The Nam Ngiep 1 Hydropower Project (NNP1) in Lao PDR consists of roller-compacted concrete (RCC) main dam, main powerhouse, RCC re-regulation dam and re-regulation powerhouse. The crest length and dam height of the main dam is 530 m and 167 m respectively. While geology at the main dam site consists of alternation of sandstone and mudstone, there remains a few horizontal weak layers below the dam foundation bedrock associated with folding activities and a folded zone lies nearly vertical on the right bank forming anticline to syncline. Foundation treatment was carried out carefully and a shear key was placed at the bottom of the dam body by penetrating the weak layers to reinforce the dam stability against sliding. An initial impounding of the main dam was started in the middle of May 2018. The reservoir water level reached 315.8 m in Aug 2018 and 316.8 m in Dec 2018, which are slightly lower than the NWL 320.0 m. When the reservoir water level approached the NWL, several issues such as unanticipated behavior of the dam and a rise of the underground water at the downstream abutment were observed. The initial impounding was suspended based on the Emergency Action plan, and subsequently hydro-geological investigation such as drilling, water level monitoring, water quality testing and tracer testing were implemented. By conducting measures such as additional grouting, the above issues were resolved and impounding has resumed this dry season.

1. Geological condition

The geological features of the dam consist of an alternation of sandstone and mudstone with a gentle inclination of around 8 degrees towards the downstream. The surface of the rock is weathered, with joints along the horizontal bedding and vertical fissures. There exists a remarkable box-shaped folded zone with a near-vertical inclination on the right bank. In addition, it was revealed during riverbed excavation that there were several clay mediated low angle weak layers inclined toward the downstream which were considered to have a negative impact on the safety of the dam. The photos of the foundation bedrock are shown in Fig.1 and geological profile along the dam axis is shown in Fig.2. The flexural slip seems to be developed following fracturing at the boundaries between sandstone and mudstone layers associated with folding activities when rock was not consolidated. And then a part of fracture materials transformed into clay and formed weak layers due to weathering, mainly fluctuation of the underground water.

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