Elsevier Encyclopedia of Geology

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Sand boils are caused by the tendency of the excess

pore water pressures to dissipate upwards, towards

the free surface, carrying soil particles up from the

liquefied layer through cracks or channels in the over-

lying material and ejecting them at the ground surface

(Figure 3). The upward seepage of pore water, driven

by the earthquake-induced excess pore pressures, can

be impeded by less permeable overlying soil layers,

which can result in water accumulating near the inter-

face between the liquefied soil and the overlying lower

permeability soil. The accumulation of water can

loosen the soil and possibly even result in the forma-

tion of water films, either of which greatly reduces the

available shear resistance along the interface (Figure

4). This phenomenon is referred to as ‘void redistri-

bution’. A key consequence of void redistribution is

that the residual shear strength of liquefied soil does

not just depend on the pre-earthquake properties of

the soil, but also depends on those factors affecting the

dissipation and movement of pore water following

earthquake shaking.

Figure 2 Stress–strain behaviour of contractive (top) and dilative (bottom) soils under monotonic and cyclic loading. t

= initial

(static) shear stress. In all cases, the steady-state undrained shear strength is ultimately reached. Reproduced from Castro (1976).

Figure 3 Sand boil in liquefied soil following an earthquake;

Japan, 1983.

Figure 4 Schematic illustration of void redistribution resulting

from spreading of pore pressure and global volume changes.

Reproduced from the National Research Council (1985) Liquefac-

tion of Soils during Earthquakes. Report by the Participants in the Work-

shop on Liquefaction, Committee on Earthquake Engineering, National

Research Council, Report No. CETS-EE-001

. Washington, DC: Na-

tional Academy Press.

526 ENGINEERING GEOLOGY/Liquefaction

Another by-product of triggering of liquefaction is

the potential effect it has on the characteristics of the

strong ground motion. A number of accelerograms

have clearly shown the triggering of liquefaction

beneath the instrument (e.g., Figure 5).

Assessment of Liquefaction Hazard

The potential for liquefaction to cause damage is

assessed in three stages: the susceptibility to liquefac-

tion, the likelihood of liquefaction being triggered by

the design earthquake scenario, and the consequences

related to liquefaction (Figure 6). The variables that

influence the onset of liquefaction include environ-

mental factors such as the location of the water table,

site stratigraphy, and depositional and seismic

loading history of the soil; soil characteristics such

as relative density, grain size distribution, mineralogy,

and the presence of any cementing agents; and the

characteristics of the earthquake under considera-

tion, mainly the amplitude of ground shaking and

its duration.

Liquefaction Susceptibility

The most obvious evidence that a particular deposit is

susceptible to liquefaction is historical precedence,

either at the same site or in similar conditions else-

where; it is not unusual for liquefaction to re-occur in

the same location. In regions of potential hazard

where there is no historical evidence, other informa-

tion must be used to make an initial assessment of the

liquefaction risk. The criteria in Table 1, classifying

different levels of liquefaction susceptibility for vari-

ous types and ages of soil deposits, are widely used by

engineers and engineering geologists for the purposes

of either producing regional maps of liquefaction

hazard for planning purposes or for preliminary

assessment of potential hazards for site selection pur-

poses. Saturated loose cohesionless soils are the most

Figure 5 Horizontal accelerograms recorded in Niigata, Japan in the 1964 earthquake. The area surrounding the recording station

was heavily damaged by liquefaction. The onset of liquefaction appears to have occurred at approximately 10 s after strong ground

shaking commenced.

Figure 6 Flow chart for the three stages of assessing the

potential of earthquake-induced liquefaction to damage struc-

tures, lifelines, and other facilities.

ENGINEERING GEOLOGY/Liquefaction 527

vulnerable to liquefaction. Older deposits generally

have higher densities and thus greater resistance to

liquefaction.

The mode of deposition is also an important factor

in liquefaction susceptibility, as can be seen in Table 1.

Alluvial, fluvial, marine, deltaic, or wind-blown de-

posits generally have a higher susceptibility than do

either residual soils or glacial tills; materials that are

more highly consolidated. There are few case histories

of liquefaction occurring at depths greater than about

10–15 m. This is partly due to the fact that soils at

greater depths tend to be older, and therefore more

resistant to liquefaction, and also that the earthquake-

induced cyclic shear stress ratio (see eqn [1]) will be

lower at greater depths. In addition, liquefaction at

greater depths beneath level ground surfaces would

not necessarily be evident at the ground surface, and

thus may go undetected. The issue of a maximum

depth for liquefaction is of particular importance

for the construction and evaluation of large earth

dams, where the presence of significant overburden

above natural deposits should not be construed as a

reduction in liquefaction susceptibility.

When some site-specific information is available,

compositional criteria can be used to make prelimin-

ary assessments of liquefaction susceptibility. The

fines content and clay content, the Atterberg limits,

and the particle size distribution are all relevant.

Though clays are generally resistant to liquefaction,

it is also understood that plastic silts and clays may

develop significant shear strains under sufficiently

strong and sustained earthquake loading, leading to

strength loss and rapid deformation or even instabil-

ity. Compositional characteristics are used in various

criteria, such as the often-cited ‘Chinese criteria,’ to

evaluate a soil’s liquefaction susceptibility, but a clas-

sification of ‘non-liquefiable’ should not be equated

with the absence of a problem. Thus, liquefaction

evaluation procedures may not be appropriate for

plastic silts and clays, but the potential for such soils

to fail and induce damage under earthquake loading

should not be discounted.

Evaluation of Liquefaction Potential

The state-of-the-practice approach for evaluating liq-

uefaction potential at a specific site, often referred to

Table 1 Susceptibility of sedimentary deposits to liquefaction during strong seismic shaking

Type of deposit

Distribution of

cohesionless sediments

in deposit

Likelihood that cohesionless sediments, when saturated, would be

susceptible to liquefaction

<500 years Holocene Pleistocene Pre-Pleistocene

Continental

River channel Locally variable Very high High Low Very Low

Flood plain Locally variable High Moderate Low Very Low

Alluvial fan and plains Widespread Moderate Low Low Very low

Marine terraces and plains Widespread — Low Very low Very low

Delta and fan delta Widespread High Moderate Low Very low

Lacustrine and playa Variable High Moderate Low Very low

Colluvium Variable High Moderate Low Very low

Talus Widespread Low Low Very low Very low

Dunes Widespread High Moderate Low Very low

Loess Variable High High High Unknown

Glacial till Variable Low Low Very low Very low

Tuff Rare Low Low Very low Very low

Tephra Widespread High High ? ?

Residual soils Rare Low Low Very low Very low

Sebka Locally variable High Moderate Low Very low

Coastal zone

Delta Widespread Very high High Low Very low

Estuarine beach Locally variable High Moderate Low Very low

High wave energy Widespread Moderate Low Very low Very low

Low wave energy Widespread High Moderate Low Very low

Lagoonal Locally variable High Moderate Low Very low

Fore shore Locally variable High Moderate Low Very low

Artificial fill

Uncompacted fill Variable Very high — — —

Compacted fill Variable Low — — —

Reproduced with permission from Youd TL and Perkins DM (1978) Mapping liquefaction-induced ground failure potential. Journal of

the Geotechnical Engineering Division

, ASCE 104(GT4): 443–446.

528 ENGINEERING GEOLOGY/Liquefaction

as the ‘simplified procedure’, was first published by

H B Seed and I M Idriss in 1971. This procedure

compares the shear stresses induced by an earthquake

with those required to cause liquefaction in the soil

profile, where the liquefaction resistance is based on

empirical data. The procedure is based on the

following relationship:

CSR ¼

¼ 0:65 

max



MSF

½1

where CSR is the cyclic shear stress ratio, t

/ s

,at

the depth of the soil layer under consideration, a

max

the peak ground acceleration at the surface, g is ac-

celeration due to gravity, s

/ s

is the ratio of total to

effective vertical stress in the soil layer, and r

is a

stress reduction coefficient (Figure 7). MSF is a mag-

nitude scaling factor, equal to unity for M ¼ 7.5 earth-

quakes, and greater than unity for M < 7.5, to correct

in an implicit manner for the duration of shaking.

Recommended procedures for the evaluation of r

and MSF for use in conjunction with eqn [1] are

described in a 2004 paper by Idriss and Boulanger.

The factor of safety (F) against liquefaction is then

determined as follows:

F ¼ CRR =CSR ½2

where CRR is the cyclic resistance ratio of the soil

layer. The evaluation of CRR can be determined dir-

ectly by laboratory tests on high-quality undisturbed

samples and reconstituted samples (for new fills), or

indirectly by in situ testing. An effective method

(albeit an expensive one) of evaluating the cyclic

resistance involves collection of high-quality un-

disturbed soil samples, using techniques such as

ground freezing, and undertaking cyclic testing in

the laboratory (e.g., cyclic simple shear or triaxial

tests). The most common approach used by engineers

to determine the CRR is based on in situ testing, and

in particular the standard penetration test (SPT) and

the cone penetration test (CPT). Shear wave velocity

) measurements and the Becker penetration test

(BPT) are also used in some specific applications.

Several publications provide guidance on the esti-

mation of CRR using in situ measurements. Such

correlations are obtained through back analysis of

case history data, and as such are related to the vari-

ables used to define the CSR (eqn [1]) for the case

histories, which can vary considerably among differ-

ent authors. For this reason, it is important that

practitioners are consistent in the methodology

they adopt, because the combination of different

recommendations could lead to incorrect results.

An example of a recently completed correlation relat-

ing CRR to (N

)

for clean sands (i.e., fines content

5%) is presented in Figure 8. The parameter (N

)

represents the SPT blowcount corrected to a ham-

mer energy level of 60% (to compensate for vari-

ations in testing equipment and procedures) and

normalized to an effective vertical stress of 1 atmos-

phere (to compensate for variations in the depth

of the in situ testing between the site and the case

history data). Similar curves for cohesionless soils

with higher fines content are also available,

based on the same data used to construct the curve

in Figure 8.

Permanent Ground Deformation

The extent of the permanent ground deformation as a

result of liquefaction will dictate the engineering solu-

tions in terms of mitigation or redesign. The interaction

between liquefied and non-liquefied soils, foundations

Figure 7 Schematic for determining maximum shear stress (t

max

) and the stress reduction coefficient (r

). Reproduced with

permission from Seed HB and Idriss IM (1971) Simplified procedure for evaluating soil liquefaction potential.

Journal of Soil Mechanics

and Foundations Division, ASCE

97: 1249 –1273.

ENGINEERING GEOLOGY/Liquefaction 529

and superstructures, and soils and embedded struc-

tures (e.g., pipelines, tunnels, tanks) will determine

whether the potential for liquefaction-induced damage

is unacceptable.

Flow failure Liquefaction flow failure occurs when

the static shear stresses exceed the residual shear

strength of a liquefied soil. Normally such failures

occur on sloping ground that was stable in its static

condition but became unstable due to the reduced

shear resistance of the liquefied soil. Once movement

is mobilized, displacements can be very large (up to

tens of metres) and very rapid. The potential for flow

failure can be assessed using standard slope stability

analyses, substituting the residual undrained shear

strength (discussed later) of the liquefied layer for its

static properties, where a factor of safety below 1

indicates a flow failure hazard.

Lateral spreading Lateral spreading is also a down-

slope failure mechanism, related to cyclic mobility

(Figure 9). The disturbing forces are a combination

of the gravitational static downslope forces and the

inertial loads generated by the earthquake. Lateral

spreads can occur on gentle slopes or where there is

a free face. The factor of safety against slope failure

may remain above 1, and the ground deformation is a

result of the progressive movement of surface layers

as a result of the oscillation of the ground. Lateral

spread-induced movements of as much as 10 m have

been observed in past earthquakes.

The amount of horizontal movement can be

estimated using relationships developed from empirical

data, soil mechanics theory, or numerical modelling.

Simplified relationships developed using empirical data

are the simplest to employ, but users should consider

the uncertainty associated with any simplification of

this very complex phenomenon; simplifications neces-

sarily neglect three-dimensional effects, local effects, or

redistribution of pore water pressures, for example.

Ground oscillation Ground oscillation, another

form of lateral ground deformation, can occur

where the underlying soil has liquefied but there is

no slope or free face for permanent lateral deform-

ations to occur. Ground oscillation will manifest itself

as large-amplitude transient ground waves with little

or no resultant permanent deformation.

Settlement Liquefaction is a result of the tendency

for saturated granular soils to densify under earth-

quake shaking. The eventual manifestation of this

behaviour is settlement at the ground surface as

excess pore pressures dissipate after the earthquake.

Settlements are usually estimated using free-field,

one-dimensional relationships for the volumetric

strain induced in the soil as a function of both the

relative density of the soil and the maximum shear

Figure 8 Correlation between cyclic shear stress ratio (for an

earthquake of magnitude 7.5) causing liquefaction and standard

penetration resistance for clean sands (FC, fines content 5%).

Figure 9 The failure of the upstream slope of the Lower San

Fernando Dam shortly after the 1971 San Fernando earthquake.

California narrowly avoided catastrophic flooding of 80 000 down-

stream homes. Liquefaction of the hydraulic fill caused loss of

strength and slope instability, which left less than 1 m of free-

board after the earthquake. This picture shows the slide after

emptying the reservoir; the paved road to the left indicates the

former dam crest. Photograph by Leslie F. Harder, Jr.

530 ENGINEERING GEOLOGY/Liquefaction

strain induced by the earthquake (Figure 10). In prac-

tice, the degree of settlement is complicated by the

heterogeneity of the soil, the interaction between ver-

tical and lateral movements, and the presence of

the structure, and most simplified methods for pre-

dicting volumetric strains have quite high associated

uncertainties.

Bearing failure If the residual shear strength of

the liquefied soil beneath is sufficiently low, then

bearing capacity failure will occur, causing the struc-

ture to settle or tilt uniformly (Figure 11). These types

of failures are generally accompanied by heaving of

the ground around the foundation. The potential

for a bearing-capacity failure can be determined

using simple static bearing-capacity formulas, substi-

tuting the residual undrained shear strength of the

liquefied layers. Failure will occur if the factor of

safety is less than unity. However, structures that are

safe against bearing failure can still develop excessive

settlements (total and differential), depending on

the strains (shear and volumetric) that develop in

the underlying soils.

Residual undrained shear strength The residual

shear strength of a liquefied soil can be determined

in the laboratory or can be estimated using correl-

ations between in situ test data and liquefied shear

strength obtained through back analysis of field case

histories. Field data, compared to laboratory tests,

tend to indicate much lower shear strengths. As well

as the difficulty in obtaining true, undisturbed

samples, some phenomena cannot be replicated in

laboratory tests. The layered nature and contrasting

permeability of in situ soil deposits can impede the

flow of water as earthquake-induced excess pore

water pressures dissipate; this can lead to localized

weakening of the soil at permeability interfaces (see

Figures 4 and 12). A further in situ phenomenon that

cannot be replicated by testing small samples is the

potential intermixing of soil layers with different

characteristics, due to the shear deformation, which

Figure 10 Chart for the estimation of volumetric strain as a

functionofthefactorofsafetyagainstliquefaction(F;eqn[2])

and the relative density of the liquefied soil layer (D

). (N

)

the standard penetration test blowcount normalized to an

equivalent overburden pressure of 1 atmosphere and hammer

energy of 60%.

Figure 11 Tilted buildings with shallow foundations in Niigata,

Japan (1964) as a result of liquefaction-induced bearing-capacity

failure.

Figure 12 An elevated transport link in Kobe following the

17 January 1995 earthquake. The bridge piers were piled, and

the structure is generally undamaged despite the settlement of

the surrounding soil by up to 1 m.

ENGINEERING GEOLOGY/Liquefaction 531

can reduce the shear strength. Field data, however,

are implicitly likely to include such phenomena. Con-

sequently, the use of the empirical correlations to

estimate postliquefaction shear strengths is recom-

mended.

Consequences of Earthquake-

Induced Liquefaction

The importance of liquefaction from an engineering

aspect is damage to the built environment, as repre-

sented by the following examples:

Settlements, which can be of the order of 5% of the

thickness of the liquefied soil, causing uniform or

differential settlement of foundations, pipelines, or

transportation routes.

Damage to piled foundations as result of loss of

support and flow of soil around piles that extend

into or through the liquefied soil layer(s).

Loss of bearing support to shallow or piled foun-

dations causing settlement or tilt.

Lateral spreads damaging building foundations,

bridge piers, highways and railways, river banks,

or embankment dams or pipelines.

Lateral flow failures.

Embankment failures due to liquefaction of under-

lying material or embankment fill.

Service interruption to buildings due to connection

damage caused by foundation displacements.

Increased lateral pressures against retaining struc-

tures such as quay walls.

Induced hazards, such as fire caused by gas pipeline

rupture, spread of fire due to interrupted water

supply, or floods caused by dam failure.

The severity of the damage is dependent on the

strength and duration of the earthquake ground

shaking, the thickness of the liquefied layer, the ma-

terial properties of the liquefied soil, and the proxim-

ity of free faces such as slopes or retaining structures,

as well as other variables related to the structures. The

response of a building to liquefaction-induced vertical

or lateral permanent ground deformation depends

to a large extent on the building’s foundations

(Figures 13–15). Bridge embankments are vulnerable

to lateral spreading due to the combination of suscep-

tible river channel deposits and sloping or free faces.

Bridge piers in liquefied ground can rotate, twist, or

displace laterally. In extreme cases, when liquefaction

is combined with poor design in the form of insuffi-

cient bearing width of the bridge deck, excessive

movement can cause loss of support to the bridge

deck and hence collapse (Figure 15). Damage to

roadways or railways due to liquefaction most

commonly comprises settlement or lateral spreading

of embankments, highway fills, or natural soil, which

causes cracking and uneven surfaces. Such damage is

relatively easy and inexpensive to repair but, nonethe-

less, the disruption and the indirect losses must be

considered.

The performance of a region’s lifelines networks,

including water, electricity, and gas distribution

systems, is an essential factor in the immediate emer-

gency response and the subsequent recovery from the

earthquake impact. Pipeline damage can be induced

both by transient ground oscillations and by vertical

Figure 13 Tilted apartment block in Adapazari, Turkey, due to

extensive liquefaction in the 1999 magnitude 7.4 Kocaeli earth-

quake. This building had thick, continuous, shallow foundations,

which caused it to rotate as a rigid body. There is very little

damage to load-bearing elements, walls, or ceilings; nonethe-

less, the building would have been demolished. Photograph by

Beyza Taskin.

Figure 14 Damage to the Marine Laboratory at Moss Landing,

Monterey Bay, California, during the 1989 magnitude 6.9 Loma

Prieta earthquake. Lateral spreading caused the building pad

foundations to spread apart. The laboratory was subsequently

demolished. Photograph by Leslie F. Harder, Jr.

532 ENGINEERING GEOLOGY/Liquefaction

or lateral permanent ground deformations, which

can shear, compress, or pull apart pipelines at their

joints. Foundation displacements are likely to damage

pipeline connections to buildings, which can lead to

building shutdown even in the absence of structural

damage. Ruptured gas and water pipelines can trigger

fires and impede the control of fires, respectively,

creating a significant induced hazard related to lique-

faction damage to pipelines. Buried pipelines, as well

as other buried vessels such as tanks, can become

buoyant in liquefied soils. Ports and harbours tend

to be particularly vulnerable to liquefaction damage

due to the combination of high water tables,

reclaimed soils, and retained vertical faces. Dam fail-

ures due to liquefaction at the toe, or flow failure of

the embankment, can cause catastrophic damage and

massive loss of life as a result of the flooding induced

by such a failure (see Figure 9). Mitigation of lique-

faction hazard in the design of dams is therefore of

the utmost importance.

Mitigation

When the potential liquefaction risk is considered to

be unacceptable for the performance requirements of

an engineering project, the three principal options are

relocation, prevention, and effective design. Relocat-

ing a structure or facility to avoid susceptible zones

may be the most straightforward option in terms of

mitigating the effect of liquefaction. For regionally

distributed facilities such as lifeline or transportation

networks, this option is unlikely to be practicable.

The use of ground improvement to limit or prevent

the occurrence of liquefaction has been shown to be

effective in many past earthquakes. Ground improve-

ment may involve increasing the density of the lique-

fiable soil through compaction, vibration, or

replacement; reinforcing and densifying the soil

through jet grouting or deep soil mixing; or providing

additional drainage to allow excess pore water pres-

sures to dissipate more rapidly (Figure 16). Often a

combination of several techniques is adopted. Again,

this is unlikely to be an appropriate solution for

regionally distributed facilities.

Accommodation through design is the third option

to manage liquefaction risk. Foundations can be

designed to withstand expected ground deformations

Figure 16 Installation of stone columns to mitigate liquefaction through densification and strengthening of liquefiable layers

(courtesy of Hayward Baker, Inc.).

Figure 15 This span of the Nishinomaya Bridge collapsed

following the 1999 magnitude 6.9 Kobe earthquake in Japan as

a result of liquefaction-related foundation deformations. Ground

cracks behind the quay walls and parallel to the water edge are

indicative of the lateral ground movements that occurred. Sand

boils are visible on the ground surface. Photograph by Leslie

F. Harder, Jr.

ENGINEERING GEOLOGY/Liquefaction 533

and to ensure that movements are not translated to the

superstructure. Piled foundations can be designed to

accommodate additional lateral loads imparted by

soil movement, or to have sufficient vertical capacity

even in the case of negative skin friction due to settle-

ment. Shallow foundations can be designed to be suf-

ficiently strong so as to behave as a rigid body when

subjected to ground deformations. In the case of life-

line networks, the accommodation of the expected

movements, and the implementation of appropriate

response and repair measures, are frequently the only

available solution.

Further Reading

Berrill J and Yasuda S (2002) Liquefaction and piled foun-

dations: some issues. Journal of Earthquake Engineering

6(Special Issue 1): 1–41.

Boulanger RW and Idriss IM (2004) State Normalization

of Penetration Resistance and the Effect of Overburden

on Liquefaction Resistance, pp. 484–491. 11th Inter-

national Conference on Soil Dynamics and Earthquake

Engineering and 3rd International Conference on Earth-

quake Geotechnical Engineering, University of Califor-

nia, Berkeley.

Harder LF, Jr (1997) Application of the Becker Penetration

Test for Evaluating the Liquefaction Potential of Grav-

elly Soils. Proceedings of the NCEER Workshop on

Evaluation of Liquefaction Resistance of Soils, NCEER-

97-0022.

Idriss IM and Boulanger RW (2004) Semi-empirical Pro-

cedures for Evaluating Liquefaction Potential during

Earthquakes, pp. 32–56. 11th International Conference

on Soil Dynamics and Earthquake Engineering and 3rd

International Conference on Earthquake Geotechnical

Engineering, University of California, Berkeley.

Ishihara K (1993) Liquefaction and flow failures during

earthquakes. (Rankine Lecture). Geotechnique 43:

351–415.

Ishihara K and Yoshimine M (1992) Evaluation of

settlements in sand deposits following liquefaction

during earthquakes. Soils and Foundations 32(No. 1):

173–188.

Kramer SL (1996) Geotechnical earthquake engineering.

Upper Saddle River, NJ: Prentice Hall.

National Research Council (1985) Liquefaction of Soils

during Earthquakes. Report by the Participants in the

Workshop on Liquefaction, Committee on Earthquake

Engineering, National Research Council, Report No.

CETS-EE-001. Washington, DC: National Academy

Press.

Seed HB (1987) Design problems in soil liquefaction.

Journal of Geotechnical Engineering, ASCE 113(8):

827–845.

Seed RB and Harder LF, Jr (1990) SPT-based analysis of

cyclic pore pressure generation and undrained residual

strength. In: H. Bolton Seed – Volume 2. Memorial Sym-

posium Proceedings, pp. 351–376. Vancouver, BC:

BiTech Publishers Ltd.

Seed HB and Idriss IM (1971) Simplified procedure for

evaluating soil liquefaction potential. Journal of Soil

Mechanics and Foundations Division, ASCE 97:

1249–1273.

Seed HB and Idriss IM (1982) Ground Motions and Soil

Liquefaction during Earthquakes. Monograph No. 5.

Oakland, CA: Earthquake Engineering Research Institute

(Note: A new edition of this monograph is expected

in 2005).

Tokimatsu K and Seed HB (1987) Evaluation of settlements

in sand due to earthquake shaking. Journal of Geotech-

nical Engineering, ASCE 113(8): 861–878.

Youd TL, Hansen CM, and Bartlett SF (2002) Revised

multilinear regression equations for prediction of

lateral spread displacement. Journal of Geotechnical

and Geoenvironmental Engineering, ASCE 128(12):

1007–1017.

Youd TL and Idriss IM (2001) Liquefaction resistance of

soils: Summary report from the 1996 NCEER and

1998 NCEER and NSF workshops on evaluation of li-

quefaction resistance of soils. Journal of Geotechnical

and Geoenvironmental Engineering, ASCE 127(4):

297–313.

Youd TL and Perkins DM (1978) Mapping liquefac-

tion-induced ground failure potential. Journal of the

Geotechnical Engineering Division, ASCE 104(GT4):

433–446.

534 ENGINEERING GEOLOGY/Liquefaction

Made Ground

J A Charles, Formerly Building Research

Establishment, Hertfordshire, UK

ß 2005, Published by Elsevier Ltd.

Introduction

The term ‘made ground’ is used to describe ground

that has been formed by human activity rather than

by natural geological processes. The material of which

made ground is composed is described as ‘fill’, and in

practice the terms ‘made ground’ and ‘fill’ are often

used interchangeably. Fill materials include not only

natural soils and rocks but also the waste products of

mining and industrial processes, and commercial and

domestic refuse.

Throughout history, mankind has deliberately

adjusted the topography of the Earth by excavating

soil and rock and placing the excavated material in

more convenient locations. The casual disposal of

waste materials has also changed landforms in urban

areas. These processes greatly accelerated during the

twentieth century, and in some localities, such as

major conurbations and mining areas, a significant

proportion of the surface area is now made ground.

It is therefore important that the extent, depth, and

nature of such deposits are reliably mapped and that

the geotechnical behaviour of the fill materials is

adequately understood.

A simple classification system has been developed

for describing ground affected by human activity, and

this classification is used in the geological mapping of

the UK.

Made ground – areas where it is known that fill

material has been placed.

Worked ground – areas where excavations have

been made in natural ground.

Disturbed ground – areas of surface and near-sur-

face disturbance (including ground that has sub-

sided), typically associated with mining.

Landscaped ground – areas where the ground sur-

face has been remodelled, but made ground and

worked ground cannot be distinguished.

A broad distinction can be made between ‘engin-

eered fill’ and ‘non-engineered fill’. The distinction is

essentially one of purpose: engineered fill is placed for

a specific purpose, whereas non-engineered fill is a

by-product of the disposal of waste material. Engin-

eered fill is selected, placed, and compacted to an

appropriate specification in order that it will exhibit

the required engineering behaviour; engineering

design focuses on the specification and control of

filling. Non-engineered fill is placed with no subse-

quent engineering application in view. The distinction

between engineered and non-engineered fill is clear in

principle, but not always in practice.

History

Early earthmoving activities were undertaken to meet

practical objectives, although in some cases that ob-

jective is not obvious to the modern mind. Large-scale

earthmoving has provided many durable reminders of

previous ages.

Silbury Hill, situated in Wiltshire in the south of

England, is the largest prehistoric manmade mound

in Europe and a remarkable early civil-engineering

achievement. Archaeological evidence suggests that

the 40 m high mound (Figure 1) is more than 4000

years old. It was carefully engineered in a series of six

stepped horizontal layers created from concentric

rings of chalk block walls. There are radial as well

as circumferential walls, and the compartments be-

tween them are filled with chalk rubble. The steps in

the outer slope were infilled to give a smooth slope. It

has been suggested that the hill is a burial mound, but

no graves have been found.

One practical reason for placing fill is to increase

the area of land suitable for building on within the

confines of a town or city. This has been achieved in

hill country by cut-and-fill earthworks on hillsides

and in low-lying areas by raising the level of marshy

ground. Some 3000 years ago rock-fill platform terra-

cing was formed on the eastern slopes of Jerusalem,

thereby substantially increasing the building area.

This is thought to be King Solomon’s Millo, the

Figure 1 Silbury Hill, England.

ENGINEERING GEOLOGY/Made Ground 535

Elsevier Encyclopedia of Geology - vol I A-E

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