Mammo, Tilahun (Professor)Aman, Ali2022-04-072023-11-092022-04-072023-11-092020-02-01http://10.90.10.223:4000/handle/123456789/31189Dams are critical facilities, which require special consideration to ensure their longterm safety. Among the safety concerns for large storage dams, seismic safety plays an important role, particularly for dams located close to the seismically active regions like the East African Rift System. Large dams in such seismically active regions must be capable of resisting severe earthquake ground motion expected at the dam site without uncontrolled release of water impounded in the reservoir. This can be achieved by conducting a comprehensive site-specific seismic hazard analysis, proper seismic design, adequate construction quality control, and appropriate operation and timely maintenance for upgrading any seismic deficiency, particularly for older dams. The main factor contributing to the risk of large storage dams is the water stored in the reservoir. Some of the reservoirs in Ethiopia are very large. In this dissertation, the seismic safety evaluation of large Ethiopian dams is analysed, which includes the review of previous works, site-specific seismic hazard evaluations, seismic risk analysis and detailed seismic safety analysis. A review of the seismic design criteria used for large dams in Ethiopia shows that different criteria were considered and, in some cases, the poorly known seismic activity in the project region was ignored. In some dams, modern design and safety criteria were used whereas in other projects out-dated seismic design guidelines and codes were employed. As a result, the existing, under construction and planned dams require detailed seismic safety reviews to comply with modern seismic safety criteria. The dam sites are located in variable geological and tectonic settings, which are responsible for the spatial variability of the seismicity at dam sites. The main tectonic structure in Ethiopia is the Main Ethiopian Rift (MER), which is characterized by its extensional tectonic nature, seismically active faults and major fractures that affect the safety of dams and may cause water losses from the reservoirs. This requires an extensive geological investigation beyond the footprint of the dam. For the seismic hazard study, the seismogenic source zones were modelled by integrating the information developed from the regional geology, tectonics, seismic energy release map, and observed seismicity. The seismic hazard analyses were conducted based on the probabilistic approach and a seismic hazard map is developed for the horizontal component of the peak ground acceleration (PGA) for a return period of 10,000 years. Six seismic zones are delineated in this map. In addition, for the different seismic zones, an estimation of the future power and irrigation potential of Ethiopia is made. Moreover, the dam sites are ranked according to the PGA-values for return periods of 10,000 years. These results are used as input for the seismic risk analysis. The seismic risk of 30 large Ethiopian dams was evaluated. In the risk analyses, the levels of seismic hazard for which the dam is exposed, the vulnerability of the dam, and the consequences in the case of uncontrolled release of water from the reservoir were considered. Based on the risk analysis results, the following five dams Gibe GERD Saddle dam, Gidabo, Tendaho, and Tekeze dams were selected for detailed site-specific hazard evaluation and seismic safety analysis. In the site-specific seismic hazard analyses, multiple earthquake effects were taken into account. For the nonlinear stress, deformation and stability analyses acceleration time histories were used, which match the acceleration response spectra obtained from the seismic hazard analysis. Gibe III dam is an RCC gravity dam with a height of 243 m. It is the world's highest RCC dam. The dam site is located at the border of the seismically active MER. Seismic stability analyses are carried out to check the response of the dam for the updated ground motion parameters of the safety evaluation earthquake (SEE). The static and dynamic analyses are performed using a two-dimensional (2D) plane stress finite element model of the highest cross-section of the dam. First, a linear-elastic dynamic analysis is carried out followed by the dynamic sliding stability analysis of different detached concrete blocks. The foundation rock is assumed massless, which implies that only the kinematic interaction effects are considered. The hydrodynamic pressure acting on the upstream face of the dam was represented by an added mass according to Westergard, assuming incompressible water in the infinite reservoir. The spectrum-matched acceleration time histories are used as input in the dynamic analysis. All dynamic analyses are done by direct time integration of the equations of motion. Grand Ethiopian Renaissance Dam (GERD) is the largest hydropower station currently under construction in Africa. The main dam is a 145 m high roller compacted concrete (RCC) gravity dam. It will create a reservoir with a volume of about 74 km3. Besides the RCC dam, there is a 5.2 km long saddle dam with a maximum height of 65 m and a volume of 17 Mm3. It is one of the longest concretefaced rockfill dams (CFRD) in Africa. The seismicity in the project area is assumed to be low. However, the information on historical seismicity is scarce in the region. Because of the size and importance of the GERD project, the seismic stability of the saddle dam is checked for the ground motion with a return period of 30,000 years. The dynamic analysis of the saddle dam is carried out by the equivalent linear method using a 2D dam model of the highest cross-section. The results show that the dam is safe under the worst-case earthquake loading and the crest settlement is insignificant compared to the available freeboard. Gidabo dam is a central core earth-fill dam with a height of 27 m. The project includes an intake tower supported by a pile foundation in the upstream part of the dam, which is connected with a diversion conduit laid on a weak compressible foundation passing through the dam body. During dam construction (2016), the conduit that was laid on the soil foundation settled, creating a vertical offset of more than 50 cm at the joint between the pile-supported intake tower and the conduit due to the static loads from dam construction. Moreover, the dam site is located in the seismically active MER with several destructive earthquakes recorded in the past. The seismic stability analysis was conducted to determine the maximum deformation of the dam and settlement of the conduit when it is subjected to the SEE ground motion with 10,000 years return period. The total settlement estimated during SEE is tolerable. The dam is safe against overtopping, as sufficient freeboard is provided. However, cracking of the clay core along the conduit due to differential settlement may lead to internal erosion. Moreover, the offset of the conduit will increase and the conduit may not be adequate for lowering of the reservoir after the SEE. vi Tendaho dam is one of the largest irrigation dams located in a region of high seismicity in Ethiopia. Movements along tectonic faults and other discontinuities in the footprint of the dam that can be activated by strong earthquakes close to the dam are expected to be the worst-case seismic effects for the dam. The present study aimed to check the seismic safety of the dam when it is subjected to both ground shaking and tectonic fault movements. The dynamic analyses were carried out by a 2D model of the highest cross-section using the equivalent linear analysis method. The results of the dynamic analyses show that the maximum loss of freeboard is 1.89 m due to slope movement, seismic densification of the embankment and fault displacement in the footprint of the dam that can be accommodated by the available freeboard of 3 m. Seepage along the fault in the dam foundation due to damage of the grout curtain and erosion along the dam-abutment contact due to seepage are possible. The 188 m high Tekeze dam is the highest arch dam in Africa. The project area is characterized by undulating topography with steep slopes and deep valley, which is different from the other dam sites. Therefore, mass movements into the reservoir that generate impulse waves are possible during strong earthquakes. The seismic stability of the critical slopes is checked for the horizontal component SEE ground motion with a conventional pseudo-static procedure. The landslide is modelled as a solid mass and three-dimensional (3D) free radial propagation of the impulse wave was considered for estimating the wave generation and wave propagation parameters. The parameters controlling the impulse waves on dams were computed and the size of resulting impulse waves in the reservoir was determined. The maximum wave run-up and the possibility of dam overtopping were estimated. The results show that the maximum wave run-up under worst earthquake action can be accommodated by the freeboard allowance of 5 m adopted in the design. As a result, there is no overtopping risk expected from landslides generated impulse waves. However, the overall result of the present study highlights the importance of reviewing the seismic safety of the dam for the increased level of earthquake ground motion.enSeismic Vulnerability AnalysisMajor Ethiopian DamsEmphasis on DamSafety EvaluationThesis