Elsevier Encyclopedia of Geology

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Some hazards are natural, others are purely

anthropogenic, and many have both natural and

anthropogenic causal elements.

There is a need to consider geohazards from a

sociological as well as a scientific viewpoint, as the

perception and response of the local populace and

all tiers of government are as important as tech-

nological understanding. Geohazards can also only

be successfully evaluated with a full interdisciplin-

ary approach: an international, possibly even a

global, approach is also called for.

Definitions

There are two published definitions of geohazards.

A geohazard is a hazard of geological, hydrological

nature which poses a threat to Man and his activities.

(McCall & Marker 1989)

and

A geohazard is one that involves the interaction of man

and any natural process on the planet. (McCall and

others 1992)

The second is wider, but the first brings in the

essential element of human vulnerability, which is

well expressed by MR Degg in his diagram for

the disaster equation (Figure 1) – the relationship

between the hazard, disaster, and vulnerability. The

larger and more severe the event and the more vulner-

able the population, the greater the disaster; there is

a threshold below which there is no disaster. If

there are no human’s present, strictly speaking there

is no hazard.

The words ‘hazard’ and ‘risk’ are often used inter-

changeably, but the risk is a quantification. RJ Blong

defined ‘risk’ as the product of ‘hazard’ and ‘vulner-

ability’. The word ‘disaster’ refers rather loosely to

the consequence of hazards.

Types of Natural Geohazard

RJ Blong lists the following as the principal natural

geohazards:

earthquakes (r);

volcanic eruptions (r);

landslides (r);

tsunamis (r);

subsidence (s);

coastal erosion (s);

coastal progradation (s);

soil erosion (s);

expansive soils (s).

Already, we see two distinct classes of geohazard:

the rapid-onset, intensive hazards (r) and the slow-

onset, pervasive hazards (s). Attention has been trad-

itionally focused by scientists, the media, and the

populace on the former, but the latter can be equally

damaging.

SA Thompson separated ‘natural geohazards’ from

‘meteorological hazards’:

cyclones;

tornadoes;

floods;

heatwaves;

thunderstorms and gales.

All of these have some geological consequences,

especially the first three.

GJH McCall later added to the list and separated

those that are potentially catastrophic and cannot

at present be reliably predicted, unless in general

terms or not at all. These can be treated mainly by

emergency planning, warning systems, education

and communication, and evacuation. Mitigation

and prevention procedures are of limited application.

The list is:

earthquake hazards (e.g., Kobe 1995);

volcanic hazards (lava, pyroclastic flows, nue

ardentes, ash falls, lahars, blasts, and gas releases)

(e.g., Etna, Martinique, Montserrat, Nevado del

Ruiz 1985, Lake Nyos 1986);

tsunamis (related to either of the above) (e.g., Hilo,

Hawaii, 1960);

extraterrestrial impacts (e.g., Tunguska 1908).

Figure 1 The ‘disaster equation’. After Degg MR, in McCall

GJH and Marker BR (1989)

Earth Science Mapping for Plan-

ning, Development and Conservation, Engineering Geology Special

Publication No. 4

. London: Kluwer.

516 ENGINEERING GEOLOGY/Natural and Anthropogenic Geohazards

The remainder can be reasonably counteracted by

engineering procedures at a cost. The list is:

mass movements (landslides, avalanches, debris

flows, ice-related movements) (e.g., Ventnor, Swiss

Alps, Cotopaxi, Cordillera Blanca);

subsidence (swallow holes, karstic processes,

gypsum-related sink holes, sinking cities) (e.g., Pen-

nines, Wuzhan, Ripon, Venice);

flooding after abnormal rainfall and cyclones (Ban-

gladesh);

coastal erosion (sea-level rise) and coastal progra-

dation (China, Burma);

riverbank failure and silting up of rivers (Missis-

sippi River);

expansive and collapsing soils, thixotropic sands

(e.g., Anchorage in Alaska);

permafrost (e.g., Canada, Siberia);

hazardous gas emission (radon) (e.g. Cornwall).

A third category was listed of those that fall be-

tween the two:

combustion and wildfire (e.g., Australia);

neotectonic deformation and fissuring (e.g., Xian);

desertification (e.g., Sahel).

K Hewitt and I Burton, in 1975, published a list of

parameters for ‘selecting’ hazard events:

property damage extending to more than 20 fam-

ilies, or economic loss (including loss of income, a

halt to production, costs of emergency action) in

excess of US$50 000;

major disruption of social services, including com-

munications failure and closure of essential facil-

ities of establishments of economic importance;

a sudden, unexpected or unscheduled event, or

series of events, which puts excessive strain on es-

sential services (police, fire service, hospitals, public

utilities) and/or requires the calling in of men,

equipment, or funds from other jurisdictions;

an event in which 10 or more persons are killed or

50 or more injured.

Such quantification of thresholds is applicable to

rapid-onset, intensive hazards, but of little use for the

slow-onset, pervasive hazards, such as karstic pro-

cesses or coastal erosion. Also, it appears to be ap-

plicable to developed countries, but of little relevance

to less developed countries, such as islands in the

Pacific Ocean. Monetary loss and body count alone

are thus poor indicators of the magnitude of an effect

of a hazard on an afflicted community. Vulnerability

should take into account not merely the risk, but

also the endemic conditions inherent in the society.

Deaths due to starvation consequent on hazards are

not considered in this scheme.

SA Thompson, in 1982, produced a table com-

bining frequency with deaths per event (Table 1).

This table shows that earthquakes and cyclones

are the most lethal per event on a global scale and

both are of high frequency; however, flooding,

because of its very high frequency, is nearly as

lethal. The low frequency and number of deaths

per event for landslides can be misleading – for

example, in Basilicata, Italy, virtually all of the

dense cluster of hilltowns are threatened by land-

slides, and it is unquestionably a major hazard,

even if not a great killer. Likewise, in Nepal and the

Pamirs (Figure 2), the steep topography means that

landslides repeatedly wipe out villages and com-

munications and present a major and intractable

hazard problem.

SA Thompson also produced a table of fatalities

covering Asia and Australasia between 1947 and

1981, showing how regional statistics can reveal

extraordinary contrasts (Table 2). Asia had 85.8%

of the global count of 1 208 044 global deaths

from the hazards listed in Table 2 during this

period. However, this figure is a combination of

both geological and meteorological disasters, and,

for the geological disasters, the figure is slightly

below the global average. Australasia accounted

for only 0.4% of the global deaths. The death counts

and magnitude of the event cannot be correlated,

because of factors such as variations in populat-

ions at risk, ground conditions, and building con-

struction quality and type. In the case of volcanic

eruptions, there is a wide variation in the expected

magnitude and type of eruption according to the

classification of the volcano and its petrological

products.

Volcanoes and earthquakes are covered by separate

entries in this encyclopedia (see Tectonics: Earth-

quakes, Volcanoes).

Table 1 Frequency of hazards and deaths per event. From

Thompson SA (1982)

Hazard

frequency

Deaths

per event

Geological hazards

Landslides 29 2.7 190

Tsunamis 10 1.0 856

Volcanoes 18 1.7 525

Earthquakes 161 15.2 2652

Meteorological hazards

Cyclones 211 19.9 2373

Tornadoes 127 12.0 66

Floods 343 32.3 571

Heatwaves 22 2.1 315

Thunderstorms and gales 36 3.4 587

ENGINEERING GEOLOGY/Natural and Anthropogenic Geohazards 517

Procedures for the Mitigation of

Natural Hazards

These include:

the study of existing conditions and evidence of

past activity, including mapping;

prediction in time and space (when?, where?, and

on what scale? risk assessment);

prevention or mitigation by engineering or other

methods;

planning of land use;

monitoring and installation of warning systems;

emergency planning; proactive, including planning

for evacuation; post-event relief.

Each of the above will have variable relevance to

a particular hazard and subject site/area/region, and

may have none at all.

Types of Anthropogenic Geohazard

JD Mather and others included:

surface movements – subsidence (Figure 3) related

to mineral or fluid (hydrocarbon, water) extraction

and surface collapse into voids left by mineral

workings;

contaminated land – left by mineral workings,

chemical plants, gas works, etc. (‘brownfield sites’);

rising groundwater levels;

modification of groundwater quality (unsewered

sanitation, organic solvent pollution);

waste disposal.

GJH McCall and others also included:

loss of soil and agricultural land (including the

effects of urbanization on the soil resource);

reduction in biodiversity.

The anthropogenic hazards are mostly of long

build up – even surface collapses due to old mine

workings (Figure 4), which appear abruptly without

warning, as the roof of the mined-out cavity gradually

moves towards the surface due to the fall-out of

blocks.

Three of these quiet, pervasive, slow build-up

hazards are mentioned below to illustrate this type

of geohazard.

Figure 2 Beneath this peaceful scene in the Pamirs, Tadzhikistan, is buried Xait, one of 33 villages buried by landslides triggered

by earthquakes in 1948, with 50 000 lives lost which geological knowledge could have saved. de Mulder EFJ, Holland, International

Union of Geological Sciences ‘COGEOENVIRONMENT’ photograph.

Table 2 Fatalities per event in Asia and Australasia. From

Thompson SA (1982)

Event Asia Australasia

Landslides 3576 0

Tsunamis 7864 44

Volcanoes 2806 4000

Earthquakes 33 3623 133

518 ENGINEERING GEOLOGY/Natural and Anthropogenic Geohazards

Rising Groundwater

Rising groundwater was described in 1992 by DJ

George – a problem that has affected many cities in

the Middle East (Kuwait, Doha,Cairo, Riyadh, Jeddah,

Madinah), and is a major hazard in terms of cost, and

loss of and injury to human life. A typical cause is the

installation of new water supply systems piped in from

outside a city area, which previously relied on numer-

ous small wells within it. The water table under the city

is no longer utilized and the water table rises. This

produces:

damage to buildings and structures;

damage to services and roads;

overloading of sewer systems and treatment

plants;

salting and waterlogging of soils;

public health hazards.

This is essentially anthropogenic but, like all such

hazards, is due to interference with the natural

system.

Sea-Level Change on the China Coast

S Wang and X Zhao covered this hazard, which

is likely to become more significant with global cli-

mate change – warming related to the ‘greenhouse

effect’–which has now been shown to have been

unquestionably influenced since the beginning of the

Industrial Age (see summary report by McCall GJH

(2003); Solar System: The Sun). On the long, sinuous,

and island-dotted coast of China, there are many

large, industrialized cities and regions – e.g., Shang-

hai, Tianjin, Guangzhou – located on coastal plains

and deltas. The evidence reveals a complex picture of

overall oscillations of the order of a thousand years

and different effects of Quaternary crustal move-

ments, up and down in different regions; however,

subtracting the latter, a general rise of 0.65 mm per

year has been recognized. A future rise of several

centimetres, which is predicted (also allowing for

the effect of global warming), will have disastrous

effects on the coastal and coastal hinterland environ-

ment and the industry and agriculture there. The

wetlands will be inundated. Engineering construc-

tions proposed in the Yangtze delta will be under the

following main headings:

control of land subsidence due to human activity;

prevention of seawater seepage and windstorm tides;

drainage of low-lying land and land improvement;

taking into account sea-level rise in planning new

urban areas.

Global Soil Loss: Biodiversity Loss

WS Fyfe has emphasized the fact that the explosive

growth of the human population (a taboo subject as

WI Stanton has emphasized) (Figure 5) is at the root

of most of the quiet, pervasive hazards. He indicates

that topsoil loss is occurring at a global rate of 0.7%

per year. The global climate bioproductivity is vital

to the functioning of the global thermostat, and

species continue to be wiped out wholesale due to

the activities of humankind.

As described by GJH McCall et al., in an original

definitive account, urban geohazards are becoming

more and more significant as urbanization mush-

rooms in the early twenty-first century (Figure 6).

These include natural and anthropogenic hazards.

Hazard and Risk Mapping

JG Doornkamp, in 1989, published a concise account

of hazard mapping. He covered:

landslips and avalanches;

natural ground subsidence;

Figure 3 Leaning church tower due to differential settlement in

soft Holocene sediments, a foundation problem near Lake Ijssel,

The Netherlands. de Mulder EFJ, Holland, International Union

of Geological Sciences ‘COGEOENVIRONMENT’ photograph.

ENGINEERING GEOLOGY/Natural and Anthropogenic Geohazards 519

hazards of quarrying and mining;

erosion and deposition;

flooding;

saline soils;

permafrost;

seismological hazards;

volcanic hazards.

The third in this list is an anthropogenic hazard –

the general principles apply to both natural and an-

thropogenic hazards. JG Doornkamp noted that the

distinction is ambiguous.

Every hazard has three key properties:

magnitude;

frequency;

location.

The last of these is most amenable to mapping.

Mapping may be taken for different reasons:

to define the nature and extent of a historical event;

to define the nature and extent of a recent event;

to define present conditions in order to assess the

likelihood of a recurrent event.

The social and economic content of the affected

subject must be included in any such programme.

A hazard cannot in itself be mapped, as it is a process.

Rather, the results of a hazard are mapped. The re-

sultant landforms, the effects on soil and rocks, and

the effects on constructions are mapped.

JG Doornkamp devised a useful diagram showing

the context of hazard mapping (Figure 7).

Aids to mapping include:

satellite imagery;

airborne multispectral scanning;

photogrammetry;

shallow geophysics;

global positional systems.

DKC Jones has illustrated well the procedures in

hazard mapping in an account of the application to

landslides. The graphical methods are only part of the

exercise, which includes:

graphical methods, mainly involving mapping;

the analysis of the empirical relationships be-

tween landslides and individual causal factors,

such as slope steepness, material type, vegetation

cover, rainfall intensity, and human constructions;

multivariate analysis of the causes of landslides.

True hazard maps display the extent of past haz-

ardous events (zoned), together with an internal div-

ision reflecting the magnitude frequency distribution

Figure 4 The result of a void migrating to the surface in old mine workings beneath a suburb of Glasgow. British Geological Survey

photograph, in McCall GJH, de Mulder EFJ, and Marker BR (1996) Urban Geoscience. Rotterdam: Balkema.

520 ENGINEERING GEOLOGY/Natural and Anthropogenic Geohazards

Figure 5 The escalation of the world population. After Stanton WI (2003) The Rapid Growth of Human Populations: 1750–2000. Brent-

wood: Multi-science Publishers.

ENGINEERING GEOLOGY/Natural and Anthropogenic Geohazards 521

of past events (hazard rating). Evidence of past fail-

ures must be used to predict the likelihood of future

events, their scale, and frequency. Risk maps are quite

different in that they assess the potential losses that

may be incurred by society. They may involve

numerical quantification (for example, the table for

the Central Pacific by Blong RJ (1988)) (Table 3).

They may be followed by maps showing geo-

graphical divisions of recommended action (for

example, the map of Basilicata; Figure 8). Hazard

mapping may be very complex and involve numerous

maps of different type. For example, DKC Jones, in

1992, listed the following as all used in landslide

mapping:

geological structure;

chronostratigraphy of rocks;

lithostratigraphy of rocks;

lithostratigraphy of soils;

rockhead contours;

geotechnical properties of soils;

geotechnical properties of rocks;

hydrological conditions;

geomorphological processes;

geomorphological history, palaeodeposits, surfaces,

or residual conditions;

seismic activity;

climate (including precipitation);

ground morphology (especially slope height, length,

and angle);

land use;

vegetation type, cover, root density, and strength;

pedological soils, type and thickness of regolith;

past landslide deposits;

past landslide morphometry.

Wider Responses

JG Doornkamp listed four wider responses beyond

mapping considerations that can be adopted to miti-

gate hazards:

land use planning: place development in less haz-

ardous places;

economic planning: investment in the right places

and maintenance of financial reserves to cover

disasters; adequate insurance schemes;

development control: designing built structures to

cope with known hazards (e.g., earthquake-resistant

designs);

emergency planning: evacuation procedures to be

planned and rehearsed in all areas at risk from ma-

jor hazards; rescue teams trained and tested with

modern equipment, able to move quickly to the site

of a disaster.

Engineering and Geohazards

Experience has shown that geologists, on the one

hand, and engineers and developers, on the other,

Figure 6 The escalation of urbanization. After de Mulder EFJ,

in McCall GJH, de Mulder EFJ, and Marker BR (1996) Urban

Geoscience

. Rotterdam: Balkema.

Figure 7 The context of hazard mapping. After Doornkamp JG,

in McCall GJH and Marker BR (1989) Earth Science Mapping for

Planning, Development and Conservation, Engineering Geology Special

Publication No. 4

. London: Graham and Trotman.

522 ENGINEERING GEOLOGY/Natural and Anthropogenic Geohazards

tend to have rather different viewpoints concerning

geohazards. The differences concern site investiga-

tion, which is the keystone of any engineering project.

There is a tendency amongst engineers and developers

to omit consideration of the hazards pertaining to a

particular site from previous site investigations, with

the view that the problems will only be addressed

if and when they arise. For the past three decades,

environmental geologists have stressed that these

factors must be evaluated as part of the previous

site investigation, and this is becoming, certainly in

the UK, a regulatory condition for construction and

development.

Engineers are greatly involved in investigation,

mitigation, and repair programmes related to both

natural and anthropogenic hazards. For example,

Figure 8 Landslide hazard assessment map, Basilicata, Italy. After Jones DKC, in McCall GJH, Laming DJC, and Scott SR (1992)

Geohazards – Natural and Man Made. London: Chapman and Hall.

Table 3 Geological risk rating for selected towns in Papua

New Guinea. After Blong RJ (1988)

Town

Earthquake

rating

Volcano

rating

Tsunami

rating

Risk

rating

Arawa 6 2 0.1 8

Daru 2 0.1 0.03 2

Goroka 5 0.1 – 5

Kavieng 3 0.1 0.1 3

Kiet 6 2 0.1 8

Kimbe 10 4 0.1 14

Kokopo 10 4 0.2 14

Lae 5 0.1 0.2 5

Madang 5 1 0.2 6

Mt Hagen 3 0.1 – 3

Port Moresby 3 1 0.01 3

Rabaul 10 5 0.2 15

Wewak 6 2 0.2 8

ENGINEERING GEOLOGY/Natural and Anthropogenic Geohazards 523

they design earthquake-proof buildings in Japan and

deviation works to prevent lava flows or lahars from

reaching villages and towns (as in Sicily). To illustrate

the variety of engineering applications to geohazards,

a list is given below of the types of work in which

engineers have been involved, and which are repre-

sented by reports to Special Publications of the

Engineering Group of the Geological Society:

liquefaction of sediments in the Fraser River Delta

(Canada);

flooding at Ladysmith (South Africa);

fluvial hazards (Bihar, India);

ice- and snow-related high-altitude problems in

glacial lakes (Himalayas, Peru);

cliff erosion (Isle of Wight, UK);

lessons from the Kobe earthquake (Japan);

risks from low-seismicity earthquakes (Holland);

gypsum-related subsidence at Ripon, Yorkshire

(UK);

subsidence related to solution in chalk (south-east

England);

groundwater recharge under rapid urbanization

(Mexico, Thailand);

organic solvent pollution of groundwater (Coven-

try, UK);

landfill disposal of urban wastes (Tanzania,

Gambia, Mauritius);

acid mine drainage (Transvaal);

heavy metal contamination (Cornwall, UK);

foundation conditions and site investigations (UK);

land restoration, brownfield sites;

detection of karst features by remote sensing

(England).

Conclusion

There have been great advances in hazard-related

earth science and engineering in recent decades, but

these hazards are far from understood and there is an

ongoing need for more data and statistics. Countries

such as China, with its immense population, many of

whom live in sites of high risk, and Colombia or

Nepal, where the climate and physiography militate

to give a situation in which natural geohazards are

quite unavoidable, but can be mitigated, if only by

emergency planning, are extreme examples; however,

no country is free from these problems. With the

growth of megacities, urban geohazards are assuming

increasing importance (Figure 6).

Further Reading

Appleton JD, Fuge R, and McCall GJH (1996) Environ-

mental Geochemistry and Health, Special Publication

No. 113. London: Geological Society.

Blong RJ (1988) Assessment of eruption consequences.

Kagoshima International Conference on Volcanoes Pro-

ceedings, pp. 569–572.

Bullock P and Gregory PJ (1991) Soils in the Urban Envir-

onment. Oxford: Blackwell Scientific Publications.

Culshaw MG, Bell FG, Cripps JC, and O’Hara M (1987)

Planning and Engineering Geology, Engineering Geology

Special Publication No. 4. London: Geological Society.

de Mulder EFJ COGEOENVIRONMENT, International

Union of Geological Sciences (IUGS) (undated leaflet)

Planning and Management, the Human Environment –

the Essential Role of the Geosciences.

Maund JG and Eddleston M (1998) Geohazards in Engin-

eering Geology, Engineering Geology Special Publication

No. 15. London: Geological Society.

McCall GJH (2003) Global climate change – a view from

the floor. Geoscientist 13(6): 18–20.

McCall GJH, Laming DJC, and Scott SR (1992) Geoha-

zards – Natural and Man Made. London: Chapman and

Hall.

McCall GJH and Marker BR (1989) Earth Science Map-

ping for Planning, Development and Conservation, En-

gineering Geology Special Publication No. 4. London:

Graham and Trotman.

McCall GJH, de Mulder EFJ, and Marker BR (1996) Urban

Geoscience. Rotterdam: Balkema (see, especially, papers

by McCall, Simpson, and Mather).

Stanton WI (2003) The Rapid Growth of Human Popula-

tions: 1750–2000. Brentwood: Multi-science Publishers.

Thompson SA (1982) Trends and Developments in Global

Natural Disasters 1947–1981. University of Colorado

Institute of Behavioural Science Natural Hazards.

Research Working Paper No. 45.

Wang S (1997) Engineering Geology, Proceedings of the

30th International Geological Congress, Beijing, 1997.

Vol. 2–3. Utrecht: VSP.

Zhang Z, de Mulder EFJ, Liu T, and Zhou L (1997) Geo-

sciences and Human Survival, Environment, Natural

Hazards, Global Change. Proceedings of the 30th Inter-

national Geological Congress, Beijing, 1997. Vol. 2–3.

Utrecht: VSP.

524 ENGINEERING GEOLOGY/Natural and Anthropogenic Geohazards

Liquefaction

J F Bird, Imperial College London, London, UK

R W Boulanger and I M Idriss, University of

California, Davis, CA, USA

Introduction

Liquefaction-induced ground failure has caused

widespread damage and devastation; a recent exam-

ple is the closure of the Port of Kobe, Japan’s busiest

port, following the 17 January 1995 earthquake,

largely due to the liquefaction-related failure of

reclaimed land (Figure 1). The potential consequences

of liquefaction are far reaching, ranging from settle-

ment or tilt of individual building foundations to

the spread of fire when the water supply system is

damaged by permanent ground deformations.

Liquefaction is the loss of shear strength of a satur-

ated cohesionless soil due to increased pore water

pressures and the corresponding reduction in effec-

tive stress during cyclic loading. Liquefaction and its

associated ground deformation are very complex phe-

nomena, and the term ‘liquefaction’ has been used to

describe a wide range of soil behaviour. This article

focuses on the susceptibility, triggering, and conse-

quences of earthquake-induced liquefaction; liquefac-

tion caused by non-earthquake-related loading is not

included. Although the emphasis here is mainly the

liquefaction of saturated cohesionless deposits such

as sands or silty sands, it is important to consider that

strength loss in fine-grained soils under earthquake

loading can also pose a significant hazard. The assess-

ment of liquefaction hazard is an essential part of any

engineering project in seismic regions and is needed to

make informed decisions with respect to mitigation

options, foundation design, and emergency response

and recovery plans based on what is considered to be

an acceptable level of risk.

The Principles of Liquefaction

As a saturated cohesionless soil is cyclically loaded,

its particle structure can tend to collapse to a denser

arrangement. If the soil’s permeability and the site

stratigraphy are such that drainage cannot occur im-

mediately, then, as the collapse occurs, stresses will be

transferred from the soil grain contacts to the pore

water, leading to an increase in pore water pressure.

In simple terms, when the pore water pressure in-

creases, the effective stress (total stress minus pore

water pressure) on the particle structure will reduce,

and as it approaches zero, the shear resistance of the

soil will also approach zero. This loss of effective

stress and shear resistance is known as liquefaction.

The stress–strain behaviour of liquefying sand

depends strongly on its relative density. When loose

sand liquefies, the gravitational static shear stresses

may exceed the shear resistance of the soil and rapid

deformation with very large shear strains can com-

mence; this is referred to as flow deformation. The

soil behaviour is termed ‘contractive’, and the shear

resistance exhibited by the liquefied soil during flow

deformation is termed the ‘residual strength’. When

moderately dense granular soils are cyclically

sheared, pore pressures may similarly rise and lique-

faction can be triggered. However, rather than under-

going flow deformation, the soil particle structure

may try to expand as it reaches a certain level of

shear strain, resulting in what is termed ‘dilative’

behaviour. For undrained conditions, this leads to a

reduction in the pore water pressures and a corres-

ponding increase in effective stress and shear resist-

ance. A shear stress reversal, however, such as will

occur many times during earthquake shaking, may

cause the soil particle structure to be incrementally

contractive and the state of zero effective stress may

be temporarily reached once more. This continued

cycle of zero effective stress and strength regain is

termed ‘cyclic mobility’. The cumulative deforma-

tions can be significant, particularly if the duration

of shaking is long, but dilative soils do not exhibit very

large flow deformations in the way that contractive

soils do (Figure 2).

Figure 1 Liquefaction of reclaimed fills at the Port of Kobe in

1995 caused complete suspension of operations. The lateral dis-

placement of the quay walls in this picture pulled apart the crane

legs, causing collapse. Photograph by Leslie F. Harder, Jr.

ENGINEERING GEOLOGY/Liquefaction 525

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