Stability analyses of dams using multidisciplinary geoscience approach for water reservoir safety: case of Mpu Damsite,
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ORIGINAL PAPER
Stability analyses of dams using multidisciplinary geoscience approach for water reservoir safety: case of Mpu Damsite, Southeastern Nigeria S. N. Ukpai 1 Received: 19 April 2020 / Accepted: 18 September 2020 # Springer-Verlag GmbH Germany, part of Springer Nature 2020
Abstract Geological, hydrological, and geotechnical conditions affected water reservoir safety at the studied damsite. Geophysical and hydrogeological surveys were integrated with hardness test, geological strength index (GSI), rock mass rating (RMR), and shear and compressive strength analyses of bedrocks. Fresh bedrocks were identified at minimum depth of about 10 m with resistivities ranging from 1670 to 4539 Ohm m for the respective argillaceous Ezeaku Group and indurate Asu River shales; both depicted 54 and 47.5 of RMR consecutively. Further results showed very blocky and slightly weathered bedrock, uniaxial compressive strength (UCS) ≥ 20 MPa, mean point load index of 9.3 MPa, and allowable bearing capacity (qal) ˃ 21 MPa. The bedrock qualities were certified fair for bearing dam loads, hence, signified competence as stable bearing surfaces, but devastated due to exposure to oxidation processes introduced by seepages through clay/siltstone that overlies bedrocks as composite layer. Then, uplift pressure (Pu) was initiated, resulting in failure modes. Predictably, occurrence of critical exit gradient and eventual general failure is envisaged due to vigorous leakage into the foundation zone from settling tail water portion. Panaceas should include Pu ˂˂ Dw (dam weight), grouting the leakage zones with concretes of UCS ≈ 20 MPa, and piling into deep foundations. Keywords Damsite . Seepages . Uplift pressure . Dam foundation . Composite bedrock . Reservoir safety
Introduction Background to the study Climatic change due to global warming drives hydrologic system to extreme events like flooding and drought (Sivakumar 2011). Adverse effects of these components have been impeding the economy of developing nations, in terms of sustainable freshwater supply for food production, in energy sector and domestic works. For instance, over the past 105 years that temperature has increased by 1.1 °C, annual rainfalls have decreased by 81 mm (Akpaodiogaga and Ovuyovwiroye 2010) resulting in longer period of drought than annual duration of flood characteristic rain falls in Africa. Consequently, perennial streams and rivers are
* S. N. Ukpai [email protected] 1
Department of Geology, Ebonyi State University, Abakaliki, Nigeria
increasingly modifying to ephemeral conditions, thus lessened crop production, slowed both services in industries and economic activities, particularly around southeastern Nigeria. The region is the economic stronghold of the nation through palm oil, kernel oil, and rice production. Because the underlying geology is dominated by mineralized rocks (Obaje 2009) and low permeable sediments that spanned from Lower Benue Trough to Anambra Basin (Appendix 1), occurrences of alternative water supply from groundwater resourc
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