Geology Reference
In-Depth Information
complicated by the way that individual structures respond to repetitive
dynamic loading, which is a matter of harmonic resonance. Thus,
whilst for the M5.2 design earthquake at 10 km the peak acceleration
can be predicted from the equation presented earlier as 0.12g and for
the M8 design earthquake at 200 km as 0.04g, other characteristics
will be very different. Figure 6.31 shows the predominant period in
ground acceleration records for western USA (Seed et al ., 1968), which
indicates that for a near-
eld M5.2 quake the predominant period
might be less than 0.2 secs, whereas for the distant M8 quake the
predominant period could be more than 0.8 secs. Furthermore, the
duration of shaking will be signi
cantly longer for the large magnitude
earthquake (e.g. Bommer &Martinez-Pereira, 1999). The duration of
strong shaking for a M5.2 earthquake might be a few seconds. For the
Christchurch Feb 2011 M6.3 earthquake, the strong shaking lasted
about 12 seconds. For an earthquake of M8, the duration could be
over a minute. With longer duration, the potential for ampli
cation
will be much greater and fatigue-type failure can occur.
6.10.2 Liquefaction
Liquefaction is a common failure mode in natural soils,
fill and some-
times in embankment dams during earthquakes. It occurs in loose
saturated cohesionless sand and silt, which, when disturbed, loses its
structure and collapses. Because of its low permeability, water cannot
escape so natural piping and even general liquefaction occurs as the
effective stress and thereby friction reduces to zero. There are many
classic examples of whole apartment blocks tilting over and buildings
settling. Elsewhere,
service pipes
oat
to the surface and sea
walls collapse as the retained
ll
flows into the sea. The potential for
liquefaction is readily identi
ed during site investigation. The general
rules are:
1.
fill or geologically recent soil.
2. The most susceptible soils are cohesionless (sands and silts) with a
liquid limit less than 35%and water content greater than 0.9 times
the liquid limit (Seed & Idriss, 1982).
3. It generally occurs at depths shallower than about 15m.
4. Generally, SPT N value (corrected) less than 30 (Marcusson et al .,
1990) or CPT cone resistance less than 15 MPa (Shibata &
Taparasaka, 1988).
It occurs in un-cemented deposits
-
Analysis of the hazard might be re
ned by considering the liquefaction
potential vs. the characteristics of a design earthquake, but generally if
the area has high seismicity and the granular soil at a site is relatively
loose and groundwater table high, then it is probably wise to carry out
 
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