(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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