EARTHQUAKE SAFETY EVALUATION OF ATATURK DAM

In view of the randomness of the earthquake excitation and the uncertainties concerning the dynamic
soil properties, a number of cases listed in Table 1 were analysed.
Table 1: List of analysed cases and some principal results
Case
Dynamic shear moduli
(Gmax value and G/Gmax
curve)
Damping ratio curve
Fundamental
eigenfrequency
(Hz)
Peak absolute
acceleration at
crest level (g)
Horz. Vert.
A Average Average 0.49 0.64 0.33
B Average Lower bound 0.46 0.75 0.35
C Average Upper bound 0.51 0.55 0.30
D Lower bound Average 0.31 0.42 0.26
E Upper bound Average 0.67 0.93 0.49
Table 1 lists the fundamental eigenfrequencies in the various cases. The fundamental eigenfrequency
of the dam subjected to the MCE lies in the range of 0.31 Hz to 0.67 Hz, and is equal to about 0.50 Hz
for the estimated average material properties. It should be noted that the dam eigenfrequencies also
depend on the earthquake size, i.e. they are smaller for a larger earthquake due to the straindependence
of the dynamic shear moduli.
The peak value of the horizontal component of the absolute crest acceleration is in the range of 0.42 g
to 0.93 g and that of the vertical component in the range of 0.26 g to 0.49 g. The highest and lowest
earthquake accelerations are, respectively, associated with the estimated upper and lower bounds of
the dynamic shear moduli of the embankment materials. This is related to the fact that the spectral
acceleration increases when the fundamental frequency becomes higher in the frequency range up to
3.3 Hz due to the shape of the input response spectrum.
DYNAMIC SLOPE STABILITY ANALYSIS
If the inertial forces acting on a potential sliding mass on a dam slope during an earthquake become
sufficiently large, the total (static plus dynamic) driving forces would exceed the available resisting
forces. As a result, the factor of safety would drop below 1.0, implying that the sliding mass starts to
move because equilibrium is no longer possible. The permanent displacements of such potential
sliding masses on the dam slopes were computed by the Newmark sliding block analysis.
Calculation of yield accelerations
For the Newmark sliding block analysis, first of all, the yield acceleration of a potential sliding mass
has to be determined. This corresponds to the pseudo-static horizontal earthquake acceleration for
which the factor of safety against a sliding failure is equal to 1.0.
The yield acceleration depends mainly on the following factors: (i) gradient of slope, (ii) shape of
sliding surface, (iii) shear strength parameters (angle of sliding friction and cohesion), and
(iv) distribution of pore water pressures.
The slope stability analysis for the calculation of the yield acceleration was performed using the
Spencer method. In this method, the factor of safety is obtained by taking into account both the force
and moment equilibrium conditions, and the resultant interslice forces are assumed to have a constant
slope throughout the sliding mass.