(1) Safety factor no less than 1.5 for permanent or sustained
loading conditions.
(2) For foundations of structures, a safety factor no less than 2.0
is desirable to limit critical movements at foundation edge. See DM-7.2,
Chapter 4 for detailed requirements for safety factors in bearing capacity
analysis.
(3) For temporary loading conditions or where stability reaches a
minimum during construction, safety factors may be reduced to 1.3 or 1.25 if
controls are maintained on load application.
(4) For transient loads, such as earthquake, safety factors as low
as 1.2 or 1.15 may be tolerated.
6. EARTHQUAKE LOADING. Earthquake effects can be introduced into the
analysis by assigning a disturbing force on the sliding mass equal to kW
where W is the weight of the sliding mass and k is the seismic coefficient.
For the analyses of stability shown in Figure 9a, k+s,W is assumed to act
parallel to the slope and through the center of mass of the sliding mass.
Thus, for a factor of safety of 1.0:
Wb + k+s,Wh = FR
The factor of safety under an earthquake loading then becomes
FR
F+Se,= ))))))))
Wb + k+s,Wh
To determine the critical value of the seismic efficient (k+cs,) which
will reduce a given factor of safety for a stable static condition (F+So,)
to a factor of safety of 1.0 with an earthquake loading (F+Se, = 1.0), use
b
k+cs, =
)
(F+So, - 1) = (F+So, -1) sin [theta]
h
If the seismic force is in the horizontal direction and denoting such
force as k+ch, W, then k+ch, = (F+So,-1) tan[theta].
For granular, free-draining material with plane sliding surface (Figure
9b): F+So, = tan[phi]/tan[theta], and k+cs, = (F+So, -1)sin[theta].
Based on several numerical experiments reported in Reference 7, Critical
Acceleration Versus Static Factor of Safety in Stability Analysis of Earth
Dams and Embankments, by Sarma and Bhave, k+ch, may be conservatively
represented as k+ch, [approximately] (F+So, -1)0.25.
The downslope movement U may be conservatively predicted based on
Reference 8, Effect of Earthquakes on Dams and Embankments, by Newmark as:
V.2-
A
)))))))))) [multiplied by] )))))
U
=
2g k+cs,
k+cs,
7.1-329
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