EARTHQUAKE SAFETY EVALUATION OF ATATURK DAM

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The earthquake analysis of the dam consisted of the following steps: 1. Generation of input ground motion (in the form of artificial spectrum-compatible accelerograms) corresponding to the MCE, which was selected on the basis of the available information about the seismic hazard at the dam site 2. Assessment of the following dynamic properties of each dam material • Maximum dynamic shear modulus (i.e. shear modulus for very small dynamic shear strains), which depends mainly on the mean effective static stresses and the relative density in the shells, and on the undrained shear strength in the clay core • Strain-dependence of dynamic shear modulus and damping ratio • Shear strength properties: angle of internal friction and cohesion • Other properties: Poisson’s ratio and mass density 3. Dynamic analysis of the dam subjected to the earthquake ground motion by means of a finite element model using the equivalent linear method first developed by H.B. Seed and co-workers; this method is the state-of-the-practice in the earthquake analysis and design of embankment dams (several cases were analyzed in view of the random nature of the earthquake shaking and uncertainties concerning the dynamic material properties; the main result of interest from the dynamic analysis is the distribution of the earthquake accelerations in the dam, in particular, in potential sliding blocks) 4. Selection of potential sliding surfaces and calculation of yield accelerations by performing slope stability calculations (note: the yield acceleration of a potential sliding mass is the pseudo-static horizontal earthquake acceleration for which the factor of safety against a sliding failure is equal to 1.0) 5. Calculation of the permanent earthquake-induced displacements of the potential sliding masses based on the knowledge of their yield accelerations and the distribution of the earthquake accelerations (Newmark sliding block analysis) 6. Seismic settlement analysis to determine the effect of vibration-induced densification of the dam materials during the earthquake shaking 7. Determination of total loss of freeboard due to the sliding displacements caused by the dynamic slope instabilities as well as the seismic settlements 8. Assessment of effect of the earthquake-induced deformations on safety against internal erosion in the dam. MAIN FEATURES OF ATATURK DAM The Ataturk Dam is a zoned rockfill dam with a central core and is located on the Euphrates River. The main features of the dam are as follows: • Dam height: 170 m • Crest length: 1670 m • Crest level: 549 m • Maximum base width: approx. 900 m • Dam volume: 84 × 10 6 m 3 • Reservoir volume: 48 km 3 • Installed capacity: 2400 MW • Annual energy generation: 8100 GWh The construction of the cofferdam lasted from 1985 to 1987. The fill work for the main dam began in 1987 and was completed in 1990. The reservoir level reached 535 m a.s.l. in March 1994 and has varied between 526 m and 537 m a.s.l. since then. The maximum and minimum operation reservoir levels are 542 m and 526 m a.s.l., respectively.
DESIGN EARTHQUAKE GROUND MOTION The Ataturk Dam is situated about 50 km southeast of the East Anatolian fault. The most recent damaging seismic activity associated with the East Anatolian fault took place on 27 June 1998 during the Adana-Ceyhan earthquake, which had a surface wave magnitude of 6.2 and an epicentral distance of about 270 km from the Ataturk dam site. To estimate the peak ground acceleration (PGA) of MCE at the dam site, the seismic hazard data from the Disaster Management Implementation and Research Centre of the Middle East Technical University (METU/DMC, 1999) were used as a basis. Accordingly, the peak horizontal ground accelerations at the dam site (rock surface) for various return periods are as follows: Return period (years) 100 225 475 1000 Peak ground acceleration 0.22 g 0.27 g 0.33 g 0.39 g The maximum credible earthquake (MCE) is defined as the largest possible earthquake that can be reasonably expected to occur in the given tectonic setting. The return period of the MCE may be several thousand years. It is clear that the PGA of the MCE should be larger than that of an earthquake with a return period of 1000 years. In view of the above-mentioned information about the seismic hazard at the dam site, the PGA of horizontal shaking at the free surface of the rock due to the MCE was taken as 0.50 g. The vertical earthquake acceleration was also considered in this study. The PGA of the vertical component of the input earthquake motion was assumed to be equal to 2/3 of the PGA of the horizontal component. The elastic response spectrum defined by in the Turkish code was used (Ministry of Public Works and Settlement, 1998). In view of the random nature of the ground motion, three independent sets of artificial accelerograms were employed in the dynamic analysis. These spectrum-compatible accelerograms were generated with the computer program SIMQKE (Gasparini and Vanmarcke, 1976). The strong ground motion (i.e. the stationary part of the input motion) was assumed to last for 25 s, 30 s and 35 s for the three artificial earthquakes, respectively. TWO-DIMENSIONAL FINITE ELEMENT MODEL OF HIGHEST DAM SECTION A two-dimensional finite-element model of the dam was used for the dynamic analysis. Four-node plane-strain elements were employed. The two-dimensional model represents the central cross-section of the dam. The model geometry corresponds to the actual shape of the dam surface. The roads built on the dam surface were also modelled. Some adjustments made in the topmost part of the dam during the crest reinstatement were also incorporated. The horizontal and vertical degrees-of-freedom were fixed at the base of the dam, where the input seismic motion defined on the rock surface was applied. DYNAMIC MATERIAL PROPERTIES The behaviour of geotechnical materials subjected to an earthquake loading is highly nonlinear. In particular, the dynamic shear moduli and damping ratios of such materials are strongly influenced by the amplitudes of the dynamic shear strains. The dynamic characteristics of the dam materials had not been specially investigated by means of dynamic triaxial tests. Such tests would provide the necessary information about the dynamic properties of these materials. The material properties required for the dynamic analysis were obtained from the geotechnical literature on the dynamic characteristics of
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Page 5 and 6: Fig. 1: Dependence of dynamic shear
Page 7 and 8: Table 2: Material properties for sl
Page 9 and 10: Danger of leakage through core Rela

EARTHQUAKE SAFETY EVALUATION OF ATATURK DAM

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