The liquefaction of soils, recently recognized as an earthquake

phenomenon, turns out to be a hazard well worth

reckoning with.

26 MOSAIC July /August 1979

I 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

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

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

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

• T h e r epor t s of d i sappear ing is lands , sand geysers, and other exotic geological phenomena in the New Madr id-Southern Mississippi Valley earthquakes of 1811-1812.

Seed, who 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 when the prosperous Greek coastal town of Helice slid into the sea dun . 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 ea r thquake ," says Seed. "I t seems reason­able to conclude that only the entrapment of the inhabitants in collapsed buildings and temporar i ly l iquefied and f lowing soils could have led to the recorded facts that no one survived and no dead were found ." It happened again during the Helice earth­quake of December 26 ,1861. 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 may not have liquefied during the shaking. This upward flow of water then liquefies near-surface layers, which behave like quicksand. (Quicksand is merely the result of an u p ­ward flow of water in a sandy deposit, keep­ing the sand in suspension.)

Liquefaction in confined zones at large depths might be of little importance if the s t ructure-support ing layers above were not affected, says Seed. The upward flow of water from an underlying liquefied layer " m a y well be the cause of the surface mani­festations of liquefaction, such as sand boils, quicksand conditions or a general condition of water seepage, causing inundat ions that can cause major damage to structures sup ­ported on the near-surface soils." In fact, Seed says, near-surface soils are more likely to become liquefied by the upward flow of water from lower layers of liquefied soils

than they are from the direct effects of the ear thquake shaking.

Niigata Nowhere have the effects of liquefaction

been more dramatically demonstrated than in the ea r thquake at Ni igata , Japan , on June 16, 1964. The epicenter of the 7.3-magnitude earthquake was 55 kilometers from Niigata, a city of 300,000 on the west coast of Japan, where the Shinano River enters the sea. The river built up 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. The ground cracked and subsur­face water flowed up and out onto it. Sand vents, like giant gopher holes, popped into existence. Some were surrounded by rings

On a sand foundation Liquefact ion occurs pr imari ly , b u t not

exclusively, in sandy soils over high water tables. W h e n water-saturated sand is sub­jected to s trong ground 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 fric-tional 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. The 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, now with the San Francisco engineering firm Wood­w a r d - C l y d e Consu l t an t s , l iquefact ion 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.

There may also occur, Seed points out, a secondary kind of liquefaction in the upper layers of a deposit. This would not be caused by the ground motions themselves, bu t by the development of liquefaction in an under­lying layer.

Suppose , for instance, a layer of saturated sand 10 meters below the surface liquefies du r ing an e a r t h q u a k e . The water , u n d e r high pressure, tries to flow. It cannot flow down or sideways because of similar condi-

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

28 MOSAIC July/August 1979

of sand carried to the surface by the upward-flowing water.

As liquefaction developed over extensive areas, buildings settled, some by as much as a meter. Cars and trucks settled engine-deep into the flowing sand. A large, rectangular sewage treatment tank, originally with its top at g round level, tilted and rose two to three meters.

One of the apartment buildings at Kawa-gishi-cho tipped over, intact, almost onto its side. The people were able to get out by walking on the side of the building. Later, many of these buildings were jacked back u p , reinforced and reoccupied. The lique­faction also caused major damage to bridges, h ighways , utilities, dock areas, oil refineries and railroads.

Niigata is the classic case of loss of load-bearing capacity caused by liquefaction in an ear thquake. In his Berkeley laboratory, Seed has used a soil profile closely simulating

condit ions in Niigata during the ear thquake to show 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, under lying 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 depths of 4.5 and 12 meters during the 50-second earthquake. Three minutes after the shaking had stopped, the ground had lique­fied to within three meters of the surface. A minute 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 The Alaska earthquake, at magnitude 8.3

to 8.5, was and still is the largest ear thquake on the North American continent since the existence of recording instruments . It be­came the most studied U.S. earthquake in his­tory. It produced a variety of dramatic land-collapse effects, many caused by liquefaction.

At Valdez, a coastal town built on a delta of silt, fine sand and gravel, a violent and sudden landslide dropped into the harbor some 75 million cubic meters of soil, moving the shoreline 150 meters inland. Harbor fa­cilities and nea r - shore facilities were de­stroyed. In some sections, soil 60 meters deep slid hundreds of meters into the bay. Seed says the slide was the consequence of liquefaction of the sediments on which the facilities were built. (When the communi ty 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 ear thquake continued, also left dramatic changes in the landscape.

T h e Valdez, Kenai Lake and Seward events, Seed says, are all consequences of lateral liquefaction-induced flow slides. So was the famous post-ear thquake 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 common types of ground failure caused by liquefac­tion dur ing e a r t h q u a k e s . These fai lures 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.

Latera l s p r e a d i n g f rom l i q u e f a c t i o n played a large role in Nor th America's most famous disaster, the San Francisco earth­quake- f i re of 1906. Accord ing to Youd , every major pipeline break in the city oc­curred in areas of lateral spreading.

During the 1964 Alaska ear thquake, 266 bridges were damaged to an extent requiring substantial repair and replacement. Almost all of the bridge damage resulted when the structures were compressed as a result of lateral spreading of l iquefied f lood-p la in deposits toward the river channels, Youd

MOSAIC July/August 1979 29

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

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.

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.

30 MOSAIC July/August 1979

Some of the houses moved laterally as much as 150 to 200 meters during the sliding.

The Turnagain 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 down-slope mud slides, is a ground failure not associated with the liquefaction phe­nomenon.)

Most 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. Most clays have sensitivity ratios of four or less. Anyth ing above eight may be prone to failure. The 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 Turnaga in Heights land failure, or of clay failures generally. Some, such as Dwight A. Sangrey , professor of civil and env i ron­mental engineering at Cornell University, consider Turnagain to have been a promi­nent example of clay collapse. O n the other hand, 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 suppor t of his view, Seed, who made some 20 trips to Alaska to study effects of the ear thquake, points to several lines of evi­dence. Among 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 up 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 up from be low in to the living r o o m . " C o m m e n t s Seed: "I t 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 made 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 proponent of the

need to give more attention to the strength deterioration of clays. "I t is impor tant ," he argues, "to ask whether fine-grained soils do not also experience strength deterioration dur ing ear thquakes ."

One of the best-documented cases of clay soils failing during an earthquake, according to Sangrey, was an ear thquake in 1944 that affected both sides of the St. Lawrence River Valley, at Cornwall in Ontar io and Massena in New York. "There 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 shown 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 rength de­terioration," rather than "l iquefact ion," to describe the behavior of both sands and clays. The term "l iquefact ion" is usually applied only to sandy soils, he points out, and detracts from the fact that the effects are all part of a cont inuum.

Sangrey and his colleagues have attempted to develop one model to describe the re­sponse to disturbance of all saturated soils, inc luding sands , silts (midway be tween 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

Though sands fail first, they find, with both 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, " tha 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. The 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 ear thquake can.

The increased importance of offshore oil drilling makes this an important practical consideration, and Sangrey has been devot­ing considerable attention to it. " In 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 to rm-caused 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 Company ' s off­shore oil platform known as South Pass 70 B, about 25 kilometers southeast of the mou th of the Mississippi River. The platform was found on its side in 100 meters of water, its base displaced some 25 meters downslope and against the direction of wave motion.

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. The clay soils, Sangrey says, collapsed due to s t rength 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, pu t up 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 important 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 sandy soils. The 1971 San Fe rnando e a r t h q u a k e b r o u g h t the hazard 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 ear thquake of February 9, 1 9 7 1 . "

The embankment dam, about 40 meters high, provided a reservoir capacity of 24.5 million cubic meters of water. It had a core of fine clay sur rounded by a hydraulic fill of sand. At the moment of the ear thquake, the water in the reservoir was 10 meters below the crest. When all the post-earthquake slide movements were over, the top of the c rumb­ling dam was barely 1.5 meters above the water level,. The ups t ream part of the em-

32 MOSAIC July/August 1979

bankment , 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.

The 1.5 meters of remaining freeboard was obviously precarious. The embankment was cracked. Some 80,000 people l iving downstream from the dam had to be evac­uated until the reservoir could be drained to safe levels. "It was almost catastrophic," re­calls Seed. "The margin by which the em­bankment 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 Depar tment of Water Resources, the Los Angeles Depar t ­ment 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 durat ion of shaking or the water level in the reser­voir, the Lower Dam could have failed, re­sulting in a sudden release of [10 million metric tons] of water over a heavily popu­lated urban residential area."

Laboratory and field studies carried out by Seed and his colleagues allowed them to re­construct the mechanisms of failure of the dam. T h e y pa ins tak ing ly identified and 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 what had happened.

The foundation had remained intact. The sliding originated in the embankment itself. A large, wedge-shaped segment of sand fill inside the dam had liquefied.

"After about 12 seconds of strong shak­ing ," reports Seed, "very 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 movements de­veloped. The slide mass moved outwards on the liquefied soil, breaking into blocks and removing suppor 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 movement 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 enough sur­rounding material retained its s trength 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 undergone stabilizing pro­cedures, such as the construction of large upstream buttresses, and in some cases a second dam has been built downst ream.

New methods for evaluating the safety of dams have also been developed. One 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 dur ing simulated ear thquakes. Then 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 robab ly l iquefy in an ear th­quake ," says Seed.

Seed recently completed a detailed s tudy of the 195-me te r -h igh Orovi l le D a m in northeastern California, the largest earth-fill dam in Nor th America. The region in which the dam was built in the nineteen-sixties was thought to be one of low seismic-ity. Then, on Augus t 1, 1975, a magni tude 5.7 ear thquake occurred only 10 kilometers southwest of the dam. There was no major damage, but there was concern. Seed studied the dam's safety margins against a 6.5-mag-nitude ear thquake occurring very close to the dam. The work indicates the dam would easily resist such an ear thquake. " In fact," says Seed, " i t ' s safe even for one of 8.0."

What 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 but is restricted to cer­tain geologic and hydrologic environments , specifically layers of relatively loose, cohe-sionless sediments [usually sand and silt] and a high water table [generally within 10 meters of ground 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 hydro -logic studies."

In this way, Youd and his U.S.G.S. col­leagues have prepared and published lique­faction-potential maps for the southern San Francisco Bay region. Areas there, under ­lain by recent stream deposits and bay m u d conta in ing sand layers , have the h ighes t liquefaction risk. A similar map for the San Fernando Valley shows a number of areas just north of the Santa Monica Moun ta ins 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 number of characteris­tics that influence the liquefaction poten­tial of cohesionless soils. Best recognized is the density of the soil. The 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 young, loose deposits. This heightened resistance seems to be the result of some form of cementation or welding over time. Also, soils that have been sub­jected to the strains of some small earth­quakes in the past seem to be somewhat more resistant to l iquefact ion t h a n are freshly deposited sands, even though 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 s tandard head and driven into the ground. The less resis­tant the soil, it has been found, the more prone it is to liquefaction. Over the years this standard penetrat ion resistance test has been refined till the correlations between penet ra t ion res is tance and poten t ia l for liquefaction are good.

Solutions What can be done once a high liquefac­

tion potential is identified? As Idriss says: "There are solutions. One is not to build. Another is to build and accept the r isk."

If an important , specific site is involved, the liquefaction-susceptible soil can be phy­sically removed and replaced with a more cohesive soil. Loose sands can sometimes be made more dense by compacting.

Many 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 new San Fernando Valley Juveni le Hall l iquefied during the 1971 ear thquake there, engineers

cut two trenches across the site deep enough to intersect the liquefiable zones. They then backfilled the trenches with well-compacted soil to prevent any future lateral movement . They also grouted the soils beneath existing buildings.

The liquefaction of the fill beneath the Jensen Water Filtration Plant during the same ear thquake called for a different engi­neer ing so lu t ion . Engineers have recom­mended that gravel drainage columns be in­stalled t h r o u g h the fill and unde r ly ing 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 ty , 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 Wi l l i am 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 tudy of land-use planning responses to ear thquakes, "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 young de­posits susceptible to l iquefaction." And po­tentially liquefiable soils are being sought and found under congested metropolitan areas in many parts of the world. Youd dis­covered one such in a recent analysis of the situation along the nor thern coast of Puerto Rico, including parts of San Juan. And two Japanese engineers—Kenji Ishihara of the University of Tokyo and Kaihei Ogawa of the Tokyo Metropol i tan Office—have re­ported liquefaction susceptibility at a third of the more than 1,000 sites they investi­gated in downtown Tokyo .

But the lessons are being learned. In Seed's words, the efforts of many workers "have 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 much greater awareness than before. " N o w nobody would design a major project without investigating the liquefac­tion problem," says Seed. "There 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.

34 MOSAIC July/August 1979