landslides in loess
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Some thoughts on the nature of landwslides in loess groundTRANSCRIPT
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
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.
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