京 都 大 学 防 災 研 究 所 年 報 第48号 B 平成17年4月
Annuals of Disas. Prev. Res. Inst., Kyoto Univ., No. 48 B, 2005

Excess Pore Water Pressure Change Necessary to Cause Flow Liquefaction Failure of
Sands Loaded in Undrained Condition

Ogbonnaya IGWE*, Kyoji SASSA, and Hiroshi FUKUOKA*Graduate School of Science, Kyoto University

Synopsis
A theoretical model predicting the pore pressure change necessary for liquefaction
failure of saturated soil masses in undrained condition is assessed. It is shown that athreshold pore pressure, uc, derived from the Mohr Coulomb failure criterion when porepressure at failure is equal to the corresponding shear resistance is enough to initiateliquefaction type of failure in sandy masses. Loading tests to failure on source-area sandy
soils from a catastrophic landslide location, undertaken to verify the model, show that
under definite conditions of loading, a threshold state, characterized by the equality andsubsequent constancy of pore pressure and shear resistance from a few seconds after thecommencement of shearing until failure, develops in the sands at a given density. Samples
in which the threshold pore pressure was exceeded readily liquefied while those in which
the pore pressure built-up was below the limit gained strength by tendencies to dilate. This
paper demonstrates that while the stability of a slope founded on sandy soils may bebreached when the pore pressure exceeds a certain limit, it is possible to make estimates ofthe limit. It is shown that where such estimates are accompanied with adequate fieldmeasurements of pore pressure, the efficiency of landslide prevention projects may beenhanced because only slopes whose stability is proven to constitute a real public threat are
reinforced and reinforced adequately.
Keywords: landslides, sliding-surface liquefaction, peak strength, phase transformation,pore pressure

1. Introduction
Landslides are vicious slope movements
accounting for inestimable amount of loss, waste anddamage in virtually every part of the world. Triggeredby earthquake, volcanic eruption, intense rainfall,
rapid snowmelt, changes in water level, and even by
the activities of man himself, it is very difficult, if not
impossible, to overestimate their threat to public
safety. They are known to have frequently breached
the peace of cities and towns, sacked communities
and villages, buried the wealth of rural and urban
dwellers, wrecked countless hopes and dreams,
harshly punished some sloppy structure designing,defied some inadequate preventive measures, andproduced endless catalogs of carnage. Landslides do
not only destroy homes and hopes, they also deface
and devalue historical, cultural, and entertainment
facilities so dear to man. Taming their aggression andruinous impacts, thereby rescuing the environment
from a potential crisis, should, in point of fact,
become a priority. Liquefaction of saturated soils,
often regarded as the fundamental cause of flow
slides, has been responsible for many of the tragedies
resulting from slope failures. The intense mobility ofliquefied soils, which permits movements that range
from several tens of meters to several thousands ofmeters, almost always ensures that huge amount ofresources is lost in the wake of a landslide disaster.
Sound knowledge of the mechanism of liquefaction,
the factors that influence the liquefaction potential ofa mass of soil, and the characteristics of liquefiablesoils, is a potent tool not only in landslide
investigation and mitigation but also in the civilengineering industry. For indeed careful and rigorous
assessment of the liquefaction potential of sands
when selecting them for embankments, dams,
foundations, and roads is a tradition of immense
importance in the construction industry. And because
a great deal of failures of earth structures,
foundations, and slopes founded on sands have been
attributed to the liquefaction of the sands,
stakeholders in environmental protection and urban
development seem to have elevated the importance ofliquefaction-evaluation by placing it at the heart oftheir management policies. This elevation ofimportance has, in part, inspired intense researchleading to, for instance, better knowledge of thefactors and dynamics behind the failure of Fort Peck
Dam in Montana in 1938, Calaveras Dam in
California in 1920, the Lower Lan Norman Dam, thefoundation failures induced by the 1964 earthquake in
Alaska, USA, and Niigata, Japan, and the flow slides
in the province of Zealand in Holland and Mississippi
River. Ever since the widespread destruction arisingfrom the 1964 earthquakes in Alaska, U.S.A. andNiigata, Japan dramatically brought the subject of soilliquefaction to public awareness, considerable
amount of research has been undertaken by several
researchers, including Sassa, and colleagues at theDisaster Prevention Research Institute, Kyoto
University, Japan, who have used one of the mostrefined ring shear apparatuses to simulate, as closely
as possible, the stress-strain conditions that develop
on a mass of soil when it is subject to conditions
capable of triggering liquefaction.
2. The problem
Although liquefaction phenomenon has been thesubject of a barrage of investigations and publications
for decades now, its mechanism leading to large
lateral displacements has yet to be fully understood.
Questions such as – why do some soils collapse andliquefy whereas others, under identical stressconditions, dilate and gain some measure of strength;
and what are the primary factors triggeringliquefaction and flow failures, especially, in loose
cohesionless soils, only serve to underscore theincompleteness of what researchers as yet know
about soils liquefaction. Finding perfect answers has

led to the emergence of a good number of beneficial
concepts including Casagrande’s critical voids ratio
concept. In spite of the emergence of these concepts,
questions still remain, especially as to effective waysof relating the critical state of soils with essential soil
parameters, such as pore pressure and shear
resistance; there does not seem to have been any
previous attempt to relate collapse and liquefaction toan experimentally-verifiable limit or critical value ofpore pressure, above which collapse occurs andbelow which it does not. In this paper, two newconcepts – the concepts of least dilation, and criticalpore pressure – are introduced to interpret theundrained shear behavior of granular soils at athreshold density. It is shown that the characteristicsof the soils so interpreted tend to define the boundary
between contraction in loose, and dilation in dense
soils held under same effective normal stress. This
paper also demonstrates that while the stability ofslopes founded on sandy soils may be breached when
the pore pressure exceeds a certain limit it is possible
to estimate the limit. Where such estimates areaccompanied with adequate field measurements theefficiency of landslide prevention projects may beenhanced because only slopes whose stability isproven to constitute a real public threat are reinforced
and reinforced adequately.
2.1 Liquefaction and limited liquefaction
Since not all slope failures are due toliquefaction, establishing a standard that enhances theprediction and identification of flow-type failures in
the field will not only shed more light on themechanism of soil liquefaction but will also improvethe efficiency of slope stability analysis. Literature is
replete with studies, including that by Ishihara(1993), attempting to establish such a standard.
Following a summary of a good number of field andlaboratory data, Ishihara (1993) proposed a threshold
SPT N-value to distinguish between flow-type andnon-flow type failure. Although researchers havemade quality efforts at drawing a boundary between
liquefaction and non-liquefaction, they have yet tofind a common ground over what behaviors of sand,as observed in the laboratory, should be recognized as
an important mechanism determining the occurrenceor otherwise of flow-type failures in the field. It may
be important to note that even though beneficial
concepts, hypotheses, and postulates explaining theundrained behaviors of sands whose voids ratio
exceed or fall below the critical density exist in theliterature, there has yet to be a distinctive behavior

associated with sand at critical density. Determining
how sand at a critical density behaves during
undrained loading may be important in understanding
more about soil liquefaction.
Three basic undrained behaviors of granular materials
are very commonly referred to in geotechnicaldiscourse: dilation, limited or partial liquefaction, andliquefaction, Fig. 1. The phase transformation line
(PT line) as recognized by Ishihara 1993, is a line
passing through points where contractive behaviors
terminate and dilative behaviors begin, in specimensthat first contract, and then dilate. Although thevalidity of limited liquefaction as a true soil behavior
has been subjected to a considerable amount of doubt,
debate and controversy, the three basic behaviors
sketched above are not only a very useful means ofcharacterizing granular soils but also an effectivemeans of understanding the mechanism of slope
failures. The occurrence of, and practical implicationof the so-called limited liquefaction have been acontentious issue with two opposite views
increasingly gaining currency. Sutter and Smith(1980) have reported that the occurrence of limitedliquefaction is a function of how close the void ratioof a given material is to a critical void ratio. They
have noted that whereas specimens with voids ratio
considerably higher than the critical would almost
certainly suffer complete liquefaction, those whosevoids ratio are marginally higher or nearly equal tothe critical would experience limited liquefaction.
Sutter and Smith’s results are supported by those ofCastro and Poulos (1977), and Poulos et al. (1985)
who, while assessing procedures for evaluating theundrained steady-state strength of sands with resultsof undrained triaxial tests, have reported that theundrained strength of sands was dependent on only
insitu void ratio; and independent of either soil
fabrics or loading methods. They conclusively
showed that sands whose voids ratio exceeded acertain threshold value suffered liquefaction insteadof the so-called partial liquefaction.
Evidences from other works, however, seem toindicate that the occurrence of limited liquefaction
does not depend wholly on the proximity of material
density to the critical, but in part on the constraints
offered by the testing apparatus, and test conditions(Mathew and John, 1991; Jude, 1998). It is this partial
dependence on apparatus constraints that hascompelled some (like Jude 1997, Love 2000) toquestion the validity of limited liquefaction as a true
soil behavior. In their elaborate argument, doubts
have been raised over the possibility of observing, inthe field, a material flowing and at the same time

undergoing hardening. Those who support limitedliquefaction as a true soil behavior have, however,tried to make sense out of the frequency at which the
behavior is observed. Relying heavily on the rate at
which the behavior is observed on loose specimens
during testing, they have vigorously demonstrated
that the behavior is indeed a true characteristic
associated with the deformation of granular materials
in undrained shear. In spite of these divergent views,
however, there seems to be a consensus that there
exists a boundary between a purely dilative behavior
and liquefaction, whether complete or limitedliquefaction. But, that boundary has yet to be clearly
assessed.

τ

Failure line
Limitedliquefaction
PT lineDilation
Liquefactionσ′Fig.1 A sketch of the three basic behaviors of
granular materials (after Castro 1969)

2.2 Objective and methodology
In the light of the above, it is possible then to ask
whether or not there should be a boundary between
dilation and liquefaction for material under same
confining stress – limited liquefaction or completeliquefaction; and what the defining parameters ofsuch a boundary should be. The approach employedin this paper was to carefully alter the void ratio of
specimens held under same confining stress inattempts to identify stress paths whose peak strengths
would nearly coincide with their strength values at
the PT line. Any specimen whose peak strength
equals its strength values at the PT line will beidentified as the least dilating at a given normal stressbecause its phase transformation line will be the sameas its failure line. The characters of such a specimen
will then be used to define the boundary between
dilation and liquefaction. Such a definition will
permit adequate and logical interpretation of the
behavior of soils as density is varied from dense toloose. It may instantly become obvious that if thedensity of sand is gradually decreased, a density
reaches where failure line and phase transformation

line will coincide. Further decrease in density may
lead to flow liquefaction behavior; with the ultimate
consequence of having only a failure line as its
prominent feature.

(a)τ
PT lineNormally consolidated soils (Figure 2a, b) atsame confining stresses will follow stress paths SXand SY respectively depending on the material stateof the samples. For these samples, the conditions at
PT line are such that a dilation potential index, rf, (rf =∆up/∆τp) are < and = 1 respectively. The conditions
prevailing at 2b are recognized in this paper ascritical. If however, the soil is made in such a waythat ensures the stress path follows SZ as in Fig. 2c,the specimen will not go through the phasetransformation stage because its rf would clearly begreater than one. The specimen will, instead, collapse
and liquefy. It may be beneficial to note that a dilative
specimen (Fig.2a) should have distinct phasetransformation and peak stress states while
contractive specimens (Fig. 2c) may be easily
identified by just a distinct failure state. In between
these two fundamental behaviors is a relative density
at which the phase transformation and peak stressstates should coincide to form a threshold state(Fig.2b). The present theory underlines the fact thatthe magnitude of excess pore pressure from the outsetof any undrained test determines whether or not agiven specimen will pass through the phasetransformation stage. The fate of specimens whose
excess pore pressures are not big enough to induce
outright liquefaction and avoid reaching the PT line,
depends on the ratio ∆up/∆τp at the phasetransformation point. If this ratio is unity, porepressure and shear resistance should remain the same
until failure occurs, meaning that the sample will
experience the least dilation possible at a given
effective stress. The PT line of such a specimen will
be approximately equal to its failure line because thestate of stresses at the PT point approximately
coincides with those at failure. This condition willdefine a critical situation. All other stress paths above
this critical should dilate, while other stress paths
below it should show contractive behavior. To
enhance comprehension, the pore pressure at which
this critical is observed will be called a critical porepressure. If the ratio as seen above is less than one at
the phase transformation line, the material will dilate
significantly and its PT line will be different from its
failure line.

upX

3. The solution: hypothesis

Total stresspath

Effective
stress path
σ′

S

(b)
τ

Failure andcoincide

Y

PT

line

up

Total stresspath

Effective
stress path
σ′

(c)

τ

Only failure, no PT line

Total stress
path

up
Z

ESP.
S

σ′

Fig 2 Schematic diagrams illustrating the concepts ofleast dilation and critical pore pressure (a) dilation (b)critical (c) liquefaction
3.1 Experimental verification
3.1.1 When failure and PT lines coincide
Artificially constituted silica sands and natural
samples taken from the 1995 Takarazuka landslide(Fig. 3) that killed 34 people in Kobe, Japan wereused to verify the concepts. Both the artificial andnatural samples have similar physical properties, andindeed have almost the same friction angle.

remained same value until failure because while theformer would have decreased, the latter would haveincreased. The same behavior was found to be true in
the Takarazuka specimens confined at 372 kPa andconsolidated to a void ratio of 0.77, Fig. 5a and b.

100
Failure line
same with PT
line

50

0

81 kPa81 kPaShear resistance (kPa)

(a)

45

0

50

100
150
Effective stress200

Normal stress, pore pressure,and shear resistance (kPa)(b)

Normal stress

150
100

Pore pressure

50

Shear resistanceThreshold state

0
-50
0.1

1

10
100
1000
Shear displacement (mm)

10000

(a)
200

166 kPa

Failure line
same withPT line150

166 kPa

Shear resistance (kPa)

Figures 4a and b are stress path and stress versus
shear displacement respectively of a normallyconsolidated gap graded silica sand material confinedat 196 kPa with a void ratio of 0.89. The figures
illustrate what happens whenever pore pressure at
failure is equal to the corresponding shear resistancesuch that there is no distinction between the phasetransformation stage and failure state because thespecimen appeared to have experienced the leastdilation possible at the given confining stress. Excess
pore pressure and shear resistance became equal at
the phase transformation point and not only remained
equal but essentially constant until failure, thus
establishing a threshold state at a small shear
displacement (Fig. 4b). The equality and subsequent
constancy of excess pore pressure and shear
resistance, which started at about 2 mm andcontinued until the sample failed at 10 mm shear
displacement, are typical characteristics of specimensthat tend to form a transition region by demarcating
the contractive from the dilative behavior.
Theoretically, it may be easy to see that any stress
path below this critical will liquefy while any above
will dilate. If the specimen had dilated significantly,
pore pressure and shear resistance would not have200

Fig. 4 (a) Stress path showing a threshold condition(b) Pore pressure and shear resistance behavior in asilica sand specimen

100
50

45

0
0

50

100 150 200 250 300 350 400
Effective stress (kPa)(b)
Normal stress, pore pressure
and shear resistance (kPa)

Fig. 3 Picture of the Takarazuka landslide that
followed the Great Hanshin earthquake of 1995.

250

400
300

Normal stress
Pore pressure

200
Shear resistance

100
0
-100
0.1

Threshold state
1

10
100
1000
Shear displacement (mm)410Fig. 5 (a) Stress path (b) Pore pressure and shear
resistance behavior in a Takarazuka specimen

u c = σ tan φ /(1 + tan φ )150
Failure line
Shear resistance (kPa)Two cases, among many others, that typify
situations where pore pressure at the phasetransformation stage is not equal to the corresponding
shear resistance are illustrated in Figs. 6 and 7. The
consequence of this situation is that the specimens
dilated and ensured that the phase transformation line
remained different from the failure line. Fig. 6billustrates the mechanism of dilation in a silica sandspecimen consolidated to a void ratio of 0.82. The
figure shows that because pore pressure at PT line isdifferent from the corresponding shear resistance, thespecimen dilated, expressed as a decrease in porepressure and a corresponding increasing in shear
resistance (highlighted in the circle). These changes
continued until failure occurred. For a denser
specimen, the changes would even be more
remarkable although they follow the same pattern (asin Fig. 7). Because the difference between porepressure and shear resistance at the PT line would begreater in a denser specimen, the dilation would also
be higher than in Fig. 6a and b. As density increases,
the difference increases too. A decrease in the densityof a material will decrease the difference between
pore pressure and shear resistance at the PT point. As
density is decreased further, a time reaches when thepore pressure and shear resistance at the PT point will
have the same value; and will remain the same until
failure takes place (Fig.8) This situation establishes athreshold state and unambiguously defines atransition condition for all specimens under the same
confining stress. Specimens denser than that for
which a critical condition was defined would dilate,while those looser than the critical would collapse. At
any confining stress, there is only one stress path thatwill define this critical condition; meaning that only
one specimen will dilate the least and as a result haveits pore pressure and shear resistance equal from the
PT point until failure. It is the opinion of this paperthat this is one of the conditions that can lead to a
failure line coinciding with the PT line. The
coincidence of the PT line with the failure line is a
new phenomenon that might become very useful in
predicting and characterizing the behavior of granularmaterials held under same confining stress.
Liquefaction occurs in loose soils because porepressures generated in them during static loading tend
to exceed the threshold. The critical pore pressure, uc,
as used in this paper may be calculated from thefollowing equation:

Where u c is the critical pore pressure, σ is thenormal stress used in the undrained test, and φ is thefriction angle of the material at a given normal stressσ. This is the amount of excess pore pressure thatmust be generated before liquefaction failure of thesands can be expected. The beauty of the newconcepts lies in the fact that the defining parametersconsidered critical may be adequately represented in a
stress-strain-void ratio space, and interpreted withreferences to some experimentally measurable
quantities. Unlike abstract analogies, the reference
parameters in the concepts under consideration may
be directly observed and measured. Such aquantitative analytical procedure may be easily
verified by colleagues elsewhere.
(a)Peak (107 kPa)9.3 mm

100

PT
line
3 mm

50

0

0

50

100
150
Effective stress (kPa)

200

(b)

Normal stress, pore pressure,and shear resistance (kPa)

3.1.2 When failure and phase transformation lines
do not coincide

200
150

Normal stress100

Pore pressure

50
0
-50
0.1

PT
1

FailureShear resistance

10
100
1000
Shear displacement (mm)10000

Fig. 6: behavior of a silica sand specimen with Dr =44 % (a) stress path (b) stress versus displacement
Equally important is the fact that the critical porepressure (the pore pressure required at the phasetransformation line to ensure that a material dilates
the least) can be quickly but conditionally calculatedwith as small as one good laboratory test at a known
confining stress. The conditions for a reliablecalculation include: 1) the specimen must be fully
saturated; 2) the test must be monotonically loadedundrained; 3) Mohr Coulomb failure criterion must
be applied.
250
Failure line

Shear resistance (kPa)

200

Peak (198 kPa)
at 8 mm

150
PT
line

100

50
40 kPaat 10 mNormal stress, pore pressure,and shear resistance (kPa)

0

0

50

100
150
200
Effective stress (kPa)

250

250
200
150

Normal stress

100

Shear resistance50

Pore pressure

0
-50
0.1

Failure

PT
1

4

10
100
1000
Shear displacement (mm)10Fig. 7: (a) stress path (b) stress versus displacementof silica sand specimen with Dr = 50 % illustrating
inequality between pore pressure and shear resistanceat failure

4. Conclusions
1. Test results have shown that there is a critical or
limit value of pore pressure, above which the sandy
samples suffered sudden collapse and liquefaction,and below which they dilated and gained somemeasure of stability.
2 Any specimen whose pore pressure at the phasetransformation point equals the corresponding shear
resistance will dilate the least among other specimens
held under the same confining stress.
3. Once pore pressure becomes equal with thecorresponding shear resistance at the PT point, theyremain the same until failure and ensure that the
specimen dilates the least at a given confining stress.
4. The new concepts highlight the fact that there exist
a fundamental relationship between changes in
effective stress at failure and the shear displacementof the materials. The displacement of a material willremain at a ‘safe’, small value until the critical isexceeded. Once exceeded, the material suffers verylarge displacement, very rapidly.
5. Considering that all changes in shear resistance areentirely due to changes in effective stress, thechanges in stress (shear resistance and effective
stress) that give rise to the ratio of pore pressure atand shear resistance at failure being unity should beconsidered a crucial boundary distinguishing twovery important soil behaviors – liquefaction anddilation.
6. One of the implications of these results is thatsome slopes are still sitting safe probably because acertain threshold pore pressure has yet to be
exceeded. If such slopes must keep sitting safe, in situ
pore pressure measurements followed with adequate
drainage regime should be a grave necessity.

References

Shear resistanceand pore pressure (kPa)

250

Shear resistance
1a

200
150
100

2a
3a

4a and 4b

50
0Corresponding pore pressure

Critical

-50

1b
2b

-100
0.1

1

3b
10
100
1000
Shear displacement (mm)10000

Fig. 8 Shear resistances and the corresponding porepressures of some sands as density changes(specimens at 196kPa confining stress)

Ambrasseys, N. N. (1973): Dynamic and response offoundation materials in the epicentral regions ofstrong earthquakes. 5th World Conf. Earthquake
Engineering Rome.
Casagrande,A.
(1936):
Characteristics
ofcohesionless soils affecting the stability of slopesand earth fills. Journal of the Boston Society of
Civil Engineers, Vol. 23, No. 1, pp. 13-32.
Casagrande, A. (1976). Liquefaction and cyclic
mobility of sands: a critical review. Harvard Soil
Mechanics Series No.88, Harvard University,
Cambridge, Mass.
Castro, G. (1969): Liquefaction of sands. PhD. Thesis,
Harvard University, Cambridge, Mass.
Castro, G. (1975): Liquefaction and cyclic mobility ofsaturated sands. Journal of the GeotechnicalEngineering Division, ASCE, Vol. 101, No. GT6,
pp. 551-569.
Eckersley, J. D. (1985): Flowslides in stockpiled coal.
Engineering Geology, Vol. 22, pp. 13-22.
Gilbert, P. A. (1976): Case histories of liquefaction
failures. Misc. Paper S-76-4, U.S. Army Engineer
Waterway Experiment Station, Vicksburg, Miss.
Gilbert, P. A., Marcuson, W. F. (1988): Density
variation in specimens subjected to cyclic andmonotonic loads. Journal of GeotechnicalEngineering, Vol. 114, No. 1, pp. 1-20.
Ishihara, K. (1993): Liquefaction and flow failure
during earthquakes. Geotechnique, Vol. 47, No. 3,
pp. 349-451.
Ishihara, K., Okusa, S., Oyagi, N., and Ischuk, A.
(1990): Liquefaction-induced flowslide in thecollapsible loess deposit in Soviet Tajik. Soils andFoundations, Vol. 30, No. 4, pp. 73-89.
Kramer, S. L., and Seed, H. B. (1988): Initiation ofsoil liquefaction under static loading conditions.
Journal of Geotechnical Engineering, Vol. 114,
No. 4, pp. 412-430.
Kutter, B. L. (1982): Behavior of embankments underdynamic loading. Part of PhD thesis, Universityof Cambridge.
Marui, H. (1996): Preliminary report on the
Gamahara torrent debris flow of 6 December
1996, Japan. Journal of Natural Disaster Science,Vol. 18, pp. 89-97.
McRoberts, E. C., and Sladen, J. A. (1992):
Observations on static and cyclic sandliquefaction
methodologies.
Canadian
Geotechnical Journal, Vol. 29, pp. 650-665.
Poulos, S. J. (1981): The steady state of deformation.
Journal Geotech. Eng. Division, ASCE 107, No.
GT5, pp. 553-562.
Poulos, S. J., Castro, G., and France, J. W. (1985):
Liquefaction evaluation procedure. Journal of
Geotech. Eng. Division, ASCE 111, No. 6, pp.
772-792.
Okada, Y., Sassa K, and Fukuoka H. (2004):
Excess pore pressure and grain crushing of sands
by
means
of
undrained
andnaturally drained ring-shear tests. Eng. Geo.

Journal, 75(3), pp.325-343.
Sassa, K. (1996): Prediction of earthquake induced
landslides. Special Lecture of the 7th International
Symposium on ‘’Landslides’’, Rotterdam:
Balkema, Vol. 1, pp. 115-132.
Sassa, K., Fukuoka, H., Scarascia-Mugnozza, G., andEvans, S. (1996): Earthquake-induced landslides:
Distribution, motion, and mechanisms. Special
Issue for the Great Hanshin Earthquake Disaster,Soils and Foundations, pp. 53-64.
Sassa, K. (1997): A new intelligent type of dynamic
loading ring shear apparatus. Landslide News. No.
10, pp. 33.
Sassa, K., Fukuoka, H., and Wang, F. W. (1997b):
Mechanism and risk assessment of landslidetriggered debris flows: Lessons from the 1996
Otari debris flow disaster, Nagano, Japan.
Landslide Risk Assessment (ed. Cruden and Fell),
Proceedings of the International Workshop on
Landslide Risk Assessment, pp. 347-356.
Sassa, K. (1998a): Recent urban landslide disasters inJapan and their mechanisms. Proceedings of the
2nd International Conference on Environmental
Management, ‘’Environmental Management’’,Rotterdam: Balkema, Vol. 1, 47-58.
Seed, H. B. (1966): Landslides during earthquakes
due to soil liquefaction. Journal of Soil Mechanics
Foundations Division, ASCE, Vol. 94, No. 5, pp.
1055-1122.
Seed, H. B. (1979): Soil Liquefaction and cyclic
mobility evaluation for level ground during
earthquakes. Journal of Geotech. Engineering
Division, ASCE, Vol. 105, pp. 201-255.
Seed, H. B., and Idriss, I. M. (1971): Simplified
procedures for evaluating soil liquefaction
potential. Journal of Soil Mechanics Foundation
Engineering Am. Soc. Civ. Engrs. Vol. 109, GT3,
pp. 458-482.
Seed, H. B., and Lee, K. L. (1966): Liquefaction ofsaturated sands during cyclic loading. Journal of
Soil Mech. Fdn. Engng. Am. Soc. Civ. Engrs. Vol.
92, SM6, 105-134.

非排水せん断による液状化の原因となる過剰間隙水の変化
Ogbonnaya IGWE*・佐々恭二・福岡浩
京都大学大学院*
要旨
本研究では、非排水条件における飽和土塊の液状化崩壊に必要な間隙水圧の変化予測モデルを評価した。モール・クーロン破壊基準おける土のせん断抵抗が、これと等しくなる時に発揮されるしきい間隙水圧が液状化崩壊した試料で示された。崩壊性地すべり地の滑落崖の試料を用いて様々な条件下で試験し、崩壊後に示される数秒間のせん断抵抗が土の密度を発達させた。しきい間隙水圧によりせん断開始直後に液状化した試料は、崩壊以前にもダイレタンシー傾向の強度が得られた。
キーワード:地すべり, すべり面液状化, ピーク強度, 変相点, 間隙水圧