Soil Liquefaction in Earthquakes — Its effects on Structures and How to Avoid it

Delong Zuo

Abstract

Cases of structure failure caused by soil liquefaction during earthquakes are

reviewed. The mechanism of soil liquefaction is summarized. Methods to

identify soil liquefaction are introduced and mathematical models to simulate

soil liquefaction are presented. Based on the historical facts and the nature of soil

liquefaction, ways by which to reduce soil liquefaction potential during

earthquakes are suggested, with the emphasis on the structural design aspect.

Key Words: Soil Liquefaction, Earthquake, Structural Failure

Introduction

During earthquakes, soil failures, especially soil liquefaction, can cause

devastating effects on structures, such as land-sliding, lateral spreading, large

ground settlement and so on. This phenomenon has been observed for many

years. In fact, many of the structural failures in ancient earthquakes can be

associated with soil liquefaction based on the knowledge we process today. But

this phenomenon was not brought to the attention of engineers until after the

Niigata earthquake and the Alaska earthquake, both of which occurred in 1964

and demonstrated lots of typical soil liquefaction effects. Since then, careful

observations and in-depth research on this phenomenon have been carried out

by engineers and scientists all over the world. The mechanism of this

phenomenon has been studied and principles drawn from these studies have

been applied to practical engineering designs and construction. However, due to

the complex nature of soil and liquefaction, this phenomenon is far from

thoroughly understood.

Soil Liquefaction in Some Major Earthquakes

All strong earthquakes are accompanied by the phenomena of soil liquefaction

of some kind. Liquefaction can cause the failure of structures of any form in

many modes. Some typical cases of structure failure caused by soil liquefaction

in some major earthquakes are summarized below.

1. The Niigata Earthquake

Fig. 1 shows overturned buildings in the

Niigata Earthquake, which occurred on June 20,

1964 in Japan with a Richter magnitude of 7.7.

The buildings in the picture remained relatively

intact but rotated as whole structures because of

the land-sliding under their foundations. The

land-sliding was determined to have been caused

by soil liquefaction.

2. The Alaska Earthquake

The Alaska Earthquake, which occurred on March 27, 1964 in the Gulf of

Alaska, had a Richter magnitude of 8.5. It was one

of the largest earthquakes in the 20th century and

caused many structural failures due to soil

liquefaction. It is earthquake, together with the

Niigata Earthquake that called the attention of

engineers to the phenomenon of soil liquefaction

in earthquakes. Fig. 2 shows the failure of a road

embankment caused by soil liquefaction. The

failure of the roadbed caused the embankment to

spread to the two sides of the road, thereby tore the embankment apart.

Fig 1.Tilted Buildings1

Fig 2. Cracking of Road Embankment1

3. The Loma Prieta Earthquake

The October 17, 1989 Loma Prieta Earthquake

had a Richter magnitude of 7.1. It was also not

exempted from significant structure failures

caused by soil liquefaction. Fig. 3 shows a sand

boil at the Oakland International Airport caused

by soil liquefaction in the earthquake.

4. The Kobe Earthquake

Liquefaction caused by strong dynamic ground

motion during the Kobe earthquake (7.2, Richter)

also contributed greatly to the structure failure,

especially the failure of the bridges and viaducts

on the Hanshin Expressway. Fig. 4 shows the

Nishinomiya Bridge with one span of its deck

fallen to the ground. The supports of the bridge

were not damaged, but large ground deformation occurred. Soil liquefaction

played a key role in the failure of this bridge.

5. The Izmit Earthquake

The Izmit earthquake is a more recent large

earthquake, which hit Turkey on August 17,

1999, with a Richter magnitude of 7.4. This

earthquake caused many deaths because of

residential building failure. One common

failure type of buildings is caused by soil-

liquefaction-induced loss of bearing strength

beneath shallow mat foundations. Fig. 5 shows one typical example of these

phenomena.

Fig. 3 Sand boil at an airport1

Fig. 4. Fallen bridge deck

Fig 5. Toppled Building2

6. The Taiwan Earthquake

One other effect of soil liquefaction—large

ground settlement, can be found in the

September 21, 1999 Taiwan Earthquake (7.6,

Richter). Fig. 6 shows the damage of the

Taichung Harbor caused by soil liquefaction.

Large ground settlement can be seen in the

picture as well as the water burst out from

the deeper soil layer can be easily discerned.

Mechanism of Soil Liquefaction

Given the examples above, it is necessary to understand the mechanism of soil

liquefaction, where it occurs and why it occurs so often in earthquakes.

Liquefaction of soil is a process by which sediments below the water table

temporarily lose strength and behave more as a viscous liquid than as a solid3.

Liquefaction occurs in saturated soils, especially clay-free sand and silts. The

water in the soil exerts pressure upon the soil particles. If this pressure is low

enough, the soil stays stable. But once the

water pressure exceeds a certain level, it forces

the soil particles to move relative to each other,

thus causing the strength of the soil to decrease

and failure of the soil follows. During

earthquakes, when the shear wave passes

through saturated soil layers, it causes the

granular soil structure to deform and the weak

part of the soil begins to collapse. The

collapsed soil fills the lower layer and forces the pore water pressure in this layer

to increase. If the water pressure cannot be ready released, it will continue to

Fig 7. Shear Deformation Caused by Earthquake4

Fig. 6 Large Ground Settlement3

build up until it can sustain the total weight of the soil layer above, thus the

upper layer soil are ready to move and behave as a viscous liquid. It then is said

that soil liquefaction has occurred. Fig 7 shows the shear deformation of soil

caused by dynamic earthquake load.

Although soil liquefaction is usually followed by significant structural

failures, it does not happen everywhere. There are some places that are more

susceptible to soil liquefaction. Generally, the more loose the soil and the higher

the underground water level, the more likely liquefaction is to occur at this site

during earthquakes. The degree of ground deformation caused by soil

liquefaction is always depended on the age, density, and depth of the soil. The

slope of the ground, as well as the characteristic of the structure sitting upon the

ground, will also affect the soil deformation caused by liquefaction.

Soil liquefaction is a very complex

phenomenon, but it generally can be put into

two major categories, that is flow liquefaction

and cyclic mobility.

Flow liquefaction occurs in large soil areas

where the strength of the liquefied soil is

extremely low and the ground has a rather deep

slope. When the static equilibrium is destroyed

by dynamic earthquake load or sometimes even

a small load, the huge soil body can “flow” as a whole and in some instances

travel a very long distance (even of tens of miles) with very high velocity.

Because of this characteristic, this type of soil liquefaction is always the most

disastrous. Fig. 8 shows the mechanism of this type of soil liquefaction and the

damages to the buildings shown in fig. 1 and fig. 5 are typical damages caused

by flow liquefaction.

Cyclic mobility is triggered by cyclic loading at places where the shear stress of

the soil is lower than the soil strength and the ground slope is moderate. During

Fig. 8. Flow Liquefaction3

earthquakes, cyclic mobility of the soil increases steadily with the dynamic load,

and finally triggers the soil to fail. One common result of cyclic mobility is lateral

spreading, which occurs when the subsiding soil cannot bear the surface layer

any longer. Thus the gravitational forces, together with the inertial forces built

up by the dynamic earthquake load cause the surface soil layer to fail. Fig. 2 and

3 show cases of soil liquefaction caused by cyclic mobility.

Identification of Soil Liquefaction

Most soil liquefaction has devastating effects, which can be very easily

identified. But there are cases where liquefaction has occurred in some

earthquakes without structure failure. It is necessary to identify these cases and

reinforce the soil below the structure to avoid possible future failure.

Several techniques have been developed to do this job. One is to place two

accelerometers at the site, with one at some depth in the soil and one in the

surface layer. If the ground acceleration recorded at the surface level is

significantly smaller than that recorded by the one underneath, and the upper

layer ground exhibits apparently longer period of motion, it can be determined

that soil liquefaction has occurred to some extent at this site. This vtechnology

has been used at the Higashi-Kobe Bridge which is near the epicenter of the 1995

Kobe earthquake. During the earthquake, the bridge site underwent very strong

ground motion. Fig. 9 shows the time histories of the earthquake recorded by

two accelerometers, one on the ground level (G2) and the other in a depth of

34m. Compare the two time histories, it can be seen very clearly that the ground

acceleration at the surface layer is much smaller than that of the depth of 34m.

And the period of the ground acceleration is apparently much longer at the

surface. This suggests strongly that soil liquefaction has occurred to a certain

extent at this site during the earthquake, although the earthquake produced no

severe damage to the bridge.

Simulation of Soil Liquefaction in Earthquake Design

Because of its devastating effects on structures, it is very important to include

liquefaction as a consideration in earthquake design for soil sites that are

susceptible to this phenomenon so that measures can be taken to reduce the

potential hazard.

Many mathematical models have been developed based on different soil

properties, different structure types and foundations to simulate soil liquefaction

induced by dynamic load. In finite element modeling, two kinds of models are

generally adopted. One uses soil springs to simulate the soil around the structure

foundation. Different stiffness of the springs is adopted during different stages of

the soil behavior during earthquakes. In this model the super-structure is

simplified as concentrated masses that are connected by stiff rod elements. The

other model uses large scale finite element simulation, in which the soil, the

Figure 3. Time History of Earthquake at Different Ground Level4

foundation and the super-structure are all simulated using different types of

finite elements reflecting their relative properties. In particular, special soil

elements are used to simulate the soil-foundation boundary and parameters of

the soil elements and the boundary elements are adjusted according to different

stages of soil deformation. The second model has some advantages over the first

model in that it takes into consideration the effects of soil-structure interaction.

The soil no longer acts alone, which is closer to reality. Furthermore, it effectively

distinguishes the soil closer to the foundation from the soil farther away from it.

It has been discovered that the soil near the structure foundation has more

complex properties and is more likely to liquefy because of the dynamic

disturbance from the foundation and water pressure is more difficult to dissipate

in this region. The difficult part of the soil liquefaction modeling is soil parameter

identification. There has been considerable work done on this subject and more

actively underway.

Methodologies to Reduce Soil Liquefaction Potential

Soil liquefaction does occur and it is rather hard to predict it. Yet it is the

engineer’s responsibility to reduce the potential for soil liquefaction and the

catastrophic results it brings to structures and. Although there is still a lot to

learn about soil liquefaction, some general rules should be followed to reduce the

potential of soil-liquefaction-induced structure failures. Some suggestions are

summarized below.

First, avoid building structures in areas that are susceptible to soil liquefaction,

which is the best policy. These areas incluide*:

1. areas known to have experienced soil liquefaction during historic

earthquakes;

2. all areas of uncompacted fills containing liquefaction susceptible material

which are saturated, nearly saturated, or can be expected to become

* Based on “Earthquake Basics”, Earthquake Engineering Research Institute

saturated;

3. areas where sufficient existing geotechnical data and analyses indicate that

the soils are potentially liquefiable;

4. areas underlain with saturated geologically young sediments (younger

than 10,000 to 15,000 years old); and

5. areas that has a relatively steep ground slope.

Second, improve the soil if it is absolutely necessary to build structures in

liquefaction-susceptible areas. Such approaches include soil exchanging,

dynamic soil compaction, concrete grouting and installing stone or concrete

column and1.

Third, structures can be built to be liquefaction resistant. Some areas may not

seem liquefaction-susceptible, but the structure may be very important, examples

include densely populated residential buildings, long bridges and large dams on

major rivers. It is especially important to put soil liquefaction validation into the

design process for these structures.

Based on the mechanism and forms of soil liquefaction and the lessons learned

from past earthquakes, some suggestions can be provided as follows for

structural design:

1. Provide sufficient drainage at the foundation so that water pressure in the

soil will not easily build up under dynamic load.

2. Design strong foundation mats so that the structures do not fail even

though liquefaction occurs under part of their foundations. The building in

Fig. 5 experienced failure because it did not have a strong enough mat.

3. Include angled piles if pile foundations are adopted. Because, if soil faction

occurs in earthquakes, the piles will sustain not only vertical load, but also

lateral load .

4. Reinforce weakened soil around structures after earthquakes or some other

dynamic load; repair damaged foundations.

Conclusions:

Soil liquefaction is a common phenomenon during earthquakes. Its effects on

structures are devastating and it occurs in many forms. The mechanism of soil

liquefaction is very complicated due to the nature of soil, which renders it

difficult to fully understand. Because of the consequences it can bring to

structures, soil liquefaction should be an important factor considered in

earthquake design, especially for important structures. Research has been

performaed to identify and model the phenomenon of soil liquefaction so as to

provide references for earthquake design. To reduce the potential for structure

failures caused by soil liquefaction, some general rules have to be followed.

Recommendations

Soil is a material that has the one of the most diverse forms and most complex

properties. Much research has been performed to study this material, both

computational and experimental. But there seems little work done to understand

the properties of the part of soil at the soil-structure interface, which has different

properties as compared to the ordinary soil and could play a decisive role in the

behavior of both the soil and the structure during such severe loads as

earthquake, including soil liquefaction. This may be a future focus of study.

One other possible field of study concerning soil liquefaction is to combine this

phenomenon with the other commonly observed phenomena during

earthquakes, such as soil-structure separation and soil softening. There has been

some work done in this field. A critical part of this area of research is soil

parameter identification, which has always been a difficulty.

References:

1. Soil Liquefaction Web site, University of Washington,

www.ce.washington.edu/~liquefaction/html/main.html

2. “The Izmit (Kocaeli), Turkey Earthquake of August 17, 1999”, EERI Special

Earthquake Report, Oct 1999, www.eeri.org/Reconn/Turkey0899/Turkey0899.html

3. “The Chi-Chi, Taiwan Earthquake of September 21, 1999”, EERI Special

Earthquake Report, Oct 1999, www.eeri.org/Reconn/Turkey0899/Turkey0899.html

4.“Earthquake Basics––Liquefaction, What it is and what to do about it”, Earthquake

Engineering Research Institute, www.eeri.org/eq_Basics/liq/LIQUEFAC.html

5. ” Response Analysis of the Higashid-kobe Bridge and Surrounding Soil in the 1995

Hyogokendnanbu Earthquake”, Todor Ganevi, Fumio Yamazaki, Hiroshi Ishizak and

Masahiko Kitazawa, ”Earthquake Engineering and Engineering Dynamics”, 557-

576,1998