Stability analyses of dams using multidisciplinary geoscience approach for water reservoir safety: case of Mpu Damsite, Southeastern Nigeria

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.