THE ROLE OF MODELING IN
GEOTECHNICAL EARTHQUAKE ENGINEERING **
by
I. M. Idriss, Professor
Department of Civil & Environmental Engineering
University of California, Davis, CA 95616-5294
e-mail: imidriss@aol.com
Numerical models, as well as physical models, have always played a strong role in all
aspects of geotechnical engineering, including geotechnical earthquake engineering.
Certainly, numerical modeling was the cornerstone of geotechnical earthquake
engineering in its early development in the late 1950's and early to mid 1960's. The ongoing "physical modeling" in geotechnical earthquake engineering at that time was
mostly derived from observations during actual earthquakes where only "after-the-fact"
information could be obtained. That is, very little, if any, instrumentation had been
placed to monitor and record the performance during or following the earthquake.
The need for "controlled" physical modeling became apparent in the 1960's and several
experiments were initiated to: (a) provide data to calibrate (if not validate) numerical
models; and (b) provide insights about the phenomenon under investigation. Most of the
initial experiments were conducted on small shaking tables. It was not long thereafter
that centrifuge tests were initiated followed by large shaking tables and more recently
field tests involving mobile shakers or explosions.
At this time (2001), there are several large shaking tables in the US and in Japan and
numerous small shaking tables in several countries, and several large to medium and
numerous small centrifuge facilities in several countries. Field tests have been recently
completed in the US and in Japan. Instrumentation of some existing and many recently
completed structures (e.g., earth and rockfill dams) will also provide very useful data
during future earthquakes. The installment of downhole arrays in Japan, the USA and
other parts of the world is among the most useful activity to be undertaken. It is hoped
that many more downhole arrays will be installed in the coming years.
The state of physical modeling appears healthy.
Numerical modeling has also continued to flourish in geotechnical earthquake
engineering, and numerous approaches & computer codes have been developed over the
past 35 or so years. I should note that I am including empirical approaches as part of the
numerical modeling space.
The state of numerical modeling appears possibly healthy (some may even say it is
bloated!).
Presented at the NSF International Workshop on Earthquake Simulation in Geotechnical Engineering,
Case Western University, November 8 – 10, 2001
Researchers today find themselves at a very unique juncture. The computational ability
available today is several orders of magnitude greater and faster than it was in the early
stages of geotechnical earthquake engineering. The past decade has been the most
prosperous period in history, and, as a result, many new and improved physical modeling
facilities have been completed in the past few years, are under construction, or are in the
final planning stages with significant funding already allocated to them. (We will be
hearing about several of these facilities during this workshop).
That is why this Workshop is so timely and hopefully will provide the means for the
participants to share and discuss the latest developments in modeling techniques in
geotechnical earthquake engineering, including 1g shaking table tests, centrifuge tests,
field simulation, numerical simulations, instrumentation of selected full scale earth
structures, foundation elements and special soil-foundation-structure systems, and
installation of downhole arrays.
It is hoped that the Workshop participants will see value in initiating and completing
collaborative research activities between, or among, researchers using different
approaches and techniques. In addition, there is a need to discuss the future directions of
research utilizing the latest technology. In particular, I trust that we will come away from
this gathering with some concrete recommendations regarding utilization of the best and
most useful technology in collecting and analyzing the data.
As we move forward, it is important that we strive to conduct physical as well numerical
simulations that are needed to satisfy the objectives noted above for vital and pressing
issues. I hope that we strive to avoid conducting a research just because "we can". I trust
that all research will be conducted because "we should".
In reading many of the reports and articles that have appeared in the past few years, I
have noted some serious deficiencies. Quite often (possibly because of page number
limitations in many technical journals), the authors provide us with the results of their
simulations and a great deal of analysis of the results forgetting to tell us how these
results were obtained. It is essential to know how the results were obtained in order to
appreciate their meaning and usefulness. Therefore, I trust that we will all strive to
provide such information to remove the deficiencies I have noted in recent reports and
articles.
Finally, I want to raise a question to the participants from the academic institutions. Are
we preparing our graduate students (i.e., the future researchers) adequately in the field of
engineering mechanics, particularly in vibration theory? I believe that we are not doing
enough; we certainly can do more.
I am very grateful to Professor Zeng and the other members of the Organizing Committee
to have given me this opportunity to share some of these thoughts with you during the
opening talk at this Workshop.