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
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Date
2023-11
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Addis Ababa University
Abstract
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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Keywords
gravity dams, seismic design, seismic performance evaluation, roller-compacted concrete, conventional mass concrete, dynamic analysis, time-history analysis, operating basis earthquake, safety evaluation earthquake, PEER-NGA.