EARTHQUAKE SAFETY EVALUATION OF ATATURK DAM

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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.
Table 2: Material properties for slope stability calculations Description Total unit weight (kN/m³) Cohesion (kPa) Angle of friction Peak Residual φ't φ'res Upstream shell (submerged) 24.0 0 43° 39° Downstream shell and top of upstream shell above water surface 22.1 0 43° 39° Upstream filter (submerged) 23.3 0 39° 37° Downstream filter and top of upstream filter above water surface 21.9 0 39° 37° Core (below reservoir level) 20.3 30 10° 10° Core (above reservoir level) 19.8 30 10° 10° Note: The φ't values listed above are representative peak friction angles for relatively high confining stresses. The friction angles for low confining stresses are higher. As the stress-dependence of the friction angles of the shell and filter materials has not been taken into account, the above friction angles are conservative. From the viewpoint of dam safety, the sliding surfaces of interest are those involving the dam crest because any sliding movement in the crest area would lead to a reduction of the freeboard. The dynamic slope stability calculations were made for a large number of potential sliding blocks on both the upstream and downstream sides of the dam crest. The material properties assumed for the slope stability calculations are listed in Table 2. A distinction is made between the peak and residual values of the angles of friction of the various dam materials. This reflects the fact the shear strength usually drops to some extent after attaining a maximum value as shear strains increase. The maximum operation reservoir level was taken as 542 m a.s.l. for the calculation of pore pressures in the upstream shell and filter. The pore pressures in the clay core were obtained from the estimated distribution of pore pressure ratios ru over the dam height (note: ru = u / γ h, where u is the pore water pressure and γ h is the total vertical pressure expressed as a product of the total unit weight γ and the depth h). The yield accelerations of each potential sliding block were calculated for the following three cases: i) Using peak angles of sliding friction and ignoring influence of vertical acceleration ii) Using residual angles of sliding friction and ignoring influence of vertical acceleration iii) Using residual angles of sliding friction and considering influence of vertical acceleration Newmark sliding block analysis As soon as the absolute horizontal acceleration of a potential sliding mass exceeds the yield acceleration, it starts to move. The relative velocity of the sliding mass grows as long as the earthquake acceleration remains above the yield level. When the acceleration falls below the yield level, the motion gets braked and, after some time, the sliding mass sticks to the underlying material again. The permanent displacement due to the earthquake shaking is determined by integrating the time history of the relative sliding velocity produced by the acceleration pulses exceeding the yield level. For the Newmark analysis, the time history of the absolute horizontal acceleration averaged over each potential sliding mass is needed. The maximum, mean and minimum values of the permanent horizontal displacements of the critical sliding block at the upstream slope of the dam are listed in Table 3 for the average dynamic material properties, as well as for the upper and lower bounds of the dynamic material properties. In this table, the resulting vertical displacements, estimated on the basis of a geometrical consideration, are also
Page 1 and 2: 1 st National Symposium and Exposit
Page 3 and 4: DESIGN EARTHQUAKE GROUND MOTION The
Page 5: Fig. 1: Dependence of dynamic shear
Page 9 and 10: Danger of leakage through core Rela

EARTHQUAKE SAFETY EVALUATION OF ATATURK DAM

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