EARTHQUAKE SIMULATION IN GEOTECHNICAL ENGINEERING:
APPLICATIONS AND RESEARCH NEEDS TO IMPROVE STATE OF PRACTICE
K. Arulmoli
Earth Mechanics, Inc., Fountain Valley, California
ABSTRACT
During the past decade or so, significant progress has been made in the area of understanding
soil behavior during earthquakes. With the advent of faster and cheaper microcomputers, use of
recent advanced computer programs for site response and soil-structure interaction evaluation
has become more affordable thus making it possible to accommodate these analyses into projects
with limited budgets and demanding schedules. However, clarification or additional work is still
needed in many areas to improve the current state of practice. In the future, as the geotechnical
profession is challenged to develop more efficient designs, researchers will be faced with the
task of supplying the profession with appropriate tools to face these challenges. An additional
challenge for geotechnical engineers is to understand the structural design implications of the
geotechnical recommendations since in most cases the end users of geotechnical
recommendations are structural engineers. Efforts should be focused on overcoming these
challenges through effective collaboration between geotechnical and structural researchers and
practitioners.
INTRODUCTION
Significant strides have been made in the area of understanding soil behavior during earthquakes
in the recent past. Under the sponsorship of the National Science Foundation, significant
improvements have been made in the area of earthquake simulation in geotechnical engineering
allowing practitioners to better evaluate geotechnical earthquake engineering problems. In the
early 1990s, the VELACS program was instrumental in highlighting the use of centrifuge
modeling to verify numerical codes through collaborative efforts among geotechnical researchers
and practitioners (Arulanandan and Scott 1993). For the first time ever, consensus has been
reached by the geotechnical community regarding the use of cone penetration testing and shear
wave velocity measurements, in addition to standard penetration testing, to evaluate liquefaction
potential, as documented in a 1997 NCEER publication (Youd and Idriss 1997) and later
published in the ASCE Geotechnical and Geoenvironmental Engineering Journal (Youd et al.
2001). With the advent of faster and cheaper microcomputers, use of recent advanced computer
programs for site response and soil-structure interaction evaluation has become more affordable
making it possible to accommodate these analyses into projects with limited budgets and
demanding schedules. Recently, guidelines for evaluating and mitigating earthquake-induced
induced hazards such as liquefaction (Martin and Lew 1999) and landslides (Blake et al. 2002)
have been developed to help the practicing geotechnical engineers and the reviewing agencies of
Southern California.
In spite of the significant progress in geotechnical earthquake engineering practice in the recent
past, clarification or additional work is still needed in certain areas to improve the current state of
practice. Some areas needing immediate attention to develop guidelines include, (1)
Liquefaction potential and seismic settlement of silty soils, (2) Liquefaction potential of deep soil
deposits, (3) Lateral soil springs (p-y curves) during seismic loading, (4) Seismic earth pressures,
and (5) Faster computer programs for seismic soil-structure interaction evaluations. Some
example situations are highlighted in the following paragraphs where guidelines are needed for
the practicing engineers to tackle real-world geotechnical earthquake engineering design
problems.
LIQUEFACTION POTENTIAL AND SEISMIC SETTLEMENT OF SILTY SOILS
While the 1997 NCEER publication provides recommended procedures for evaluation of
liquefaction potential of clean sands and sandy soils with fines, the authors are not aware of any
clear guidelines for liquefaction potential evaluation for silty soils, especially containing nonplastic fines. The same is also true for evaluation of seismically induced settlements of silty
soils. Evaluation of liquefaction potential and seismically induced settlement of silty soils
deserves attention from the geotechnical earthquake engineering research community.
LIQUEFACTION POTENTIAL OF DEEP SOIL DEPOSITS
The geotechnical engineer often encounters subsurface conditions where Holocene soil deposits
deeper than 15 m (50 ft.) are present. Recommended procedures for evaluation of liquefaction
potential of deep soil deposits (Youd and Idriss 1997; Youd et al. 2001) are based on simplified
procedures verified using case histories of known liquefaction (or no liquefaction) of soils within
depths less than 15 m (50 ft.). Using the same procedures to evaluate liquefaction potential of
deep soil deposits often leads to questionable results. As an example, a clean sand deposit
located 30 m (100 ft.) below ground surface with a field SPT blow count (N60) of 50 or more
would be characterized as “dense to very dense” implying that this layer would be nonliquefiable under a moderate earthquake shaking.
Using the overburden correction
recommended by the NCEER document, the overburden-corrected blow count (N1)60 was
estimated to be about 22 (assuming the water table depth of 6 m and typical soil unit weights).
22
FIG. 1 SPT Clean Sand Base Curve for M=7.5 Earthquakes (Youd and Idriss 1997)
It can be seen in Figure 1 that the sand deposit would be considered liquefiable, contradicting the
generally accepted notion that a field blow count of 50 or more is a very competent soil. The
implication of this conflict becomes even more significant when pile foundations are tipped into
deep “dense to very dense” sand deposits.
LATERAL SOIL SPRINGS DURING SEISMIC LOADING
This is another area where guidelines are needed for the practitioners. Lateral soil springs are
used in soil-structure interaction problems where pile foundation is used to support structures
such as bridges, buildings, wharves, and other structures. Selection of appropriate lateral soil
springs (p-y springs) during seismic loading should consider various factors that affect the lateral
behavior of piles. They include, pile diameter effects, pile group effects, depth effects,
liquefaction and soft soil effects, gapping effects, and sloping ground conditions (level ground,
upslope, and downslope). Although significant research efforts have been and are on going,
recommendations regarding the use of lateral soil springs in practice are lacking.
SEISMIC EARTH PRESSURES
Seismic earth pressures on retaining structures (walls, abutments etc.) are estimated using the
Mononabe-Okabe (M-O) equation (Seed and Whitman 1970). One of the limitations of the M-O
equation is that it “blows up” for cases of large ground acceleration and/or steeper slopes. In
such cases, the geotechnical designers are left with the option of having to select seismic earth
pressures based on the individual’s experience and judgment. Clarification is needed on the use
and limitations of the M-O equation for estimating seismic earth pressures. When the M-O
equation is not applicable, alternate recommendations are needed.
The seismic passive earth pressure is used in the design of bridge abutments in California
(Caltrans 2001). At present, Caltrans recommends 239 kPa (5.0 ksf) as the maximum passive
pressure during earthquake for a 1.7 m (5.5 ft.) high abutment. This value is based on the results
of limited full-scale tests performed on a 1.7 m (5.5 ft.) high abutment at the University of
California, Davis (Romstad et al. 1995). For abutment heights other than 1.7 m (5.5 ft.), the
earth pressures are estimated by interpolation. Due to the limited amount of data that the current
passive pressure recommendation is based on, it is desirable to have additional data points to
verify or modify this value.
FASTER COPMUTER PROGRAMS
With the advent of faster and cheaper microcomputers, use of recent advanced computer
programs for site response and soil-structure interaction evaluation has become more affordable
thus making it possible to accommodate these analyses into projects with limited budgets and
demanding schedules. However, time-history analyses can still be slow and time consuming.
Furthermore, the results from time-history analyses are voluminous and can overwhelm the user.
Faster and user-friendly computer programs for seismic soil-structure interaction evaluations
with effective pre-processors for data input and post-processors/animation techniques for
visualization and presentation of results are needed to advance the state of practice.
CONCLUSIONS
Additional research effort must be focused on many areas of geotechnical earthquake
engineering to improve the current state of practice. Some areas needing immediate attention to
develop guidelines were discussed in this paper. In the future, as the geotechnical profession is
challenged to develop more efficient designs, researchers will in turn be challenged to focus their
efforts on new areas in order to supply the profession with appropriate tools. Potential long-term
research areas include, evaluation of unsaturated soil behavior under earthquakes and dynamic
modeling of three-dimensional problems.
An additional challenge for the geotechnical engineers is to understand the structural design
implications of the geotechnical recommendations since in most cases, the end user of the
geotechnical recommendations are structural engineers. Efforts should be focused on
overcoming these challenges through effective collaboration between geotechnical and structural
researchers and practitioners.
REFERENCES
Arulanandan, K., and Scott, R. F. (1993) “Verification of Numerical Procedures for the Analysis
of Soil Liquefaction Problems,” Proceedings of the International Conference, University of
California, Davis, California.
Blake, T. F., Hollingsworth, R. A., and Stewart, J. P. (Editors) (2002) “Recommended
Procedures for Implementation of DMG Special Publication 117, Guidelines for Analyzing and
Mitigating Landslide Hazards in California,” Southern California Earthquake Center, University
of Southern California, California.
Caltrans (2001) “Caltrans Seismic Design Criteria,” Version 1.2.
Romstad, K. M., Kutter, B. L., Maroney, B. S., Vanderbilt, E., Griggs, M., and Chai, Y. H.
(1995) “Experimental Measurements of Bridge Abutment Behavior,” Department of Civil and
Environmental Engineering, University of California, Davis, California, Report No. UCD-STR95-1.
Martin, G. R., and Lew, M. (Editors) (1999) “Recommended Procedures for Implementation of
DMG Special Publication 117, Guidelines for Analyzing and Mitigating Liquefaction Hazards in
California,” Southern California Earthquake Center, University of Southern California,
California.
Seed, H. B., and Whitman, R. V. (1970) “Design of Retaining Structures for Dynamic Loads,”
Proceedings of the ASCE Specialty Conference on Lateral Stresses in the Ground and Design of
Earth Retaining Structures, Cornell University, Ithaca, New York, 103-147.
Youd, T. L. et al. (2001) “Liquefaction Resistance of Soils: Summary Report from the 1996
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