Seismic Vulnerability Analysis Of Major Ethiopian Dams With Emphasis On Dam Safety Evaluation

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Dams are critical facilities, which require special consideration to ensure their longtermrnsafety. Among the safety concerns for large storage dams, seismic safety playsrnan important role, particularly for dams located close to the seismically active regionsrnlike the East African Rift System. Large dams in such seismically active regions mustrnbe capable of resisting severe earthquake ground motion expected at the dam siternwithout uncontrolled release of water impounded in the reservoir. This can bernachieved by conducting a comprehensive site-specific seismic hazard analysis, properrnseismic design, adequate construction quality control, and appropriate operation andrntimely maintenance for upgrading any seismic deficiency, particularly for older dams.rnThe main factor contributing to the risk of large storage dams is the water stored inrnthe reservoir. Some of the reservoirs in Ethiopia are very large. In this dissertation, thernseismic safety evaluation of large Ethiopian dams is analysed, which includes thernreview of previous works, site-specific seismic hazard evaluations, seismic riskrnanalysis and detailed seismic safety analysis.rnA review of the seismic design criteria used for large dams in Ethiopia shows thatrndifferent criteria were considered and, in some cases, the poorly known seismicrnactivity in the project region was ignored. In some dams, modern design and safetyrncriteria were used whereas in other projects out-dated seismic design guidelines andrncodes were employed. As a result, the existing, under construction and planned damsrnrequire detailed seismic safety reviews to comply with modern seismic safety criteria.rnThe dam sites are located in variable geological and tectonic settings, which arernresponsible for the spatial variability of the seismicity at dam sites. The main tectonicrnstructure in Ethiopia is the Main Ethiopian Rift (MER), which is characterized by itsrnextensional tectonic nature, seismically active faults and major fractures that affect thernsafety of dams and may cause water losses from the reservoirs. This requires anrnextensive geological investigation beyond the footprint of the dam.rnFor the seismic hazard study, the seismogenic source zones were modelled byrnintegrating the information developed from the regional geology, tectonics, seismicrnrnenergy release map, and observed seismicity. The seismic hazard analyses werernconducted based on the probabilistic approach and a seismic hazard map is developedrnfor the horizontal component of the peak ground acceleration (PGA) for a returnrnperiod of 10,000 years. Six seismic zones are delineated in this map. In addition, forrnthe different seismic zones, an estimation of the future power and irrigation potentialrnof Ethiopia is made. Moreover, the dam sites are ranked according to the PGA-valuesrnfor return periods of 10,000 years. These results are used as input for the seismic riskrnanalysis.rnThe seismic risk of 30 large Ethiopian dams was evaluated. In the risk analyses, thernlevels of seismic hazard for which the dam is exposed, the vulnerability of the dam,rnand the consequences in the case of uncontrolled release of water from the reservoirrnwere considered. Based on the risk analysis results, the following five dams Gibe rnGERD Saddle dam, Gidabo, Tendaho, and Tekeze dams were selected for detailedrnsite-specific hazard evaluation and seismic safety analysis. In the site-specific seismicrnhazard analyses, multiple earthquake effects were taken into account. For thernnonlinear stress, deformation and stability analyses acceleration time histories werernused, which match the acceleration response spectra obtained from the seismic hazardrnanalysis.rnGibe III dam is an RCC gravity dam with a height of 243 m. It is the world's highestrnRCC dam. The dam site is located at the border of the seismically active MER.rnSeismic stability analyses are carried out to check the response of the dam for thernupdated ground motion parameters of the safety evaluation earthquake (SEE). Thernstatic and dynamic analyses are performed using a two-dimensional (2D) plane stressrnfinite element model of the highest cross-section of the dam. First, a linear-elasticrndynamic analysis is carried out followed by the dynamic sliding stability analysis ofrndifferent detached concrete blocks. The foundation rock is assumed massless, whichrnimplies that only the kinematic interaction effects are considered. The hydrodynamicrnpressure acting on the upstream face of the dam was represented by an added massrnaccording to Westergard, assuming incompressible water in the infinite reservoir. Thernspectrum-matched acceleration time histories are used as input in the dynamicrnrnanalysis. All dynamic analyses are done by direct time integration of the equations ofrnmotion.rnGrand Ethiopian Renaissance Dam (GERD) is the largest hydropower stationrncurrently under construction in Africa. The main dam is a 145 m high rollerrncompacted concrete (RCC) gravity dam. It will create a reservoir with a volume ofrnabout 74 km3. Besides the RCC dam, there is a 5.2 km long saddle dam with arnmaximum height of 65 m and a volume of 17 Mm3. It is one of the longest concretefacedrnrockfill dams (CFRD) in Africa. The seismicity in the project area is assumed tornbe low. However, the information on historical seismicity is scarce in the region.rnBecause of the size and importance of the GERD project, the seismic stability of thernsaddle dam is checked for the ground motion with a return period of 30,000 years.rnThe dynamic analysis of the saddle dam is carried out by the equivalent linear methodrnusing a 2D dam model of the highest cross-section. The results show that the dam isrnsafe under the worst-case earthquake loading and the crest settlement is insignificantrncompared to the available freeboard.rnGidabo dam is a central core earth-fill dam with a height of 27 m. The projectrnincludes an intake tower supported by a pile foundation in the upstream part of therndam, which is connected with a diversion conduit laid on a weak compressiblernfoundation passing through the dam body. During dam construction (2016), thernconduit that was laid on the soil foundation settled, creating a vertical offset of morernthan 50 cm at the joint between the pile-supported intake tower and the conduit due tornthe static loads from dam construction. Moreover, the dam site is located in thernseismically active MER with several destructive earthquakes recorded in the past. Thernseismic stability analysis was conducted to determine the maximum deformation ofrnthe dam and settlement of the conduit when it is subjected to the SEE ground motionrnwith 10,000 years return period. The total settlement estimated during SEE isrntolerable. The dam is safe against overtopping, as sufficient freeboard is provided.rnHowever, cracking of the clay core along the conduit due to differential settlementrnmay lead to internal erosion. Moreover, the offset of the conduit will increase and thernconduit may not be adequate for lowering of the reservoir after the SEE.rnvirnTendaho dam is one of the largest irrigation dams located in a region of highrnseismicity in Ethiopia. Movements along tectonic faults and other discontinuities inrnthe footprint of the dam that can be activated by strong earthquakes close to the damrnare expected to be the worst-case seismic effects for the dam. The present study aimedrnto check the seismic safety of the dam when it is subjected to both ground shaking andrntectonic fault movements. The dynamic analyses were carried out by a 2D model ofrnthe highest cross-section using the equivalent linear analysis method. The results ofrnthe dynamic analyses show that the maximum loss of freeboard is 1.89 m due to slopernmovement, seismic densification of the embankment and fault displacement in thernfootprint of the dam that can be accommodated by the available freeboard of 3 m.rnSeepage along the fault in the dam foundation due to damage of the grout curtain andrnerosion along the dam-abutment contact due to seepage are possible.rnThe 188 m high Tekeze dam is the highest arch dam in Africa. The project area isrncharacterized by undulating topography with steep slopes and deep valley, which isrndifferent from the other dam sites. Therefore, mass movements into the reservoir thatrngenerate impulse waves are possible during strong earthquakes. The seismic stabilityrnof the critical slopes is checked for the horizontal component SEE ground motionrnwith a conventional pseudo-static procedure. The landslide is modelled as a solidrnmass and three-dimensional (3D) free radial propagation of the impulse wave wasrnconsidered for estimating the wave generation and wave propagation parameters. Thernparameters controlling the impulse waves on dams were computed and the size ofrnresulting impulse waves in the reservoir was determined. The maximum wave run-uprnand the possibility of dam overtopping were estimated. The results show that thernmaximum wave run-up under worst earthquake action can be accommodated by thernfreeboard allowance of 5 m adopted in the design. As a result, there is no overtoppingrnrisk expected from landslides generated impulse waves. However, the overall result ofrnthe present study highlights the importance of reviewing the seismic safety of the damrnfor the increased level of earthquake ground motion.

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Seismic Vulnerability Analysis Of Major Ethiopian Dams With Emphasis On Dam Safety Evaluation

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