Introduction to Soil Liquefaction
This introduction to soil liquefaction was created by Jörgen Johansson with the
University of Washington’s Civil Engineering Department and adapted for use by
the MAST Laboratory. Supported in part by the National Science Foundation
Grant No. DUE-9950340.
http://www.ce.washington.edu/~liquefaction/html/main.html
Liquefaction Failures in 1964 Nigata Earthquake SC)
What is soil liquefaction?
Liquefaction is a phenomenon in which the strength and stiffness of a soil is reduced by earthquake shaking or other
rapid loading. Liquefaction and related phenomena have been responsible for tremendous amounts of damage in
historical earthquakes around the world.
Liquefaction occurs in saturated soils, that is, soils in which the space between individual particles is completely filled
with water. This water exerts a pressure on the soil particles that influences how tightly the particles themselves are
pressed together. Prior to an earthquake, the water pressure is relatively low. However, earthquake shaking can cause
the water pressure to increase to the point where the soil particles can readily move with respect to each other.
Earthquake shaking often triggers this increase in water pressure, but construction-related activities such as blasting
can also cause an increase in water pressure. When liquefaction occurs, the strength of the soil decreases and, the
ability of a soil deposit to support foundations for buildings and bridges is reduced as seen in the photo of the
overturned apartment complex buildings in Niigata in 1964. The type of ground failure shown above can be simulated
in the laboratory.
Liquefied soil also exerts higher pressure on retaining walls, which can cause
them to tilt or slide. This movement can cause settlement of the retained soil and
destruction of structures on the ground surface. GH
Increased water pressure can also trigger landslides and cause the collapse of
dams. Lower San Fernando dam suffered an underwater slide during the San
Fernando earthquake, 1971. Fortunately, the dam barely avoided collapse,
thereby preventing a potential disaster of flooding of the heavily populated areas
below the dam. SC
When does soil liquefaction occur?
Liquefaction has been observed in earthquakes for many years. In fact, written records dating back hundreds and even
thousands of years describe earthquake effects that are now known to be associated with liquefaction. Nevertheless,
liquefaction has been so widespread in a number of recent earthquakes that it is often associated with them. Some of
those earthquakes are listed on the following page.
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Alaska, USA, 1964 (SC)
Lome Prieta, USA, 1989 (USGS)
Kobe, Japan, 1995 (GH)
Where does soil liquefaction occur?
Because liquefaction only occurs in saturated soil, its effects are most
commonly observed in low-lying areas near bodies of water such as rivers,
lakes, bays, and oceans. The effects of liquefaction may include major sliding
of soil toward the body slumping and of water, as in the case of the 1957 Lake
Merced slide shown to the right, or more modest movements that produce
tension cracks such as those on the banks of the Motagua River following the
1976 Guatemala Earthquake.
Johannson (Above and Below)
Port and wharf facilities are often located in areas susceptible to liquefaction,
and many have been damaged by liquefaction in past earthquakes. Most ports
and wharves have major retaining structures, or quay walls, to allow large ships
to moor adjacent to flat cargo handling areas. When the soil behind and/or
beneath such a wall liquefies, the pressure it exerts on the wall can increase
greatly, enough to cause the wall to slide and/or tilt toward the water. As
shown below, liquefaction caused major damage to port facilities in Kobe,
Japan in the 1995 Hyogo-ken Nanbu earthquake.
Retaining wall damage and lateral
spreading, Kobe 1995 (KG)
Lateral displacement of a quay
wall on Port Island, Kobe 1995
(KG)
Lateral spreading caused 1.2-2
meter drop of paved surface and
local flooding, Kobe 1995. (KG)
Liquefaction also frequently causes damage to bridges that cross rivers and other bodies of water. Such damage can
have drastic consequences, impeding emergency response and rescue operations in the short term and causing
significant economic loss from business disruption in the longer term. Liquefaction-induced soil movements can push
foundations out of place to the point where bridge spans loose support (bottom left) or are compressed to the point of
buckling (bottom right).
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Why does soil liquefaction occur?
To understand liquefaction, it is important to recognize the conditions that exist in a soil deposit before an earthquake.
A soil deposit consists of an assemblage of individual soil particles. If we look closely at these particles, we can see that
each particle is in contact with a number of neighboring particles. The weight of the overlying soil particles produce
contact forces between the particles - these forces hold individual particles in place and give the soil its strength.
The length of the arrows
represent the size of the
contact forces between
individual soil grains. The
contact forces are large when
the porewater pressure is low.
Soil grains in a soil deposit.
The height of the blue
column to the right
represents the level of
porewater pressure in the
soil.
Liquefaction occurs when the structure of a loose, saturated sand breaks down due to some rapidly applied loading. As
the structure breaks down, the loosely-packed individual soil particles attempt to move into a denser configuration. In
an earthquake, however, there is not enough time for the water in the pores of the soil to be squeezed out. Instead, the
water is "trapped" and prevents the soil particles from moving closer together. [Increased water pressure is caused by
the soil particles trying to rearrange and pushing on the water. The increased water pressure reduces the contact forces
between the individual soil particles, thereby softening and weakening the soil deposit.]
Observe how small the contact forces are because of the high water pressure. In an extreme case,
the porewater pressure may become so high that many of the soil particles lose contact with each
other. In such cases, the soil will have very little strength, and will behave more like a liquid than a
solid - hence, the name "liquefaction".
How can soil liquefaction hazards be reduced?
There are basically three possibilities to reduce liquefaction hazards when designing and constructing new buildings or
other structures as bridges, tunnels, and roads.
Avoid Liquefaction Susceptible Soils
The first possibility is to avoid construction on liquefaction susceptible soils. There are various criteria to determine the
liquefaction susceptibility of a soil. By characterizing the soil at a particular building site according to these criteria one
can decide if the site is susceptible to liquefaction and therefore unsuitable for the desired structure.
Build Liquefaction Resistant Structures
If it is necessary to construct on liquefaction susceptible soil because of space restrictions, favorable location, or other
reasons, it may be possible to make the structure liquefaction resistant by designing the foundation elements to resist the
effects of liquefaction.
Improve the Soil
The third option involves mitigation of the liquefaction hazards by improving the strength, density, and/or drainage
characteristics of the soil. This can be done using a variety of soil improvement techniques.
Some things to take away…
1. The shaking from an earthquake can cause wet soil (with relative uniform particles) to momentarily
“liquefy”—turn to quicksand—lose strength and behave like a fluid.
2. Reclaimed land (e.g., where soil is brought out into the sea to create more land) might be more susceptible to
liquefaction.
Photos:
United States Geological Survey. (USGS)
Hayward Baker, Inc., Odenton, Maryland. (HB)
Earthquake Engineering Research Center, University of California, Berkeley:
Karl V. Steinbrugge Collection (SC), Kobe Geotechnical Collection (KG), Great Hanshin Bridge Collection(GH), and Loma Prieta Collection (LP)
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