SUMMARY OF GEOTECHNICAL EARTHQUAKE ENGINEERING - Geotechnical Earthquake Engineering - page 488

Engineering analyses for geotechnical earthquake engineering are even more difficult
because of both the variable nature of soil deposits and the uncertainties regarding earth-
quake parameters, such as the peak ground acceleration and earthquake magnitude. Because
of these uncertainties, the engineering analyses need to be checked against experience and
judgment to make sure the results are reasonable.
As discussed in Sec. 15.1, most soil related failures are due to the weakening of the soil
during the earthquake. Thus, the types of engineering analyses should be based on whether
or not the soil will be weakened during the earthquake. Tables 15.2 to 15.4 summarize
the commonly used engineering analyses for soil weakened during the earthquake and
Table 15.5 summarizes the commonly used engineering analyses for soil not weakened
during the earthquake. It must be recognized that the engineering analyses in Tables 15.2
to 15.5 are of limited value if the construction is not properly performed. For example,
Huang (2005) describes an interesting case study of highway retaining walls that were
severely damaged by the 1999 Chi-Chi earthquake in Taiwan. Loose soil below the
retaining walls and in the passive zone allowed the walls to collapse or deform exces-
sively. The study demonstrated the importance of proper construction such as adequately
compacted soil in the bearing and passive zones.

15.4.1 Materials Weakened during the Earthquake

Examples of materials that are likely to be weakened during the earthquake are as follows:

1. Foliated or friable rock that fractures apart during the earthquake.
2. Unstable ground, such as abandoned underground mines and tunnels that are likely
to collapse during the earthquake.
3. Unstable natural ground, such as limestone cavities, caves, or other underground
voids that are likely to collapse or form sinkholes during the earthquake.
4. Loose to very loose natural sand and gravel located below the groundwater table
and subjected to contraction during the earthquake, which results in liquefaction or
substantial increase in pore water pressure.
5. Hydraulic fill (fill placed under water) susceptible to liquefaction or increased pore
water pressure because of the loose and segregated soil structure created by the soil
particles falling through water.
6. Recently deposited alluvium, such as sediments deposited in lakes, rivers, estuaries,
etc.
7. Dry and loose sands and gravels above the groundwater table that are subjected to
volumetric compression.
8. Other types of collapsible soil, such as dumped fill and debris fill, that have un-
stable soil particle arrangements.
9. Loose rock debris, such as talus deposits at the base of a slope.
10. Sensitive silts and clays that are susceptible to strain-softening. These types of soils
lose shear strength if the peak shear strength is exceeded during the earthquake.
11. Soft and compressible silts and clays that have high water contents. These soils can
be susceptible to cyclic softening. They can also be overloaded and subjected to
plastic flow during the earthquake.
12. Highly organic soils, such as peat.
13. All types of waste deposits, such as rubbish, waste tailings, mine waste, wastewater
effluent deposits, etc.