Geotechnical Earthquake Engineering

Geotechnical
Earthquake Engineering
25.1 Introduction
25.2 Earthquake Strong Shaking
25.3 Site-Specific Amplification
Illustrative Site Response Problem
25.4 Soil Liquefaction
Illustrative Soil Liquefaction Problem
25.5 Seismic Slope Stability
25.6 Summary
25.1 Introduction
Geotechnical factors often exert a major influence on damage patterns and loss of life in earthquake
events. For example, the localized patterns of heavy damage during the 1985 Mexico City and 1989 Loma
Prieta, California, earthquakes provide grave illustrations of the importance of understanding the seismic
response of deep clay deposits and loose, saturated sand deposits. The near failure of the Lower San
Fernando dam in 1971 due to liquefaction of the upstream shell materials is another grave reminder that
we must strive to understand the seismic response of critical earth structures. The characteristics and
distribution of earth materials at a project site significantly influence the characteristics of the earthquake
ground motions, and hence significantly influence the seismic response of the constructed facilities at a
site. Moreover, the composition and geometry of earth structures, such as earth dams and solid waste
landfills, significantly affect their seismic response. Geotechnical considerations therefore play an integral
role in the development of sound earthquake-resistant designs. In this chapter, geotechnical earthquake
engineering phenomena such as site-specific amplification, soil liquefaction, and seismic slope stability
are discussed. Case histories are used to illustrate how earthquakes affect engineered systems, and established,
simplified empirical procedures that assist engineers in assessing the effects of these phenomena
are presented. The field of earthquake engineering is quite complex, so the need for exercising engineering
judgment based on appropriate experience is emphasized.
25.2 Earthquake Strong Shaking
The development and transmission of earthquake energy through the underlying geology is quite complex,
and a site-specific seismic response study requires an assessment of the primary factors influencing
the ground motion characteristics at a site. They are
• Earthquake source mechanism
• Travel path geology
Jonathan D. Bray
University of California at Berkeley
25
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The Civil Engineering Handbook, Second Edition
• Topographic effects
• Earthquake magnitude
• Distance from zone of energy release
• Local soil conditions
Earthquakes are produced in a particular geologic setting due to specific physical processes. A midplate
earthquake (e.g., New Madrid) will differ from a plate margin earthquake (e.g., San Andreas) [see Nuttli,
1982]. The principal descriptive qualities of the earthquake source are the type of fault displacement
(strike-slip, normal, or reverse), depth of the rupture, length of the rupture, and duration of the rupturing.
The characteristics of the rock which the seismic waves travel through influence the frequency content
of the seismic energy. Significant topographic features (e.g., basins) can focus and hence amplify earthquake
motions. The
magnitude
of an earthquake is related to the amount of energy released during the
event. The difference between earthquakes of different magnitudes is significant; for example, a magnitude
7 earthquake event releases nearly a thousand times more energy than a magnitude 5 event. The potential
for seismic damage will typically increase with earthquake events of greater magnitude. Seismic energy
attenuates as it travels away from the zone of energy release and spreads out over a greater volume of
material. Hence, the intensity of the bedrock motion will typically decrease as the distance of a particular
site from the zone of energy release increases. A number of
attenuation relationships
based on earthquake
magnitude and distance from the earthquake fault rupture are available [e.g., Nuttli and Herrmann, 1984;
Joyner and Boore, 1988; Idriss, 1991]. Local soil conditions may significantly amplify ground shaking,
and some soil deposits may undergo severe strength loss resulting in ground failure during earthquake
shaking. The last three factors listed above (magnitude, distance, local soil conditions) are usually the
most important factors, and most seismic studies focus on these factors.
There are several earthquake magnitude scales, so it is important to use these scales consistently. The
earliest magnitude scale, local magnitude (
M
L
), was developed by Richter [1935] and is defined as the
logarithm of the maximum amplitude on a Wood-Anderson torsion seismogram located at a distance
of 100 km from the earthquake source [Richter, 1958]. Other related magnitude scales include surface
wave magnitude,
M
s
, and body wave magnitudes,
m
b
and
m
B
[Gutenberg and Richter, 1956]. These
magnitude scales are based on measurement of the amplitude of the seismic wave at different periods
(
M
L
at 0.8 s,
m
b
and
m
B
between 1 s and 5 s, and
M
s
at 20 s), and hence they are not equivalent. The
moment magnitude,
M
w
, is different from these other magnitude scales because it is directly related to
the dimensions and characteristics of the fault rupture. Moment magnitude is defined as
(25.1)
where
M
0
is the seismic moment in dyne-cm, with
M
0
=
m
·
A
f
·
D
;
m
= shear modulus of material along
the fault plane (typically 3
¥
10
11
dyne/cm
2
),
A
f
= area of fault rupture in cm
2
, and
D
= average slip over
the fault rupture in cm [Hanks and Kanamori, 1979]. Heaton et al. [1982] has shown that these magnitude
scales are roughly equivalent up to
M
w
= 6, but that magnitude scales other than
M
w
reach limiting values
for higher moment magnitude earthquake events (i.e., max
m
b
ª
6, max
M
L
ª
7, max
m
B
ª
7.5, and
max
M
s
ª
8). Thus, the use of moment magnitude is preferred, but the engineer must use the appropriate
magnitude scale in available correlations between engineering parameters and earthquake magnitude.
The earthquake motion characteristics of engineering importance are
• Intensity
• Frequency content
• Duration
The intensity of ground shaking is usually portrayed by the
maximum horizontal ground acceleration
(MHA)
, but since velocity is a better indicator of the earthquake energy that must be dissipated by an
engineered system, it should be used as well. The MHA developed from a site-specific seismicity study
Mw
2
3
= -- log M0 – 10.7
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Geotechnical Earthquake Engineering
25
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should be compared to that presented in the U.S. Geological Survey maps prepared by Algermissen et al.
[1991] and the seismic zone factor (
Z
) given in the Uniform Building Code. The frequency content of
the ground motions is typically characterized by its
predominant period (T
p
)
. The predominant period
of the ground motions at a site tends to increase with higher magnitude events and with greater distances
from the zone of energy release [see Idriss, 1991]. Earthquake rock motions with a concentration of
energy near the fundamental periods of the overlying soil deposit and structure have a greater potential
for producing amplified shaking and seismic damage. Lastly, the
duration of strong shaking
is related
to the earthquake’s magnitude and is typically described by the duration of the earthquake record in
which the intensity is sufficiently high to be of engineering importance (i.e., MHA exceeding around
0.05
g
) or the equivalent number of cycles of strong shaking [see Seed and Idriss, 1982].
A seismic hazard assessment generally involves those items listed in Fig. 25.1. Earthquake engineering
is a multidisciplinary field that requires a coordinated effort. The success of the geotechnical evaluation
depends greatly on the results of the geological and seismological evaluation, and the results of the
geotechnical evaluation must be compatible with the requirements of the structural design.
25.3 Site-Specific Amplification
The localized patterns of heavy damage during the 1989 Loma Prieta earthquake in northern California
demonstrate the importance of understanding the seismic response of deep soil deposits. Well over half
of the economic damage and more than 80% of the loss of life occurred on considerably less than 1% of
the land within 80 km of the fault rupture zone largely as a result of site-specific effects [Seed et al., 1990].
For example, in the Oakland area, which is 70 km away from the rupture zone, maximum horizontal
ground accelerations were amplified by a factor of 2 to 4 and spectral accelerations at some frequencies
were amplified by a factor of 3 to 8 [Bray et al., 1992]. The dramatic collapse of the elevated highway I-
880 structure, in which 38 people died, is attributed in part to these amplified strong motions [Hough
et al., 1990]. Hundreds of buildings in the San Francisco Bay area sustained significant damage because
of earthquake strong shaking. These observations are critical to many cities as deep soil deposits exist in
many earthquake-prone areas around the world. For example, records of the January 31, 1986 northeastern
Ohio earthquake suggest that similar site-specific amplification effects could occur in the central U.S. and
produce heavy damage during a major event in the New Madrid seismic zone [Nuttli, 1987].
Response spectra are typically used to portray the characteristics of the earthquake shaking at a site.
The
response spectrum
shows the maximum response induced by the ground motions in damped singledegree-
of-freedom structures of different
fundamental periods
. Each structure has a unique fundamental
period at which the structure tends to vibrate when it is allowed to vibrate freely without any external
excitation. The response spectrum indicates how a particular structure with its inherent fundamental
period would respond to the earthquake ground motion. For example, referring to Fig. 25.2, a low-period
structure (say,
T
= 0.1 s) at the SCT building site would experience a maximum acceleration of 0.14
g
,
whereas a higher-period structure (say,
T
= 2.0 s) at the SCT site would experience a maximum acceleration
of 0.74
g
for the same ground motions.
The response spectra shown in Fig. 25.2 illustrate the pronounced influence of local soil conditions
on the characteristics of the observed earthquake ground motions. Since Mexico City was located approximately
400 km away from the earthquake’s epicenter, the observed response at rock and hard soil sites
FIGURE 25.1
Seismic hazard assessment.
Geologic and
Seismologic Evaluation
• Identify Seismic Sources
• Potential for Surface Rupture
• Size and Frequency of Events
• Develop Rock Motions
• Site Response
• Liquefaction Potential
• Seismic Stability
• Soil-Structure Interaction
• Dynamic Analysis
- Pseudo-static
- Time History
• Design Considerations
Geotechnical Evaluation Structural Design
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25
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The Civil Engineering Handbook, Second Edition
was fairly low (i.e., the spectral accelerations were less than 0.1
g
at all periods). Damage was correspondingly
negligible at these sites. At the Central Market site (CAO), spectral accelerations were significantly
amplified at periods of around 1.3 s and within the range of 3.5 s to 4.5 s. Since buildings at the CAO site
did not generally have fundamental periods within these ranges, damage was fairly minor. The motion
recorded at the SCT building site, however, indicated significant amplification of the underlying bedrock
motions with a maximum horizontal ground acceleration (the spectral acceleration at a period of zero)
over four times that of the rock and hard soil sites and with a spectral acceleration at
T
= 2.0 s over seven
times that of the rock and hard soil sites. Major damage, including collapse, occurred to structures with
fundamental periods ranging from about 1 s to 2 s near the SCT building site and in areas with similar
subsurface conditions. At these locations, the soil deposit’s fundamental period
matched
that of the overlying
structures, creating a resonance condition that amplified strong shaking and caused heavy damage.
The 1991 Uniform Building Code (UBC) utilizes site coefficients in its pseudostatic design base shear
procedure to limit damage due to local soil conditions (see Table 25.1). For example, the site coefficient
for soil characteristics (
S
factor) is increased to 2.0 for the soft soil profile
S
4
, and an
S
factor of 1.0 is
used at rock sites where no soil-induced amplification occurs. However, a deposit of stiff clay greater
than 61 m thick, such as those that underlay Oakland, would be categorized as soil profile
S
2
with an
S
factor of only 1.2. The seismic response of the deep stiff clay sites during the 1989 Loma Prieta
earthquake with spectral amplification factors on the order of 3 to 8 suggest that we may be currently
underestimating the seismic hazard at these sites. Earthquake engineering is a relatively young field of
study, and additional research is required to support the evolution of safer building codes.
The UBC also allows dynamic analyses of structural systems and provides the normalized response
spectra shown in Fig. 25.3. The spectral acceleration of a structure can be estimated from this figure
given an estimate of the system’s fundamental period (
T
), the peak ground acceleration (MHA) of the
design event, and the classification of the subsurface soil conditions. At longer periods (
T
> 0.5 s), the
spectral accelerations for deep soil sites (soil type 2) and soft soil sites (soil type 3) are significantly higher
than that for rock and stiff soils (soil type 1). The engineer can also use wave propagation analyses [e.g.,
SHAKE91; see Idriss and Sun, 1992] to develop a site-specific design response spectrum based on the
FIGURE 25.2
Acceleration response spectra for motions recorded in Mexico City during the 1985 Mexico City
earthquake (after Seed, H. B., Romo, M. P., Sun, J., Jaime, A., and Lysmer, J. 1987. Relationships between Soil
Conditions and Earthquake Ground Motions in Mexico City in the Earthquake of Sept. 19, 1985. Earthquake
Engineering Research Center, Report No. UCB/EERC-87/15, University of California, Berkeley).
0.9
0.8
0.7
0.6
0.5
0.4
SCT Site
(depth to hard layer, D ≈ 37m)
CAO Site
(D ≈ 58m)
5% Damping
Rock and
hard sail
0.3
0.2
0.1
0
0 1 2 3 4 5
Period-seconds
Spectral Acceleration-g
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Geotechnical Earthquake Engineering
25
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geologic, seismologic, and soil characteristics associated with the project site. The seismic response of
earth materials is dictated primarily by geometric considerations and by the soil’s dynamic properties
(e.g., shear modulus and damping characteristics). The shear modulus gives an indication of the stiffness
of the soil system, whereas the
damping ratio
provides a measure of the soil system’s ability to dissipate
energy under cyclic loading. Soils exhibit strain-dependent dynamic properties so that as earthquake
strong shaking increases and the strain induced in the soil increases, the material’s damping ratio increases
and its shear modulus decreases [see Seed et al., 1984; Vucetic and Dobry, 1991]. As material damping
increases, the soil-induced amplification tends to decrease. As the material stiffness decreases, however,
the fundamental period of the soil system increases, and this may affect the amplification of higherperiod
motions.
TABLE 25.1
1991 UBC Site Coefficients
a,b
Type Description
S
factor
S
1
A soil profile with either (a) a rock-like material characterized by a shear wave velocity greater than
762 mps or by other suitable means of classification, or (b) stiff or dense soil condition where the
soil depth is less than 61 m
1.0
S
2
A soil profile with dense or stiff soil conditions where the soil depth exceeds 61 m 1.2
S
3
A soil profile 21 m or more in depth and containing more than 6 m of soft to medium stiff clay but
not more than 12 m of soft clay
1.5
S
4
A soil profile containing more than 12 m of soft clay characterized by a shear wave velocity less than
152 mps
2.0
a
The site factor shall be established from properly substantiated geotechnical data. In location where the soil properties
are not known in sufficient detail to determine the soil profile type, soil profile
S
1
shall be used. Soil profile
S
4
need not
be assumed unless the building official determines that soil profile
S
4
may be present at the site, or in the event soil profile
S
4 is established by geotechnical data.
b The total design base shear (V) is determined from the formula V = Z · I · C · W/Rw, where C = 1.225S/T2/3 £ 2.75.
Z = seismic zone factor, I = importance factor, S = site coefficient, T = fundamental period of structure, W = total seismic
dead load, and Rw = reduction coefficient based on the lateral load-resisting system (see UBC). Hence V μ S if C < 2.75.
FIGURE 25.3 1991 UBC normalized acceleration response spectra.
4.00
3.00
2.00
1.00
0.00
0.00 1.00
PERIOD, T
(Seconds)
SOFT TO MEDIUM CLAYS AND
SANDS (SOIL TYPE 3)
DEEP COHESIONLESS OR STIFF
CLAY SOILS (SOIL TYPE 2)
ROCK AND STIFF SOILS
(SOIL TYPE 1)
SPECTRAL ACCELERATION/
EFFECTIVE PEAK GROUND ACCELERATION
2.00 3.00
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25-6 The Civil Engineering Handbook, Second Edition
The amplification of higher-period ground motions that may match the fundamental period of the
building located at the site is one of the most critical concerns in seismic site response studies. If the
building’s fundamental period is close to that of the site, an earthquake with a concentration of energy
around this period would have the potential to produce heavy damage to this structure. The matching
of the building and site fundamental periods creates a resonant condition that can amplify shaking.
Consideration of a different structural system whose fundamental period does not match that of the
underlying soil deposit might be prudent. Otherwise, the design criteria should be more stringent to
limit damage to the building during an earthquake event.
Illustrative Site Response Problem
Problem 25.1
An eight-story building will be constructed at a deep soil site
in Tennessee. As shown in Fig. 25.4, the deep soil site contains
surficial deposits of loose, saturated sand overlying a thick clay
deposit. The design earthquake is a magnitude (mB) 7.5 event
occurring at a distance of 130 km from the site. Develop a
preliminary estimate of the maximum horizontal acceleration
(MHA) at the site and evaluate the potential for soil-induced
amplification of earthquake shaking near the building’s fundamental
period.
Earthquake Strong Shaking
Limited data is available to develop attenuation curves for the
deeper-focus eastern-U.S. earthquakes, and hence this is an
area of ongoing research. Nuttli and Herrmann [1984] proposed
the attenuation curve shown in Fig. 25.5 for earthquakes
likely to occur in the eastern and central U.S. For a site
located 130 km from the zone of energy release for an mB =
7.5 event, the bedrock MHA would be on the order of 0.1g.
This magnitude of MHA is comparable with what established
building codes (e.g., BOCA, SBCCI, and UBC) would recommend
for the central part of the state of Tennessee. At this
distance (130 km), a magnitude 7.5 earthquake would tend to
produce bedrock strong shaking with a predominant period
within the range of 0.4 s to 0.7 s [Idriss, 1991].
Site-Specific Amplification
Comparisons between MHA recorded at deep clay soil sites and those recorded at rock sites indicate that
MHA at deep clay soil sites can be 1 to 3 times greater than those at rock sites when MHArock ª 0.1g
[Bray et al., 1992; Idriss, 1991]. The results of one-dimensional (columnar) dynamic response analyses
also suggest MHA amplification factors on the order of 1 to 3 for deep clay sites at low values of MHArock.
A proposed relationship between the MHA at soft clay sites and the MHA at rock sites is shown in
Fig. 25.6. The estimated rock site MHA value of 0.1g would be increased to 0.2g for this deep clay site
using the average site amplification factor of 2.
The fundamental period of typical buildings can be estimated using the 1991 UBC formula: T = Ct ·
(hn)3/4, where T = building’s fundamental period (s), Ct = structural coefficient = 0.020 for a typical
building, and hn = height of the building (ft). A rough estimate of a level site’s fundamental period can
be calculated by the formula Ts = 4D/Vs, where Ts = the site’s fundamental period, D = the soil thickness,
and Vs = the soil’s average shear wave velocity. Computer programs such as SHAKE91 can calculate the
site’s fundamental period as well as calculate horizontal acceleration and shear stress time histories
throughout the soil profile.
FIGURE 25.4 Subsurface conditions of
project site discussed in Problem 25.1.
0 m
3 m
6 m
12 m
18 m
27 m
39 m
54 m
BEDROCK
Vs = 1250 m/s
VERY STIFF CLAY
Vs = 480 m/s
STIFF CLAY
Vs = 330 m/s
STIFF CLAY
Vs = 250 m/s
STIFF CLAY
Vs = 180 m/s
CLEAN SAND
N1 = 8-12; Vs = 150 m/s
MEDIUM STIFF CLAY
Vs = 130 m/s
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Geotechnical Earthquake Engineering 25-7
Assuming a story height of 13 feet (4 m), the eight-story building would be 104 feet (32 m) high,
and T = 0.020 · (104 ft)3/4 = 0.65 s. The soil deposit’s average initial shear wave velocity is estimated to
be Vs = [(150 m/s)(6 m) + (130 m/s)(6 m) + (180 m/s)(6 m) + (250 m/s)(9 m) + (330 m/s)(12 m) +
(480 m/s)(15 m)]/54 m = 300 m/s. The site’s fundamental period at low levels of shaking is then
approximately Ts = 4 · (54 m)/300 m/s = 0.72 s. As a check, SHAKE91 analyses calculate the site’s
fundamental period to be 0.71 seconds, which is in close agreement with the first estimate (Ts = 0.72 s).
Since the building’s fundamental period (T = 0.65 s) is close to that of the site (Ts = 0.72 s), an
earthquake with a concentration of energy around 0.6 to 0.7 s would have the potential to produce heavy
damage to this structure. In fact, SHAKE91 results indicate a maximum spectral acceleration amplification
factor of almost 15 at a period around 0.7 s. In comparison, this site would be classified as an S3
site with a site coefficient (or site amplification factor) of 1.5, and the design base shear would be
FIGURE 25.5 MHA attenuation relationship proposed by Nuttli and Herrmann [1984] for mB = 7.5 earthquake in
the central U.S.
FIGURE 25.6 Variations of peak horizontal accelerations (MHA) at soft soil sites with accelerations at rock sites
(after Idriss, I. M. 1991. Earthquake Ground Motions at Soft Soil Sites. Proceedings, Second International Conference
on Recent Advances in Geotechnical Engineering and Soil Dynamics, March 11–15, St. Louis, pp. 2265–2272).
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
1 10 100
EPICENTRAL DISTANCE (km)
PEAK HORIZONTAL ACCELERATION (g)
Acceleration on Soft Soil Sites - g
0.6
0.5
0.4
0.3
0.2
0.1
0.0
0.0 0.1 0.2 0.3
Acceleration on Rock Sites - g
0.4 0.5
based on calculations
0.6
median relationship
recommended for use
in empirical correlations
1985 Mexico City
1989 Loma Prieta
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25-8 The Civil Engineering Handbook, Second Edition
multiplied by an amplification factor of C = 2.5 for this site (see Table 25.1). Alternatively, using the
normalized response spectrum for deep stiff clay soils in the UBC (Fig. 25.3), the spectral response at
T = 0.7 s would be twice the MHA value of 0.2g.
25.4 Soil Liquefaction
Unlike many other construction materials, cohesionless soils such as sand possess negligible strength
without effective confinement. The overall strength of an uncemented sand depends on particle interaction
(interparticle friction and particle rearrangement), and these particle interaction forces depend on
the effective confining stresses. Soil liquefaction is a phenomenon resulting when the pore water pressure
(the water pressure in the pores between the soil grains) increases, thereby reducing the effective confining
stress and hence the strength of the soil. Seed and Idriss [1982] present this qualitative explanation of
soil liquefaction:
If a saturated sand is subjected to ground vibrations, it tends to compact and decrease in volume; if
drainage is unable to occur, the tendency to decrease in volume results in an increase in pore water
pressure, and 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 completely, and it develops a liquefied
state.
If the strength of the ground underlying a structure reduces below that required to support the
overlying structure, excessive structural movements can occur and damage the structure. The pore water
pressure in the liquefied soil may be sufficient to cause the liquefied soil to flow up through the overlying
material to the ground surface producing sand boils, lateral spreading, and ground breakage. This
dramatic seismic response of a saturated, loose sand deposit can pose obvious hazards to constructed
facilities, and the potential for soil liquefaction should be assessed in seismic regions where such soil
deposits exist.
An example of the structural damage that can result from soil liquefaction is shown in Fig. 25.7. The
Marine Research Facility at Moss Landing, California was a group of modern one-and two-story structures
founded on concrete slabs. This facility was destroyed beyond repair by foundation displacements
as a result of liquefaction of the foundation soils during the 1989 Loma Prieta earthquake [Seed et al.,
1990]. The facility settled a meter or two and lateral spreading of the structure “floating” on the liquefied
soil below the slab foundation stretched the facility by 2 m, literally pulling it apart.
Engineering procedures for evaluating liquefaction potential have developed rapidly in the past twenty
years, and a well-accepted approach is the Seed and Idriss [1982] simplified procedure for evaluating soil
liquefaction potential. The average cyclic shear stress imparted by the earthquake in the upper 12 m of
a soil deposit can be estimated using the equation developed by Seed and Idriss [1982]:
(25.2)
where
(t/s ¢0) d = average cyclic stress ratio developed during the earthquake
MHA = maximum horizontal acceleration at the ground surface
g = acceleration of gravity
s0 = total stress at depth of interest
s¢0 = effective stress (total stress minus pore water pressure) at depth
rd = reduction in acceleration with depth (rd ª 1 – 0.008 · depth, m)
For a magnitude (Ms) 7.5 event at a level ground site, the cyclic stress ratio required to induce
liquefaction, (t/s ¢0 ) l , of the saturated sand deposit can be estimated using the empirically based standard
penetration test (SPT) correlations developed by Seed et al. [1985]. The SPT blowcount (the number of
hammer blows required to drive a standard sampling device 1 foot into the soil deposit) provides an
index of the in situ state of a sand deposit, and especially of its relative density. Field measured SPT
t s0 ¢ ( § )d 0.65 MHA § g s0 s 0 ¢ § rd ª ◊ ◊ ◊
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Geotechnical Earthquake Engineering 25-9
blowcount numbers are corrected to account for overburden pressure, hammer efficiency, saturated silt,
and other factors. A loose sand deposit will generally have low SPT blowcount numbers (4–10) and a
dense sand deposit will generally have high SPT blowcount numbers (30–50). Moreover, changes in
factors that tend to increase the cyclic loading resistance of a deposit similarly increase the SPT blowcount.
Hence, the cyclic stress required to induce soil liquefaction can be related to the soil deposit’s penetration
resistance. Well-documented sites where soil deposits did or did not liquefy during earthquake strong
shaking were used to develop the correlation shown in Fig. 25.8. Correction factors can be applied to the
(t/s¢0)l versus SPT correlation presented in Fig. 25.8 to account for earthquake magnitude, greater
depths, and sloping ground [see Seed and Harder, 1990].
Finally, the liquefaction susceptibility of a saturated sand deposit can be assessed by comparing the
cyclic stress ratio required to induce liquefaction, (t/s¢0)l , with the average cyclic stress ratio developed
during the earthquake, (t/s¢0)d . A reasonably conservative factor of safety should be employed (ª 1.5)
because of the severe consequences of soil liquefaction.
Illustrative Soil Liquefaction Problem
Problem 25.2
Evaluate the liquefaction susceptibility of the soil deposit shown in Fig. 25.4.
Soil Liquefaction
At this site, MHA ª 0.2g (see Problem 25.1). At a depth of 5 m, s0 = rt · g · z = (2.0 Mg/m3)(9.81 m/s2)
(5 m) = 98 kPa; s ¢0 = s0 – gw · zw = 98 kPa – (1 Mg/m3)(9.81 m/s2)(3 m) = 68 kPa; and rd ª 1.0 – 0.008(5m) =
0.96. Hence, using Eq. (25.2), the average cyclic stress ratio developed during the earthquake is
(25.3)
FIGURE 25.7 Liquefaction-induced damage of the Marina Research Facility at Moss Landing, California, caused
by the 1989 Loma Prieta earthquake (after Seed, R. B., Dickenson, S. E., Riemer, M. F., Bray, J. D., Sitar, N., Mitchell,
J. K., Idriss, I. M., Kayen, R. E., Kropp, A., Harder, L. F., and Power, M. S. 1990. Preliminary Report on the Principal
Geotechnical Aspects of the October 17, 1989 Loma Prieta Earthquake. Earthquake Engineering Research Center,
Report No. UCB/EERC-90/05, University of California).
t s0 ¢ ( § )d 0.65 (0.2g § g) 98 kPa
68 kPa
ª ◊ ◊ ---------------- ◊ 0.96 = 0.18
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25-10 The Civil Engineering Handbook, Second Edition
The corrected penetration resistance of the sand at a depth of 5 m is around 10, and using Fig. 25.8,
for a clean sand with less than 5% fines, (t/s¢0)l ª 0.11.
The factor of safety (FS) against liquefaction is then
(25.4)
Since the factor of safety against liquefaction is less than 1.0, the potential for soil liquefaction at the
site is judged to be high. Modern ground improvement techniques (e.g., dynamic compaction) may be
used to densify a particular soil deposit to increase its liquefaction resistance and obtain satisfactory
performance of the building’s foundation material during earthquake strong shaking.
25.5 Seismic Slope Stability
Considerable attention has been focused over the last few decades on developing procedures to analyze
the seismic performance of earth embankments [e.g., Newmark, 1965; Seed, 1979; Marcuson et al., 1992].
The first issue that must be addressed is an evaluation of the potential of the materials comprising the
earth structure to lose significant strength under cyclic earthquake loading. Saturated cohesionless materials
(gravels, sands, and nonplastic silts) that are in a loose state are prime candidates for liquefaction
and hence significant strength loss. Experience has shown that cohesionless materials placed by the
hydraulic fill method are especially vulnerable to severe strength loss as a result of strong shaking. A
modified version of the Seed and Idriss [1982] simplified method has been employed to evaluate the
liquefaction potential of cohesionless soils in earth slopes and dams [see Seed and Harder, 1990]. Certain
types of clayey materials have also been shown to lose significant strength as a result of cyclic loading.
If clayey materials have a small percentage of clay-sized particles, low liquid limits, and high water
contents, the material’s cyclic loading characteristics should be determined by cyclic laboratory testing
[Seed and Idriss, 1982].
The potentially catastrophic consequences of an earth embankment material that undergoes severe
strength loss during earthquake shaking is demonstrated by the near failure of the Lower San Fernando
FIGURE 25.8 Relationship between stress ratios causing liquefaction and (N1)60-values for silty sands for Ms = 7½
earthquakes (after Seed, H. B., Tokimatsu, K., Harder, L. F., and Chung, R. M. 1985. Influence of SPT Procedures in
Soil Liquefaction Resistance Evaluations. Journal of the Geotechnical Engineering Division, ASCE. (111) 12:1425–1445).
29 37
25
10
20
12
20
31
50+
50+
50+
18
17
11
10
12
16
12
1013
75
75
31
0
0
10
0.1
0.2
0.3
0.4
0.5
0.6
20 30
(N1)60
40 50
27
27
22
30
80 12
10 10
10 10
10
13
25
10
30
30
20
20
20
2025
48
12
27
20
20
50
60 20
Percent Fines = 35 15 ≤ 5
FINES CONTENT ≥ 5%
Modified Chinese Code Proposal
(clay content = 5%)
Pan-American data
Japanese data
Chinese data
Marginal No
Lique- Lique- Liquefaction
faction faction
tav
s¢o
FS t s0 ¢ ( § )l t s0 ¢ ( § )d = § = 0.11 § 0.18 = 0.6
© 2003 by CRC Press LLC
Geotechnical Earthquake Engineering 25-11
dam as a result of the 1971 San Fernando earthquake [Seed et al., 1975]. The center and upstream sections
of the dam slid into the reservoir because a large section within the dam liquefied. The slide movement
left only 1.5 m of earth fill above the reservoir level. Fortunately, the reservoir level was 11 m below the
original crest at the time of the earthquake. Still, because of the precarious condition of the dam after
the main shock, 80,000 people living downstream of the dam were ordered to evacuate until the reservoir
could be lowered to a safe elevation.
Surveys of earth dam performance during earthquakes suggest that embankments constructed of
materials that are not vulnerable to severe strength loss as a result of earthquake shaking (most wellcompacted
clayey materials, unsaturated cohesionless materials, and some dense saturated sands, gravels,
and silts) generally perform well during earthquakes [Seed et al., 1978]. The embankment, however, may
undergo some level of permanent deformation as a result of the earthquake shaking. With well-built
earth embankments experiencing moderate earthquakes, the magnitude of permanent seismic deformations
should be small, but marginally stable earth embankments experiencing major earthquakes may
undergo large deformations that may jeopardize the structure’s integrity. Simplified procedures have been
developed to evaluate the potential for seismic instability and seismically induced permanent deformations
[e.g., Seed, 1979; Makdisi and Seed, 1978], and these procedures can be used to evaluate the seismic
performance of earthen structures and natural slopes.
In pseudostatic slope stability analyses, a factor of safety against failure is computed using a static limit
equilibrium stability procedure in which a pseudostatic, horizontal inertial force, which represents the
destabilizing effects of the earthquake, is applied to the potential sliding mass. The horizontal inertial
force is expressed as the product of a seismic coefficient, k, and the weight, W, of the potential sliding
mass (Fig. 25.9). If the factor of safety approaches unity, the embankment is considered unsafe. Since
the seismic coefficient designates the horizontal force to be used in the stability analysis, its selection is
crucial. The selection of the seismic coefficient must be coordinated with the selection of the dynamic
material strengths and minimum factor of safety, however, as these parameters work together to achieve
a satisfactory design. For earth embankments, case histories are available which have guided the selection
of these parameters. For example, Seed [1979] recommends using appropriate dynamic material
strengths, a seismic coefficient of 0.15, and a minimum factor of safety of 1.15 to ensure that an
embankment composed of materials that do not undergo severe strength loss performs satisfactorily
during a major earthquake.
A significant limitation of the pseudostatic approach is that the horizontal force, representing the
effects of an earthquake, is constant and acts in only one direction. With dynamically applied loads, the
force may be applied for only a few tenths of a second before the direction of motion is reversed. The
result of these transient forces will be a series of displacement pulses rather than complete failure of the
slope. Normally, a certain amount of limiting displacement during an earthquake event is considered
tolerable. Hence, if conservative strength properties are selected and the seismic coefficient represents
the maximum disturbing force (i.e., maximum shear stress induced by the earthquake on the potential
sliding surface; see Seed and Martin, 1966), a factor of safety of one is likely to ensure adequate seismic
performance. Other conservative combinations of these parameters could be developed, but their use
must be evaluated in the context that their use in analysis ensures a design that performs well during
earthquakes (i.e., tolerable deformations).
FIGURE 25.9 Pseudostatic slope stability analysis.
kW
W
s FS =
Resisting Forces
Driving Forces
© 2003 by CRC Press LLC
25-12 The Civil Engineering Handbook, Second Edition
Seismically induced permanent deformations are generally calculated using a Newmark-type procedure.
The Newmark [1965] analysis assumes that relative slope movements would be initiated when the
inertial force on a potential sliding mass was large enough to overcome the yield resistance along the slip
surface, and these movements would stop when the inertial force decreased below the yield resistance
and the velocities of the ground and sliding mass coincided. The yield acceleration is defined as the
average acceleration producing a horizontal inertial force on a potential sliding mass which gives a factor
of safety of one and can be calculated as the seismic coefficient in pseudostatic slope stability analyses
that produces a safety factor of one. By integrating the equivalent average acceleration [see Bray et al.,
1993] acting on the sliding surface in excess of this yield acceleration as a function of time, the displacement
of the slide mass can be estimated. A commonly used procedure for calculating seismically induced
permanent deformations has been developed by Franklin and Chang [1977] and computer programs are
available [e.g., Pyke and Beikae, 1991]. Simplified chart solutions have been developed by Makdisi and
Seed [1978] for earth embankments.
An emerging area of concern in many regions of the world is the seismic performance of waste fills.
For example, recent U.S. federal regulations (40 CFR 258-USEPA 1991: Subtitle D) require that municipal
solid waste landfills located in seismic impact zones be designed to resist earthquake hazards. Since these
designated seismic impact zones encompass nearly half of the continental U.S., these regulations are
having a pronounced impact on the design of new landfills and the lateral expansion of existing landfills.
The results of a comprehensive study of the effects of the characteristics of the waste fill, subsurface soils,
and earthquake ground motions are presented in Bray et al. [1993]. The investigators found that the
seismic loading strongly depends on the dynamic properties and height of the waste fill. General design
considerations regarding the seismic stability of solid waste landfills are discussed in Repetto et al. [1993].
25.6 Summary
Geotechnical earthquake engineering phenomena such as site-specific amplification, soil liquefaction,
and seismic slope stability are important aspects of earthquake engineering, and these aspects must be
adequately addressed in the development of sound earthquake-resistant designs. Seismic risk assessments
of a facility, community, or region must incorporate engineering analyses that properly evaluate the
potential hazards resulting from these phenomena. Deep soil deposits can amplify the underlying bedrock
ground motions and produce intense levels of shaking at significant distances from the earthquake’s
epicenter. Under sufficient cyclic loading, loose, saturated sand deposits may suddenly liquefy, undergo
severe strength loss, and fail as a foundation or dam material. Seismically induced permanent deformations
of a landfill’s liner system can jeopardize the integrity of the system and potentially release pollutants
into the environment. Simplified empirical procedures employed to evaluate these hazards have been
presented and they provide a starting point. The field of earthquake engineering is quite complex,
however, and there are many opportunities for future research.
Defining Terms
Attenuation relationship — Provides the value of an engineering parameter versus distance from the
zone of energy release of an earthquake.
Damping ratio — An indication of the ability of a material to dissipate vibrational energy.
Duration of strong shaking — Duration of the earthquake record in which the intensity is sufficiently
high to be of engineering importance (i.e., MHA ≥ 0.05g).
Fundamental period — Period at which a structure tends to vibrate when allowed to vibrate freely
without any external excitation.
Liquefaction — Phenomenon resulting when the pore-water pressure within saturated particulate material
increases dramatically, resulting in a severe loss of strength.
© 2003 by CRC Press LLC
Geotechnical Earthquake Engineering 25-13
Magnitude — Measure of the amount of energy released during an earthquake. Several magnitude scales
exist (e.g., local magnitude, ML, and moment magnitude, Mw).
Maximum horizontal ground acceleration (MHA) — Highest horizontal ground acceleration
recorded at a free-field site (i.e., not in a structure) during an earthquake.
Predominant period (Tp) — Period at which most of the seismic energy is concentrated, often defined
as the period at which the maximum spectral acceleration occurs.
Response spectrum — Displays maximum response induced by ground motions in damped singledegree-
of-freedom structures of different fundamental periods.
Shear wave velocity (Vs) — Speed that shear waves travel through a medium. An indication of the
dynamic stiffness of a material. Note that G = rV2
s , where G = shear modulus and r = mass
density.
References
Algermissen, S. T., Perkins, D. M., Thenhous, P. C., Hanson, S. L., and Bender, B. L. 1991. Probabilistic
earthquake acceleration and velocity maps for the United States and Puerto Rico. U.S.G.S. Misc.
Field Stud. Map MF2120.
Allen and Hoshall. 1985. An Assessment of Damage and Casualties. Federal Emergency Management
Agency Report, Sections V and XI.
Bray, J. D., Chameau, J. L., and Guha, S. 1992. Seismic response of deep stiff clay deposits. In Proc. 1st
Can. Symp. Geo. Nat. Hazards, Vancouver, BC, May 6–9, pp. 167–174.
Bray, J. D., Repetto, P. C., Angello, A. J., and Byrne, R. J. 1993. An overview of seismic design issues for
solid waste landfills. In Proc. Geosynth. Res. Inst. Conf., Drexel University, Philadelphia, December.
Building Officials & Code Administrators International, Inc. 1992. National Building Code. Country Club
Hills, IL.
Franklin, A. G., and Chang, F. K. 1977. Earthquake resistance of earth and rockfill dams. Misc. Pap. S-71-
17, U.S. Army Waterways Experiment Station, Vicksburg, MS, November, 1977.
Gutenberg, B., and Richer, C. F. 1956. Earthquake magnitude, intensity, energy and acceleration. B. Seism.
Soc. Am. 46(2):143–145.
Hanks, T. C., and Kanamori, H. 1979. A moment magnitude scale. J. Geophys. Res. 82:2981–2987.
Heaton, T. H., Tajima, F., and Mori, A. W. 1982. Estimating Ground Motions Using Recorded Accelerograms.
Report by Dames & Moore to Exxon Production Res. Co., Houston.
Hopper, M. G., ed. 1985. Estimation of Earthquake Effects Associated with Large Earthquakes in the New
Madrid Seismic Zone. U.S. Geological Survey Open-File Report 85-457.
Hough, S. E., Friberg, P. A., Busby, R., Field, E. F., Jacob, K. H., and Borcherdt, R. D. 1990. Sedimentinduced
amplification and the collapse of the Nimitz Freeway, Nature. 344:853–855.
Hynes, M. E., and Franklin, A. G. 1984. Rationalizing the seismic coefficient method. Misc. Pap. GL-84-
13. U.S. Army Engineer WES, Vicksburg, MS.
Idriss, I. M. 1985. Evaluating seismic risk in engineering practice. In Proc. 11th Int. Conf. Soil Mech.
Found. Eng. August 12–16, 1985, San Francisco, Vol. I, pp. 255–320.
Idriss, I. M. 1991. Earthquake ground motions at soft soil sites. In Proc. 2nd Int. Conf. Recent Adv. Geo.
Eng. Soil Dyn. March 11–15, St. Louis, pp. 2265–2272.
Idriss, I. M., and Sun, J. I. 1992. User’s Manual for SHAKE91 — A Computer Program for Conducting
Equivalent Linear Seismic Response Analyses of Horizontally Layered Soil Deposits. Center for Geotechnical
Modeling, Dept. of Civil and Environ. Eng., University of California, Davis, November.
International Conference of Building Officials. 1991. Uniform Building Code. Whittier, CA.
Joyner, W. B., and Boore, D. M. 1988. Measurement characteristics and prediction of strong ground
motion: state-of-the-art reports. In Proc. Spec. Conf. Earthquake Eng. Soil Dyn. II, ASCE, pp. 43–102.
Makdisi, F. I., and Seed, H. B. 1978. Simplified procedure for estimating dam and embankment earthquake-
induced deformation. J. Geotech. Eng., ASCE. 104(7):849–867.
© 2003 by CRC Press LLC
25-14 The Civil Engineering Handbook, Second Edition
Marcuson, W. F., III, Hynes, M. E., and Franklin, A. G. 1992. Seismic stability and permanent deformation
analyses: The last twenty-five years. In Proc. ASCE Spec. Conf. Stability and Performance of Slopes
and Embankments — II. Berkeley, CA, June 28–July 1, pp. 552–592.
Newmark, N. M. 1965. Effects of earthquakes on dams and embankments. Geotechnique. 15(2):139–160.
Nuttli, O. W. 1982. Advances in seismicity and tectonics. In Proc. 3rd Int. Earthquake Microzonation Conf.
Vol. I, pp. 3–24.
Nuttli, O. W., and Herrmann, R. B. 1984. Ground motion of Mississippi Valley earthquakes. J. Tech. Top.
Civ. Eng., ASCE. 110:(54–69).
Nuttli, O. W. 1987. The Current and Projected State-of-Knowledge on Earthquake Hazards. Unpublished
report presented in St. Louis, MO.
Pyke, R., and Beikae, M. 1991. TNMN. Taga Engineering Software Services, Lafayette, CA.
Repetto, P. C., Bray, J. D., Byrne, R. J., and Augello, A. J. 1993. Seismic design of landfills. In Proc. 13th
Cent. Pa. Geotech. Semin., Penn. DOT & ASCE, April 12–14.
Richter, C. F. 1935. An instrumental earthquake scale. B. Seism. Soc. Am. 25(1):1–32.
Richter, C. F. 1958. Elementary Seismology. W. H. Freeman, San Francisco.
Seed, H. B., and Martin, G. R. 1966. The seismic coefficient in earth dam design. J. Soil Mech. Found.,
ASCE. 92(3):25–58.
Seed, H. B., Lee, K. L., Idriss, I. M., and Makdisi, F. I. 1975. The slides in the San Fernando dams during
the earthquake of February 9, 1971. J. Geotech. Eng., ASCE. 101(7):889–911.
Seed, H. B., Makdisi, F. I., and DeAlba, P. 1978. Performance of earth dams during earthquakes. J. Geotech.
Eng., ASCE. 104(7):967–994.
Seed, H. B. 1979. Considerations in the earthquake-resistant design of earth and rockfill dams. Geotechnique.
29(3):215–263.
Seed, H. B., and Idriss, I. M. 1982. Ground motions and soil liquefaction during earthquakes. Monograph,
Earthquake Engineering Research Institute, Berkeley, CA.
Seed, H. B. 1983. Earthquake-resistant design of earth dams. Paper presented at ASCE National Convention,
Philadelphia, May 16–20, pp. 41–64.
Seed H. B., Wong, R. T., Idriss, I. M., and Tokimatsu, K. 1984. Moduli and damping factors for dynamic
analyses of cohesionless soils. Earthquake Engineering Research Center, Report No. UCB/EERC-
84/14, University of California, Berkeley, October.
Seed, H. B., Tokimatsu, K., Harder, L. F., and Chung, R. M. 1985. Influence of SPT procedures in soil
liquefaction resistance evaluations. J. Geotech. Eng., ASCE. (111)12:1425–1445.
Seed, H. B., Romo, M. P., Sun, J., Jaime, A., and Lysmer, J. 1987. Relationships between soil conditions
and earthquake ground motions in Mexico City in the earthquake of Sept. 19, 1985. Earthquake
Engineering Research Center, Report No. UCB/EERC-87/15, University of California, Berkeley.
Seed, R. B., Dickenson, S. E., Riemer, M. F., Bray, J. D., Sitar, N., Mitchell, J. K., Idriss, I. M., Kayen, R.
E., Kropp, A., Harder, L. F., and Power, M. S. 1990. Preliminary report on the principal geotechnical
aspects of the October 17, 1989 Loma Prieta earthquake. Earthquake Engineering Research Center,
Report No. UCB/EERC-90/05, University of California.
Seed, R. B., and Harder, Jr., L. F. 1990. SPT-based analysis of cyclic pore pressure generation and undrained
residual strength. H. Bolton Seed Memorial Symp., Vol. II, May, pp. 351–376.
Seed, R. B., and Bonaparte, R. 1992. Seismic analysis and design of lined waste fills: Current practice. In
Proc. ASCE Spec. Conf. Stability and Performance of Slopes and Embankments — II. Berkeley, CA,
June 28–July 1, pp. 1521–1545.
Southern Building Code Congress International. 1992. Standard Building Code. Birmingham, Alabama.
United States Code of Federal Regulations, Title 40. 1991. Protection of the Environment, Part 258, Solid
Waste Disposal Facility Criteria.
Vucetic, M., and Dobry, R. 1991. Effect of soil plasticity on cyclic response. J. Geotech. Eng., ASCE.
117(1):89–107.
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Geotechnical Earthquake Engineering 25-15
Further Information
“Ground Motions and Soil Liquefaction during Earthquakes” by Seed and Idriss [1982] provides an
excellent overview of site-specific amplification and soil liquefaction. Seed and Harder [1990] present an
up-to-date discussion of soil liquefaction. “Evaluating Seismic Risk in Engineering Practice” by Idriss
[1985] provides an excellent discussion of seismicity and geotechnical earthquake engineering. Seismic
stability considerations in earth dam design are presented in “Considerations in the Earthquake-Resistant
Design of Earth and Rockfill Dams” by Seed [1979]. Seismic design issues concerning solid waste landfills
are presented in Bray et al. [1993].