of vis• cosity, some of the following examples still lack confirmation. As far as the Norwe[!:ian coarse clays are concerned, on the basis of numerous ｩｮｶ･ｳｴｴｾ｡ｴｩＰｮｳ already available (I listed only a few of them in 1948 - Table I) thixotropic properties may qUite generally be assumed. Furtbermore Boswell's results (1948) after examining a very wide variety of fine-grained rocks Justify the assumption of thixotropic properties in all sufficiently fine-grained, damp and loose sediments. Slow downhill movements of multi-storey bUildings on the post• glacial blue clay of Oslo attained a distance of 74 em. in the M611ergate, and in an extreme case on the Holbersgate of 138 em. (G. Holmsen 1938, p. 64). According to Bjorliykke (1921) these motiOns took place essentially in the liqUid-plastic zone of high water content, which "'TaS often found just belOW the dry clay crust at a depth of 4 - 6 m. (Eckstrom 1925, p. 10). The yielding movements of softly plastic clays during an em• bankment filling operation at the ｇｾｲｴｮ･ｲ･ｬ Fornebu (Oslo) after the sand fill had reached a height of 8 m. were somewhat more rapid. First an eloneated bUlge of the kind frequently observed in ground failures by heave appeared along the western foot of the fill. After several days this elevation had attained a height of 1.5 m. The motion was brought to a halt in the usual way by the imposition of a counterweight. Then, a few metres ahead of the eastern foot of the embankment small cracks appeared which gradually Widened. These ｬｮ､ｩ｣｡ｴ･ｾ an upward pressure (literally: vaulting or welling- -36-

up) corresponding to a measurable settlement of the beams of the adjacent forms. After the yielding movements Jf the plastic clays in this direction had been halted in turn by counterweight fills, very slow movements to the side began to develop - similar to the downhill movement of the houses mentioned above - until counter• weight fills were undertaken at the southern end of the embankment. Here the frontal walls of stone greenhouses were pushed in like folding doors up to a distance of 2 to 3 dm. The pipe lines of the heating system were bent outwards in a curve. The side walls of nearby wooden greenhouses were squeezed in, the boards forced outwards between their anchoring points in curves. The deformations took place qUite gradually over several months of the years 1943-44. As in the case of the upward movements at the sides of the embankment, the lateral movements at its ends can be attributed to a slow, subcrustal plastic flow due to pressure. The direction of the flow was the only one available in view of the trough-shaped character of the sUbgrade after the counterweight material had been placed; that is to say, the flow was towards the fjord, to the lower part of the basin sinking in that direction. Because of the disturbance to the deposition structure, the bearing str€ngth was so reduced that even the thixotropic increase of strength to he expected after arresting and retaining the upward pressures was insufficient to prevent the movement which had been initiated. To state in geological terms, a quick clay or soft plastic bed which has already been in place undisbnbed for some time will be displaced sideways if unevenly loaded between its underlying and overlying beds (cf. Island] in the ship channel of the Mississippi River, p. 43). The resulting deformation pattern

• should be not merely turbulent (Hadding 1931), but under certain conditions might resemble the convolutions hitherto attributed to subaquatic slides. Similar upheavals of thixotropic clays of plastic consistency alongside Sinking embankments have occurred in Sweden (Statens ｊａｲｮｶｾｧ｡ｲｳＬ etc. 1922). ------_.. 'T

-37-

Sapropels and bog limes are also thixotropic and tend to under• go such plastic changes of form without sliding planes. Of course, such displacements, in Terzaghi's sense, take place not only in the presence of excess local stresses due to imposition of extra loads, but also as a result of the relief of stresses over a limited area, e.g. in the Rapperswyl railroad cutting on Lake Zurich (Moser 1894). After removal of a series of covering deposits 5.5 m, thick, the underlying "soft mud" 15 m, thick was pushed up ... into the cutting and the mobility of the mud was increased by the .. vibrations from lumps of the covering deposits being further broken .. up, a process observed also in other thixotropic materials elsewhere• Demonstrably thixotropic displacements took place at the upper boundary of a terrace of blue clay with a dry crust about 2 m. thick at BrOther (east of Oslo). Here in April 1944 an unusual kind of ground movement was started by exploding bombs (Fig. 11). The slope of the terrace was pushed outwards about half a metre and the consequent extension of the surface produced a ditch-like de• pression in the dry crust. At the same time a wave 1 - 2 m. high rose up at the bottom of the slope. At the end of this wave, as an extension of the lateral upcast, quick clay emerged which after a few days (on sampling) had lost water by evaporation (hence its low positive consistency number in no. 64 of Table II) and was covered by a few centimetres of dry crust. Here we observe, on a smaller scale, phenomena which are known from general geotectonics and which are associated with the subcrustal. displacement of an incompetent stratum by plastic ｦｬｯｾＱ (cf. the data given by G. Holmsen 1946). Along the northern coast of the Island of Java, where the premiocene surface of the geanticline veers towards the geosyncline, are the vo1crrnoes of the Ardjoeno-Welirang group. Their cones, ac• cording to Van Demmelen (1937) are bedded on the marine geosynclinal sediments of the Neogene. These sediments were squeezed out be• neath the load of the volcanic masses at the base of the cones and folded up towards the Bangi anticline. At the same time the old volcanic cone of G. Ringgit sank from a height of about 3000 m. at the 7 km. long Alas Ridge disturbance down to about 2474 m., while -.'38-

shifting about 3 km. to the northwest. The ArdJoeno Volcano then formed at the former crater pcsition of the G. Ringgit (Fig. 12). As shown by repeated triangulations in the Karangkobar Region of central Java, the volcanoes there sink under similar geological conditions several declmetres per year. This continuous sillking and lateral movement - gravity tecton• ics in van Bemmelen's sense - is due less to the settlement of the unconsolidated basal sediments than to a slow plastic flow, aided by the thixotropic properties which are undoubtedly present in the young clays. The start of this movement can probably be traced to vibrations due to earthquakes or volc"'liic explosions. The alterna• tion of periods of sinking with periods of accumulation of volcanic products determines the development and structure of the Tenger Mountain range and many other volcanic regions on Java. If the plastic clays are banded, as in the interior parts of some Norwegian f.1orcls ana in southern Sweden, the movements will be influenced by the different behaviour of the sandy and clayey strata. The flow is no longer constant, as in the thicker, quasi• homogeneous blue-clay zones. The motion is differentiated, it prefers either the sandy or the clayey strata, depending on the con• sistency of the individual bands. At the same time the strata of softer consistency become thicker in the vicinity of folds, while the strata that are less mobile nre carried along. The resulting motion patterns are frequently well preserved in the folds of recent contortion moraines (e.g. Gripp 1926, Table 9; 1927, Table 8), and in some contortion folds of coal-bearing tertiary strata in central and eastern Germany, as well as the flowage folds in fossil sub-aquatic slides (e.g. Hadding 1931).

Recent Spread of Thixotropically Influenced Flow Movements

We expect to find ground movements of a type determined by the thixotropic liquefaction of the soil masses only in geologically young, loose-textured deposits which still contain considerable moisture. In general this involves particularly the upper parts

1 -39-

which have not yet acquired plastic consistency by superposition and diagenesis. In the sub-aquatic deposits, therefore, these form

a 3 - 10 m. thick "thixotropic zone II as described by Boswell (1948). However, the quick soils are not confined to these zones, nor even to sub-aquatic regions. As the Scandinavian examples show, there are extensive flow• prone qUick soils above the sub-aquatic ｾｯｮ･ of deposition as well, which are already protected by a dry crust. A third possibility is found in the qUick soils which are em• bedded in plastic layers or which occur at their bases (Ackermann 1948, p. 21). In contrast to the first-mentioned qUick SOils, the latter are no longer in the primary thixotropic state. Rather, they have undergone a transition to the plastic state during an earlier diagenetic stage, along with the material superimposed on them. In these particular strata, the water seeping over the water-impermeable strata or through intermediate sand or boulder beds (shore ridges or the like) as described by Rosenquist (1944) results in the leach• ing out of the salt content. In 1934 G. Holmsen had already noted the considerable salt content of the waters emerging from the blue clays or ｯ｢ｴ｡ｩｮ･ｾ from them by drilling, as well as the changes of concentration that occur when they flow for some time. In a personal conversation with H. Meixner in 1944 in the Lerkedal mentioned above (page 14) he kindly informed me that water had the lowest salt content in the flow-prone qUick clay zone. These "secondary qUick cl8.ys", therefore, were only transformed from their plastic state to their present thixotropic conSistency by "leaching • diagenesis". QUick clays originating in this way are aSSOCiated, of course, with marine deposits. In the latter they may be various• ly distributed, depending on the varying ground-water conditions, electrolyte content and other decisive factors. ',fuen we further consic1er that the flow-proneness of such clays has also been influenced, as mentioned above (page 19), by the grain size distributlon during deposition, we begin to understand why , for example, certain parts only of deposits maintained in terrace form tend to produce flow movements of a landslide char-act er, -40-

Therefore, if quick soils 0.0 occur outside the "thixotropic zone" their numbers will be limited so that even characteristic earth flows are possible only at places where several unfavourable factors coincide. Such ･｡ｲｴｾｭｯｶ･ｭ･ｾｴｳＬ whose thixotropic character has been demonstrated 1n many cases by special sOil-physics methods (Ackermann 1948), were first found in various Norwegian coastal regions. The earth movements which have beccme known in historical times up to 1944 in the vicinity of Trondheim are indicated in Fig. 13. The greater frequency of occurrence after 1940 can be as• cribed to construction work which resulted in greatly overloading the quick-clayey sUbgrade. But even when we disregard these more recent movements in the immediate vicinity of the city of Trondheim a considerable number of such events are noted. These regions ex• tend along with the marrne blue clay deposits of southern Norway into the adjacent parts of Sweden. Several earth movements dis• covered at BohusLan (Statens ｊｾｲｮｶｩ｜ｧ｡ｲｳ 1922), e. g. Harnmar-bugaar-d 1920, can probably be attributed essentially to thixotropic struc• tural collapses and flOttl movements, a Lthough in 1922 the flow pro• cess was not recognized as an essential condition. As far as their geological conditions of occurrence are con• cerned, the Norwegian flow movements can be considered a special type. They occur in the coastal parts of a shield-like uplift region. The soft plastic clays of the former ocean bed were ele• vated during the post-glacial period, while those areas now close to the coast line, have risen above sea level only in historical times. Here loose se11ments reached the stage of denudation and transference soon after heing deposited. The flow movements con• stitute a special form of the latter. Thelr subaerial deposits are • subject to repeated denudation processes. Only rarely can tlJey be maintained. Fossil transference products of such flows are to be expected only from sub-aquatic soil movements. Many of these move• ments begin on the shores of bodies of water. A wide variety of shoreline deposits are carried off with the flowing mud. They may be transported hundreds and even tlJousands of metres into distant -11- depressions, thus smoothing out the unevenness of the terrain (e.g. Thamsnavn).. Shoreline sediments may thus reach a deposition con• sequence some distance from the coast line and remain there. Soil movements of a similar kind occur in Alpine lakes where bog limes and so-called, "silty sands" (whLch also contain orgamc substance) SUddenly flow away. Albert Heim's (1932) impressive de• scription of the Zug catastrophe of 1887, has recently been supple• mented by v. Moos (1945) with some soil-physics data. At a shore• line subsidence on the. Lake of Gerardmer in 1944, the structural collapse of the bog lime which flowed away (v , Hoos and Butsch 1944) was initiated by the dynamiting of tree roots. The present author demonstrated thixotropy in a bog lime of the Walchensee. The conditions for such flow movements are probably present also in glacially carved regions containing closed-in basins and lakes, e.g. in Sweden and Finland, in the Baltic and in the drift areas of northern Germany as well as similar regions in North America. In Canada flow movements of an apparently stable mud bot• tom of an Ontario lake have been described by Legget (1948). In the Cge1see (Brandenburg Province) Potonie (1913) has described how an island formed in the upper part ofa three-metre-thick calcareous sapropel deposit which was closed off by a bed of sand ｾ m. thick. The latter was burst open explosively by gases generat• ed in the sapropel and at the same time was raised together with the plastic parts of the underlying ooze above the surface of the water, while at the same time the sapropel which had been liquefied by the sudden release of ｾ､ｓ flowed from the side into the gap that was left and thereby prevented the island from sinking back into the lake. The mud acquired its bearing capacity for this by rapid thixotropic stiffening. Smaller flow movements must also he reckoned with in the mud deposits of shalloHs on the steep banks of tidal inlets. Larger redepositions are to he expected as a result of sub-aquatic soil flows in maritime regions, especially at the edge of the continental block - Hull's (1899) t1Great Declivity" - and also on the slopes of depressed trou[Shs (r.:scher 1916), and especially on ocean floors -42-

which have been s t.c epened by irregular secUmentation or tectonic movement s , Actually,. some of the many ocean Gable hreaks 0111ne 1897) were associ0ted with sudden superpositions of sediment and took place on days when there were earthquake shocks , The se probably lnitiated structural breakdowns and flo",! movements in loose deposits of the thixotropic zone (e.g. 0n the eastern siele of the Grand Banks of Newfoundland and at the shelf drop of Africa, 240 km. west of St. Louis de Senegal on the lOth of March, 1895). Finally, spontaneous liquefaction can occur under certain con• ditions in such artificial clay-sand "depositions ll as hydraulic fills, embankment fills, etc. Ehrenberg (1933) reports that about: 1,000,000 cu. m. of loosely deposited fine sand (chiefly 0.2 - 0.5 mm , grain size \'lith about 4% clay particles less than 0.01 mm ,) from a sand dump, followine a rapid rise of ground-Hater level and collapse of the base of the fill, flowed Within a few minutes into a coal pit 700 m. away. Plastic flow movements viittl thixotropic appearance are more widely distributed in fine-grained, loosely bedded soils than flows of quick SOils. Natural conditions are more favourable to the retention of plastic consistency than to tiiat of quick consistency. Plasticity is still present in many loose r-ocks which have been above sea level for some time. The oozing out phenomena occurring in cOnstruction projects during subsidenGe and the like, as described by v. Terzap-,hi (1925), are widely distributed throughout the demonstrably thixotropic coarse clays of Norway (cf. p. 35), onc1 in other regions, also, .. they afford examples of thixotropically influenced, plastic flow movements. Movements of. this kind are encountered primarily on • f'ounda t Lcn soils which were laid down only in geologically early times. In addition to a few coastal depressions and maritime bays in the extensive uplift regions of the Scandinavian and Canadian shields, there is the example of the Malay Archipelago and similar zones which show active earth crust movements. Along sinking coast lines and in their river mouths (e.g. at Hamburg) plastic flow or yielding movements are observed under local loading (Loos 1937, -43-

p •. 163). Settlement regions above zones of salt leaching (e.g. Or1atal, Scheidig 1941, p. 154) are also involved as well as the bor-der- areas around filled in lakes (Lake Zug, v , Moos 1948 • Bodensee, Scheidig 1941, - 6gelsee, Potonie 1913, - Singer 1932), the parts of former lal\es and extinct river tributaries left dry by filling or drainage and territory subject to intermittent flood• ing. Delta fills with their irregular compositions constitute a special group. Not only recent anrt post glacial, but also diluvial and ter• tiary deposits can show a tendency towards thixotropic structural collapse, provided they are not too greatly compressed and dia• genetically altered by pressure from ice or later covering stratao Examples of this are afforded by soil movements in neocene, pre• dominantly pliocene clays of Roumania (Scheidig 1941, p. 184) and Java (Loos 1932). Thus even clay-sand strata with only a thin cov• ering, especially the neocene littoral regions, e.g. the Hessian Basin, the Rhenish rift valley, and the. Pannonian a.nd Viennese basins, come within the scope of our consideration. Various locally confined loads over extensive plastic strata occur under special conditions in the course of natural deposition processes. On the Baltic Sea coast, considerable sand loads ac• cumulate on the Kurische Nahrung, in the form of narrow dunes, and the clay marl is squeezed out ahead of the tallest parts of the dunes (K. Andree 1932). Local sedimentary mounds produced by wind and water currents can thus exert similar one-sided pressure effects, as can volcanic deposits (e.g. Java, Van Bernme1en 1937). Delta fillings belong to such locally restricted deposits also. At the end of the r-assissippi delta (according to Shaw 1913) • islands rise in the channel whose muddy substance is squeezed out from the delta deposits. The soil on these islands, for the most part, has plastic consistency. Only an insie;nificant part of the mud moved emerges from within the islands in the form of thinly liquid mud in small craters, like the salinelles of mineral oil fields. Host of it is forced up as a plastic paste from the older deposits, Nhich were formerly a long Nay from the coast and are -4.4-

therefore richer in clay, by the weight of the more recent delta deposits which anvance at the rate of about 100 m. annually. They rise at places where the surface covering has been weakened by dredging of the channel. The upwards pressures occur especially during and shortly afte;r flood periods under the added pressure of the water and new deposits. Durinc the flood from March to May 1913 over the entire region a total of about 2.5 million cubic yards were deposited, copresponding to an added ground pressure of about 0.4 kgm. per ｳｾＮ cm. about 200 m. above the mud island zone. Only artificial disturbances of the natural deposition conditions in the Mississippi produce these unusual upward plastic movements. It is possible that erosive ocean currents can produce similar effects. It would be scarcely possible, however, to trace the displacement forms thus a.risine; in fossil examples back to the initiating cause. To the best of the author's knowledge no recent examples of the sub-aquatic movements with flow contorsion often preserved in fossil form have been observed, but their presence can be inferred in certain deposition regions. Other folding phenomena showing probable thixotropic influences are the known contorsions which have formed in young plDstic deposits (e.g. SpitZberGen, Gripp 1927, Table VIII) in varved clays and in tertiary clays under the pressure of advancing ice messes. However, as in many cases proof of the fact that the strata in question actually had thixotropic properties is lacking.

Summary

.. Most ordinary landslides are not associated with thixotropic phenomena. Only under ｱｾｩｴ･ exceptional circumstances does thixo• .. tropic liquefaction of qUick soils dominate the motion so that the "ideal case of earth flow" develops. Sliding and flowing movements frequently participate together in the landslide type movements of mOist, fine-grained lOOse sediments. Both forms of motion can be affected by thixotropy. In either case it is necessary to decide whether the temporary change of viscosity termed thixotropy goes as far as liquefaction, as in the case of soils haVing qUick consist- -45-

ency, or merely involves a (partially) reversible change of strength, as with the plastic soils. The compositions of loose sediments subject to thixotropic movement vary from fat clays to sands of low clay content and also include calcareous mud ,deposits and fine-gralne

st-itisl water pressure in one c1irection. Quick clays which ｢ｾ｣ｯｭ･ viscous result in local displacements which may involve both sliding and flowing movements.

ｾｵｩ｣ｽｻ soils flow under their own weigilt alone I i. e , , subject only to the action of gravity. Loose sediments in the plastic state flow under preSSUJ'8 from one sloe applied locally. In this case the thixotropic viscosity change results not in liquefaction but merely in a temporary reduction of strength. Thus plastic flow takes place under pressure in a given direction where there is a sudden reduction of internal friction. Thixotropy facilitates the process. The loss of strength at the beginning of the flowing movement is due to a structural collapse which for the most part is not thixotropic and is therefore non-reversible; a small part only is rendered gradually recoverable ｡ｾ｡ｩｮ by thixotropic resolidifi• cation. There is no difference between the movement processes of sub• aerial and sub-aquatic flowso However, in future it will be neces• sary, with reference to the extent of the mass transport and the character of the re8ulting deposition, to decide between thixo• tropically influenced flows, settlement I Lows Hi thout any thixotrop• ｩｾ component and the ordinary sliding movements. Thixotropic flows differ from settlement flows in that they show resolidification without change of water content, and they differ from creeping earth glaciers in the rapidity of their move• ment and their independence of water acquisition and loss. Thlxotropically influenced movements are restricted to geo• • logically young, loose sediments with sufficient fine-grain anrl mOisture content, but they occur not only 11'1 sub-raquat i c deposits but in nry-land deposits as well. -47-

Li terature

ａｃｋｅｒｾｉａｎｎＬ E.: Thixotropie und Fl iefsuigcnscha Hen Ieiukorniaor IWdcn. Geol. Rdsch. 36, H118. - Die quicke Konsistenz. Manuskript 1930. - Das Hcwccuncsl.il.l ei nes aktiven Erclrutschcs nebst cincr Ft-inst rat igraphie des unteren Wellenkalks. Manu• skript 1950 (Akad, d. Wiss, Go tti ntrcn). -- ANDERSSON, J. G.: Solifluclion, a com• ponent of subaerial denudat ion. Journ. of Geol. 14, 1906. - ANDREE, K.: Ilnu urul Ent• stchunz der Kurischen Nehrung. Kiinigsberg ｬＡｮｾＮ -- ATTERBlma. A.: Di« I'Iastizi tat del' Tone. Internat. Mitt. f. Bodenkunde 1, 1911. - BENDEL, L. &:: Hl:CKLI: Die Erd• rutsche Emruencgg unr] Dallenwi l, StraBe lind Verkchr Nr. 1.), 1!J;17. - VAN HIDDfELEN, R. W.: The Volcano-Tcctonic Structure of the Residence or Malana (Eastern Java). De Iricenieur in Nederlurnlsch-Indic IV. Batavia In:J7. - J:.JilnLYKKE, K. 0.: Om Under• grunnsbanen ojr flrsaken til sprekke.lannelscu in h uscne i Kristiani n by. If. Kr isti ania 1£)21. - HaswELL, P. G. H.: The Thixotropy of certain sedimentary rocks. Science Pro• gress 31i, London t818, ]/.112-J22. _A preliminary ex a mi nation of the thixotropy of some sedimentary rocks. Quart. Journ. Gcol. Soc. 104, S.I!l\J-:i:W, London 1ｈｾＸＮ • OASAGRANDE, A.: The structure' of clav and its importance in foundation engineering. J. Boston Soc. Civ. Irurr», 19, Nr. ·1, 1\);12. - CLOOS, H.: Flid3Pll lind Hrcchcn in del' Errlkruste und im gcologischen Experiment. Pla-a isch« Masson in Wiss, u. Teehn. Tros• dorf bei Kiiln 1831. - ECKSTROM, G., och FLODKVlST. II.: Hv.lrulonisku undersdkniugar av aakerjord inom Orebro Iiin, Svcr. gcol. Unclcrs. Arsbok HI) 1\323. - EHHENllEIW, J.: Das Ausflieflcn ci ner Sandkippe in einer Brauukohlengrubc. Dautechnik 11, 1933, S.254 bis ｾＵＷＮ - EUCKIiN, A.: Lelirbuch del' chcrnischcn Physik. Leipzig l[J·14. - ESCHER, B. G.: lJeschouwingen over het Opullings Mechanisme van Diepzeeslenken. Verh. Geol.

Mijnbouwkundig Gcn. wor Nederland U3W. Geol. Sev., .Iuni lnu.i. --0 FENNER, R: Unter• suchungen zur Erkcuntuis des Gebirgsdrucks. Ghickauf 74, 1\);\8, S.681/95 u. 705/15. • FREUNDLICH, II.: Thixotropy. Paris 1935. - FREUNDLICH, H., and JULIUSIltJRGER, F.: Quicksand as a th ixot ropic System. Transact. of the Faraday Soc. 31, 769. Edinburgh 1935. - GRIPP, K.: Glaziologische ' und geologische Ergebnisse del' Hamburgischen Spitzbergen-Expedition ＱＹｾＷＮ Abh, aus d. Geb. d. N aturwiss., Nat.urwiss. Vel'. Ham• burg 32, II. ;)-1, S.145-24\J. Hamburg ＱＹｾＹＮ - GRIPP, K., & 'l'ODTMANN, K: Die End• morane des Green-Hay-Glctschers auf Spitzbergen usw, Mi tt. d. Geogr. Ges. in Ham• burg 37, 1926. - HADDING, A.: On subaqueous Slides. Geol. Forcn Forh. 53, 1931, S.377-39;). - HAEFELI, R.: Mechanische Eigenschaften yon Lockcrgcsleinen. Schweiz. Bauzeitg.111, S. 2H9-;101. Ziirich 1938. - HAEFELI, R., & v. Moos: Drei Lockergesteine und ihre tcchnischcn Problemo. Schweiz.lJauzeitg.112, HI3H. - Hxuo, Se.: Svevendc trepeles baereevne i Leirc. Medd. Ira Norges Staatsbaner Nr.2. Oslo 1840. - HElM, ALB.: Berg• sturz und Menschcnleben. Vierteljahresschr, N atf. Ges. Zurich 77,1 \);)2. - HElM, ARN.: Uber rezente u. fossile subaquatische Rutschungen u. dercn lithologisehe Bedeutung. N. J b. f iir Min., 1908, Bd. II, 8. 136--1:'l7.-HELLAt

- REUSCH, H.: Nogle optegnelser fra Vaerdalen. Norges Geol. Unders. Nr.32, Aarbok for 1900. Kristiania 1901. - POTONIE, H.: Eine im Ogelsee (Prov. Brandenburg) pldtz• lieh neu entstandene Insel: Jb. d. Pr. Geol. L.-A. Bd.32, Berlin 1913 fiir 1911, 8.187. • 8IUFERT-EHRENBERG-TIEDEMANN, ENDELL, HOI"FMANN- WILM: Bestehen Zusarnrnen• Hinge zwischen Rutschneigung u. Chemie von TonbOden? Mitt. PreuB. Vers.-Anst. Wasser• bau u. Schiffbau, Heft 20. Berlin 1935. - SHAW, E. W.: The mud lumps at the mouth of the Mississippi. Prof. Paper, 85-B, 1913, 8.11-27. - 8CHEIDIO, A.: Del' Loss und seine geotechnischen Eigenschaften. Leipzig 1934. - SCHEIDIO, A., in: KOGLER, F.: Bau• grund und Bauwerk, Berlin, 1. Aufl. 1938, 3. Aufl.1941. - SINGER, Y.: Del' Baugrund. Wien 1932. - Statens Jarnvagars Geotekniska Kommission 1914-1922. Slutbetankande, Stockholm 1922. - v. TERZAGHI, K.: Erdbaumechanik auf bodenphysikalischer Grund• lage. Leipzig 1925.- TISCHENDORF, C.: Die Rutsehung in Vardalen, Norwegen, 8chweiz. Bauzeitg. 23, 1894, I, 8.17 u.25. - TROLL, C.: Die Formen del' Solifluktion und die • periglaziale Bodenabtragung, Erdkunde 1. Bonn 1947. - WINKLER, H.: Thixotropie von Mineralpulvern mikroskopischer GroBe. Kolloid-Beihefte 48. Leipzig 1938.

During printing, the following appeared: Cailleux. A. and Tricart, J.: Un type de solifluction: Les coulees boueuses. (A type of solifluction: mud flows)

•

• .. • • It ..

TablU

Soil-physical indices of plastic and quick-clay deposits from Norwegian ｣ｯｾｳｴ｡ＱｲｾｧＱｯｮｳ

I Grain size distribution -, Location of in mm. I Sere I Test Place of origin sample and 0.1 to 0.02 to Consistency >0.1 <0.002 P F w k no... no. depth in m. 0.2 0.002 I form

48 Drilling 46 2.81 2 35 53 10 39.8 68.0 62.5 ｾ 49 981}982 I Bay on the inner " " 3.4 ------21.3 40.2 36.8 +0.14+0.07 Liquid 50 983 Oslo Fjord near " " 3.9 20 63 17 26.4 46.5 44.5 +0.08 plastic 51 6.3 - 21.8 42.4 41.0 +0.06 I 984 Bekkelaget " " ------I 52 I 985 II " 8.3 ------14.0 ,39.3 39.5 -0.02 c;.uick ｾ I Upper terrace on I 53 the Trondheim Drilling 02 2 ------19.8 35.6 31.2 +0.22 Liquid plastic 54 I857] Fjord: Lerkedal II 3 - 12.0 27.3 27.9 -0.05 I 858 " ------'I ｾｵｩ｣ｫ II - 55 862 in the middle of II 7 ------7.7 25.2 38.7 -1.75 j 56 863 the sliding flow " " 8 ------9.0 25.8 25.2 +0.06 Liquid 57 867 Alongside flow " " 4 1 11 58 30 9.7 29.0 27.5 +0.15 } plastic 58 5 Fornebu, low Trench 2.5 1 19 45 35 19.5 44.0 40.6 +0.17 Liquid plastic land on the Oslo Fjord

.. Serial ｮｵｾ｢ｾｲｳ follow consecutively those of Table I in Ackermann 1948. Abbreviations: P- plasticity index; F- ｬｩｾｵｩ､ limit; w - natural water content; k -p-F- w relative consistency .. • . ｾ

Table II Soil-physical indices of quick-clay samples from Norwegian earth flows

Grain size distribution in mm. Location of sample and 0.1 to 0.02 to Ser: I Test Place of origin >0.1 <0.002 Er P F w f k H no. depth in m, 0.02 0.002 1 no. i i 7 I 957/2 I Lerkedal 1944 I Lower- end of flow -- 12 52 36 37.3 8.0 24 34.5 -0.79 -1.32 I 0.47 9 935/5 I Lerkedal 1944 79/5 45.4 9.3 24,2 25.0 -0.04 -0. 08 1 i I ------1 935/9 j Lerkedal 1944 79/9 ------29.0 9.1 23.1 29.1 -1.01 -0.65 -- 59 151 I Hommelvik 1942 Sliding surface 10 23 29 28 -- 10.2 23.7 25.6 -- -0.19 -- 60 292 I Hommelvik 1942 I Slid in,!! sur-race ------8.0 23.4 Z'? .9 -- -0.55 -- 36 I 870 I Ilsviken 1944 Mound in front I -- 10 57 33 99.8 10.7 31 35 -0.05 -0.37 -- I I of sliQe baGe i I 61 805 a Lade 1944 I Front - house 20 12 53 35 -- 5.2 24:.8 33.9 -1.39 -- CJ1 --11 I -- o 62 805 b Lade 1944** Front- house 20 "48 "8" "34" "7" -.,. 24.3 48.3 , 68.0 -- -0.81 -- I 53 691 Lade 1944 ':{estern part -- -- 12.5! 24.8 32.3 -- -0.60 I ------I -- 64 730 BrOther 1944 'Ilt. emerged mass ------13.1 25 24.2 -- +0.06 -- I Foot of slope -- 65 738 D i Drilling 01 10 ------1l.3 26.9 40.9 -- -1.25 0 •.3 ＢＢｾＬｾｏ 7·38 E " 01 13 ------. 7.7 2,,).5 40.6 -- -2.22 0.3 67 739 D Fj-=llvik 1945 I 02 10 -- 6.8 21.0 22.1 -0.15 0.5 " ------.... -- 1 -- 68 I 739 ｾ i'11dd1e of " 02 1,3 I ------3.9 19.6 31.0 I -- -2.94 0.3 2 740 D earth flow " 04 4 I 29 60 10 --1 42.3 -- -- 22.4 ------5 740 E " 04 5 ------35.0 7.9 19.6 29.5 -0.67 -1.28 0.4 69 740 F " 01 13 ------5.8 18.6 35.4 -- -2.90 0.3 70 762 Rygge Drilling 1969C7 28 50 22 70.0 1304 31.6 37.8 -0.46 0.3 i -- -- 7lj Aserumvan***" -- 9 32 59 -- 7.4 29.4 62.8 -- -4.51 ｾｏＮＳＳ * Num,:,ers 1 - 47 agreeing with Table 1 in Ackermann 1948. ** Fraction >0.1 mm. consisted chiefly of cellulose fibres. Sample contained ｯｲｧｾｮｩ｣ impurities. *** According to ｡ｯｳ･ｮｾｵｩｇｴ 1946. Abbreviations: Er - stiffening limit; P- plasticity index; F- liquid limit; w - natural water content; f = ｾ : F= F- ､｡ｮｾ･ｲ［ w ｲ･ｬ｡ｴｴｶｾ - index of flow k = ---p-- = consistency; H1 relative strength of the disturbed (stirred) sample according to the Swedish cone test -51-

Undisturbed loose sediment ｱｵｾｳｩＭｳｯｬｩ､

Quasi-solid ｾｾｳｴｾｴ･ Solidification thixotropic structural Structural disturbance Ｎｲｲ｡ｮｧｾ .. Temporar:tructura:'COllapse

reduction of / viscosity liquefaction

Fig. 1 Cycle of changes of strength in a quick clay after a structural disturbance

sand

ｾＶＰｾｲＭＭＭＭＭＭＭＭＺＭ］ＭＭＺＭＢＢ［ＭＧＭｴＬＭＧＺＭＢｾＧｈｦ＼ｾｾｾｾＧＺＺＺｴＢＧ［ｮｾＢＷＭＷＢＢＷＭＺＷＧＭＭｴＺＭｲＭＭ｝ .... ｾＴＰｴＭＭＭＭＭＭＭＭＭＧＧＭＭＧＺＭＭｴＭＺＭＭＺＺＭＮｾＺＧ｟ＺＧＺ｟Ｇ｟ｾｾｾｾｾｾｾｲＭＮｾｾｾｌｲＧＧＧＭＺＭｬ >. ,Q ｾＲｖｦＭＭＭＭＭＮＮＮＮＮＺＮＮＮＮＮＭＭＺＭＺＫＭＭ［ＭＭＺＭｾｾｾｾｾﾥｾｾｾｾ＿ＷＧｓｌﾥｾ

• soils •

Fig. 2

Flow ability and zones of ｾ｡ｩｮＭｳｩｺ･ distribution of non-thixotropic and possibly thixotropic sand-clay soils, based on available test material. Denser shading indicates increased flowabll1ty

I j -52-

First Second structural structural disturbance disturbance ! Jl, Ｍｾ deposition strength of a plastic clay

- H;s deposition strength of a plastio quick clay

ｾ strength in the undisturbed state ｾｾＬ decrease or strength on ､ｩｾｾｵｲ｢｡ｮ｣･ to the structure ,I i H,-Hr increase of strength during solidification j ,i :I Thixotropic ｾｮ､ .... I strength (, ':i i ... _----jI ..... _------.Hr pla.stic clay r-l I .,'" u ... 4J ! ｾ 1," ｾＢ !/ ./' -10 F' H, I-- _ ..{ __ _ _. I lit (quick olay' II, o;so 0", 100 i60 360 DA')'S --+ Solidification time Fig. 3 Change of strength with the time of a quick clay (dotted curve) and a thixotropic plastic clay due to two structural disturbances. De• crease of strength during the first structural disturbance 1s indi• cated in common for both clays by a single line (dot - dash line). H3 - Ht = non- -lixotropic component of strength decrease • • • ! ｾ •

CONSISTENCY FIOt/ COi-JDITIONS Thixotropic sediment Limits Diagram Forms II Non-thixotropic sediment Continuous possibility of flowing due to intrinsic weight Under mechanical stress: liquid Stiffening Temporary No reduction of viscosity reduction of viscosity I limit _ILLI L&.I der own wt. only if structurally disturbed ｾｯｲ｡ｲｹ viscosity reduction = liquefaction II quick CJ.Ily number- of times "II" only once :'iquid thixotropic solidification solidification after loss I limit without loss of water of water (Jl 01 Plastic flowing only under pressure I plastic temporary reduction permanent reduction of va scosfty Plastic of viscosity limit thixotropic solidification no resolidification ------""""'"iloOOOO"I" .... II " I Flowing deformation under high pressure i solid no thixotropic changes Fig. 4

Flow conditions of loose fine sediments in various consistency ranges -54-

ｾ 0 0 '0 frJt..ｲｾＧ eAtkr "'9I! 118 1""0' ," ｾｾ .t'" 16 - '" > Ｎｾ [.....--"" " ｾ f-- +' ;:; ｱＮｾＮｾ I:-- '" 14 V -fJ 0 ｾ o . V ...-- t>I!" 12 '" -0 ,/' --- '0 'd II / ＱｾＧｙ S ､ｰｰＶｲｾ , l'irL "'Ina eAri'a / 1/ II 1// Lo.d ｪｴｾｧＫＭａｾＧ osl< -I // tl1\e i.,te,·yd, I At ,. 2. ,krj. jV I .. i9' o V o 1000 I' 11. 1'30 \" ISOO • Time from load application in min• • A - Vj epne<;e ma..;t:] - "ptnr,..] compress; on (-p=20) Fig. 5 Slow plastic flow before fracture. Test with Viennese marl (from Hvorslev 1937)

Consdt'!ftt.y ...... Mbt...r ｃｏＢｩ｜ｴｨ＼ｾ -IS-'-/UO -as., lid••ｲＢｰｾＮ r ｾ 10 JO ",6h

ＬＧ］ｾ =.= \ =...... ----\ ｾ • R w F

Fig. 6 Lerkedal cutting at Trondheim. Transverse profiles and consistency diagrams thrOUgh a slide {upper profile) and a ground shift with pre• dominantly flowing movement (lower prOfile) on 22nd Aug. 1944. Ver• tical hatching - dry crust; dashed - soft plastic clay; wavy dash• es - qutck clay; k - relative consistency; R- pla.stic limit; F• liqUid limit; w - water content (grain size distribution cf. Fig. 2 and ｾ｡｢ｬ･ I, no. 6,7,38 in Ackermann, 1948)

1 -55-

ｾ Drainage gulleys of the earth flow (.J Mud covered area o Silt [B Presumed remains of earlier landslides ｾ Sand on top of silt Q Sand

==-===------,= =---

Pig. 7

" Map and sketch of earth flow in the Vaerda1, 1893, (from H. Reusch, 1901) • • 11 • • •

HOMf'1£LVIK Road I SIN NO ｒＴｩｬｲｾＶｾ

·111 ....,,=="""..｟ｧｦｦａＧｦｦｾＧ .. · ngu. t-" ｾ -- -,;,.. -_._--- ·'0 Ｍ｟ＴＢＬＢＴＺｌｾＺｾＺＺＸｷＬｾＺｴＺＺＺｾＬﾷＭｾＭＬｷＬｾ ...... ;.·..,·;------I- JP o .. .1IJt) 100 Jsa I - en - (J) ZUG I Lake Zug ｾＭＧＦｵ｢ＭｲＺｍＭＭＭｵＭＭ［､［ＭＭｳＭ｡ｮＭｾ､ｾ

ｾＢＢＧ .. 4.o...... • • • Boulder 0180:1 o Mud flow () m:lm 1M Slide material' (n«IIAHlIM)

Fig. 8 Profiles through the flow movements of Hommelvik, Central Norway (above) and Zug, Switzerland (below). Hatched areas - slide ｾ｡ｳｳ･ｳ［ shaded areas - shifted mud

Ｎ｟ＭＭＭＭＭＭＭｾＭＭＭ｟ＮｾＭ｟Ｎ｟ＭＭＭＭＭＭＭＭＭ -57-

• ＢＢＢＢＢＢＢＢＬＬｾＱ •..

ｾＱＱＱｉＱＱｉＱｉＱＱｉＱＱＱＢＱＱＱＱＱｩＱＢＱＱＱＱＱＱＱＱＱＱＱＱＢＱＱＱ 28 lfiliiiiiiiiiijiiiiiiiiiiiiiliillillilliiiiiiiiiiiiiiiiiiiiiiiiiJiiiiiiiiiiiiiiiiiiiiiiiiiiiiliiiihi

ＯＬＯ］ＭＭＭ［ＬＭｾＭＮ 2b ＢｬｩｩｩｩｬＢｩＺＺｩｾｬｩｩｩｾｩｩｬｩｬｬｩｩｩｩｩｩｩｬｾｩｩｩｬｩｩｩｬｩｩｩｾｩｩｬｩｩｩｩｬｩｾｉｉＧＢ

Fig. 9 Diagrammatic representation of sliding (1), flowing (2a and 2b) and combined sliding and flowing movements. Hatched parts _ dry crust or non-plastic covering deposits; white part - softly plastic • clay; black parts - quick clay -58-

ff.+.•

--__ -....J-----

D..ｩｬｬｩｮｾ｟

o ｾ u ｾｾｾｾｾ " ｾｾ i

Fig. 10 Profiles of the Lade landslide. Height not exaggerated Upper profile: Section through the eastern slide region and the house at Ladeallee no. 20. Broken line: surface before slide. Solid line: surface after slide. Falling in the upper portion, ele• vation in the lower part. Increased lateral movement below the zone of fecal mud Lower profile: Section through the western slide region. Broken and solid lines as above. Remnants of quLck clay horizons shown by drillings above the slide. Bulging upheaval at the lower boundary

,... Fa.urt norma.l o Ea.rth wa.ll ｾ Quick clay outoropping :0 Bomb cra.ter o Drilling o A

Fig. 11 Ditch-like depression and wall formation by subcrustal plastic flow• ine due to the structure being disturbed by bombing -59-

Sun "om th. (j.Ptnand'/altan (Ttn99tr mountains}.

Fig. 12 View ｾｮ､ profile of the Ardjoeno-Ringit Volcanoe Group, Java (from Van ｾｬ･ｮ 1937). Sliding down of volcano cone and contortion folding of neocene clay sediments at the foot of the slide mass ..

• • • ｾＭ Ii

[: TRONDH[l115FJORD

AA5ENFJORD

S,.,., ＱＧｾ＼Ｚ＾ｾ 171'0

ＯＧＺＢＭＺｾＮＧｉｾＭＭＺＮＮｩＺＩｾｾｾ［Ｚｾ __ｾＮ '-' ｾＺＧＭＭｾＭＬ ...... - - - . ｾｾｩＭｾＭＧＭｾ .... - .. =-... "" ＬＬｾｾｾＺＺＮｪｾＭ［ｾＭ［ＭＭＺＭＭﾷｾＧｩｾ＾ﾷＬｾｾＬＺＭＯ J c, (J) ｩｾｾＭＺＺＧＩｽｾＧＢｾｾｴｾＩ o ＧＧＭｾ I , __ ",_I ,_-:" CJRock I _. ｟ｾＧＮＮＮＮＺＺＮＲｾＺＺ ...:...... - -< /:.. ＬｾＭＭ｟ＭＮＬＮｾＮｾＬ .' o Post-glacial I ＧＭ］ＭＺ＿ｾＲＭ］ＺＮｾＲｪＧ "7 ＭｩＸＶＸｕｏＧｾｃｇ and sand deposits ＧＮＬＧｾ :.::' :.._:" :'-/"" ＼ｾｾ［ｾｾＯｾｏＤＬｾＬｾｾＺｾｽｾｾＺｾｴ［ｬｾ［ｩ､･ＮＺｾ｜ｾｾｾＺｾｾｾ｟ＺＺＬＬＭＬＺ［Ｈ［ｾ｟ｾｾ ::;; -; l -,ＭＮＮＮＮＮＺＮＮｾＬ ... ＭＭＭＭＭＭＭＭＭＭＭＭｾＬＬＬｾＮＧ ::' - ':':;-,-,-,-:' Ｍ］ｾＢＧｾ］ＭＺＧｾＭＭＭＭＭＭＭＭ -=. 1,-....:. •• Q ＱｰｾｭｾＧ［ｪｾＭ］ＭＭＺＬｾ［ＭＮＯＧＧＭＧ＼ｾＷｾ｣ｾｾＬ［Ｇ

Fig. 13 Map of earth slide (including the flow motions) in the post-glacial clays near Trondhelm in historical times up to 1944

.. -61-

Ｑｉ｟ｾ｟ＭＭＭＭＭＭＭＭ !s- - __ ------/--.:::- ...... -IP 1'/ <, -, <, ""' ...... " 11- -- "..... , "------..... "" ..... ", .,.- -- - - ...... -, " ...... -, \ " '- -, \ , \ <, ,

•(\

10 Map of the slide-flow in Lade (Trondheim)

LADE LANDSLIDE

Decades old landslide ｾ Ground collapse of 11.4.1944 P 26-27.7.1944 PI> 30-31. 7.1944 ｴＮＮｾＮ Upthrust region of 11·4.1944 ｾ Quick clay and mud mixed with ｦ｡ｾ｣･ｳ r Water ponding . ｵｾ Contour lines in the area ｾＭ - of motion of the 11.4• . Contour lines outside the area of motion of the 11.4. ｾｾ Boundary between areas of subsidence and elevation A'I Is' Profile 1 ines ｾ Motion vectors .;:::: Ladabach streambe.fore and after the movement (from a map by H. Meixner, redrawn by E. Ackermann) oｾｾ｟ｾｾ｟］ｾ｟］ｾ｟］ｾＺＺＮＧＰＬｏｭＬＭＭＭＭｾｾｾｾＮＧＬＭＭＮＭｾ