EP 1110-2-12
30 Sep 95
failure mechanism to form in the dam structure which
dam. This defensive measure would be effective for
results in dam failure; however, it is more likely that
flood-control projects where the reservoir pool eleva-
the fault movement would create flow paths that
tion is low enough that the required height of the
could lead to a release of the impounded reservoir.
buttress fill is economically feasible, and does not
Seepage can erode dam or foundation materials which
impair the stability of the dam.
eventually results in failure because capability for
controlled release of the pool is lost.
2-4.
Refined Dynamic Analyses Methods
(2) An earth-fill dam with a flexible core is nor-
mally considered less susceptible to failure due to
a. Need for refinement. When the simplified
foundation fault displacement because it would tend
linear-elastic analysis methods described above for an
to conform to the displaced shape of the foundation.
existing RCC dam produce tensile stresses in excess
Although this flexibility of the dam material will
of the allowables discussed in paragraph 2-2e, more
reduce voids and flow paths in the dam and founda-
refined analyses methods shall be pursued before the
tion it will not completely eliminate them. Thus, an
dam is judged unsafe. Also, if all practical and eco-
earth-fill dam is susceptible to erosion of core or
nomical adjustments to the design of a new dam have
foundation material from water flowing through faults
been exhausted in the attempt to satisfy the allowa-
or through voids in the dam or foundation created by
bles based on simplified linear-elastic methods, the
fault movements. For this reason, an earth-fill dam is
more refined analyses methods may be pursued to
not necessarily superior to a concrete gravity dam in
better evaluate nonlinear structural behavior. Refined
resisting the effects of fault movement.
analyses consist of linear or nonlinear time history
analyses as discussed in paragraph 2-2d, with some
h. Defensive design features. Defensive design
additional details of the nonlinear analysis provided
features which can be employed in the design of an
below. The response produced by refined analyses
RCC dam susceptible to foundation displacement are
shall be evaluated in accordance with the stipulations
discussed below.
of paragraphs 2-2f and 2-2g.
(1) The arching action provided by laying out the
b. Fracture mechanics. Nonlinear dynamic
dam axis on a curve may better distribute the forces
analysis is based on fracture mechanics theory which
on a gravity dam due to foundation fault displace-
is presently in the research phase. It is also difficult
ment, and reduce the tensile stresses and cracking of
to determine just what level of structural damage can
the RCC. This defensive feature is only effective if
be sustained safely by the dam and still consider it to
the heave of the foundation block is generally in a
satisfy the performance requirements. The nonlinear
downstream direction, and providing the fault move-
attribute requires this type of dynamic analysis be
ment does not occur at either abutment.
performed in a time domain (time history analysis)
rather than a frequency domain (response spectrum
(2) Special sliding joints may also be used to
analysis), and use a direct integration solution. The
reduce cracking of the RCC due to fault displace-
analysis accounts for: energy dissipation by cracking,
ment. For example, vertical joints may be located in
strength of cracked concrete, changes in vibration
the RCC to accommodate potential strike-slip fault
characteristics caused by cracking, changes in damp-
displacements where the strike is generally in the
ing, and changes in strength due to strain rate and
upstream-downstream direction.
loading history.
(3) A design feature for controlling the reservoir
c. Nonlinear analysis requirements. Because it
release is to provide a buttress fill against the
is very complex, costly, and requires a considerable
upstream face of the dam. This requires the reservoir
amount of judgment to interpret the results, an expert
water to pass through a succession of filters and
in fracture mechanics and nonlinear analysis tech-
crack stoppers in a manner analogous to the behavior
niques should be consulted to provide guidance when
of the transitions and filters in a zoned embankment
pursuing a nonlinear analysis.
2-6
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