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complex as it depends on the soil-water-wall interaction. Secondly, excess pore pressure will build up m
saturated soil. It is difficult to estimate excess pore pressure using simple calculation and hence it is not
possible to use simple design calculation to determine the threshold acceleration or the resulting
displacement. Therefore, small-scale model simulation or comprehensive numerical codes need to be
used.
Physical Modeling
Since earthquakes in the field are unpredictable, it is difficult to record seismic response of gravity walls
during earthquakes in the field. In most cases, what geotechnical engineers have are the pre-earthquake
information and data collected during post-earthquake investigations. The most crucial information i.e.
the response of the structure during earthquakes such as the amplification of vibration, excess pore
pressure, and dynamic soil-structure interaction is generally not available. In order to calibrate or verify
analytical procedures or numerical simulation, realistic physical data are needed.
In the absence of field data recorded in earthquakes, data generated by physical model tests such as shake
table tests and centrifuge tests become very valuable. In recent years, significant progress has been made
in physical modeling technique in geotechnical earthquake engineering as reported in the 2001 NSF
International Workshop on Earthquake Simulation in Geotechnical Engineering (Zeng, 2002). Shake table
tests conduct at Ig have been used to study seismic response of gravity quay walls by Inagaki et al (1996)
after the Kobe earthquake. While the interpretation of data from Ig shake table test is complicated by the
scaling law issues, centrifuge modeling has the ability to create the same stress and strain condition as
expected in the field. Since most mechanical properties of soils are stress dependent, centrifuge modeling
has the advantage of satisfying most of the scaling laws. In the 1990s, a group of centrifuge tests of
gravity retaining walls were conducted by Zeng (1998) for the VELACS project. The result of one model
test is shown in Fig. 2.1.
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Fig, 2.1 Cross-sectional view of a gravity wall model after earthquake test (unit:m)