SIMULATION ON LATERAL FLOW MECHANISM IN LAYERED SAND
BY 1G SHAKE TABLE AND OTHER TESTS
Takeji Kokusho
Civil Engineering Department, Chuo University
Kasuga 1-13-27, Bunkyo-ku, Tokyo 112-8551 JAPAN
Abstract: Lateral flow or lateral spread movement is sometimes immensely larger than free
surface settlement and exceeds several meters even in gentle slopes of less than a few percent.
It occurs not only during but also after earthquake shaking. Mechanism of the lateral flow,
which is still poorly understood, has been examined from different viewpoints in previous
investigations. In this research, water film or void redistribution effect in layered sand deposit
is focused among other mechanisms by addressing recent research findings obtained by
simulation techniques for water film generation and associated lateral flow movement. 1G
shake table tests, 1-D tube tests, torsional simple shear tests, insitu soil investigations,
numerical analyses, etc. are used to simulate lateral flow in layered sand deposits during
liquefaction. The experimental simulations have qualitatively disclosed the mechanism for
water film generation and lateral flow passing through water films.
Introduction
Lateral flow or lateral spread movement in liquefied ground is sometimes immensely
larger than the free surface settlement and exceeds several meters even in gentle slopes of
less than a few percent. More interestingly, lateral spread or flow occurs not only during but
also after earthquake shaking. Hence, a part of lateral flow is essentially driven by the gravity
force although it is initially triggered by the seismic inertia force. The flow failure in
liquefied ground seems to have been involved also in submarine slides triggered seismically.
Slopes of sliding surfaces in those slides were not so steep but much gentler, normally less than
5o and sometimes even less than 1o (Ikehara, 2000), considerably less than the internal friction
angle of the soil.
As significant effects of large displacement of lateral flow on civil structures are
recognized more and more in recent earthquakes, greater efforts have been made how to deal
with this type of failure in structural design. Foundation design must take into consideration
the flow slide, so that a structural integrity can be sustained within a certain required limit
under the effect of lateral displacement or pressure. Pile foundations in liquefiable soils are
normally designed by considering dynamic effects of surrounding liquefied soils and
superstructures. In addition, how to design pile foundations considering the effect of the
lateral displacement has posed a new challenge to geotechnical engineers. Two different
evaluations are considered now in Japan; by applying lateral pressure directly on piles or by
applying lateral displacement via soil springs. It is by no means an easy task to estimate the
flow-induced lateral pressure or lateral displacement of ground, which may be highly
dependent on site, soil and seismic conditions. The experience in Kobe may be one of the
references to rely on because there are few other case histories. It should be noted in these
evaluations that a non-liquefiable surface layer which normally caps liquefied soils has
enormous effects on the structural behavior of piles in terms of lateral displacements, bending
moments, etc. If the flexural rigidity of piles is not so large, the displacement of the pile
head is almost equal to that of the surface layer. Consequently, the estimation of lateral
displacement in surface non-liquefiable soil by understanding the lateral flow mechanism is
1
essential in designing pile foundations against flow failure and secure the ultimate safety of
superstructures.
Lateral Flow Mechanism
Although lateral flow failures occurred in past earthquakes frequently and caused
significant damages on civil structures, its mechanism leading to large lateral displacements
is known very little, yet, and is still very much controversial. While the ground
displacement in current design methods may be assigned based on experiences in previous
earthquakes based on the thickness of liquefied layer, ground slope, etc., it lacks a theoretical
basis from the soil mechanical point of view. There have been proposed several soil
mechanical views so far on the lateral flow mechanism in liquefied ground as indicated
below;
(a) Cyclic loading by seismic inertia force,
(b) Residual undrained shear strength of liquefied soil,
(c) Effect of small tremors during aftershocks,
(d) Effect of water film or void redistribution in layered sand.
Dobry et al. (1995) made centrifuge tests for gently inclined submerged uniform sand
layers with the angle of i = 1.3o ~ 5o made from clean Nevada sand of Dr=45%. They found
that, for i = 1.6o , the flow displacement Df amounts about 0.3m in prototype under the
application of 20 cycles of sinusoidal motion of 0.2 to 0.5G in the sloping direction. For the
thickness of liquefied soil of 3.5m in prototype, the shear strain was about 9%. A system
identification analysis for the test results indicated a clear dilative stress-strain response
occurring only in the down-slope direction after some strain has accumulated. The dilative
response restored effective stress and shear resistance in sand, allowing stepwise increase in
lateral displacement. This model test clearly explains how stepwise downslope movement
takes place in liquefied soil by the cyclic inertia effect. However, this mechanism by the
undrained cyclic shearing of liquefied sand under the influence of initial shear stress may not
be able to explain large flow displacements of several meters unless the dilative response is
completely diminished. This dilatancy effect is actually associated with the undrained
stress-strain behavior measured in laboratory soil element tests. It firstly depends on the
relative density of sand. Laboratory undrained shear tests indicate that the dilative response
still appears even for loose sands with Dr as low as 30%.
Castro and Poulos (1977) and Poulos, Castro, et al. (1985) presented a procedure for
evaluating undrained steady-state strength for clean sands based on undrained triaxial tests.
The undrained strength was defined as a function only of insitu void ratio, independent of soil
fabrics or loading methods. They demonstrated that sand samples with a relative density
smaller than some threshold void ratio exhibits flow-type failure without showing dilative
response and that the steady-state strength can be determined by undrained triaxial tests on
sand samples with insitu void ratio. On the other hand, Seed (1987) summarized case
history data of lateral flow or lateral spread failures and established an empirical relationship
between the back-analyzed residual shear strength and the insitu penetration resistance.
Comparing with the relationship by Poulos et al (1985), the same author found that the
procedures using the steady-state strength leads significantly higher values of residual
strength than those estimated by the case studies.
Ishihara in his Rankine lecture (1993) made a comprehensive review on sand behavior
under undrained monotonic loading including a concept of quasi-steady state strength in
addition to steady-state strength. It was found that the effect of soil fabrics will actually
disappear under large deformation and the residual strength of fines containing sands
considerably differs from that of clean sands. He summarized a number of laboratory and
insitu data back-calculated from case studies for the steady state residual strength and
proposed threshold SPT N-values differentiating the occurrence and non-occurrence of
flow-type failure.
Meneses, Ishihara, et al. (1997) disclosed that the residual strength will considerably
decrease on account of a superimposed complementary cyclic shear stress representing small
aftershocks persisting after a strong main shock. In one of their test results using a hollow
cylindrical torsional/axial loading test apparatus for sand specimens made by wet tamping,
2
normalized residual strength for Toyoura sand with Dr=50% reduced to zero with the
application of 25 cycles of stress ratio t cy s v¢ =0.08 or 125 cycles of t cy s v¢ =0.06 for each
increment of 1% shear strain. It seems to indicate that several hundreds or thousands
equivalent cycles of aftershocks with the above stress ratios are needed during sustained full
liquefaction (probably within a few hours after the cease of a main shock) to realize typical
large flow of several meters. This test results clearly show that seismic shaking can greatly
contribute to lubricate soil contacts and impede the soil dilatancy. This impediment effect
seems to restrict the development of flow displacement only during shaking, which is actually
demonstrated in a model shake table test of a uniform sand layer as will be addressed later in
Fig.4(b). Although it may be highly probably that aftershock small tremors contribute to
decrease the residual strength to some extent, a more quantitative study still remains to be
done for identifying this mechanism as a major cause for large flow displacements.
Kokusho (1999) recently demonstrated, based on model tests, numerical analyses and site
investigations, that the water film or void-redistribution effect plays an important role in
post-earthquake large lateral flow in liquefied ground. This idea had been introduced in a
committee report in US (National Research Council, 1985) and also discussed by Seed (1987)
by using a special term “water interlayer”. In this view, fine soil layers sandwiched in sand
deposits are considered to play a key role in flow failure. If the layers are thick enough they are
identified as silty or clayey layers in a normal soil survey. However, a sand layer, which is
dealt with as a single uniform layer in normal engineering practice, may also be composed of
sublayers with different grain size and, hence, different permeability. This may be able to
explain, as Seed (1987) already pointed out, why the steady-state strength leads significantly
higher values of residual strength than those estimated by the case studies in the field.
Simulation for Lateral Flow Mechanism
Study on lateral flow mechanism focusing on the significant effect of water filming or void
redistribution has been undertaken by utilizing various simulation techniques such as soil
investigations of insitu deposits, 1-D liquefaction tests in a tube, 1G shake table tests, case
history studies in Niigata, laboratory soil tests, numerical analyses etc. as listed in Table 1.
They will be systematically combined to clarify a basic mechanism for large lateral flow
movement and to propose how to evaluate its displacement.
Table 1 Simulation techniques for lateral flow mechanism focusing on water film
generation or void redistribution
Investigator
Chuo University
Simulation technique
Goals
1G shake table test
Qualitative understanding on flow mechanism in layered sand
1-D model test
Basic understanding on water film generation mechanism
Insitu soil investigation
Down-hole video observation
Degree of layering in insitu sand deposits
Possibility of involvement of water film in flow displacements during
Niigata EQ
Mechanism of void redistribution in layered sand in element test
1-D numerical simulation of post-liquefaction water film generation in
layered sand
Visual demonstration of water film generation in insitu sand deposit
Centrifural shake table
Quantitative understanding on flow mechanism in layered sand
Case history study
Laboratory soil test
Numerical analysis
UC Davis
Soil inveastigations
3
Fig.1 illustrates sieve test results for sand deposits carried out at two sites. Fig.1(a) is for an
artificially reclaimed ground, formed by hydraulic filling, which shows that the soil is obviously
sand but very variable in terms of particle size along the depth. The fines content
corresponding to the mesh size of 0.075mm is fluctuating almost periodically by intervals
shorter than 2m. This periodical fluctuation of soil particle size can be seen continuously in the
horizontal direction. The second site investigated is a natural sand deposit at the mouth of the
Shinano River in Niigata city, where extensive liquefaction took place during the 1964 Niigata
earthquake. Fig. 1(b) indicates that down to EL.-5.6m the sand is rather uniform and consists
of clean sand. Below EL-5.6m, silty or clayey layers appear which are continuous in the
horizontal direction at least within 10m in both sides. These insitu soil investigation results
vividly demonstrate that even in apparently uniform sand, it is layered with its grain size
: 0.075mm
: 0.106mm
: 0.250mm
(a)
: 0.425mm
: 0.850mm
: 2.000mm
: 0.075mm
: 0.106mm
: 0.250mm
(b)
: 0.425mm
: 0.850mm
: 2.000mm
2
-2
1
-3
Elevation (m)
Elevation (m)
0
-1
-2
-3
-4
-5
-6
-4
-7
-5
0
20
40
60
80
0
20
40
60
80 100
Percentage finer by weight (%)
100
Percent finer by weight (%)
Fig.1
Humus
Vertical variations in sieve test result for sand deposits carried out at two sites:
Reclaimed ground by hydraulic filling (a) and Alluvial natural ground (b).
(b)
Soil settlements and
thickness of water film (cm)
(a)
(c)
z
4
3
Hu
●
■
2
△
: z = 96cm
: z =200cm
: Water film
Upper sand
layer
Hm
1
Middle layer
iu i m
Water film
0
0
p1 p2
Hl
p3
100
200
p4 p5
Elapsed time (s)
300
Lower sand
layer
Excess pore pressure
Fig.2 One-dimensional tube test demonstrating water film development beneath a
sandwiched fine soil sublayer (a), time-histories of soil settlement and water film
thickness (b) and mechanism for water film generation (c).
4
fluctuating in the vertical direction.
1-D tube test
Based on the observations that insitu sand deposits are often horizontally layered sandwiching
fine soil sublayers, 1-dimensional model tests were carried out in a Lucite tube as shown in Fig.
2(a). In this model, loose saturated sand sandwiching a thin silt seam in the middle was
liquefied by a hammer blow. The test result clearly indicated (Kokusho, 1999) that a water film
is readily formed just after the onset of liquefaction beneath a sandwiched sublayer with smaller
permeability and stays there much longer than re-sedimentation of liquefied sand particles as
depicted in Fig.2(b). This indicates that the liquefied sand is actually in the drained condition
locally, allowing the void redistribution to occur. Various soil layered models tested in the
same way disclosed (Kokusho, 2000b) that the mechanism generating a stable water film is
explained by an excessive hydraulic gradient, im, introduced in the sandwiched layer due to the
overlying layer solidified earlier from liquefaction as illustrated in Fig.2(c). Soil layering
structures providing this mechanism seems to be abundant in the field.
By a simple 1-D numerical analysis, it was shown that the water film generation in the model
tests is readily simulated considering the difference in settlement velocity of soil particles
between different sublayers. Also shown was
400
that skin friction between the tube and sand Rectangular lucite box (unit：mm）
800
makes the lifespan of water films longer than in
the field, although test results can apply to
insitu sand deposits qualitatively (Kokusho and
Kojima, 2001).
Accelerometer
500
2-D shake table test
In a 2-dimensional shaking table test shown
in Fig.3, it was demonstrated that a soil mass
flows along a continuously formed water film
beneath sandwiched fine soil sublayer still after
the end of shaking (Fig.4(a)), while in a
uniform sand a major lateral flow takes place
only during shaking (Fig.4(b)) (Kokusho, 1999).
This indicates that the sand can experience large
(a) case 1
(cm)
50
Pneumatic
actuator
Silt arc
Sloping sand layer
Fig.3 Two-dimensional shaking table
test of saturated slope with a silt arc.
30
20
20
10
10
10
20
30
40
50
60
70
(cm)
80
0
0
(cm)
50
20
30
40
50
60
70
(cm)
80
60
70
(cm)
80
: end of shaking
: end of all deformation
40
30
30
20
20
10
0
0
10
(cm)
50
: end of shaking
: end of all deformation
: arc of silt
40
: before shaking
: end of shaking
40
30
0
0
(b) case 2
(cm)
50
: before shaking
: end of shaking
: arc of silt
40
Shake table
10
10
20
30
40
50
60
70
(cm)
80
0
0
10
20
30
40
50
Fig.4 Soil displacements in a saturated slope with a silt arc (a) and without a silt arc (b).
5
shear strain without the recovery of shear strength due to the dilatancy effect during shaking
although it stops moving after the end of shaking if there exists no silt seam as indicated in
Fig.4(b). In another shaking table test, a soil mass flowed even on a very gently inclined water
film, which broke at weak points of the overlying sublayer, triggering the boiling failure in the
sand above and a mud avalanche of the upper layer (Kokusho, 2000a). A basic question may
arise that sand which can be so dilative if sheared under a low confining stress may absorb
ambient excess pore water and hence block the water film development. It was pointed out,
however, based on the comparative observation in Figs.4(a) and (b) that a water film formed
beneath a silt seam can serve as a shear stress isolator which shields the deeper soils from
development of shear strain and dilatancy (Kokusho, 2000a).
Shear stress (kPa)
Shear stress (kPa)
Torsional simple shear test
The void redistribution effect
( b)
Loadi ng
can be simulated not only in the ( a )
pl at e
model tests but also in a hollow
cylindrical torsional undrained
shear test as indicated in Fig.5(b). Gr ound sur f ace
Wat er t abl e
The test specimen is supposed to
Si l t seam
Set t l ement No
simulate a soil element extracted
ver t i cal
from a level or gently inclined
movement
sand layer beneath a low
permeable
sublayer
as
schematically
illustrated
in
Fig.5(a). In this test method,
vertical movement of a top
Fig.5 Hollow cylindrical torsional test apparatus (b)
loading plate above the specimen
simulating a soil element in a level or gently inclined
is restricted so that the plate can
sand layer beneath a sandwiched low permeable
represent a low-permeability
sublayer under cyclic loading (a).
non-liquefiable sublayer covering
a liquefied sand. A smooth face
of the plate, in which metal edges planted in
normal soil tests are removed, simulates a
flat interface between the fine soil sublayer
20
(a)
and the sand. It may be readily understood
that the height of the test specimen, much
smaller than the actual thickness of a
0
prototype sand layer, has nothing to do with
the speed of the sand settlement or the water
film development.
Figs.6(a) and (b)
-20
T=100S　Dr=53%
exemplify stress-strain relationships of the
Stress ratio=0.2
Toyoura clean sand with relative density of
-15
-10
-5
0
5
10
15
about 50%, in which the two cases without
Shear strain (%)
and with the vertical restraint are compared
30
to each other. Obviously, the dilative
response, which appears during all loading
20
(b)
cycles in the normal test without the vertical
restraint in Fig.6(a), diminishes in the case
10
of the vertical restraint after the moment of
0
water film generation as demonstrated in
Fig.6(b). Thus, the element test result can
-10
vividly tell a significant effect of the void
-20
redistribution or water film generation
T=100ｓ　Dr=53%
Stress Ratio=0.2
beneath an impermeable sublayer on
-30
-15
-10
-5
0
5
10
15
undrained stress-strain behavior and
Shear strain (%)
residual strength.
Fig.6 Stress-strain relationship of Toyoura
sand: without vertical restraint (a) and with
Case history study
vertical restraint (b).
6
Flow displacement normal to
elevation contour:Dfn(m)
5
4
3
2
1
0
-1
-2
0
0.5
1
1.5
2
Surface inclination:imax(%)
Fig.7 Lateral displacement vectors measured by Hamada(1992) compared with
elevation contours.
In order to demonstrate the significance of the water film effect in the field, large lateral
displacements, which occurred during the 1964 Niigata earthquake, were reviewed in an
investigated area in Niigata, which gave the following findings (Kokusho and Fujita, 2001).
a) By comparing elevation contours of 0.1m pitch and lateral displacement vectors measured
by Hamada (1992) by air-photographs as shown in Fig.7, it is found that the soil behaved
like liquid, flowing on a very gentle slope in the downward direction mostly normal to the
contours.
b) Continuous sublayers of fine soils near the surface as well as liquefiable loose sand
EAST
WEST
SHINANO
RIVER
WALL
HOTEL
NIIGATA
bore-hole
number
Less permeable
sublayer
Liquefiable sand
Sand
Water table
Clay
Silt
Fig.8 Two-dimensional soil profiles indicating liquefiable sands beneath silty sublayers.
7
beneath them are located in the investigated area, where large lateral flow occurred during
the Niigata earthquake, implying that water films may have developed during the
earthquake (Fig.8).
c) A clear correlation can be identified between the lateral displacement and the ground
surface inclination as illustrated in Fig.9, indicating that even an inclination smaller than
1% has a great influence on the post-liquefaction lateral displacement.
If water films are formed continuously in a liquefied sand deposit, they will tremendously
reduce its resistance against a sliding failure. Its reduction may, however, be greatly
influenced by how much the sliding surface can exploit the water films. If the sliding of an
infinitely long slope can occur all the way through a continuous water film, the shear
resistance becomes zero. This however seems unrealistic because a slope is actually of a
finite dimension and capped with a non-liquefiable layer through which the sliding surface
must pass. Furthermore, a water film may not be so straight but more or less winding. In
(a)
order to know the readiness of a sliding failure in such cases, some simple calculation
may be
possible for a typical soil profile in Fig.10(a) based on the data in Niigata city. The
thickness of non-liquefied cap layer is assumed as 2m as in Niigata city on average. The
front and back ends of this surface layer are postulated to fail in the passive and active modes,
respectively. If the internal friction angle and the cohesion of the cap layer is assumed as
f ¢= 30o and c¢= 0 , respectively, the
Fig.9
Correlation
between(b) lateral
minimum dimension for the movable
displacement and ground surface inclination.
block will be 300m for imax = 1.0% .
This may indicate that, if the
non-liquefiable cap layer remains intact,
it is not so easy for the sliding to occur
im a x
for a soil block with such large
Intact surface layer
dimensions because water films may not
be so continuous or straight but more or
Wat er f i l m
Hypothetical slip surface
less discontinuous or winding. This
deterrent effect of the non-liquefiable
cap layer should be kept in mind not
im a x
only in the water film mechanism but
also in all other mechanisms addressed
previously to explain large flow
Wat er f i l m
displacements by finite segments.
Local liquefaction
However, an additional mechanism
may possibly help lateral flow occur
Fig.10 Conceptual soil profile with water
even under this circumstance.
The
film (a) and local liquefaction in surface
surface soil resting on a continuous
soil (b).
water film may deform, even if it is
confined at the down-slope edge, due to
the lateral component of its own weight.
This may then introduce a local compression in sublayers. Furthermore, fine soil sublayers
are inherently non-uniform and variable in thickness from point to point. They may
disconnect at some places. In such cases, excess pore-water tends to rush to such weaker
points, triggering boiling or re-liquefaction of the overlying sandy layer as demonstrated in
the model test carried out by the present author (Kokusho, 2000a). Such local liquefactions
will considerably reduce the shearing strength in the cap layer as illustrated in Fig.10(b),
enabling a block by block lateral movement to occur.
Based on the above considerations, the formation of water films beneath continuous fine
soil sublayers seems indispensable to explain large flow displacement in gentle slopes after
the end of shaking, because no other mechanism alone is likely to provide such a low shear
resistance against flow. The above discussions on the water film mechanism still remain
partially qualitative, however, and further quantitative studies are certainly needed by means
of model tests or case study analyses in which a soil movement by a finite segment is well
documented together with its soil conditions.
Down-hole video observation
Based on the above discussions it seems immensely important to grasp actual behavior of
8
insitu liquefying deposit and to demonstrate significant role of the water film or void
redistribution effect in the field during actual liquefaction. For that purpose, down-hole
video observation is currently planned in a blast-induced liquefaction test site in Hokkaido,
Japan. Three boreholes of about 9 m depth will be drilled and protected with acrylic
transparent casings. The casings are filled with clear water, in which CCD cameras with
LED lights are suspended to take video movies of liquefied sand prier, during and after
liquefaction. The cameras are moved up and down in a liquefied zone to record every small
change of soil profiles to be seen through the transparent casings.
CONCLUSIVE REMARKS
Among possible mechanisms for lateral flow failures in liquefied ground, a special focus
has been placed on water film or void redistribution effect due to soil layering. Several
experimental studies were carried out up to this time for simulating this effect such as 1G
shake table tests, 1-D tube liquefaction tests, insitu soil investigations, laboratory soil tests,
etc., which have yielded following major findings;
1) Sand deposits investigated in the field are rich of sublayers with different particle
sizes and permeability which are continuous in the horizontal direction..
2) A uniform sand deposit can develop flow displacement only during shaking because
of the dilatancy effect even if its relative density is rather low.
3) Model tests demonstrates that water films are very easy to be formed beneath less
permeable sublayers and serves as a sliding surface even after the end of shaking.
4) In layered sand deposits, the water film or void redistribution mechanism can
facilitate large flow displacements without the mobility of the dilatancy effect
because the water film serves as a shear stress isolator.
5) According to case studies in Niigata City, large lateral displacements during the 1964
Niigata earthquake can be correlated with ground inclination of less than 1%,
indicating possible involvement of water films.
6) Thus, the water film mechanism in a layered sand deposit seems highly responsible
for large flow failures, which occur even in a gentle slope after the end of earthquake
shaking.
At this stage of the research, more quantitative tests using a centrifuge shaking table are
needed in order to acquire more direct results to be incorporated in actual design. US-Japan
Collaborative Research is now undertaken between the present author of Chuo University and
Professors Kutter and Boulanger in UC Davis on the subject “Effects of void redistribution
on liquefaction flow of layered soils”. This will greatly advance the design methodology
how to deal with the failure by lateral flow or lateral spread by considering the void
redistribution effect.
It is also scheduled to take a down-hole video movie of insitu liquefied soil during
blast-induced liquefaction tests which are planned by Prof. Ashford in UC San Diego in near
future. By that movie, actual behavior of liquefied sand possibly including void
redistribution and water film generation will hopefully be demonstrated.
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of Geotechnical Engineering, ASCE, 103(6), 501-516.
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