Quick Soils and Flow Movements in Landslides

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Quick Soils and Flow Movements in Landslides

Ackermann, E.

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QUICK SOILS AND FLOW MOVEMENTS IN LANDSLIDES

/

TECHNICAL TRANSLATION 839

BY

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

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Qu1ck s01ls and flow movements 1n landslides

(Qu1ck erden und Fllessbewegelngen bel Erdrutschen) Ernst Ackermann

Zeitschr1ft der deutschen geolog1schen Gese11sohaft,

100: 427-466, 1948

•

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 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,

September 1959

H.F. Legget, Director

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

Thixotropy. ''Ii • • • • • • • • • • • • • • • • • • • • • _,_ • • • • • • • • • • • • • • • • • • • • • • • • 3

The quLck consistency ••••••••••••••.•••..••••••••.•••••••• 5

Flow-prone deposits •••••••••••••.••••••••••••••••••••••••• 6 Structural collapse and strength changes in

thixo-tropic and non-thixothixo-tropic 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-aqua tlc 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

L1terature •••••••••••.•••••••.••••.••.••••••.••••••••••.••••••47

Tables •••••••••••••.•••••.•...••.•.•.•.•••••••••••••••••••• 49

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-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

If

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(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 im-portance 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 •

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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,

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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 resolidificaliquefac-tion to the quasi-solid state takes

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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 •

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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 as sociat.ed with loose bedding, it can also occur in cohe s i.on Les 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)

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

•

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.

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,

-8-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" (H₃) and is a multiple of the value after

structural collapse (H1 ) . When the deposition structure is

dis-turbed the strength falls to

a

H₃ to H₁) . Immediately after termination of the disturbance a

cer-tain rearrangement of the loose structure begins, and hence there

is a small increase of strength (H₁ -+ H

t in Fig. 3). In the event

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

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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 vi scosi ty 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

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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

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.

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Where ground movements occur as flowing processes it is

necessary to distinf,Uish between those flows which take place as a

resul t of gravi ty 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

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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 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

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

,

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

=

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

•

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 re-moved 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 todown-wards 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

,

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

8.5 km.2 • The river, dammed by the clay for half a day, formed a

lake which covered an area of 3.2 km.2 •

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

•

•

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 toto-wards 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

•

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

only 0.4 kgm./cm2

• 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).

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.

..

..

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 (o r , 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 (Holm sen 1934) shows, SUb-aquatic flow motions of the thixotropic nature also begin with the suddenness of an avalanche.

Rapidity of motion and the flow movements of quick sible to rescue people from occurred (Hommelv1k 1942). the regions have drowned in

breVity of the process are typical of

soils. Only in a few caSes was it

pos-the areas in which pos-the soil movements Frequently the surprised 1nhabitants of

•

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

If the flow occurs in a terrace at higtl al ti tude 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}

the clay masses (generally 1 to 3 million m..3 ) 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.

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.

•

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

•

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 thickcomparative-ly 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

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

=

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

•

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

•

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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:

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,

•

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

*

rleposits (silts and coarse clays) which occur only once, as