Dam Engineering

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    Comparative Performance Evaluation of Roller- Compacted Concrete and Conventional Vibratory Concrete Gravity Dams Under Earthquake Loads: A Case Study of Gibe III Dam in Ethiopia
    (Addis Ababa University, 2023-11) Alemayehu Mamo; Asie Kemal (PhD)
    Ethiopia is currently planning to build many large, medium and small dams as a step forward to the development of the hydropower and irrigation potentials in the different regions of the country. Rolled Compacted Concrete (RCC) dams are among the technologies that have been recently introduced for hydroelectric projects. The recently completed Gibe III dam, a 243m high dam, located on the Omo river in the south-western part of Ethiopia, is located in the vicinity of the seismically active region of the main East African Rift System (EARS). Many existing concrete dams, including Gibe III dam, have been in operation for decades in several countries, however their capacity to perform well during major seismic events is yet to be confirmed when seismic performance criteria have changed and/or new developments have taken place in seismic hazard assessment, in defining design/evaluation earthquakes, in methods of seismic analysis, and in the dynamic behavior of materials. In this study, Gibe III RCC dam was selected as a case study and its seismic performance was compared with an idealized equivalent CVC dam. Project information for the existing dam and previous site-specific seismic studies were reviewed; design/evaluation earthquakes, as recommended by the ICOLD (2010), were defined; ground motion acceleration-time histories were selected from PEER-NGA database, and then modified (scaled) using SeismoMatch 2022. The comparative seismic performance evaluation was carried out using linear-elastic time history analysis procedures with the help of finite element simulation program, ANSYS Workbench 2023 (Transient Structural). The seismic responses (deformations and stresses) were evaluated and compared in accordance with the acceptance criteria provided in the US Army Corps of Engineers (USACE, 2007) guidelines. Based on the analyses results, it was found that both types of dams have demonstrated high degree of similarities in deformations and stresses when subjected to critical earthquake load combination cases comprising Operating Basis Earthquake (OBE) and Safety Evaluation Earthquake (SEE). In terms of deformation behaviors, however, the results for the CVC dam were found slightly lower in magnitude than that for the RCC dam type, which resulted from the use of a homogeneous and higher grade of mass concrete (18MPa compressive strength) in the CVC dam as opposed to the use of variable compressive strengths (8 to 18MPa) in different regions of the RCC dam. The study also concluded that the tensile stress responses of both types of dams to OBE ground motions in reservoir empty condition are falling inside the performance limit, indicating “little or no damage.” However, their responses to OBE and SEE ground motions in reservoir full conditions are found to exhibit nonlinear responses, indicating the need for nonlinear analysis using 2-D and/or 3-D finite element modelling. In the Ethiopian context, considering the potential advantages that are possibly gained in large proportions (time, economy, methodology) from the RCC method of construction, the choice of RCC method of construction is highly recommended as this method allows the use of wide range of material properties and variable strengths of concrete in different regions that are sometimes unacceptable or unsuitable in the case of CVC method.
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    Comparison of Modal Analysis and Time History Finite Element Method for Analysis of Concrete Gravity Dams: A Case Study of Gilgel Gibe III Dam
    (Addis Ababa University, 2023-10) Esubalew Molla; Asie Kemal (PhD)
    The Gilgel Gibe III Dam is a 243 m-tall roller-compacted concrete dam with a hydroelectric power plant On the Omo-Gibe River in Ethiopia. Concrete gravity dams are stable structures that can withstand external stresses with the help of their weight, shape, and strength. Stresses affect these complex structures, which is developed due to the action of dead loads, reservoir and tail water loads, uplift pressure, earth pressure, and silt pressure. Because it is frequently long and straight, a concrete gravity dam can be conceived of as a plane strain structure. The analysis is carried out using two-dimensional (2D) numerical modeling since the majority of the loads operating on the dam are distributed along the same plane. The design is based on rigid body analysis with "no tensile stress permitted" standards. Nonlinearity in shape, material, and the boundary condition of dams, foundations, reservoirs, and dynamic systems, however, may now be accounted for using finite element analysis methods. This study examine assessments on a roller compacted concrete dam (RCC) and analyze the effects of seismic loading using 2D numerical modeling in ANSYS software with comparisons between modal analysis and time history finite element analysis are compared in the case of the Gilgel Gibe III RCC dam. The distribution of stress under static and dynamic loading situations is also evaluated. The analysis of the Gigel Gibe III dam is used as a control case, and the number of mode shapes required for each modal approach to produce an accurate analysis is determined. For the findings, the study provides the time histories of a few significant parameters including directional deformation, the maximum principal stress and minimum principal stress at the dam heel and the horizontal component of displacement at the dam crest. For comparison's sake, the direct method's outcome is also displayed in each graph. The results of each analysis method will also be compared to show how they differ in their correlation concerning the design of gravity dams. In this work, the researcher visualizes stress distributions and dam displacement patterns using ANSYS software. The study's overall conclusion demonstrates that both analysis methods (time history and modal) have their effective areas like modal in terms of time while the time history method includes non-linearity in the dam structure.
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    Geotechnical Evaluation and Remediation Strategies for Seepage, Pipping and Liquefaction Hazards at Yanda Dam
    (Addis Ababa University, 2023-09) Mesfin Kere; Tezera Firew (PhD)
    The Yanda Dam project is a proposal for a dam on the Yanda River in the Southern Nations, Nationalities, and Peoples' Region, Ethiopia. Its purpose is to irrigate a 5,500-hectare command area. However, the dam's foundation poses challenges due to its deep alluvium deposit and shallow groundwater conditions. This makes it susceptible to issues such as seepage, piping, and liquefaction potential. To address these concerns, this research paper presents a methodologies (empirical and numerical methods) for studying and simulating the dam's alluvial foundation, as well as suggesting remedial measures for the deep alluvial soil. The alluviums at the Yanda dam site consist of loose sand and silty sand soils. These soils have been classified into two categories: cohesion soils and cohesion-less soils. The cohesion soils include silty clay with a trace of sand, silty clay with a trace of sand and gravel, and clayey silt with a trace of gravel. The cohesion-less soils include fine-grained silty sand and coarse gravel. Due to the lateral variability in the soil condition of the foundation, three representative cross-sections were selected for the analysis. This approach helps to address the issue of soil heterogeneity in both the vertical and horizontal directions, providing a more accurate understanding of the foundation's behavior. To control excessive seepage and piping in the alluvium foundations, various remedial measures have been considered. These include the use of an upstream blanket, a cutoff wall, and a clay core trench. To determine the effectiveness of these seepage control methods, a comparison was made by conducting seepage analysis using Geo-Studio’s SEEP/W software, and liquefaction assessment of the foundation was also conducted using Quake/W. The method of extending the clay core key trench to create a barrier against seepage and piping. In addition to the clay core key trench, a soil-bentonite slurry trench is also constructed. This slurry trench helps further prevent piping by creating a low-permeability barrier. To perform these remedial measures, equipment such as long (12.8m) and short (5m) armed excavators, roller compactors, and dozers are used. To address liquefaction hazards, the method of removal and replacement is used for upstream issued part and for downstream sections, densification by deep blasting compaction would be recommended. This method involves using controlled explosions to compact the soil and increase its stability, thus minimizing the risk of liquefaction.
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    Overtopping of Dam by Landslide Wave Action a Case Study of Sego Dam
    (Addis Ababa University, 2024-02) Amdemariam Shiferie; Netsanet Nigatu (PhD)
    Since dam failure occurrences have resulted in numerous recorded human casualties and severe economic crises throughout history, it is without a doubt necessary to prepare safety plans and hazard management strategies. It is crucial to set up risk management, emergency action plans, or evacuation planning systems to safeguard people and property during abrupt dam failure phenomena brought on by landslide impulse wave causes. In the context of a case study, this thesis examined the landslide impulse wave in a dam under a number of pre-established scenarios. The case topic was the sego dam, which is situated in Ethiopia's Gamo zone SNNPRS. Landslide zonation map of the area around the reservoir rim has been prepared using ILWIS tool. Considering existing historical landslide spot, susceptibility map and geological structures three main landslide point has been identified for impulse wave analysis. Different maps such as historical inventory map, land use and land cover, soil, slope, aspect, lithological maps, distance from road and distance from rivers have been prepared using ArcGIS. Using the actual topographic map of the area, the geometry of the dam and from prominent literatures design inputs, the wave generation and propagation, wave run-up and overtopping has been estimated by excel spreadsheet templates. Three main landslide impact zones have been chosen after taking into account the landslide susceptibility map and discovered geological faults. In terms of overtopping volume and depth ,the impact of the event is minimal whenever the slide position is far from the dam (landslide three), and the worst-case scenario occurs when the landslide impact point is close to the dam (landslides one and two).In landslide one with bulk slide volume of 24,000m3 and drop height of 50m, the wave height ,period, amplitude, length and wave run-up height found to be 6.94m,18.04s,5.55m ,332.14m and 7.34m respectively and overtopping volume per unit length dam crest and across the dam axis length found to be 50.61m3/m, 13,564.3 m3 respectively. In land slide number two, which had a bulk slide volume of 27,000 m3 and a drop height of 35 m, the overtopping volume per unit length dam crest and across the whole axis was determined to be 1.8 m3/m, 482.4 m3 respectively, and the wave height, period, amplitude, length, and run-up height were all estimated to be 5.75 m, 18.53 s, 4.6 m, and 4.49 m, respectively. Finally, the wave height, period, amplitude, period, and wave run-up height for landslide three, which situated at the end of the reservoir and has a bulk slide volume of 115,200 m3 and a drop height of 25 m, were found to be 0.1 m, 6.93 s, 0.07 m, 155.5 m, and 0.11 m, respectively. In conclusion, because the dam's freeboard is 4 meters in the analysis, overtopping of the flood by landslides one and two has observed in the computation.
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    Stress and Deformation Analysis of Genale Dawa III Hydropower Concrete Faced Rock Fill Dam, Ethiopia
    (Addis Ababa University, 2023-10) Tegegn Filfilu; Asie Kemal (PhD)
    The deformation of the dam is a serious problem for the faced rockfill dam, so it is very important to analyze the stress and deformation of the concrete faced rockfill dam. Genale DawaIII hydropower CFRD is the dam in Ethiopia in which the study is conducted which aims to analyze the maximum stress and deformation. The finite element software Midas GTS NX is used to establish the model to analyze the stress and deformation of the Genale Dawa III hydropower concrete face rockfill dam. The stress and deformation characteristics of the concrete faced rockfill dam at the normal water storage condition, designed flood level condition, and normal water storage condition during the earthquake are obtained through analysis. Settlements, horizontal displacements, and principal stresses are evaluated for the above conditions, and these results are compared with each other in detail. The result shows that the maximum horizontal displacement is 60cm at the crest and vertical displacement 103cm occurs in normal water pressure with earthquake conditions. The computed maximum tensile stress at the concrete facing was found to be 1.94MPa, which is greater than the tensile strength of concrete (1.67MPa). This indicates that the tensile capacity of concrete is insufficient in the linear-elastic range, and hence non-linear analysis is required to ensure that no collapse would occur during the dam's life span. Additionally, the computed maximum compressive stress is 42.12MPa, which is significantly greater than the concrete's compressive strength (25.3MPa), justifying the need for additional studies using a non-linear analysis approach. The maximum major and minor principal stresses are located near the perimetric joint or on the concrete facing at the heel of the dam.