landslides in loess

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A Discussion on Landslides in Loess; Some Large Movements in Metastable Ground Ian Smalley, Ken O’Hara-Dhand Giotto Loess Research Group, Geography Department, Leicester University, Leicester LE1 7RH, UK ([email protected] ) Tom Dijkstra Civil and Building Engineering Department, Loughborough University, Loughborough LE11 3BU, UK ([email protected] ) Abstract Two large and famous landslides were loess landslides. Both occurred in China. The 1920 Gansu slide was triggered by a large earthquake; the Saleshan slide was on a slightly smaller, more accessible, scale. Five factors are considered which relate to loess slides: 1. moderate thickness of deposit, rather than great thickness; 2. some 1

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Some thoughts on the nature of landwslides in loess ground

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Page 1: Landslides in Loess

A Discussion on Landslides in Loess; Some Large Movements in Metastable Ground

Ian Smalley, Ken O’Hara-Dhand

Giotto Loess Research Group, Geography Department,

Leicester University, Leicester LE1 7RH, UK ([email protected])

Tom Dijkstra

Civil and Building Engineering Department, Loughborough

University, Loughborough LE11 3BU, UK

([email protected])

Abstract

Two large and famous landslides were loess landslides. Both

occurred in China. The 1920 Gansu slide was triggered by a

large earthquake; the Saleshan slide was on a slightly

smaller, more accessible, scale. Five factors are considered

which relate to loess slides: 1. moderate thickness of

deposit, rather than great thickness; 2. some relief, some

potential energy is required; 3. An impervious sliding plane

for initial movement, bedrock or palaeosol; 4. Open soil

structure, high voids ratio, metastability, this is the critical

loessic factor; 5. Wet base, water is significant for initial

action; 6. earthquake trigger, may be necessary, may be

observed. Loess landslides are best known in China but

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observed in Danube bank locations, and in Central Asia. The

remarkable nature of loess ground contributes to landslide

activity, in particular the open airfall structure which allows

the initial structural collapse (like hydroconsolidation) which

allows the slide to develop. The Saleshan slide is considered

in moderate detail; the 1920 slide was so huge that it cannot

be considered typical, and little detail is available about slide

mechanics. The open structure and hydroconsolidation

action are the critical factors underpinning the special nature

of a loess flowslide. The aspect of loess which allows

hydroconsolidation also controls the typical loess landslide.

Keywords: loess flowslides, loess material, soil structure

collapse, hydroconsolidation, short range bonds, primary

mineral particles

Introduction

Comprehensive studies (Derbyshire et al 1994,1999,

Derbyshire 2001, Dijkstra et al 1994, Dijkstra 2000) have

provided an excellent overview of landslide problems in the

Chinese loess, particularly loess in the region of Lanzhou, in

Gansu. The large monograph (Derbyshire et al 1999) gives a

remarkably detailed discussion of loess and the Gansu

region and the classic large loess landslides so that it is

probably the most complete treatment of any particular

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landslide ‘type’ so far published. This is the touchstone for

further studies on loess landslides; this current review and

study will focus on mechanisms and structures; this is a

study of ground material, and a consideration of factors for

further investigation, such as particle arrangements and the

nature of loess material and the links between

hydroconsolidation and landslide behaviour. The nature of

loess material plays a large part in determining the nature of

the landslide events, but the well known slides are so large

that the purely loessic factors may be obscured by the

general enormity of the event.

Gansu was the site of the huge earth movements in 1920

when a large earthquake(Richter 8+) mobilised large

amounts of ground, said to be equivalent in area to the

nation of Belgium. The Chinese loess still offers a landslide

hazard(which Chinese investigators have recently been

emphasising; Zhang et al 2002, Zhang & Wang 1995, Wu &

Wang 2006, Xu et al 2007), and benefits from continued

investigation.The Dijkstra(2000) monograph touches on

many material problems related to the Chinese landslide

situation and our considerations follow from his

observations. Another location where loess landsliding

presents continuing problems is in the Danube basin, in

Central Europe. The Danube basin is classic loess country

(see Smalley & Leach 1978) and the Danube bluffs are the

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sites for much landslide activity. The slide at Dunauvarous in

1969 gave rise to much literature(Pecsi 1971, Pecsi &

Scheuer 1979, Pecsi et al 1979, Ujvari et al 2009) and these

Danubian slides provide another thread for our review. The

third region to be identified is Central Asia, largely because

of the recent comprehensive study of Havenith and

Bourdeau (2010), but also the studies in Tajikistan by

Ishihara et al(1990) and Evans et al(2009).

In the USGS world overview of large landslides there are two

loess slides on the current list, the 1920 Gansu slide at no.4

and the 1983 Saleshan slide at no.27 (the slides are listed

chronologically); because of their ranking among the most

significant landslides they must receive attention in this

review. For convenience they will be referred to as the 1920

slide and the Saleshan slide. Both were long-run-out

flowslides, and their existence as dynamical events

depended on their occurring in loess ground. The Saleshan

slide can be the main focus because there is more detail

available about this particular slide event, and it could be

considered that the 1920 event was so vast that it was

atypical; all sorts of features and factors were involved in

this slide, which may have obscured the loessic contribution.

The relationship between the loess ground and the nature of

the slide is the topic of this review, and it is a reductionist

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view with a focus on loessic properties which are central to a

particular flow.

There is a large amount of loess ground in the world but the

relevant regions in North America, South America and New

Zealand do not seem to suffer particularly from landslides.

Loess landslides present problems where loess and people

coincide, in fact loess presents geotechnical problems

wherever loess and people coincide. The tunnel gullies which

form in the New Zealand loess are essentially a problem

when they form beneath someones house. This is why most

of the literature on geotechnology and engineering geology

of loess is in Russian (read Trofimov 2001 for a flavour of

this). The western part of the Soviet Union was a region with

much loess and many people, but relatively few loess

landslides. The major problem in the Soviet Union was

subsidence, the loess ground structure collapsed on loading

and wetting and foundation failures ensued. These were

ground failures caused by the peculiar nature of the loess

ground; the question that has to be asked with respect to

loess landslides is- how much does the peculiar nature of

loess ground specify the type and nature of slope failure that

occurs in it?

The large flowslide such as Gansu 1920 or Saleshan 1983 is

relatively well known although it is only recently that

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discussion has taken place on any large scale about the

exact mechanisms of these very large flowslides, and there

is a perceived danger in parts of China that the threat of

large landslides is still present. There might be some benefit

to be gained by comparing loess flowslides to quick clay

flowslides; identifying common factors may aid in the

understanding of the failure and flow mechanisms. We need,

in particular, to examine the nature of loess and consider the

types of landslide that arise because of the peculiar nature

of loess. There is a tradition in geotechnology of generating

failures by not understanding the nature of the ground

material which is being used in construction. Perhaps the

most famous of these is the failure to understand the

problems of loess material construction when building the

Teton Dam in Idaho (Smalley & Dijkstra 1991), and this was

remoulded loess material, the soil-structural danger had

been removed.

The Saleshan landslide

Cao(1986) has provided an excellent study of the 1983

Saleshan slide, and more attention should be directed at this

particular paper. The large slide at Saleshan occurred

suddenly at 17-46 on 7 March 1983. This came as a great

shock to the local people (no precursor effects). Cao reports

that there was a sound like thunder, and the soil and smoke

went up into the sky and the giant mountain body quickly

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rushed to the foot of the mountain 100m away, and blocked

the Baxie river. The devastating slide destroyed four

villages, killed 237 people, buried large amounts of

cultivated land, and seriously damaged to small nearby

reservoir.

Cao has produced a classification model for the slides in the

Baxie river basin, with four basic types of slide defined. In

our limited view of loess slides it is the type 1 slide which is

of concern; this is the slide which takes places wholly in the

loess layers, free of complicating geological factors. Cao

calls it the single-layer structure slide; it is close to our

default model loess slide, where the slide is initiated by

structural collapse of a hydroconsolidating lower layer. Cao

provides a macro-vision of loess slides (a detailed and

perceptive vision) but he does not discuss failure at the soil

structure level.

Loess material and loess ground

Loess is a silty, quartz rich, airfall, Quaternary, unsaturated,

metastable, collapsible, landscape-draping sediment. In

some parts of the world striking soil formation has occurred

in the upper regions and chernozem zones such as the Black

Earth region of Ukraine and S.W.Russia have formed. The

defining characteristics can be viewed from a variety of

directions. The defining characteristic of a loess deposit is

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the open, metastable packing of silt-sized primary mineral

particles. The default loess particle is a quartz fragment with

a diameter in the order of 30um; other mineralogy is present

but the vision of loess is based on quartz silt. The typical

quartz silt particle has a flat, blade shape; studies on particle

shape (Smalley 1966, Rogers & Smalley 1993, Howarth

2010) have suggested that a typical particle will have a side

ratio of 8: 5: 2- which gives a remarkably flat particle. These

flat particles can form a very open structure after deposition

by an airfall mechanism- this is the essence of a loess

deposit.

The open metastable structure means that, relative to other

ground, there is an excess energy available for geodynamic

activity. In a loess landslide, and in a quick clay landslide,

the initial structural collapse initiates the whole consequent

activity. The structural nature of loess ground makes it

worthwhile to consider loess as a special material when

investigating ground movements and failures.

Particle arrangements in loess deposits

The arrangement(packing) of the particles in a sediment or

engineering soil has an effect on properties. Packing is an

elusive property and a satisfactory method of measuring

packing and manipulating the parameters has never been

devised. But is is readily apparent that in certain ground

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systems the particle packing has a large part to play. (for

reviews of packing in ground systems see Rogers et

al.1994b, Allen 1982, Dijkstra et al.1995).

It can be argued that packing defines loess; and that

changes in packing define subsidence and flowsliding.

Dijkstra(2000) has considered packing relative to Chinese

loess ground failures and devised some conceptual

approaches to develop an approach to the critical nature of

loess packing. Model studies on loess packing have been

attempted by Assallay et al(1997) and Dibben et al(1998).

In the world of particle packing the seminal work of Graton

and Fraser(1935) still casts a long shadow, and still tends to

be the only packing work referred to in textbooks on soil

mechanics. There are ways forward from Graton & Fraser

and it is possible that the ideal world may eventually meet

the real world of the loess packing. Loess relates well to

packing studies because it is the ground material in which

psacking is the most impotant salient feature.

Ways to move on from Graton & Fraser include: making their

system more logical and complete (Allen 1982, Dijkstra et

al.1995, Rogers et al.1994a, Smalley 1980); trying to

introduce randomness rather than regularity; particles other

than spheres, closer to reality; explorations of packing in

other dimensions hoping to find data which has three

dimensional relevance. Some elegant 2-dimensional

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packings have been produced by simple Monte Carlo

methods which can simulate loess deposits(Dibben et al

1998) but they do in fact form a better simulation for lake

bottom deposits which eventually become airborne dust

(Evans et al 2004).

Loess landslides in the Danube Region

Loess is widespread in Europe (Haase et al 2006) but it

cannot be claimed that there is a significant problem with

loess landslides. There are always subsidence problems

wherever loess occurs but in Europe the necessary relief is

lacking for there to be a major slide problem. The one place

where slides occurred and where there has been some

investigation and discussion is the loess bluffs along the

Danube river. The Danube is a great loess river (see Smalley

& Leach 1978, Smalley et al 2000) and it has moved and

distributed loess material throughout the Quaternary period.

A short distance aeolian transportation from floodplain to

inshore location has produced some significant loess

deposits close to the river. A very well known slide occurred

at Dunaujvaros in Hungary in 1966 and this will be

discussed. There is some current interest in Danube bluff

slides because the Danube bluffs at Kozloduy in Bulgaria are

being cionsidered as a site for a radioactive waste

repository. The thick loess deposit contains several clay rich

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palaeosols and these are useful in isolating the waste zone.

The loess could make a good repository for radwaste but

obviously there has to be no danger of landsliding and slope

failure.

The flowslide concept

In a flowslide failure there is an effective change from solid

ground to a liquid system. This appears to be the situation in

the very large loess landslides, and the transition from solid

to liquid merits some consideration. There are two obvious

ways in which solid ground can turn to liquid (1) via a

mudslide (2) via a flowslide; the distinction needs to be

made. A true mudslide is a clay mineral controlled event, it

is a phenomenon associated with clayeyness. As the water

content of the clay soil system increases the liquid limit is

exceeded and the rheological state of the system changes-

from plastic solid to free flowing but non-Newtonian liquid;

and a mudslide can result. It could be seen as a gradual

transition as very plastic solid becomes viscous liquid. In this

viscous liquid the involved particles still have contact via

long range bonds and this modifies the behaviour of the

deforming ground.

The transition to flowslide failure is seen as relatively

sudden; a brittle ground suddenly loses strength, and if it is

a wet system the disaggregated system can be supported

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and a flowing suspension is produced. The brittle ground

which participates in a flowslide is dominated by short-range

interparticle bonds, which are strong when undisturbed but

lose all strength when disturbed and broken. Where the

mudslide system depends on a predominance of active clay

minerals and will have a high plasticity index, the flowslide

system has a low PI and lacks clay minerals. The two classic

flowsliding grounds are the very sensitive post-glacial marine

clays of Canada and Scandinavia, the so-called quick clays,

and loess. In each situation a particular set of

geomorphological circumstances is required and the major

flowslide is a relatively rare occurrence.

Derbyshire (2001) has commented that mass movements in

loess are frequently complex, with distinct block movements

in the upper parts gradually changing into mass flowage akin

to the flowslide type in the classification of Varnes (1958,

1978, see Hungr et al (2001) for refinements of flowslide

classification).

A slide usually occurs when a thin layer at the base of thick

loess collapses because it is saturated. The extensive flow

lobes cause the greatest damage to the human environment

in the loess area of China. A good example of this type is the

Saleshan slide(but focussing on the Cao 1986 type 1

system). The dependence of the slide mechanism on the

structural collapse in a wet ground region at the base of the

slide suggests that the classic collapse mechanism of

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subsiding loess and the great loess flowslides are very

closely related. It points to the key loess factor/property

being the open structure and the developed nature of the

interparticle contacts. The fundamental nature of loess

ground, dominated by particle nature and open airfall

structure, controls both subsidence and large wscale

landsliding. The fundamental mechanism of loess collapse

needs to be understood if subsidence and landsliding are to

be understood fully.

Smalley and Derbyshire (1991) attempted to compare loess

flowslides to other types of flowslide, in particular to the

quck clay slides. The basic idea was to find common factors

in the various types of slides and to possibly identify key

variables. They did discuss the ‘sliding-consolidation’ model

of Hutchinson (1986) which is essentially the model

supported in this review. Hutchinson postulated the

existence of a zone of excess pore-fluid pressures at the

commencement of the flowslide in at least the basal part of

the debris sheet. Under the influence of the basal excess

pore-fluid pressures, the debris sheet accelerates downslope

by basal sliding, in a plug-flow mode (that is with velocity

varying constantly with depth). During this process, the

basal excess pore-fluid pressure is successively decreased

by consolidation, through simple, upward drainage in the

simplest case, until the leading element is brought to rest.

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The position where this occurs defines the runout of the

flowslide.

Smalley and Derbyshire (1991) proposed that the

Hutchinson mechanism would apply to a wet base loess

flowslide, and they went on to consider what they called

‘external factors’ which influenced the default flowslide.

They considered five major factors and produced ‘flowslide

profiles’ to assist in comparing actual flowslides. They drew

the flowslide net quite widely and included events such as

the great Niigata earthquake liquefaction event of 1964,

mine waste tip failures (Smalley 1972), failures in halloysitic

ground in New Zealand (Smalley et al 1980) and a range of

Canadian and Scandinavian failures (Maerz & Smalley 1986).

The Niigata event, in retrospect, looks like an interesting

inclusion. It is possible to see a likeness between the ground

failing at Niigata and the large apartment blocks tumbling

and a basal loess failure mobilising the ground and the large

blocks above falling and tumbling, as described by

Derbyshire (2001).

The five factors were M the mass factor; S the supporting

medium; E the energy factor; B the bond factor and H the

landscape factor.

M mass factor. The major particles in a loess flowslide are

loess particles with diameters of the order of 20-50um. In a

quick clay flowslide the mode particles will be primary

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mineral particles perhaps an order of magnitude smaller, say

2-5um. The particle needs to be carried by the supporting

medium, and it needs to be light enough to be mobilised by

the initial slide causing events.

S supporting medium. Water or air; Smalley and Derbyshire

(1990) were concerned to offer alternatives, but this is

probably unnecessary. They thought that large loess

flowslides may be air supported but it seems likely now that

this is not an important part of the flowslide process. It is the

wet base slide which is of major concern. The loess flowslide

is more like the quick clay flowslide than was appreciated.

They are both essentially wet events. And the loess flowslide

initial failure relates well to classic hydroconsolidation in

loess. The remarkable relationship between loess and water

extends to flowslide failures.

E energy factor. The 1920 slide was a consequence of a very

large earthquake and the largeness of the earthquake and

the widespread landsliding activity and the subsquent

interest in the event may have obscured the role of the

landslide in the flowslide proceedure.

B bond factor. In a simple distinction the interparticle bonds

in engineering ground can be divided into two types. This

dichotomy appears to be basic and fundamental but it must

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be emphasized that this is a simplifying concept. There are

two bond types; long range bonds and short range bonds.

The long range bond is the clay mineral bond in which

electrically charged particles interact via a medium which

contains positively charged ions. These largely determine

the properties of clay mineral soils; they provide the c term

in the classic Coulomb equation, they deliver cohesion, and

plasticity. The short range bond operates between primary

mineral particles; the bond can be quite strong but breaks

when the system is disturbed. A chemical analogy might be

that the short range bond is like the covalent bond while the

long range bond is like the metallic bond. A short range

bonded system can be cohesive, but it will not be plastic. A

great leap forward in flowslide understanding occurred when

the short range bond concept was proposed (Cabrera &

Smalley 1973) and it has moved easily from the quick clays

where it was first applied on to loess flowslides.

The very simple system needs to be modified somewhat to

fit loess slides. Loess is made essentially of silt-sized primary

mineral particles, but the interparticle contacts are

influenced by accumulations of clay mineral particles. The

initial metastable deposit becomes more collapsible by the

accumulation of clay mineral particles at the major particle

contacts. A certain level of clay mineral content is required

for collapse to occur.

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H landscape factor. Within the landscape system an

impermeable sliding surface is required. Within the deposit a

palaeosol or a lithological contact surface is required. In the

landscape system at large some relief is required, but not

vast relief. Potential energy is required for any slide system

to progress; the loess slide is not associated with large relief.

Levels

The loess flowslide can be examined and appreciated at

various levels- these are conceptual levels, rather than

physical levels, essentially size/dimensional levels.

1. At particle contact level. The primary mineral particles

form the basic packing. It is modified by clay minerals at the

contacts; so overall it is essentially rigid, but plastic material

has gathered at the contacts; trapped there by needle

crystals of calcite which grew soon after loess deposition. A

connection has been proposed between loessification and

hydroconsolidation (Smalley & Markovic 2011) and this

concentration of clay mineral particles at primary mineral

contacts is a key related event, and thus is fundamental for

the critical stages of loess flowsliding.

2. At soil structure level. The structure will collapse on

hydroconsolidation. The metastable structure is collapsible;

it becomes collapsible as clay minerals accumulate during

loessification, a certain level of clay is required for the

efficient collapse to occur. The soil structure must stay open

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before the collapse event; too great a superimposed load

would cause collapse, hence the slide requirement of

moderate thickness. This collapse is the key initiating factor

in the loess slide event; this is the part of loessic nature

which determines the nature of the slide.

3. Block movement in flow stream. The ground is

undermined by hydroconsolidation, and then there is

liquefaction of the wet base level; these represent the

critical beginning of a loess flowslide failure. A large super-

posed fissured brittle mass moves by breaking into large

blocks which are carried by the flowstream, possibly

breaking up as they progress and contributing to the

material of the flowstream. But the blockflow is not really

fundamental to the loess flowslide; it is an important event

but the key actions occur at levels 1 and 2.

Commentary & Conclusions

There are some specific requirements for a large flow slide

type failure in loess, and some of the controls involved in

these requirements can be explained, or at least discussed.

The main conclusion has to be that the property of loess

which allows hydroconsolidation also plays a key part in

initiating loess landslides, which because of the overall

nature of loess tend to become flowslides; and because of

the huge amounts of loess in particular parts of the world

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tend to become huge flowslides. The critical loessic property

is its open metastable, collapsible soil-structure, allied to a

predominance of primary mineral particles and the

consequent short-range bonding (low PI). Some features can

be listed:

1. Moderate thickness, rather than great thickness. A

feature of the Chinese loess is the great thickness of some

deposits. Thicknesses of around 400m have been reported

from Jiuzhaotai near Lanzhou; a long history of deposition

has ensured that large areas of north China are covered by

very thick deposits of a quite remarkable ground material. It

is observed that the big slides tend to occur in deposits of

moderate thickness rather than in deposits of enormous

thickness. This presumably has a soil-structure related

explanation. We are proposing that the open structure of the

loess deposit is closely involved in the ground failure and

sliding and this open structure is more likely to be preserved

at the base of a modest deposit rather than at the base of a

large deposit where enormous superposed loads will have

caused structual collapse.

2. Some relief; some potential energy is required. The large

thicknesses of the loess deposits automatically provide some

load situations to drive ground movement. Also the terrain of

north China is very hilly in parts and deposits form on

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sloping ground; the Saleshan slide was noticeably related to

hilly terrain; there was plentiful potential energy to drive the

ground moving event.

3. An imprevious sliding plane, bedrock or palaeosol. Since

these large flowslides are wet-base events some impervious

sliding plane is indicated. There is a choice in the Chinese

loess systems. Within the loess system itself are many clay-

rich palaeosols which could provide the requisite impervious

surface; there is the Pliocene red clay formation which

underlies the entire loess system, and there can be

alternative lithologies within the landscape systems.

4. Open structure; e > 1; an open particle packing. Perhaps

this can be cited on two counts; to provide the basal

collapsing system which will initiate the slide, and to provide

the open structured ground mass above which can collapse

on initial deformation and provide material for the

consequent flowslide. This may be the point at which a loess

slide becomes special; this may be where a separate

category for loess slide is indeed indicated. The

characteristic feature of a loess deposit is the open

structure, the high voids ratio, e > 1. This is central to the

whole discussion of loess material, and it is the defining

factor which has to be explained when a theory of loess

ground formation is being advanced. Loess is aeolian

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ground, loess deposits are airfall systems and this affects

their landsliding behaviour.

5. Wet base; the role of water. Water seeps down through

loess deposits, there are drainage paths. This is another

factor which favours failure in deposits of modest thickness,

rather than in thick deposits. The combination of water and

an open structured loess prone to hydroconsolidation can

cause the initial failure which cause the whole system to fail.

The overall failure may be very complex eventually invoving

classic circular slips and large block movements of the

cohesive loess but the initial action is at the wet base. The

fauilure is the classic loess collapse failure; the same failure

which brought down the Atommash factory in Volgodonsk

initiates the Saleshan landslide. This hydroconsolidation

process is much studied (see Rogers et al 1994a, Trofimov

2001)- this is another central feature of the loess world; the

loess deposit (the classic deposit of primary loess) suffers a

structural collapse when loaded and wetted. What happens

is best described by the ‘small-clay model’ of failure. The

particle contacts in the loess ground are controlled/modified

by the presence of clay minerals. These soften when wetted

allowing the main load bearing contacts to fail, and collapse

ensues. A modest amount of clay is required in the system

for this to occur; too little or too much clay prevents failure.

Collapsibility grows as the clay mineral content increases;

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the clay mineral content essentially controls the collapse

behaviour of the loess system- and this is the case for

relatively straightforward building failures, and for large wet-

bottom slides.

6. Trigger energy; earthquakes. As in the sensitive clay

situation an earthquake can trigger a slide event. The 1920

slide was earthquake triggered. An input of energy into a

metastable system can cause failure; if an activation energy

is exceeded failure can follow. Here is a key defining factor

for a loess system, it is metastable, another consequence of

the airfall process which produced the loess deposit. It is

important to recognise metastability and collapsibility;

metastability is required for collapse to occur but

collapsibility is a different property. The huge energy of the

1920 earthquake may have mobilised ground which was not

essentially collapsible; basal slides would have been

activated but large blocks were in motion and very complex

sliding behaviour occurred.

Acknowledgements

Thanks are due to the late Liu Tung sheng and fellow

members of the UNEP group in North China- for practical

assistance, and for scientific, intellectual and philosophical

support. We also thank Professors Edward Derbyshire and

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Eiju Yatsu for continuing support and encouragement. We all

press forward with the work on loess material.

References

Allen, J.R.L. 1982. Sedimentary Structures: Their Character

and Physical Basis. v.1, Elsevier Amsterdam 593p. (Packing

ch.4, pp.137-177).

Assallay, A.M., Rogers, C.D.F., Smalley, I.J. 1997. Formation

and collapse of metastable particle packings and open

structures in loess deposits. Engineering Geology 48, 101-

115.

Cabrera, J.G., Smalley, I.J. 1973. Quick clays as products of

glacial action: a new approach to their nature, geology,

distribution and geotechnical properties. Engineering

Geology 7, 115-133.

Cao, B. 1986. The geologic characteristics of the Saleshan

type of super-landslide and a model for spatial prediction.

Proceedings 5th International IAEG Congress, Buenos Aires, 3,

1989-1997.

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