Dam Engineering

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Browsing Dam Engineering by Author "Asie Kemal (PhD)"

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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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    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.