The liquefaction of soils,
recently recognized as an earthquake
phenomenon, turns out to
be a hazard well worth
reckoning with.
26
MOSAIC J u l y / A u g u s t 1979
n the past 15 years, engineers have
learned that, under conditions that are
not all that rare, solid ground can turn
to mush. Soil that had moments before supported a high-rise office building or a suburban shopping center, subjected even to
otherwise nondestructive earthquake vibration, can suddenly become a fluid. It can
flow. It can lose its bearing capacity. Anything on it—a house, a building complex, a
bridge abutment—can slip or sink like a
horse in quicksand. Buried structures, like
storage tanks, can float to the surface.
The phenomenon is known as soil liquefaction. It occurs when friction between
I
grains of soil is lost, often because of increased water pressure between them. The
soil, in effect, goes into suspension; it flows.
Non-quake-induced soil liquefaction is
an old story. It was first identified as a civil
engineering problem in the nineteen-thirties
by Arthur Casagrande of Harvard University, recalls H. Bolton Seed of the University
of California at Berkeley; "I learned about
it from him in the late nineteen-forties."
Casagrande had identified liquefaction
brought on by static loads on the soil in the
Fort Peck Dam failure in northeastern Montana in 1936.
Soil liquefaction was not thought to be a
Before, after and reconstructed: The lower San Fernando Dam was collapsed by an upstream
earthslide, caused by liquefaction, during the 1971 earthquake. Cross-sectional drawings
at the left show it as it stood before the temblor (top), what was left after (center), and
how it was modeled and reconstructed for analysis. The blue areas liquefied; segment
numbers in the collapsed and reconstructed diagram show equivalent regions.
U.S. Geological Survey; H. Bolton Seed, K. L. Lee, I. M. Idriss and F. I. Makdisi
significant component of earthquake damage, however, until 1964, when earthquakes
wracked Niigata, Japan, and Anchorage,
Alaska, and liquefaction was identified as
a major cause of damage. Since then, geologists, engineers and planners have come increasingly to recognize soil liquefaction as
an ever-present earthquake hazard, and they
have associated it with virtually every major
earthquake studied in the last 15 years.
• The San Juan, Argentina, earthquake
of November 23, 1977: 6,000 square kilometers liquefied, the most extensive area of
liquefaction yet found, fortunately most of it
on barren and agricultural land. Towers sank
and tipped. Sand boils erupted inside houses.
Curbs were crunched, school walls extended
and stone walls offset as the soil beneath
them liquefied and shifted horizontally.
• The Bucharest, Romania, earthquake
of March 4, 1977: Liquefaction occurred
on the Danube River flood plain as far as 250
kilometers from the epicenter.
• The Tangshan, China, earthquake of
July 27, 1976, the most disastrous earthquake in four centuries: Extensive liquefaction occurred in areas of young alluvial
deposits.
• The Guatemala earthquake of February 4, 1976: An area liquefied around Lake
Amatitlan, south of Guatemala City.
• The San Fernando, California, earthquake of February 9, 1971: Lateral spreading caused by liquefaction damaged a regional water filtration plant and a local government building. Liquefaction caused the
partial collapse of an earthen dam; a catastrophic flood of historic proportions was
narrowly averted.
• The Peruvian earthquake of May 31,
1970: Liquefaction occurred 190 kilometers
from the epicenter, disrupting orange groves
and banana plantations.
Further, understanding of liquefaction
has explained many historic catastrophes:
• Lateral spreading of liquefied soil that
severed water lines and prevented firemen
from controlling the blazes that destroyed
San Francisco during the 1906 earthquake.
• The dramatic flow slides in Kansu province, China, on December 16, 1920, "The
Day the Mountains Walked."
• The sand boils and earth flows left after
the Charleston, South Carolina, earthquake
of 1886.
MOSAIC July/August 1979
27
• T h e r e p o r t s of d i s a p p e a r i n g i s l a n d s ,
sand geysers, and other exotic geological
p h e n o m e n a in the N e w M a d r i d - S o u t h e r n
Mississippi Valley earthquakes of 1811-1812.
Seed, w h o has devoted almost 30 years to
liquefaction research, has even extended the
probable history of liquefaction-caused disasters back to 373 B.C. That was w h e n the
prosperous Greek coastal town of Helice slid
into the sea d u n . g a great earthquake. " T h e
destruction of Helice may well record the
earliest k n o w n case of a major landslide resulting from soil liquefaction induced by an
e a r t h q u a k e , " says Seed. "It seems reasonable to conclude that only the e n t r a p m e n t of
the inhabitants in collapsed buildings and
t e m p o r a r i l y liquefied and flowing soils
could have led to the recorded facts that no
one survived and no dead were f o u n d . " It
h a p p e n e d again during the Helice earthq u a k e of December 2 6 , 1 8 6 1 . A strip of land
13 kilometers long and 100 meters wide slid
slowly into the water. The remaining part of
the plain sank two meters.
tions in those directions. It can go only u p ward, into the overlying soils which m a y
not h a v e liquefied during the shaking. This
u p w a r d flow of water then liquefies nearsurface layers, which behave like quicksand.
( Q u i c k s a n d is merely the result of an u p ward flow of water in a sandy deposit, k e e p ing the sand in suspension.)
Liquefaction in confined zones at large
d e p t h s might be of little importance if the
s t r u c t u r e - s u p p o r t i n g layers above were not
affected, says Seed. The u p w a r d flow of
water from an underlying liquefied layer
" m a y well be the cause of the surface m a n i festations of liquefaction, such as sand boils,
q u i c k s a n d conditions or a general condition
of water seepage, causing inundations that
can cause major damage to structures s u p ported on the near-surface soils." In fact,
Seed says, near-surface soils are more likely
to become liquefied by the u p w a r d flow of
water from lower layers of liquefied soils
than they are from the direct effects of the
earthquake shaking.
Niigata
N o w h e r e have the effects of liquefaction
been more dramatically demonstrated than
in the e a r t h q u a k e at N i i g a t a , J a p a n , on
June 16, 1964. T h e epicenter of the 7.3magnitude earthquake was 55 kilometers
from Niigata, a city of 300,000 on the west
coast of Japan, where the Shinano River
enters the sea. T h e river built u p sand deposits nearly 100 meters thick on which the
city was built. The older sections of Niigata
were built on high dunes. Newer parts of
the city rested on younger, lowland sediments and on reclaimed land near the river.
Extensive areas of the low-lying deposits
liquefied. T h e g r o u n d cracked and s u b s u r face water flowed u p and out onto it. Sand
vents, like giant gopher holes, p o p p e d into
existence. Some were surrounded by rings
On a sand foundation
L i q u e f a c t i o n occurs p r i m a r i l y , b u t n o t
exclusively, in sandy soils over high water
tables. W h e n water-saturated sand is s u b jected to strong g r o u n d vibrations, it tends
to compact and decrease in volume. If drainage cannot occur, this decrease in volume results in an increase in the pressure on the,
water between the soil grains. This increased
water pressure in effect counteracts the frictional resistance of the soil particles. If the
pore-water pressure builds up to the point
at which it is equal to the overburden pressure, the effective stress becomes zero. T h e
sand loses its strength and behaves like a
thick fluid.
Loose and medium-dense sands are more
susceptible to liquefaction than are dense
sands. But, according to Seed and former
Berkeley colleague I.M. Idriss, n o w with
the San Francisco engineering firm W o o d w a r d - C l y d e C o n s u l t a n t s , l i q u e f a c t i o n of
sand may develop at any zone of a vibrating
deposit in which conditions are right. Such
a zone may be at the surface or at some depth
below, usually in the upper 15 meters or so.
T h e r e may also occur, Seed points out, a
secondary kind of liquefaction in the u p p e r
layers of a deposit. This would not be caused
by the g r o u n d motions themselves, b u t by
the development of liquefaction in an underlying layer.
S u p p o s e , for instance, a layer of saturated
sand 10 meters below the surface liquefies
d u r i n g an e a r t h q u a k e . T h e w a t e r , u n d e r
high pressure, tries to flow. It cannot flow
d o w n or sideways because of similar condi-
28
MOSAIC July/August 1979
Anchorage, 1964. The Turnagain Heights slide area and its soil composition. Either the clays lost
strength and collapsed, sand "lenses" liquefied, or both.
National Oceanic and Atmospheric Administration
of sand carried to the surface by the upwardflowing water.
As liquefaction developed over extensive
areas, buildings settled, some by as m u c h as
a meter. Cars and trucks settled engine-deep
into the flowing sand. A large, rectangular
sewage treatment tank, originally with its
t o p at g r o u n d level, tilted and rose two to
three meters.
O n e of the apartment buildings at Kawagishi-cho tipped over, intact, almost onto its
side. T h e people were able to get out by
walking on the side of the building. Later,
m a n y of these buildings were jacked back
u p , reinforced and reoccupied. The liquefaction also caused major damage to bridges,
h i g h w a y s , utilities, dock areas, oil refineries
and railroads.
Niigata is the classic case of loss of loadbearing capacity caused by liquefaction in
an e a r t h q u a k e . In his Berkeley laboratory,
Seed has used a soil profile closely simulating
conditions in Niigata during the e a r t h q u a k e
to s h o w the probable timing and sequence"
of liquefaction events there. His reconstruction illustrates dramatically the significance
of secondary liquefaction, caused by u p ward flow of water from initially liquefied,
u n d e r l y i n g sand layers.
Seed's tests show that liquefaction, which
takes place when pore-water pressure becomes equal to the confining pressure of the
overlying soil, probably developed between
d e p t h s of 4.5 and 12 meters during the 50second earthquake. Three minutes after the
s h a k i n g had stopped, the g r o u n d had liquefied to within three meters of the surface. A
m i n u t e later, liquefaction had reached to
within a meter of the surface and, 13 minutes after the shaking stopped, to within 30
centimeters. By then structures were already
sinking; the critical layer of strength loss—
not at the surface, but just beneath the buildings' foundations—had been reached.
Anchorage
T h e Alaska earthquake, at magnitude 8.3
to 8.5, was and still is the largest e a r t h q u a k e
on the N o r t h American continent since the
existence of recording instruments. It became the most studied U.S. earthquake in history. It produced a variety of dramatic landcollapse effects, many caused by liquefaction.
At Valdez, a coastal town built on a delta
of silt, fine sand and gravel, a violent and
s u d d e n landslide d r o p p e d into the harbor
some 75 million cubic meters of soil, m o v i n g
the shoreline 150 meters inland. Harbor facilities and n e a r - s h o r e facilities were d e stroyed. In some sections, soil 60 meters
deep slid h u n d r e d s of meters into the bay.
Seed says the slide was the consequence of
liquefaction of the sediments on which the
facilities were built. ( W h e n the c o m m u n i t y
was rebuilt, it was relocated at a stable site
more than six kilometers to the northwest.
T h e southern terminus of the Alaska pipeline is at the new location.) Similar slides at
Kenai Lake and Seward, where successive
strips of land disappeared into the bay for as
long as the earthquake continued, also left
dramatic changes in the landscape.
T h e Valdez, Kenai Lake and S e w a r d
events, Seed says, are all consequences of
lateral liquefaction-induced flow slides. So
was the famous p o s t - e a r t h q u a k e scene in
downtown Anchorage, showing a large portion of Fourth Street collapsed. This slide, as
did the ones at L Street and Government Hill,
appeared to have occurred as a result of
liquefaction of layers of sand underlying an
otherwise stable mass of soil.
According to T. Leslie Youd, the U.S.
Geological Survey's specialist on liquefaction, lateral spreads are the most c o m m o n
types of ground failure caused by liquefaction d u r i n g e a r t h q u a k e s . T h e s e failures
generally develop on gentle-to-near-horizontal slopes. In contrast to flow failures,
which are catastrophic and can move long
distances at relatively high speeds, lateral
spreads involve less violent, horizontal displacements of a few or a few tens of meters.
Lateral s p r e a d i n g from liquefaction
played a large role in N o r t h America's most
famous disaster, the San Francisco earthq u a k e - f i r e of 1906. A c c o r d i n g to Y o u d ,
every major pipeline break in the city occurred in areas of lateral spreading.
During the 1964 Alaska earthquake, 266
bridges were damaged to an extent requiring
substantial repair and replacement. Almost
all of the bridge damage resulted w h e n the
structures were compressed as a result of
lateral s p r e a d i n g of liquefied f l o o d - p l a i n
deposits toward the river channels, Youd
MOSAIC J u l y / A u g u s t 1979
29
says. Bridge decks buckled or were thrust
through or over abutments.
Overall, lateral spreading caused more
than $60 million of the estimated $300 million total damage from the 1964 Alaska
earthquake. Ground failures caused about
60 percent of the total earthquake damage,
much of it by liquefaction of saturated sands
and by weakening of sensitive clays.
Liquefaction potential. Pulsating load tests show, in the lower curve (above, left), the buildup
of pore-water pressure until liquefaction occurs. Evaluation chart (above, right) scales d e p t h vertical scale—against penetration resistance—horizontal scale—for sands with a water table five
feet down. The zonation map shows probable susceptibility (H-high, M-moderate, L-low, VL-very
low) of clay-free layers to liquefaction as disclosed by site testing in the San Fernando Valley.
H. Bolton Seed; Seed and i M. Idriss; T. L. Youd et ai, "Liquefaction Potential Map of San Fernando Valley, California," from
Proceedings of the 2nd International Conference on Microzonation, Vol. 1
30
MOSAIC July/August 1979
Sensitive clays
One of the most dramatic slides during the
Alaska earthquake was the one along the
coastline opposite the city's Turnagain
Heights area. Bluffs some 20 meters high,
the site of suburban home development,
lost strength and collapsed. A 65-hectare
area, extending about 2,500 meters along
the bluffline, was involved. The original
ground surface was broken into a complex
system of ridges and depressions. The depressed areas between ridges dropped an
average of 10 meters. In some places, material moved as much as 600 meters out into
the bay. Seventy-five houses built on the
east side of the slide area were destroyed.
Some of the houses moved laterally as m u c h
as 150 to 200 meters during the sliding.
T h e T u r n a g a i n Heights bluffs were composed primarily of a thick layer of clay of a
type that is quite sensitive to disturbances.
(The simple mechanical decoupling of soil
and rock on steep slopes, a cause of dramatic
d o w n - s l o p e mud slides, is a g r o u n d failure
not associated with the liquefaction phenomenon.)
M o s t clays lose strength when disturbed.
If the strength loss is not large, the clay is
termed insensitive. The measure of sensitivity is the ratio of the strength of an intact
specimen of the soil to the strength of the
same specimen after a severe disturbance.
M o s t clays have sensitivity ratios of four or
less. A n y t h i n g above eight may be prone to
failure. T h e layer of clay beneath the T u r n again Heights had sensitivities between 10
and 40.
There is not total agreement among investigators about the exact mechanism of the
T u r n a g a i n Heights land failure, or of clay
failures generally. Some, such as D w i g h t A.
S a n g r e y , professor of civil a n d e n v i r o n mental engineering at Cornell University,
consider T u r n a g a i n to have been a promin e n t example of clay collapse. O n the other
h a n d , Seed and S. D. Wilson, whose company investigated the slide for the U.S. Army
Corps of Engineers, conclude that the cause
was probably due in large measure to liquefaction of small deposits or lenses of fine
sand within the layers of clay.
In s u p p o r t of his view, Seed, w h o made
some 20 trips to Alaska to study effects of the
e a r t h q u a k e , points to several lines of evidence. A m o n g them are samples, taken from
the slide area, showing sand and clay intermixed in a form which, he declares, "could
only have occurred as a result of the sand
possessing fluid characterisics." Also, ridges
of sand u p to a meter high, 2 meters wide and
30 meters long were formed by sand boils
within the slide area. As one of the residents described this dramatic occurrence:
" T h e floor ripped and sand came u p from
b e l o w i n t o the living r o o m . " C o m m e n t s
Seed: "It is difficult to imagine such an inflow of sand except by liquefaction." Further, says Seed, "while sand lenses were
encountered in many borings made in and
behind the slide area, very few such lenses
were observed in borings made in adjacent
areas underlain by similar clay deposits but
in which no sliding occurred."
Nevertheless, Sangrey, who since 1965
has m a d e extensive studies of the strength
change of such fine-grained soils as clays,
believes it was the clays that collapsed at
Anchorage.
Sangrey is an ardent p r o p o n e n t of the
need to give more attention to the strength
deterioration of clays. " I t is i m p o r t a n t , " he
argues, "to ask whether fine-grained soils do
n o t also experience strength deterioration
during earthquakes."
O n e of the best-documented cases of clay
soils failing during an earthquake, according
to Sangrey, was an e a r t h q u a k e in 1944 that
affected both sides of the St. Lawrence River
Valley, at Cornwall in O n t a r i o and M a s s e n a
in N e w York. " T h e r e was extensive damage
to structures, and almost all of it was due to
strength deterioration of clay," says Sangrey.
Both Seed's and Sangrey's research has
s h o w n that a broad range of soils experience
changes in behavior under dynamic loading,
with the liquefaction of sandy soils being
only the most dramatic. Sangrey argues for
Niigata, 1964. One of the most dramatic
effects of soil liquefaction ever photographed:
apartment buildings at Kawakishi-cho, after
the 1964 earthquake. Many of the buildings
were later jacked up, reinforced and
reoccupied,
use of the more general term " s t r e n g t h deterioration," rather than "liquefaction," to
describe the behavior of b o t h sands and
clays. The term "liquefaction" is usually
applied only to s a n d y soils, he points out,
and detracts from the fact that the effects are
all part of a c o n t i n u u m .
Sangrey and his colleagues have attempted
to develop one model to describe the response to disturbance of all saturated soils,
i n c l u d i n g s a n d s , silts ( m i d w a y b e t w e e n
sands and clays in grain size) and clays.
MOSAIC July/August 1979
31
San Martin, 1977. The effects on a tower
and outbuilding of liquefaction caused by the
November 1977 Argentina earthquake are
seen during (above) and after (below and
photograph) the temblor.
T. L. Youd; U.S. Geological Survey
T h o u g h sands fail first, they find, with b o t h
sands and clays repeated vibrations cause
an accumulation of pressure in the pores between soil grains until the soil fails. " I t is
important to realize," Sangrey emphasizes,
" t h a t the phenomenon of liquefaction [failure of sands] is a special case within the
more general category of strength reduction
of contractive soils."
Wave action
Failures or strength reductions of soils are
a potential problem not only on land torn by
earthquakes but offshore as well. T h e repeated, cyclic impact of ocean waves, especially during an intense storm, can affect
coastal sea-bottom soils in much the same way
that the repeated vibration of an e a r t h q u a k e
can.
T h e increased importance of offshore oil
drilling makes this an important practical
consideration, and Sangrey has been devoting considerable attention to it. " I n fact," he
says, "the present-day problems with cyclic
loading of clays are largely offshore."
At least one case of an offshore oil platform lost to s t o r m - c a u s e d failure of the
underlying soil has been documented: As
Hurricane Camille, one of the more intense
hurricanes on record, swept over the Gulf of
Mexico in August 1969, it left behind the
wreckage of the Shell Oil C o m p a n y ' s offshore oil platform known as South Pass 70 B,
about 25 kilometers southeast of the m o u t h
of the Mississippi River. T h e platform was
found on its side in 100 meters of water, its
base displaced some 25 meters d o w n s l o p e
and against the direction of wave motion.
32
MOSAIC July/August 1979
Studies by R. G. Bea and later by G. H.
Sterling and E. E. Strohbeck, all of Shell,
showed convincingly that the platform had
been toppled not by high winds and waves
but principally because the soft clay soil on
which it was erected had shifted beneath it,
down to a depth of 25 meters. T h e clay soils,
Sangrey says, collapsed due to strength deterioration caused by cyclic wave loading.
Since then, Sangrey has helped in the
design of a new oil platform, as large as the
Empire State Building, p u t u p in the Gulf of
Mexico in 1978. Sangrey is also involved in
newly initiated studies to assess the response
to earthquake and wave effects of submarine
areas in the economically i m p o r t a n t Gulf of
Alaska, and in i n d u s t r y - s p o n s o r e d field
studies there of the response of piles to
cyclic loading.
Earthen dams
Soils that underlie structures are a general
problem. There are important special cases,
however, in which liquefaction-susceptible
soils are part of the structure. This is the
case of earth dams, and the concern once
again is with s a n d y soils. T h e 1971 San
F e r n a n d o e a r t h q u a k e b r o u g h t the h a z a r d
into vivid focus.
"Probably no single event has had such
an impact on the soil-mechanical aspects of
earth dam design against earthquake effects," says Seed, "as the near-failure of the
lower San Fernando D a m in the earthquake
of February 9, 1 9 7 1 . "
T h e e m b a n k m e n t dam, about 40 meters
high, provided a reservoir capacity of 24.5
million cubic meters of water. It had a core
of fine clay s u r r o u n d e d by a hydraulic fill of
sand. At the m o m e n t of the earthquake, the
water in the reservoir was 10 meters below
the crest. W h e n all the post-earthquake slide
movements were over, the top of the c r u m b ling dam was barely 1.5 meters above the
water level,. T h e u p s t r e a m part of the em-
b a n k m e n t , including the upper 10 meters of
the crest, had moved 20 meters or more into
the reservoir. Had the reservoir been full, the
story would have had a different ending.
T h e 1.5 meters of remaining freeboard
was obviously precarious. The embankment
was cracked. Some 8 0 , 0 0 0 people living
d o w n s t r e a m from the dam had to be evacuated until the reservoir could be drained to
safe levels. "It was almost catastrophic," recalls Seed. " T h e margin by which the emb a n k m e n t stood was very small. It could
easily have been the greatest natural disaster in the history of the United States."
The study, conducted by Seed, Kenneth L.
Lee (who died in 1978), I.M. Idriss and F. I.
Makdisi for the California D e p a r t m e n t of
Water Resources, the Los Angeles D e p a r t m e n t of Water and Power and the National
Science Foundation, concluded: " A major
catastrophe was narrowly missed. Had any
one of a number of possible conditions been
slightly less favorable, such as the duration
of shaking or the water level in the reservoir, the Lower D a m could have failed, resulting in a sudden release of [10 million
metric tons] of water over a heavily p o p u lated urban residential area."
Laboratory and field studies carried out by
Seed and his colleagues allowed them to reconstruct the mechanisms of failure of the
d a m . T h e y p a i n s t a k i n g l y identified a n d
located in their models all the dislocated
pieces, like parts of a jumbled jigsaw puzzle.
Reconstructing the puzzle and conducting
dynamic analyses, they were able to learn
w h a t had happened.
T h e foundation had remained intact. T h e
sliding originated in the e m b a n k m e n t itself.
A large, wedge-shaped segment of sand fill
inside the dam had liquefied.
"After about 12 seconds of strong s h a k i n g , " reports Seed, " v e r y high pore-water
pressure had developed in an extensive zone
Historic events. The massive sand boil (left) was the consequence of liquefaction in the
Charleston, South Carolina, earthquake of 1866. Lateral spreading dueto liquefaction buckled the
curbstones and pavement on Lexington Street at Eighteenth in San Francisco, 1906.
U.S. Geological Survey
of hydraulic fill near the base of the embankment and upstream of the clay core...much
of this soil was in a liquefied condition. At
this stage, the shear resistance of the soil in
the upstream shell could not withstand the
deadload stresses caused by the weight of
the embankment, and slide m o v e m e n t s developed. T h e slide mass moved o u t w a r d s on
the liquefied soil, breaking into blocks and
removing s u p p o r t from the clay core which
was then extruded into the remaining part
of the shell material."
In other words, the primary cause of the
slide was not directly the m o v e m e n t created
by. the earthquake, but rather the loss of
strength in the sand fill as a consequence of
earthquake-induced soil liquefaction.
A second dam, upstream, also had zones
that completely liquefied, but e n o u g h surrounding material retained its strength that
complete failure did not occur. Slides there
were limited.
Lessons
Important lessons can be learned from
near disasters. Following the San Fernando
studies, California ordered the reevaluation
of all such dams. As a result, some dams were
taken out of service, some reconstructed
and some had their water levels reduced.
Still others have u n d e r g o n e stabilizing procedures, such as the construction of large
upstream buttresses, and in some cases a
second dam has been built d o w n s t r e a m .
New methods for evaluating the safety of
dams have also been developed. O n e way to
find out whether soil in a dam will liquefy,
according to Seed, is to model the soil and
calculate stresses both before and d u r i n g
simulated earthquakes. T h e n take a sample
of the soil into the laboratory and subject it
to the same stresses. "If it liquefies in the
lab, it will p r o b a b l y liquefy in an e a r t h q u a k e , " says Seed.
Seed recently completed a detailed study
of the 1 9 5 - m e t e r - h i g h O r o v i l l e D a m in
northeastern California, the largest earthfill dam in N o r t h America. T h e region in
which the dam was built in the nineteensixties was t h o u g h t to be one of low seismicity. T h e n , on A u g u s t 1, 1975, a m a g n i t u d e
5.7 earthquake occurred only 10 kilometers
southwest of the dam. T h e r e was no major
damage, but there was concern. Seed studied
the dam's safety margins against a 6.5-magnitude e a r t h q u a k e occurring very close to
the dam. T h e work indicates the d a m would
easily resist such an e a r t h q u a k e . " I n fact,"
says Seed, "it's safe even for one of 8.0."
W h a t determines the likelihood of liquefaction? Fortunately it occurs in only certain kinds of conditions, and studies of the
geologic setting can provide some broad,
general guidance.
Liquefaction criteria
As Youd, a research civil engineer for the
U.S.G.S. says, "Liquefaction does not occur
at random localities b u t is restricted to certain geologic and hydrologic environments,
specifically layers of relatively loose, cohesionless sediments [usually sand and silt]
and a high water table [generally within 10
meters of g r o u n d surface].
"Sedimentary units most likely to contain
liquef iable sediments are recently deposited
deltaic, river channel, flood plain and aeolian
(wind-borne) deposits and poorly compacted fills. Thus, preliminary assessment of
liquefaction susceptibility can often be
MOSAIC July/August 1979
33
made using low-cost geologic and h y d r o logic studies."
In this way, Youd and his U.S.G.S. colleagues have prepared and published liquefaction-potential m a p s for the s o u t h e r n S a n
Francisco Bay region. Areas there, u n d e r lain by recent stream deposits and bay m u d
c o n t a i n i n g s a n d layers, have the h i g h e s t
liquefaction risk. A similar map for the San
Fernando Valley shows a number of areas
just north of the Santa Monica M o u n t a i n s
to be at high risk.
Youd and the others in the field agree that
such appraisals can indicate only a gross
potential for liquefaction within a large area;
to determine whether liquefaction is a hazard for specific sites, detailed geotechnical
studies are necessary.
Seed, summarizing recent work on the
subject, identifies a n u m b e r of characteristics that influence the liquefaction p o t e n tial of cohesionless soils. Best recognized is
the density of the soil. T h e more dense it is,
the less it is likely to liquefy.
Soils that have been under a sustained
load for a long period are less susceptible
than are very y o u n g , loose deposits. This
heightened resistance seems to be the result
of some form of cementation or welding
over time. Also, soils that have been s u b jected to the strains of some small earthquakes in the past seem to be s o m e w h a t
more resistant to liquefaction t h a n are
freshly deposited sands, even t h o u g h this
strain history doesn't increase their density.
The classic test of liquefaction potential,
first developed after the Niigata earthquake,
measures the ability of the soil to resist penetration of a shaft fitted with a standard head
and driven into the g r o u n d . The less resistant the soil, it has been found, the more
prone it is to liquefaction. Over the years
this standard penetration resistance test has
been refined till the correlations between
p e n e t r a t i o n r e s i s t a n c e and p o t e n t i a l for
liquefaction are good.
Solutions
W h a t can be done once a high liquefaction potential is identified? As Idriss says:
" T h e r e are solutions. O n e is not to build.
Another is to build and accept the risk."
If an important, specific site is involved,
the liquefaction-susceptible soil can be p h y sically removed and replaced with a more
cohesive soil. Loose sands can sometimes be
made more dense by compacting.
M a n y of the techniques, U.S.G.S.'s Youd
points out, are expensive and thus justifiable
only at economically important or critical
sites. After the soil beneath the n e w San
F e r n a n d o Valley J u v e n i l e Hall liquefied
during the 1971 e a r t h q u a k e there, engineers
34
MOSAIC July/August 1979
cut two trenches across the site deep e n o u g h
to intersect the liquefiable zones. T h e y then
backfilled the trenches with well-compacted
soil to prevent any future lateral movement.
T h e y also grouted the soils beneath existing
buildings.
T h e liquefaction of the fill beneath the
Jensen Water Filtration Plant during the
same earthquake called for a different engineering solution. Engineers have recommended that gravel drainage columns be installed t h r o u g h the fill a n d u n d e r l y i n g
layers. These vertical gravel drains would
relieve excess pore-water pressures during
an earthquake and prevent liquefaction.
" H o w conservative you are depends on
the consequences," says Idriss. A nuclear
power plant or a dam needs to have all possible m a r g i n s of s a f e t y , a n d p r o b a b l y
shouldn't be built in an area where there is
any possibility of liquefaction. For a warehouse, the risk might well be tolerable.
"If there's a choice of another site," says
W i l l i a m S p a n g e l , p r e s i d e n t of W i l l i a m
Spangel Associates, a Portola Valley, California, firm completing a s t u d y of land-use
planning responses to earthquakes, "avoid
the hazardous area. If there is not a choice,
then mitigating measures have to be taken.
And that's a matter of economics."
Youd says he sees the problem getting
worse, not better: "People are building more
and more on low-lying areas of y o u n g deposits susceptible to liquefaction." And potentially liquefiable soils are being sought
and found u n d e r congested metropolitan
areas in m a n y parts of the world. Youd discovered one such in a recent analysis of the
situation along the n o r t h e r n coast of Puerto
Rico, including parts of San Juan. And two
Japanese engineers—Kenji Ishihara of the
University of T o k y o and Kaihei O g a w a of
the T o k y o Metropolitan Office—have reported liquefaction susceptibility at a third
of the more than 1,000 sites they investigated in d o w n t o w n T o k y o .
But the lessons are being learned. In Seed's
words, the efforts of m a n y workers " h a v e
helped to raise the state of knowledge in this
field over the past decade to a condition
where engineers can practice in an extremely
difficult area with some reasonable level of
confidence."
And there is m u c h greater awareness than
before. " N o w nobody would design a major
project without investigating the liquefaction problem," says Seed. " T h e r e is a sense
of concern." •
The National Science Foundation
contributes to the support of research discussed in
this article through its Earthquake
Hazards
Mitigation
Program.