GEOLOGICAL SURVEY CIRCULAR 688 

Liquefaction, Flow, and 

Associated Ground Failure 

Liquefaction, Flow, and 

Associated Ground Failure 

By T. Leslie Youd 

G E 0 L 0 G I C A L S U R V E Y C I R C U L A R 688 

., ... , 
United States Department of the Interior 

ROGERS C. B. MORTON, Secretary 

Geological Survey 

V. E. McKelvey, Director 

Free on application to the U.S. Geological Survey, Washington, D.C. 20244 
CONTENTS 

Abstract 

Introduction ............................................................................................................................... . 

Published definitions of liquefaction ....................................................................................... . 

Observed behavior and proposed definitions ....................................................................... . 

Consequences of liquefaction ................................................................................................... . 

Flow landslides ................................................................................................................. . 

Lateral-spreading landslides ........................................................................................... . 

Quick-condition failures ................................................................................................... . 

Methods for evaluating liquefaction and ground-failure potential ................................... . 

Page 

1 

1 

1 

2 

6 

7 

8 

9 

9 

Surmnary .................. ................................... ................................. .. .. ........................................... 10 

References cited .......................................................................................................................... 11 

ILLUSTRATIONS 

FIGURES 1-7. Graphs showing: 

1. Stress-strain and pore pressure-strain curves for monotonically loaded triaxial compression test 

on undrained sample of Ottawa sand illustrating liquefaction, flow deformation, and 

solidification ............................................................................................................................................ . 

2. Typical stress-strain curves for three monotonically loaded triaxial compression tests on undrained samples of Ottawa sand .......................................................................................................... . 

3. Stress paths for the three triaxial compression tests plotted in figure 2 ........................................... . 

4. Liquefaction and unlimited flow generated by cyclic loading ............................................................... . 

5. Repeated episodes of liquefaction and limited flow generated by cyclic loading ............................... . 

6. Test data illustrating the necessity of stress reversals for cyclic loading to produce repeated 

cycles of liquefaction and limited mobility ................................................................................... . 

7. Data showing that resistance to reliquefaction is reduced by a previous episode of liquefaction 

Page 

3 

4 

4 

5 

5 

6 

7 

liquefaction, Flow, and Associated Ground Failure 

By T. Leslie Youd 

ABSTRACT 

Ambiguities in the use of the term liquefaction and in 

defining the relation between liquefaction and ground 

failure have led to encumbered communication between 

workers in various fields and between specialists in the 

same field, and the possibility that evaluations of liquefaction potential could be misinterpreted or misapplied. 

Explicit definitions of liquefaction and related concepts 

are proposed herein. These definitions, based on observed 

laboratory behavior, are then used to clarify the relation 

between liquefaction and ground failure. 

Soil liquefaction is defined as the transformation of a 

granular material from a solid into a liquefied state as 

a consequence of increased pore-water pressures. This 

definition avoids confusion between liquefaction and possible flow-failure conditions after liquefaction. Flowfailure conditions are divided into two types: (1) unlimited flow if pore-pressure reductions caused by dilatancy 

during flow deformation are not sufficient to solidify the 

material and thus arrest flow, and (2) limited flow if 

they are sufficient to solidify the material after a finite 

deformation. After liquefaction in the field, unlimited 

flow commonly leads to flow landslides, whereas limited 

flow leads at most to lateral-spreading landslides. Quickcondition failures such as loss of bearing capacity form 

a third type of ground failure associated with liquefaction. 

INTRODUCTION 

Because of the critical effect liquefaction has on 

the safe performance of engineered construction 

and the stability of certain geologic formations, 

considerable study has been devoted to this topic 

inrecentyears. Significant progress has been made 

in understanding the liquefaction phenomenon 

and the factors controlling it; however, ambiguity 

in present definitions of the term liquefaction and 

lack of clear distinction between liquefaction and 

ground-failure conditions associated with this 

phenomenon have produced some confusion in 

usage and understanding of liquefaction and its 

consequences. This confusion has encumbered 

communication between workers in various disciplines as well as between specialists in the same 

field and allowed the possibility that analyses of 

liquefaction potential could be misinterpreted or 

misapplied. 

In this paper explicit definitions of liquefaction 

and related concepts are given on the basis of 

behavior observed in laboratory tests. These concepts are then used to define relations between 

liquefaction and the various types of ground failure commonly associated with liquefaction. Examples are given to illustrate the character of 

each type of ground failure. Finally, the applicability and limitations of existing methods for 

evaluating liquefaction potential are briefly examined with the aid of the definitions formulated 

herein. 

Appreciation is given to D. H. Gray, E. J. Reiley, Kaare Hoeg, K. L. Lee, H. W. Olsen, and 

H. B. Seed for comments and suggestions. 

PUBLISHED DEFINITIONS OF 

LIQUEFACTION 

The standard technical dictionary definition of 

liquefaction is "The act or process of transforming any substance into a liquid" (Lange and 

Forker, 1961, p. 1738). Several definitions of soil 

liquefaction are in basic agreement with this concept. For example, Terzaghi and Peck ( 1948, p. 

100) defined spontaneous liquefaction as "the 

sudden decrease of shearing resistance of a quick 

sand from its normal value to almost zero withoo&" 

the aid of seepage pressure." A generally comparable definition was published by the American 

Society of Civil Engineers ( 1958, p. 1826-22) and 

quoted by the American Geological Institute 

(1972, p. 410) as follows: "The sudden large 

decrease of shearing resistance of a cohesionless 

soil, caused by a collapse of the structure by 

shock or strain, and associated with a sudden but 

temporary increase of the pore fluid pressure [is 
liquefaction]. It involves a temporary transformation of the material into a fluid mass." Ghaboussi and Wilson ( 1973) gave the following definition: "The phenomenon of the loss of strength 

of saturated granular soils during earthquakes is 

generally referred to as liquefaction. The process 

of liquefaction transforms an element of soil from 

a state of saturated granular solid to a state of 

viscous fluid." 

Several definitions for liquefaction and related 

phenomena have been proposed by Seed and his 

coworkers on the basis of behavior observed in 

cyclically loaded shear tests on saturated samples 

of sand. Lee and Seed (1967a, p. 49) define (1) 

complete liquefaction as "when a soil exhibits no 

resistance (or negligible resistance) to deformation over a wide strain range, say a double amplitude of 20 percent"; (2) partial liquefaction as 

"when a soil exhibits no resistance to deformation over a strain range less than that considered 

to constitute failure"; and (3) initial liquefaction 

as "when a soil exhibits any degree of partial 

liquefaction during cyclic loading." In addition 

Seed and Idriss (1971, p. 1249) have used the 

term liquefaction to describe "a phenomenon in 

which a cohesionless soil loses strength during an 

earthquake and acquires a degree of mobility sufficient to permit movements ranging from several 

feet to several thousand feet." Except for initial 

liquefaction, both a flow deformation requirement and a strength loss requirement are incorporated into each of the definitions proposed by 

Seed and his colleagues. 

Castro (1969, p. 7) used the term liquefaction 

to denote the following phenomenon: "The conventional use of the term [liquefaction] as it will 

be used throughout this thesis, refers to the phenomenon which takes place in a mass of soil during flow slides. Liquefaction or flow failure of a 

sand is caused by a substantial reduction of its 

shear strength." This usage in effect defines 

liquefaction as a condition of unrestrained flow 

deformation. 

The incorporation or confusion of two different 

phenomena (strength loss leading to a material 

phase transformation and flow deformation) into 

a single definition has led to ambiguities in usage. 

For example, a map of sand deposits susceptible 

to liquefaction could be interpreted as (1) a map 

of deposits that could be transformed into a 

liquefied state, but with no assessment of type 

') 

or amount of ground-failure movement, if any, 

that might follow the transformation; (2) a map 

of deposits that could lose strength and acquire 

a degree of mobility sufficient to permit movements ranging from several feet to several thousand feet; or (3) as a map of deposits that could 

lose strength and fail in the form of flow landslides. In order to reduce this ambiguity and provide for more precise usage, explicit definitions 

are needed, and the relation between liquefaction 

and ground failure needs clarification. 

OBSERVED BEHAVIOR AND PROPOSED 

DEFINITIONS 

Laboratory test data illustrative of the behavior of saturated, undrained granular soils during 

shear loading (monotonic and repetitive) have 

been published by several investigators (Castro, 

1969; Finn and others, 1970; Lee, 1970; Lee and 

Fitton, 1968; Lee and Seed, 1967a, b; Peacock 

and Seed, 1968; Seed and Lee, 1966; 1969). 

These data are used here as a basis for formulating definitions of liquefaction and related terms, 

clarifying the relation between liquefaction and 

ground failure, and examining the character of 

various types of ground failure associated with 

liquefaction. 

Some pertinent aspects of the behavior of saturated sands during conditions of undrained, 

monotonic shear loading are illustrated in the 

curves plotted in figure 1. These data were taken 

from test 4-7 of Castro ( 1969), a saturated undrained triaxial compression test on moderately 

dense (relative density; Dr==4 7 percent1 ) Ottawa 

sand. Three distinct phases, through which the 

sample passed, are shown on the curves. ( 1) During initial loading (beginning of test to point l), 

the sample behaved as a solid (did not perceptibly flow (Lange and Forker, 1961, p. 1794) ). 

During this part of the test, pore pressures increased with applied load until a point of instability was reached at point l. This rise in pore 

pressure was caused partly by the tendency of 

all but very dense samples to compact initially 

during loading (Youd, 1972a, p. 717). (2) BelRelative density, Dr• in percent is defined by the following relationship: 

e max -e 

Dr- emax-emin (100) 

where emax and emin are void ratios of a given san~ in its loosest 

and densest states, respectively, and e is the void ratio of the sand 

at the density in question. 
!/) 

z 

o .. 1-0 

wX ~--

za: 

~~ .w 

~:!: 

ww 

a: a: 

~--~ !I):> 

a:O 

Q!/) 

1-a: 

~w 

:;:a.. 

w 

0 

!/) 

z 

0"' 

~--~ 

~X 

wa: 

1 X 105 N/m2 =2090 lb/ft2 

6 

5 

4 

3 

2 

QL---__ L__ ___ L__ ___ .L._ __ ______, 

4.----,----,----.---~ 

3 

2 

agreement with the standard technical definition 

(Lange and Forker, 1961, p. 1738), the definitions of Terzaghi and Peck (1948, p. 100), the 

American Society of Civil Engineers ( 1958, p. 

1826-22), the American Geological Institute 

(1972, p. 410), and Ghaboussi and Wilson (1973). 

For cyclic loading conditions, the definition proposed above is congruous with the definition of 

initial liquefaction given by Lee and Seed (1967a, 

p. 49). 

At the conclusion of the liquefied flow segment 

of the test 4-7 (points on the curves), a process 

opposite to liquefaction occurred-the transformation of a granular material from a liquefied 

state into a solid state. This process is defined 

here as solidification. Deformations in the solid 

state are generally referred to as elastic strain or 

Zw 

Zl-

-w 

w·:!: 

a:w 

:>a: 

!/)~ 

!/):::> 

wo 

g:!/) 

wa: a:W 

QQ. 

a.. 

0 L__ ___ L__ ___ L__ ___ .L._ __ ______, plastic strain depending on whether deformations 

o 5 10 15 20 are recoverable or not, respectively. Deformations 

AXIAL STRAIN, IN PERCENT 

FIGURE 1.-Stress-strain and pore pressure-strain curves 

for monotonically loaded triaxial compression test on 

undrained sample of Ottawa sand illustrating liquefaction, flow deformation, and solidification (after test 

4-7 Castro, 1969). 

yond the point of instability (point l), the sample 

flowed in a liquefied state (rapidly deformed 

under constant total stress). In this phase the 

sample deformed through an axial strain of 6 

percent in 1.0 sec. (wavy part of curves). During 

the latter part of the flow deformation segment, 

dilatant tendencies within the sample caused a 

reduction of pore pressure and concomitant 

strengthening of the sample. This eventually led 

to an arrest of the flow, and the sample was reconverted into a solid state (points on the curves). 

(3) Beyond points the sample continued to behave as a solid. As additional load was applied, 

the dilitant tendency continued to cause further 

pore pressure reductions and greater sample 

strength. 

On the basis of the behavior illustrated in test 

4-7 (fig. 1), the following definitions for liquefaction and related terms are proposed. Liquefaction 

is defined as the transformation of a granular material from a solid state into a liquefied state as a 

consequence of increased pore-water pressures. 

This transformation occurred during the initial 

loading phase of test 4-7 and was complete 

(liquefaction occurred) at point l on the curves. 

The proposed definition of liquefaction is in close 

~ J 

in the liquefied state are referred to as flow deformations. 

Flow deformation can be divided into two 

types. ( 1) Limited flow (equivalent to Castro's 

(1969, p. 19) limited liquefaction or, for cyclic 

loading conditions, Lee and Seed's (1967a, p. 49) 

partial liquefaction) refers to a condition in 

which liquefaction occurs and flow deformation 

ensues but is arrested by solidification after a 

finite movement without significant change of 

total stresses in the process. The sample in test 

4-7 (fig.1) underwent limited flow. (2) Unlimited 

flow (equivalent to Castro's (1969, p. 16) liquefaction failure or, in essence, equivalent to Lee 

and Seed's (1967a, p. 49) complete liquefaction) 

refers to a condition in which dilatancy-caused 

reductions in pore pressure are insufficient to 

arrest flow; thus, flow deformation continues 

unabated until the applied shear stresses are 

reduced to a level less than the viscous shear resistance of the liquefied material. At that point 

solidification occurs. The sample in test 4-4 (fig. 

2) underwent liquefaction and unlimited flow. In 

this test, flow deformation ceased only when the 

deviator stress was relieved by stopblocks in the 

loading apparatus. 

Additional insight into the behavior of saturated sands during undrained shear can be gained 

from an examination of stress paths constructed 

from published laboratory test data. For example, 

effective stress paths for the three triaxial compression tests of Castro ( 1969) listed in figure 2 
8 

1 X 1 05 N/m2 =2090 lb/ft2 

(I) 7 z 

o .. 

~~ 6 wx 

za: 

zw 

-I- 5 

•W 

!2:E 

Ww 4 a: a: 1-<( (I) ::::I 3 a: a Ocn 

1-a: 

<w 2 >a.. 

w 

0 

(I) 

z 

0"' 0 

~~ 4 

Wa: Zw 3 Zl-

-w 

w':E 

a:w 2 

:::Ia: (I)<( (I) ::::I 

~g 0.. 

wa: a:W 0 00.. 0 5 10 15 20 0.. 

AXIAL STRAIN,IN PERCENT 

FIGURE 2.-Typical stress-strain curves for three monotonically loaded triaxial compression tests on undrained samples of Ottawa sand (after Castro, 1969). 

The sample in test 4-4 underwent liquefaction and 

unlimited flow; the sample in test 4-7 underwent liquefaction, limited flow, and solidification; and the sample 

in test 4-8 did not liquefy nor flow. 

have been plotted on the p-q diagrams (Schofield 

and Wroth, 1968) in figure 3, 

where 

p==.% (ul + 2ua)- U 

q==. (ul- u3) 

p is the effective spherical pressure, q is the axial 

deviator stress, u1 and ua are the major and minor 

principal stresses, respectively, and u is the pore 

water pressure. In each of these tests pore pressures rose in response to the initial increments of 

loading. These pressures caused the stress paths 

to follow a counterclockwise, concave downward 

(strain-softening) path. The paths for tests 4-4 

and 4-7 passed through a q-maximum (point l 

on the curves) that was also a point of instability 

or, as discussed earlier, the point at which liquefaction occurred. Beyond these points flow occurred (wavy parts of curves). The path for test 4-4 

(Dr==-37 percent), which underwent u'"nlimited 

flow, continued downward until the end of the 

test. The path for test 4-7 (Dr==-46 percent),however, passed through a q-minimum and then turned 

upward along a constant q/p ratio (strain-hard0 

X 

a: 

w 1-

w 

:E 

w 

a: <( 

::::1 

a (I) 

a: 

w 0.. 

(I) 

z 

0 1-

3: 

w 

z 

~ 

fli (I) 

w 

a: 1-(I) 

a: 

0 1-<( 

> w 

0 

..J <( 

x <( 

8 

1 X 1 05 N/m2 =2090 lb/ft2 

7 

6 

5 

4 

3 

2 

OL-----~----~--~~--~----~------

0 2 3 4 5 6 

EFFECTIVE SPHERICAL PRESSURE, IN NEWTONS 

PER SQUARE METER X 105 

FIGURE 3.-Stress paths for the three triaxial compression tests plotted in figure 2. 

ening) path in response to reduced pore pressures 

caused by dilatant tendencies within the sample. 

Solidification occurred at point s on that curve, 

and beyond that point the sample behaved as a 

solid. 

The dilatant tendency became effective in test 

4-8 (Dr==-47 percent) before the stress path 

could read a q-maximum (figs. 2 and 3). Pore 

pressure decreased, and the stress path passed 

through an inflection point, became concave upward. and eventually followed a constant q I p 

ratio line congruous with the one generated during the latter part of test 4-7. Thus, neither liquefaction nor flow occurred during this test. 

Although not the first to do so (Seed and Lee, 

1966), Castro ( 1969) also demonstrated the generation of liquefaction by cyclic loading. Stress 

paths from two of those tests are plotted in figure 

4. During the first loading, stresses followed paths 

similar to those of the monotonically loaded tests 

described above; however, loading was stopped 

short of a point of instability or inflection and 

then reduced to zero. The net result of the cycle 
~ 6 ·w 

(I) a: r:3ct: 

a::> 

1- o .. CI)CI)~ 4 

a::a::x 

ow a: t-a..w 

~(I) I- >ZW 2 wO:::!: 

ct-

...J~ ct:W -Z X 0 <{ 0 

1X105 N/m2 =20901b/ft2 I 

2 4 6 8 10 

EFFECTIVE SPHERICAL PRESSURE, IN NEWTONS 

PER SQUARE METER X 105 

12 

FIGURE 4.-Liquefaction and unlimited flow generated 

by cyclic loading. Stress paths from cyclically loaded 

triaxial compression tests without stress reversals. 

(Data from Castro, 1969.) 

was a pore-pressure increase equivalent to the 

net decrease in p. Similar pore-pressure increases 

were generated during each succeeding cycle until 

liquefaction occurred on the seventh cycle of test 

4 and fourth cycle of test 6. At those points the 

samples entered states of unlimited flow, and flow 

failure ensued. 

Cyclically loaded tests in which limited flow 

developed have been reported by Lee and Seed 

(Lee, 1970; Lee and Seed, 1967a; Seed and Lee, 

1966, 1969). The granular sediments they used 

were subangular to subrounded fine uniform 

sands from the Sacramento River, Calif. The 

stress path of a typical test with stress reversals 

(test 114, see fig. 2 of Lee and Seed, 1967a) is 

plotted in figure 5. Owing to the frequency of 

loading (2 cycles per second) and the relatively 

slow speed of the recorder (2.5 em/sec (1 in/ 

sec)), stress paths can not be plotted as accurately for Lee and Seed's tests as they were for 

Castro's tests. Stresses at the conclusion of each 

loading segment were well defined, however, and 

are accurately plotted as the circled points in the 

figure. The test began with an isotropically consolidated sample (Dr==38 percent). During each 

cycle, pore pressure increased (p decreased) in 

response to the tendency of the sample to compact. During the first eight and a half cycles, deformations were negligible. However, during the 

extensional loading segment of the ninth cycle, 

liquefaction occurred, and the sample entered a 

state of limited flow and rapidly strained 17 percent in axial extension before solidification occurred (point s1 ) as a result of a dilatancy-caused 

drop in pore pressure. The inferred stress path 

during this phase of the test is shown by the inter0.6 

Cl) 

z 0.4 

0 

w 

~ .. z~ 

zx 0.2 

-.a: Cl)w Cl)l- Ww 

~::21: CI)W 0 

a:~ 

g:::> 

<(0 -CI) 

>a: -0.2 ww Ca.. 

...J 

<{ 

x 

<{ -0.4 

1 X 105 N/m2 =2090 lb/ft2 

f'"' .. s,, ' ', '-... J 

' '/, 

' ' 

EFFECTIVE SPHERICAL PRESSURE, IN NEWTONS 

PER SQUARE METER X 105 

FIGURE 5.-Repeated episodes of liquefaction and limited 

flow generated by cyclic loading. Stress path from a 

cyclically loaded triaxial compression test with stress 

reversals. (Data from fig. 2, Lee and Seed, 1967a.) 

mittent wavy line (fig. 5). Liquefaction recurred 

during the ensuing compressional loading (stress 

path through l2 and Sz). The sample again entered 

a state of limited flow, strained 29 percent in 

axial compression, and then was restabilized by 

solidification. During each subsequent loading 

the sample reliquefied, rapidly deformed, and 

solidified at the opposite extremity of the developed strain excursion. 

In another set of tests, Seed and Lee (1969) 

showed that deformations during cyclic loading 

are considerably greater for tests with stress reversals above some small threshold value than 

for tests without stress reversals or for tests with 

very small stress reversals. The behavior of a test 

with no stress reversal is shown by the stress path 

in figure 6 plotted from data (test 99) reported 

by Lee (1970, p. 321). During the first loading 

of the sample (stress path from 0 through l to 1) 

liquefaction occurred; the sample entered a state 

of limited flow, strained 5.5 percent in axial compression, and then was restabilized by dilatancycaused solidification. During unloading the pore 

pressure increased (p decreased) as the dilatant 

tendency was relaxed; however, the stress path 

remained within the stable domain (path from 1 

to 1u), thus preventing a recurrence of liquefaction. During the second loading (path from 1u to 
~ X 

a: 

w 

1-

w 

::E 

w 

a: 

c( 

:::::> 

0 

C/) 

a: 

w 

a.. 

C/) 

z 

w 

~ z 

~ 

vj 

C/) 

w 

a: 

1-

C/) 

.a: 

0 

1-

c( 

> w 

0 

.J 

~ 

X 

c( 

1X 105 N/m2 =2090 lb/ft 2 

EFFECTIVE SPHERICAL PRESSURE, IN NEWTONS 

PER SQUARE METER X105 

FIGURE 6.-Test data illustrating the necessity of stress 

reversals for cyclic loading to produce repeated cycles 

of liquefaction and limited mobility. Stress path from 

a cyclically loaded triaxial compression test without 

stress reversals. Liquefaction and limited flow occurred 

only during first loading. (Data from test 99, Lee, 

1970.) 

2), the applied shear stresses were resisted immediately by increased sample strength as a result 

of dilatancy-caused decreases in pore pressure. 

This sample hardening eliminated any possibility 

of liquefaction or flow during that segment of 

the test although some plastic straining did occur. 

During unloading the stresses again traced a path 

within the stable domain (from point 2 to about 

point 1u). This behavior was repeated during 

each additional cycle. The stress conditions at 

maximum load for several of these cycles are 

shown by the correspondingly numbered points 

on the diagram. (The values decreased because 

the sample cross-sectional area increased during 

the test as a result of plastic straining.) In each 

cycle the stress paths returned approximately to 

point 1u during unloading. Thus, other than the 

single episode of liquefaction and flow deformation during initial loading, the sample did not 

reliquefy as did those in tests with appreciable 

stress reversal. These results clearly show that 

shear-stress reversals are necessary to produce 

repeated occurrence of liquefaction and limited 

flow. 

The influence of a previous history of liq uefaction on the susceptibility of a sand to reliquefaction at a later period has been demonstrated by 

Finn, Bransby, and Pickering (1970). The approximate stress paths for two liquefaction tests 

on the same Ottawa sand sample are plotted in 

figure 7 (data from fig. 5 (test 40) of Finn and 

others, 1970). The sample was formed in a triaxial compression device, saturated, and then 

consolidated under an isotropic stress of 2.0 X 105 

N/m2 (4,100 lb/ft2 ) to a relative density of 50 

percent. Cyclically reversing loads were applied 

to the sample. Pore pressure increased with each 

loading until liquefaction occurred during the 

extensional loading segment of the 25th cycle 

(light-line stress path in fig. 7). The sample 

flowed through an axial strain of 0.6 percent, at 

which time deformation was arrested by solidification. Several additional loading cycles were 

applied (stress paths not shown). With each load 

reversal, the sample reliquified, flowed, and then 

solidified at the opposite end of the strain excursion. After a total of 29 cycles the loading was 

stopped, leaving the sample stabilized at the compressional end of the strain excursion. The sample 

was then reconsolidated under an isotropic stress 

of 2.0 X 105 N /m2 ( 4,100 lb/ft2 ). This consolidation yielded a relative density of 60 percent. 

Cyclic loading was then resumed. This time liq uefaction occurred during the extensional loading 

segment of the first cycle (heavy-line stress path 

in fig. 7). From this and similar tests, Finn, 

Bransby, and Pickering (1970) conclude that 

resistance to reliquefaction is substantially reduced by a previous episode of liquefaction. 

CONSEQUENCES OF LIQUEFACTION 

Liquefaction by itself poses no particular hazard. In fact, during seismic shaking a liquefied 

layer at depth could act as an isolator, impeding 

the transmission of vibrational energy from 

underlying layers to s-tructures founded at the 

surface (Seed, 1968; Ambraseys, 1973). Only 

when liquefaction leads to some form of permanent ground movement or ground failure does it 

become a serious problem. Three basic types of 

ground failures associated with liquefaction have 

been identified (Seed, 1968)-flow landslides, 

landslides with limited displacement, and quick-
0.8 

1 X 10' N/m2 =20901b/ft 2 

c liquefaction 

X Reliquefaction 

a: 

w 

~ 

w 

~ 0.4 w 

a: 

< 

::J 

0 

Cl) 

a: 

w 

CL. 

Cl) 

z 

0 

~ 0.0 w 

z 

~ 

~ w 

a: 

~ Cl) 

a: 

0 

~ 

< -0.4 > w 

c 

_J 

< 

x 

< 

EFFECTIVE SPHERICAL PRESSURE, IN NEWTONS PER SQUARE METER X 105 

FIGURE 7.-Data showing that resistance to reliquefaction is reduced by a previous episode of liquefaction. Stress 

paths from cyclically loaded liquefaction and reliquefaction tests on the same Ottawa sand sample. Sample 

liquefied and flowed in limited flow in both tests. (Data from fig. 5, Finn and others, 1970.) 

condition failures. In addition to these types of 

ground failure, ejection of water and sediments in 

the form of sand boils has been a source of damage associated with liquefaction during earthquakes (Ambraseys and Sarma, 1969). 

FLOW LANDSLIDES 

Where conditions are favorable, liquefaction in 

the field may lead to a state of unlimited flow. In 

this situation, if the mobilized soil is unrestrained, 

sizeable masses of earth materials may travel 

long distances in the form of liquefied flows or 

blocks of intact materials riding on liquefied 

flows. In this type of failure, flow ceases only 

when the driving shear forces are reduced (such 

as by slope reduction) to values less than the 

viscous shear resistance of the flowing material. 

The dimensions and character of typical flow 

landslides are illustrated in the few examples 

cited below. 

Several flow landslides have occurred in the 

estuary section of the Dutch province of Zeeland 

(Koppejan and others, 1948), including some 

that have caused disastrous breaches in the dikes 

of that region. Dike slopes before failure were 

typically about 36 percent (20°), with banks as 

high as 40 m ( 130 ft). After failure, many slopes 

were 7 percent ( 4 °) or less. Liquefaction of a 

loose granular layer underlying the slides areas 

was the apparent cause of the flow failures. 

A flow failure occurred in the Fort Peck Dam, 

Montana, during construction of a hydraulic-fill 

embankment (Casagrande, 1965). A 250-m 

(1,700-ft)-long section of the upstream shell 

failed and moved 460 m (1,500 ft) upstream in 

about 4 min. Before failure, the slope of the upstream embankment was about 25 percent (14°); 

after failure slopes on the failed mass were generally less than 5 percent (3°). The cause of 

failure is believed to have been liquefaction of a 
sand zone in the shell and possibly a natural 

granular layer beneath the dam (Casagrande, 

1965, p. 10). 

Flow failures commonly are triggered by seismic shaking (Seed, 1968). Crandall (1908, p. 

249) described one that occurred near San Francisco during the 1906 earthquake 

Northeast of Mount Olivet Cemetery there was an 

earth-flow in the sandy soil at the base of the San Bruno 

Mountains. The angle at which the materials slid was 

hardly more than 10 degrees [18 percent]. The sand and 

water forming this slide came out of a hole several hundred feet long and 150 feet [ 46 m] wide, flowed down the 

hill several hundred yards toward the cemetery, carried 

away a pile of lumber, and knocked the power-house 

from its foundations. The front of the mud-flow piled up 

in a bank when it reached the nearly level ground, and 

dammed up the mass behind it. 

LATERAL-SPREADING LANDSLIDES 

Where conditions are favorable for liquefaction 

but the sediments are too dense to allow unlimited 

flow, limited flow may develop. On sloping ground 

this may allow finite downslope movements followed by dilatancy-caused solidification. Other 

factors that may also limit downslope displacement include rapid drainage and liquefaction of 

materials along only part of the potential failure 

plane (Seed, 1968). In many instances, however, 

dilatancy-caused solidification appears to be the 

most important factor in limiting displacement. 

One episode of limited flow may produce displacements of negligible importance; on the other 

hand, cumulative displacements generated by 

repeated episodes of limited flow, such as might 

occur during an earthquake or other source of 

repeated loading, could be substantial. Description of limited displacement landslides generated 

by liquefaction are common in the earthquake 

literature, but they are described under different 

nomenclature. For example, "earth lurches" 

(Richter, 1958, p. 124), land spreading (McCulloch and Bonilla, 1970), and lateral spreading 

(Oldham, 1899, p. 87-90; Youd, 1973) are, in 

fact, limited displacement ground failures associated with liquefaction. The term "lateralspreading landslide" is used here to denote this 

type of failure because it is commonly used, it 

has been rigorously defined (Varnes, 1958, p. 

29-32), and many failures described in the literature fit the description of lateral spreading given 

by Varnes. 

Several important characteristics of lateralspreading landslides associated with liquefaction 

can be deduced from the laboratory behavior 

described above. ( 1) Repeated episodes of limited flow can only develop if shear stress reversals 

occur during the loading sequence. These reversals are more easily accomplished beneath mild 

slopes where static shear stresses are small than 

beneath steeper ones. However, if the slope is too 

gentle, the gradient may not be sufficient to cause 

movement. Thus mild slopes should be most susceptible to this type of failure. 

(2) With each occurrence of restabilization, 

the mobilized soil in the failure zone receives a 

dilatant impulse that could cause loosening if 

excess water were available to fill the created 

voids. Excess water is often available during 

earthquakes as a result of compaction of other 

granular materials in the section. Thus, it is possible for loosening to occur which, in turn, would 

allow displacements to increase with each episode 

of limited flow and could eventually lead to a 

condition of unlimited flow. 

(3) At the conclusion of a series of limited-flow 

episodes, the soil in the failure zone is commonly 

left in a dilatant restabilized condition. The soil 

itself may be denser or looser or the same as it 

was before the disturbance, depending on whether 

pore water migrated into or out of the liquefied 

soil during shear. In any case, and even after reconsolidation, the soil may be left in a particularly vulnerable condition for reliquefaction during a subsequent disturbance. 

( 4) Repeated episodes of limited flow can continue only as long as strong ground shaking producing stress reversals continues. Thus, lateralspreading landslides associated with liquefaction 

would normally move only during periods of 

strong shaking and restabilize immediately upon 

the cessation of shaking. 

The following few examples of lateral-spreading ground failures illustrate their typical dimensions and character. 

During the 1971 San Fernando earthquake, 

lateral-spreading ground failures occurred on 

both sides of Van Norman Lake (Youd, 1971; 

1973; Smith and Fallgren, 1973; Proctor and 

others, 1972). Northeast of the lake the landslide was tongue shaped in plan, 1.2 km ( 4,000 

ft) long, and about 0.3 km (1,000 ft) wide (average). The mean slope from head to toe of the 
failure was about 1.5 percent (0.9°). Horizontal 

displacements were as large as 1.9 m ( 5. 7 ft), 

whereas maximum vertical displacements were 

only about 0.15 m (0.5 ft). The surface layer was 

fractured into several large blocks that slid downslope with very little tilting. Sand boils, indicative of increased pore pressures at depth, erupted 

at several points on and near the slide. The soil 

profile beneath the slide consisted of a firm surface layer overlying a soft saturated layer composed of sand and sandy silt. Liquefaction in the 

lower layer presumably led to the failure by 

repeated episodes of limited flow. Relict preearthquake fissures and sand boils that were found in 

postearthq uake exploratory trenches are evidence 

that similar ground failures had occurred before 

at this site, most likely during previous earthquakes (Youd, 1972b). 

West of the lake, the surface ruptures broke 

roughly parallel to the long free face formed by 

the west shore of Van Norman Lake and severely 

disrupted the newly constructed fill for the J ensen water-filtration plant. Maximum horizontal 

displacements were about 1 m ( 3 ft) near the free 

face and decreased with distance upslope. Sand 

boils were found near the toe of the landslide and 

on the fill in the southern part of the zone. A 

layer of saturated fine sand was discovered in 

postearthquake borings at depths of 2-3 m (6-9 

ft) below the original ground surface (Proctor 

and others, 1972). The failure presumably was 

caused by liquefaction of this layer. 

Reports from recent Alaskan earthquakes include numerous descriptions of lateral-spreadingtype ground movements down mild slopes toward 

free faces; these ground movements were accompanied by fissuring and ejection of subsurface 

water. For example, McCulloch and Bonilla 

(1970, p. D-1) report that during the March 27, 

1964 earthquake 

a general loss of strength [was] experienced by wet, 

water-laid unconsolidated granular sediments (silt to 

coarse gravel that allowed embankments to settle and 

enabled sediments to undergo fiowlike displacement 

toward topographic depressions, even in flat-lying areas 

* * * Stream widths decreased, often about 20 inches 

[0.5 m] but at some places by as much as 6.5 ft [2.1 m], 

and sediments moved upward beneath stream channels 

* * * Ground cracks * * * commonly extended 500 ft 

[160 m], and occasionally 1,000 ft [320 m], back from 

streams * * * Sediment-laden ground water was discharged from the cracks. 

During the 1906 San Francisco earthquake, 

9 

several lateral-spreading ground failures occurred 

in the city of San Francisco. One, in an area of 

filled marshland, was approximately 2 blocks 

wide and extended over a 1.5-km (5,000-ft) distance from Eighth and Mission Streets to the 

vicinity of Fourth and Brannan Streets. Wood 

( 1908) described this area as follows 

The fissuring and slumping and the buckling of block 

and asphalt pavements into little anticlines and synclines (arches and hollows) , accompanied by small open 

cracks in the earth, characterize the land surface. This 

slumping movement or flow took place in the direction 

of the length of the area [down about a 0.8 percent 

(0.5°) slope], and its amount was greatest near the 

center or channel, where the street lines were shifted 

eastward out of their former straight courses by amounts 

varying from 3 ft to 6 ft [1 m to 2m]. 

These examples, which are only a few of many 

that could be cited, show that lateral-spreading 

failures on mild slopes are very common during 

moderate and strong earthquakes. 

QUICK-CONDITION FAILURES 

Seepage forces, caused by upward-percolating 

pore water, often reduce the strength of granular 

soils to a point of instability. This state is termed 

a "quick condition." Artesian pressures are one 

cause of upward seepage; seismic compaction of 

saturated granular materials at depth is another. 

This condition is generally restricted to sand 

layers of significant thickness that extend from 

below the water table to the surface. 

Loss of bearing capacity is the most common 

type of failure produced by a quick condition. 

Buoyant rise of buried tanks and other vessels is 

another common effect. Shear deformations during such failures could be of either the unlimitedor limited-flow types. Bearing-capacity failures 

illustrative of this type of failure were reported 

during the 1964 Niigata, Japan, earthquake. In 

the city of Niigata, several high-rise apartment 

buildings rotated and subsided into the liquefied 

soil (Seed and Idriss, 1967). Railroad embankments, especially the Echigo line, subsided into 

the "liquefied ground" (Kobayashi, 1969). 

METHODS FOR EVALUATING LIQUEFACTION AND GROUND-FAILURE POTENTIAL 

Existing methods for evaluating liquefaction 

and associated ground-failure potential are generally based on one of two criteria: ( 1) the number of cyclical loadings required to produce a 

liquefied condition or a certain amount of flow 
deformation considered to constitute failure; or 

(2) the smallest void ratio (critical void ratio) at 

which unrestrained liquefied flow (unlimited 

flow) can develop. Both of these criteria are 

controlled largely by the same general factors; 

however, the influence of these factors may be 

considerably different for each situation. For example, relative density is a primary factor controlling both liquefaction of sands under cyclical 

loading conditions and unlimited flow failure. 

Liquefaction can be induced by cyclical loading, 

however, at considerably greater relative densities than those at which unlimited flow can occur. 

Thus different criteria have been developed for 

each approach. 

Seed and his coworkers (Lee and Seed, 1967a, 

b; Peacock and Seed, 1968; Seed andidriss, 1967, 

1971; Seed and Lee, 1966, 1969; Seed and Peacock, 1971) have developed the criteria for repeated loading. One application of this procedure 

has been the analysis of stability and deformation 

of earth dams during earthquakes (Seed, 1966; 

Seed and others, 1969, 1973). Similar procedures 

should be generally applicable for analyzing liquefaction potential and ground-failure movements 

beneath natural slopes. 

In addition, Seed and Idriss ( 1971) formulated 

a "simplified procedure" for evaluating liquefaction potential. This method incorporates several 

simplifying approximations and is valid only for 

sediments with relative densities less than 80 

percent that lie beneath level surfaces. Although 

the definition of liquefaction given by Seed and 

Idriss (1971, p. 1249, cited earlier) seems to indicate that the simplified procedure is generally 

valid for predicting ground-failure potential as 

well as liquefaction potential, examination of criteria used in formulating the procedure shows 

that this is not true. "Initial liquefaction" (Lee 

and Seed, 1967a, p. 49) was the criterion used by 

Seed and Idriss to determine the point at which 

liquefaction occurred in laboratory tests utilized 

in formulating the procedure. (Initial liquefaction 

is defined as the point at which a granular material exhibits any degree of liquefied flow during 

cyclic loading.) Although significant groundfailure movements may occur almost simultaneously with initial liquefaction in some loose 

granular soils (for example, relative densities less 

than 65 percent), this is not generally true for 

denser soils. For example, it may require many 

cycles of loading after liquefaction (or initial 

liquefaction) to produce significant ground-failure 

movements in denser sediments (for example, 

relative densities greater than 75 percent). In 

addition, ground slope, a factor of primary importance in evaluating ground-failure potential, 

is not incorporated in the simplified procedure. 

Thus, while the simplified procedure may be valid 

for evaluating liquefaction potential it is not 

necessarily valid for evaluating ground-failure 

potential. More rigorous methods, such as those 

developed for earth dams (Seed and others, 

1973), are necessary for evaluating the latter 

potential. 

The critical void ratio approach has been developed by Casagrande (1940) and Castro (1969) 

chiefly for monotonic loading conditions, the 

principal application being the differentiation of 

conditions under which unlimited and limited 

flow may occur. With additional research, particularly on the effect of repetitive loadings, this 

method could be made more general and could 

possibly be applied to evaluating type and 

amount of ground-failure movement after liquefaction. With such development the two methods 

would materially complement each other. 

SUMMARY 

Explicit definitions for liquefaction, solidification, limited flow, and unlimited flow have been 

given and the relation between these phenomena 

and ground failure examined in an attempt to 

present a clear and integral description of liquefaction and its consequences. Also, methods for 

evaluating liquefaction and ground-failure potential have been briefly examined. 

Liquefaction is defined as the transformation 

of a granular material from a solid state into a 

liquefied state as a consequence of increased porewater pressures. Solidification is defined as the 

opposite process, that is, the transformation of a 

granular material from a liquefied state into a 

solid state. Once liquefied, a granular material is 

free to flow, provided there is sufficient gradient, 

until solidification occurs. If solidification occurs 

as a result of dilatancy with very little change in 

total stress, the condition is termed limited flow. 

If solidification occurs only when driving forces 

are reduced to a level less than the viscous shearing resistance of the material, the condition is 

termed unlimited flow. 
Three types of ground failure conditions commonly follow liquefaction in the field. (1) Unlimited flow commonly leads to flow landslides. 

(2) Limited flow commonly leads to lateralspreading landslides. One episode of limited flow 

may be inconsequential; however, a series of episodes, such as might occur during an earthquake, 

often leads to significant permanent ground 

movements. Many phenomena variously described in the earthquake literature as lurching 

or land spreading are, in fact, lateral-spreading 

landslides generated by liquefaction. (3) Quickcondition failures such as loss of bearing capacity 

form the third type of ground failures attributable to liquefaction. 

Two approaches for evaluating liquefaction 

and ground-failure potential have been proposed. 

The procedure developed by Seed and his colleagues provides a method for evaluating liquefaction and ground-failure potential under cyclical loading conditions. The procedures developed 

by Casagrande and Castro were basically designed to differentiate conditions under which 

unlimited and limited flow occur during monotonic loading conditions. With further development these two approaches should become complementary and lead to improved understanding 

of the phenomena involved. 

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11 

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12