Geotechnical Evaluation and Remediation Strategies for Seepage, Pipping and Liquefaction Hazards at Yanda Dam
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Date
2023-09
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Addis Ababa University
Abstract
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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Keywords
Dam, Foundation, Alluvial, Seepage, Piping, Liquefaction