Vinnie Hung.
Liquefaction as a Result of the
1999 Chi-Chi Earthquake
February 10th, 2012
Department of Civil & Environmental Eng.
Massachusetts Institute of Technology
Soil Liquefaction is a geotechnical phenomenon whereby
saturated soils lose strength and stiffness during ground
movement and in turn behave like a liquid. This ground
movement is usually caused by an earthquake and is most
commonly observed in saturated, loose, fine-grained
cohesionless soils. Compared to landslides, tsunamis, and other
hazards, liquefaction is less likely to cause conventional building
collapses and fatalities. Lateral spreading, on the other hand, is
the most destructive liquefaction related hazard for buildings.
This paper will focus on one particular earthquake, the 1999 ChiChi, Taiwan earthquake and its many liquefaction-induced
ground failures. Resulting in sand boils, lateral spreads, tilting
and settlement of buildings, and ground settlement, this was
one of the more detrimental earthquakes of Taiwanese history.
A Study of the Effects of Liquefaction as a
Result of the 1999 Chi-Chi Earthquake
Hung, V.
Department of Civil Engineering, Massachusetts Institute of Technology
1
Introduction
At 1:47am on the morning of September 21,
1999, the largest earthquake of Taiwan’s
recent history hit central Taiwan at 7.3
magnitude causing thousands of deaths,
building collapses, destruction to bridges,
highways, dams, and railways. Schools were
closed, power was cut, and people were
evacuated.
The Chi-Chi Earthquake (or 921 Earthquake as
it soon came to be known as) caused
widespread damage to all areas of Taiwan,
particularly in Yuanlin, Nantou, Wufeng,
2.1
Chang-Bin, as well as the Port of Taichung.
Most of the damages due to the earthquake
were from ground failure mechanisms such as
liquefaction and landslides.
Certain
geological
characteristics
of
the
aforementioned
locations,
such
as
hydraulically-filled reclaimed land and river
banks near alluvial deposits enhanced the
soil’s susceptibility to liquefaction. Damages
induced as a result of soil liquefaction include
settlement, tilting, lateral spreading, and
sinking of structures, lateral spreading being
one of the most severe.
Soil Liquefaction Phenomenon
Soil liquefaction is a phenomenon in which a
saturated soil substantially loses strength and
stiffness and the effective stress of soil is
reduced essentially to zero, in response to a
sudden applied stress such as the shaking
from an earthquake, causing the soil to
behave like a liquid. Liquefaction is most
often observed in reclaimed soils and in
saturated loose (uncompacted), fine sand or
coarse silt because they have a tendency to
compress when a load is applied, allowing the
water which initially filled their pores to flow
out from the soil and move upward towards
the ground surface.
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If the load is applied too rapidly or in too
large an amount, or repeatedly too many
times, such that it does not allow the water
within the pores to flow out in time before
the next cycle of load is applied, then the
water pressures may build up to a point
where they exceed the contact stresses
between the grains of soil that keep them in
contact with each other.
This grain
interaction is what transfers load from a
building to the different layers of soil and
rock, but an imbalance in this contact will
cause the soil to lose its structure and thus its
strength so that it may act like a liquid.
2.2
A soil in a saturated loose state and one
which may generate significant pore water
pressure is the most likely to liquefy. This is
because a loose soil has the tendency to
compress when sheared, generating a large
excess pore water pressure as load is
transferred from the soil skeleton to adjacent
pore water during undrained loading. As
pore water pressure rises, a progressive loss
of strength of the soil occurs and effective
stress is reduced.
Effects of Liquefaction as a Result of Earthquakes
When earthquakes occur, pore pressures are
produced via multiple cycles of shaking. This
may cause the liquefied sand and excess
water to force its way to the ground surface
from several meters below the ground,
observed as sand boils at the surface. Sand
boils are produced by both liquefied sand
below the ground surface moving upward as
well as non-liquefied sand above it due to
buoyancy. Another phenomenon observed as
a result of earthquake-induced liquefaction is
lateral spreading. Lateral spreading is the
ground failure via cracking and sliding
movement of unsupported ground, which
tends to slope toward sources of water.
Although compared to ground shaking,
landslides, and tsunamis, liquefaction is less
likely to cause the collapse of buildings; the
effects of soil liquefaction on the
environment can nevertheless be extremely
dangerous. Buildings whose foundations sit
on soil susceptible to liquefaction are in
danger of experiencing sudden loss of
support beneath part of their foundation,
which will result in drastic and irregular
settlement of the building. This will often
cause structural damage, ranging from
cracking foundations, to structural failure. It
is also possible that the building will tilt and
fall to one side without any structural
damage. In cases where there is a thin layer
of non-liquefied soil between the liquefied
soil and a building’s foundation, a foundation
failure may occur such that irregular
settlement in the ground combined with
upward pressure applied by liquefied soil may
result in breaks in service ducts and possible
water damage to the building and
underground utilities. Tanks and manholes
which are normally buried in the ground may
float due to the buoyancy forces present in
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the liquefied soil. Earth embankments such
as flood levees and earth dams may lose
stability or even collapse if their foundations
fail as a result of the soil liquefaction.
The effects of soil liquefaction may be
mitigated using various soil compaction
techniques including but not limited to vibrocompaction, dynamic compaction, and vibro
stone compaction. These methods result in
2.3
the densification of soil and enable buildings
to withstand soil liquefaction. Additional
methods involve ground or soil stabilization
by adding colloidal silica to the groundwater,
allowing it to flow through the soil and
replace the existing water in the pores. Over
time the colloidal silica will solidify and
thereby stiffen the soil so that it is less
susceptible to liquefaction.
Consequences of Liquefaction
These aforementioned types of ground failure
can happen on very shallow slopes and
potentially open up large cracks in the ground
often causing significant damage to buildings,
bridges, roads, and service utilities such as
water, natural gas, sewerage, power, and
telecommunications. As a result of these
roads and tunnels or bridges going out of
service and these utilities being cut, the city
suffers from significant economic losses in
addition to all of the damages in
infrastructure. Widespread damages like
these usually have multiple contributing
factors, some more so than others, which are
comparatively shown in Figure 1.
Figure 1: Causes of Earthquake Damage to Buildings
Note that both primary as well as secondary
causes are shown in these charts. The
primary cause of damages done to buildings
is due to ground shaking, while secondary
damages are predominantly caused by
liquefaction.
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Utility networks suffer significant damage
during earthquakes, thus endangering
lifelines in the city. Often gas and water
pipelines as well as sewer pipelines rupture
due to ground failure and different
mechanisms associated with earthquakes and
liquefaction.
Although they are often
unnoticed and thus unaccounted for, they
should be noted as potential dangers as a
result of different forms of ground failure.
The primary and secondary causes of such
damages are shown in Figure 2.
Figure 2: Causes of Lifeline Damages
Note that ground shaking and liquefaction
were the primary causes of damage to
lifelines and other utilities as well as the
3
secondary causes predominant among other
causes of damage.
Chi Chi Earthquake and its Effects
On September 21, 1999 at 1:47 am, millions
of people were woken from their sleep by an
earthquake which would soon be known as
one of the most severe earthquakes in
Taiwan’s history. It measured 7.3 magnitude
on the Richter scale at the epicenter, which
was at the Sun Moon Lake resort area near
Nantou, shown in Figure 3. The earthquake
had a focal depth of 8.0 km, causing
substantial damage in Taiwan. The Chi-Chi
earthquake amplified some existing problems
due to poor construction and many towns
suffered severe damages, including the
Yuanlin area, Nantou area, Wufeng area, and
Chang-Bin area.
The casualties as a result of the earthquake
were: 2,415 deaths, 29 missing, 11,305
severely wounded.
Additionally, 51,711
buildings were completely destroyed, 53,768
buildings were severely damaged, and
USD$9.7 billion worth of damages incurred.
There were a total of 132 landslides during
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and after the earthquake and its aftershocks.
Large parts of Taiwan lost power for weeks,
870 schools suffered damages, 125 severely
3.1
damaged, causing schools to close across the
nation, some closing permanently.
Yuanlin
Of the many areas affected by the Chi-chi
earthquake, Yuanlin is one of the most
severely damaged areas by liquefaction. It is
a town located approximately 15km from the
Chelengpu fault rupture, as shown in Figure 3.
Figure 3: Map of Liquefaction Sites in the Chi-Chi Earthquake
It sits on a thick alluvial deposit, which
overlies older sedimentary deposits. The
ground water table is quite shallow, ranging
from about 0.5 to 4.0 meters below the
ground surface, and there exists layers of very
loose, sandy soils that are susceptible to
liquefaction. Fortunately, these liquefiable
sandy layers are often capped by a thick layer
of clayey soils, which reduces the damages to
buildings. See
Figure 4: Simplified Soil Profile at Yuanlin
for a simplified soil profile.
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Figure 4: Simplified Soil Profile at Yuanlin
Among the 39 districts in Yuanlin, 17 were
found to have suffered significant settlement
and 8 were found to have evident
liquefaction signs in the form of sand boils.
Ground motions were recorded in the Yuanlin
3.2
area with horizontal peak ground acceleration
(PGA) of approximately 0.19g. Effects of
liquefaction observed at Yuanlin include
surface sand boils, deformed roadways, and
severe building settlement and/or tilting.
Nantou
Nantou is located in the Taichung Basin,
which is approximately 5-10km away from
the fault rupture, consisting of alluvial
sediments and deposits with a shallow
ground water table approximately 2 to 5
meters below ground surface. Deposits in the
area are mainly composed of cemented or
poorly cemented clay, silty sand, sand and
gravel. The medium-dense layer of silty sand
and sandy silt approximately 4-8m below the
ground surface is most susceptible to
liquefaction. See Figure 5 for a simplified soil
profile. Ground motions were recorded in
the area with a PGA of approximately 0.43g.
Liquefaction effects were in the form of sand
boils, building subsidence, and lateral
spreading.
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Figure 5: Simplified Soil Profile at Nantou
3.3
Wufeng
Wufeng is located 26km north of the
epicenter and within 1 km of the Chelengpu
fault rupture, mainly covered by an alluvial
plain consisting of silts, sands, gravels, and
cobbles near the surface, and stratified rock
formations at shallow depths. These shallow
layers of silty sands are susceptible to
liquefaction and responsible for much of the
effects of liquefaction at the surface. The
ground water table is approximately 0.5 to
5.0 meters below the ground surface. See
Figure 6 for a simplified soil profile. Ground
motion measured maximum horizontal PGA
as 0.79g. Effects of liquefaction observed at
Wufeng include sand boils, building
subsidence, tilting and failure, as well as
lateral spreading.
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Figure 6: Simplified Soil Profile at Wufeng
3.4
Chang-Bin
Chang-Bin is located 47 km away from the
epicenter of the earthquake and 25km from
the fault rupture. It is in Changhwa, which is
a major hydraulically filled, reclaimed land
area. Within the fill layer and upper layers of
the soil, the layers generally alternated
between silty sand and silty and sandy clays.
Within the top 10 meters below the surface,
soils mainly consisted of loose to mediumdense fine silty sands and poorly graded
sands with 8-20% fines content. The ground
water table is at a depth of approximately 1.0
to 2.6 meters below the surface. See Figure 7
for a simplified soil profile. Ground motion
measured
horizontal
peak
ground
acceleration as approximately 0.12g. Ground
improvement work was ongoing at the time
of the earthquake, thus the effects of
liquefaction were only observed at some
locations where ground improvement work
had not yet been completed.
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Figure 7: Simplified Soil Profile at Chang-Bin
3.5
Observed Liquefaction Problems
Liquefaction occurred at various locations
where loose cohesionless soils and shallow
groundwater existed. Extensive liquefaction
was observed at port facilities near Taichung
harbor, as shown in Figure 8, which is on the
western coast of Taiwan. Figure 9 shows
lateral deformation as a result of liquefaction
at the port facilities. In addition to noncoastal regions and harbors, liquefaction was
also observed near rivers and other bodies of
water.
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Figure 8: Plan view showing lateral displacements at Taichung Harbor
Figure 9: Aerial view of piers of Taichung Harbor post-earthquake
In general, buildings with fewer than three
stories did not show much settlement and
tilting, while buildings with three stories or
more had a tendency to show significant
settlement or tilting. Additionally, buildings
with multiple stories below ground showed
almost no settlement, primarily because the
majority of liquefiable soils were excavated in
order to construct the deeper basement
levels. Figure 10 shows a photo of up-heaving
and breaking floor slabs due to excessive
settlement of the building, as a result of
liquefaction.
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Figure 10: Up-heaving and breaking of floor slabs due to settlement
induced by liquefaction of the soils beneath
the complex. The building shown in Figure 11
was likely supported by a shallow mat
foundation.
Figure 11: Liquefaction-Induced Bearing Capacity Failure of
a Residential Complex
Figure 11 shows a photo of a residential
building that underwent non-uniform tilting.
The tilting was caused by bearing capacity
failure, the consequence of excessive
settlement. This excessive settlement was
Figure 12 is a photo of a technical school
building located near a river, which has also
failed due to bearing capacity failure as a
result of liquefaction.
The liquefaction
caused some lateral spreading toward the
river, and caused the building to settle
irregularly, inducing a permanent tilt toward
the river. As such, one could argue that
although only shallow foundations were
needed to support the weight of the building,
deeper foundations such as piled foundations
would have prevented much of the failures
and damages that resulted from the
earthquake.
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After the earthquake, while remediation
efforts were ongoing, large scale field
reconnaissance programs were put into
effect. Soil samples were taken from multiple
affected areas and analyzed, so that more
could be learned about the soil conditions
and the possibilities of liquefaction in the
future.
For the Yuanlin area alone, 50 boreholes, 45
CPT holes, 6 sets of cross-hole seismic
surveys, and 4 sets of reflective seismic
surveys were conducted in order to identify
zones which were potentially vulnerable to
liquefaction. The locations and some geologic
data from these holes are indicated in Figure
13 and Figure 14, respectively.
Figure 12: Liquefaction-Induced Bearing Capacity
Failure of a Technical School Building
Figure 13: Locations of geologic exploration holes in Yuanlin
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Figure 14: Geologic profile for Yuanlin area
Note that in Figure 14, the liquefiable layer is
indicated by the dotted line drawn across the
borehole logs.
There are multiple numerical methods to
determine the probability of liquefaction
from CPT data, which use similar underlying
concepts. These methods include the Olsen
Method (1997), the Robertson Method
(1998), and method of Juang et al. (2003),
which is a modification of the Robertson
Method. For the sake of being concise, and
the fact that the purpose of this paper is not
to form a comparison of the different analysis
methods, these methods will not be
described herein. Further information on
these methods may be found under
references.
Using CPT data obtained at a number of sites
where
liquefaction
was
observed,
assessments of liquefaction potential were
analyzed and cone tip resistance (qc), friction
ratio (Rf), soil type index (Ic), and probability
of liquefaction (PL) were presented with
respect to depth as shown in Figure 15.
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Figure 15: Profile of CPT data from an area west of downtown Yuanlin
It should be noted that this data was taken
from an area in Yuanlin, which experienced
the most widespread liquefaction-induced
ground failure during the Chi-chi earthquake.
It should also be noted that using the
database of collected data, the highest value
of soil type index for liquefiable soil was 2.8,
at which point according to Baez et al (2000).
fines content is equal to 100% if the soil type
index exceeds 2.82. When the soil is filled
with so many fines, it is classified as “claylike” and too “clay-rich” to be liquefied.
Notice that the depths with the highest
probability of liquefaction have significantly
lower soil index numbers as well as friction
ratios.
Also note that the thickness of the nonliquefiable layer (H1) is identified as well as
the upper liquefiable layer (H2). The thin
layer of the non-liquefiable layer as compared
to that of the liquefiable layer also
contributes to the reason for such
widespread ground failure at this site.
Further analysis of the ground postearthquake was conducted by another team
of engineers, which involved measuring the
shear wave velocity through a particular part
of Yuanlin. The results are shown in Figure
16.
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Figure 16: SCPT Sounding Profile at Yuanlin
Note that in this particular situation, a low
shear wave velocity is observed within the
layer of about 3 and 10 meters of depth from
ground surface. The low shaft friction and
cone penetration resistance, as well as low
3.6
shear wave velocity in this area suggests that
there are loose soils and sandy soils present.
This makes the area susceptible to
liquefaction ground failure during an
earthquake.
Further Information
As presented by Chih-Sheng Ku (2004), a
graphical representation of borehole data
which was collected is shown in Figure 17 by
plotting the normalized cone resistance
against the cyclic stress ratio scaled to an
earthquake magnitude of 7.5. This plot
further indicates that areas with lower cone
resistance values were more likely to
encounter liquefaction, while those which
exceeded the critical CSR boundary had not
experienced any liquefaction resulting from
the earthquake.
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Figure 17: CPT-based liquefaction boundary curve
4
Conclusion
The Chi-Chi Earthquake in Taiwan in 1999 was
one of the most destructive earthquakes
Taiwan has had to encounter in its many
years of history. It was a high intensity
earthquake of magnitude 7.3 on the Richter
scale with a focal depth of 8km below ground
surface and was of longer duration than most
earthquakes. Widespread damage ensued
after the earthquake and aftershocks had
subsided and Taiwan began evaluating its
damages.
One of the main destructive phenomenon as
a lingering result of the earthquake was soil
liquefaction. Soil liquefaction was present in
many areas, predominantly in the Central
regions of Taiwan, including Yuanlin,
Taichung, and Nantou. A commonality of
many of these areas was that they were
towns sitting on hydraulically-filled reclaimed
land. CPT data collected from these areas
showed that generally areas with low cone
resistance, low shaft friction, low shear wave
velocity, and a low factor of safety were
susceptible to soil liquefaction.
These areas also generally had soils with little
fines and no cohesion. In certain areas where
there was a capping layer of non-liquefiable
soil, there were usually more fines and thus
more clay-like structure.
As a result of the soil liquefaction, many
buildings were damaged. The damage was
not to the structure itself, but to its
foundation. The soil beneath the foundation,
if it was liquefied, lost most of its strength
and resulted in non-uniform settlement and
therefore tilting of the structure. As it was
concluded by Hwang et al., on liquefied
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horizontal ground, settlement and tilting of
the building generally increased with the
number of stories in the building while on
liquefied inclined ground, lateral spreading of
the top non-liquefied layer could occur due to
very small topographical differences. As such,
the buildings which had multiple basement
levels or which had less than three stories
above ground observed little to no settlement
or tilting. In light of this information, one
could argue that when designing buildings
with three or more stories, it would be
worthwhile to also have deeper basement
levels to remove some of the potentially
liquefiable soils beneath the otherwise
shallow foundation.
earthquake and soil liquefaction, the damage
is not only superficial. There are also other
consequences as a result of liquefaction,
namely damage to utilities and lifelines, the
loss of businesses and houses, all of which
could cripple the economy for a long period
of time.
The loss of power lines would
disable areas that may not have sustained
structural damage simply because they rely
on the continuity of the nation’s
infrastructure, which was discontinued by the
earthquake. The social and economic losses
that the country endures in the aftermath of
a natural disaster as well as the costs needed
in order to rebuild the nation are also matters
not to be overlooked.
Although structural damage is the most
immediately recognizable consequence of an
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