cycles, Neq, and the square of the natural period of the
technique that can prevent fault rupture from occurring.
time-history, T.
Therefore, if the risk posed by the hazard of surface
fault rupture is unacceptable, then the mitigation
(3) Example. An example of a detailed
options are either avoiding the hazard by resiting or
evaluation of landslide potential is given in Appendix
designing for the displacements.
G.
(1) Generally, it is not feasible to d esign for the
f.
Flooding. If a facility has possible exposure
large and concentrated displacements associated with
to earthquake-induced flooding after applying the
surface fault rupture. However, during the 1978
screening criteria in Section F-3, then further
Managua, Nicaragua earthquake, the foundation and
evaluations should be directed at assessing the
basement of the Banco Central building were
potential, severity, consequences, and likelihood of the
hazard. The evaluation of the potential for landsliding
apparently rigid and strong enough to divert a fault
into or within a body of water utilizes methodologies
slippage of several inches around the building and the
described previously in the section for assessing
building sustained only minor damage due to the
liquefaction and landsliding. Evaluation of the height
faulting (Wyllie et al., 1977; Youd, 1989). Thus, the
of waves that could be produced by a tsunami, seiche,
possibility of mitigation by designing for fault
or landslide requires special expertise in fields such as
displacement should be considered unless the
fluid dynamics and coastal engineering as well as
displacements are of a magnitude that obviously would
seismological, geophysical, and earthquake engineering
not be tolerable.
expertise in characterizing the earthquakes and ground
shaking that cause these phenomena. Similarly,
c.
Soil liquefaction. Ground modification
geological, seismological, and geophysical expertise
techniques can be considered to eliminate or reduce the
are required to assess tectonic movements such as
liquefaction potential hazard. Soil modification
uplift or tilting that could cause flooding. Such studies
techniques that can be considered include soil removal
of hazard potential and severity should be undertaken
and replacement, vibratory soil densification, soil
unless it can be concluded that the effects of flooding
grouting, installation of drains, and installation of
on the facility site are tolerable considering the
permanent dewatering systems. A number of ground
performance objective for the facility, or the probability
modification techniques are summarized in Table F-3
of occurrence of the hazard is sufficiently low that the
(National Research Council, 1985; Ferritto, 1997b).
risk can be accepted.
(1) Soil removal and replacement. Removing
(1) If a facility has possible exposure to flooding
liquefiable soil and replacing it with soil that is not
from failure of a water retention structure, the agencies
liquefiable (including recompaction of the excavated
having jurisdiction over these facilities should be
soil in lifts to a dense, nonliquefiable state) is a positive
contacted to ascertain whether the structure has been
evaluated or designed for appropriate ground shaking
method for mitigating a liquefaction hazard. However,
using modern seismic analysis and design methods.
it may not be economically feasible in many cases
The potential effects of the flooding at the site should
because of the need to dewater a site to remove the soil
also be evaluated.
as well as the need to retain the area surrounding the
site if existing facilities are nearby. The effect of
F-5.
Mitigation Techniques and Considerations
dewatering and excavation on adjacent facilities should
also be evaluated.
In the event that a significant geologic hazard is found
to exist at a facility site, alternatives for mitigating the
(2) In-place soil densification. Various
hazard should be identified and evaluated.
techniques can be considered to increase the density of
the in-place soil, thereby reducing its tendency to
a. Overall approaches to hazard mitigation. The
compact and buildup pore pressures during an
overall approaches to hazard mitigation are (1)
earthquake. A number of methods are summarized in
eliminating or reducing the hazard; (2) eliminating or
Table F-3. In-place soil densification is often the
reducing the consequences of the hazard; and (3)
resiting the proposed facility to a less hazardous
location. The following paragraphs summarize hazard
mitigation strategies that have been used or considered
for the different geologic hazards.
F-36
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