EP 1110-2-12
30 Sep 95
mechanics. This type of analysis should only be
particularly when triggered by a great earthquake on a
undertaken in consultation with and as approved by
nearby large active fault. The three general types of
CECW-ED.
fault slips are strike-slip, normal-slip (dip-slip), and
reverse-slip (thrust-slip). Refer to Figure 2-1 for
illustrations of the various types of faults and how the
2-3.
Foundation Fault Displacement
magnitude of slip is measured. The strike of the fault
is the trace the fault makes with respect to the ground
surface, and it may be at any orientation with respect
a. General. Most RCC dam sites are not
to the dam axis.
subject to any significant differential displacement of
the ground surface at the dam-foundation interface
c. Design fault displacement. The design fault
during a seismic event. Dam sites should always be
avoided when located near a major active fault
displacement (DFD) is defined as the maximum
system with the potential to trigger sympathetic
possible free field fault slip movement that could
foundation displacements at the site. Occasionally it
reasonably occur in the dam foundation as measured
is not possible to avoid these sites, and it becomes
at the ground surface. The return period that would
necessary to evaluate the response of the dam should
be associated with the DFD is similar to that of the
such a foundation fault displacement occur.
MCE. Therefore, the DFD and the free field ground
motion together specify the site-specific seismic
(1) Considerable judgment is required in the
activity associated with the MCE. To fully describe
evaluation process. At best, analysis methods for
the DFD, three factors must be specified: magnitude,
foundation fault displacement are approximate and are
type of slip, and strike of the fault.
generally unsupported by past observations of the
response of existing dams to fault displacements
(1) The geology of the dam foundation is
occurring at the dam foundation. Furthermore,
complex, and the foundation may be crossed by a
considerable judgment is required in the prediction of
number of discontinuities with fault displacement
future fault movement and in the magnitude of the
potential. Experts in the fields of geology and
fault displacement. For example, the estimate of the
seismology should be consulted to study the
magnitude of potential fault displacement provided by
foundation fault system, determine which faults are
different experts for a specific site could vary from a
capable of surface displacement, and finally
few inches to several feet. This necessitates
recommend which faults are critical and specify the
consulting several geotechnical firms to provide site-
DFD for each critical fault.
specific fault displacement estimates, and then
carefully scrutinizing these estimates before finally
(2) Normally, foundation fault displacements are
establishing the design fault displacement.
not considered to occur concurrently with strong
motion shaking associated with the OBE. The active
(2) Experts in plate tectonics, geology,
fault near the dam site that produces a seismic event
seismology, and finite element analysis techniques
of OBE magnitude is not likely to trigger sympathetic
should be consulted to provide guidance for any dam
slippage in the fault system in the dam foundation.
located on a site subject to foundation fault
The probability of sympathetic foundation fault
displacement. Because of the many uncertainties and
displacement is normally several orders of magnitude
the risk involved, approval by CECW-ED is required
less than the recurrence rate for the strong motion
for any RCC dam which is located on a site subject
shaking associated with the OBE; therefore, the
to foundation fault displacement.
probability of the OBE being accompanied by
significant foundation displacement is usually
considered negligible.
b. Types of faults. Fault slip is the relative
displacement of two adjacent tectonic plates with
(3) On rare occasions, the probability logic
respect to each other. This refers to large active fault
discussed above may not apply when considering if it
systems such as the San Andreas or Hayward faults
is appropriate to combine foundation fault
in California. On a smaller scale, the foundation rock
displacement with ground shaking in specifying the
mass beneath a dam contains various discontinuities,
OBE. For example, unusual geology of the
joint sets, and shear and fault zones. Normally this is
foundation could make it susceptible to a reservoir-
a system of historically inactive discontinuities;
induced foundation fault displacement or to other
however, there is a potential for fault slippage
2-2
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