Investigating anomalous seepage trends in selected earth dams: a comprehensive study

Abstract

Previous geotechnical reports established that the selected three dams, Igbeti, Awon, and Asa dams’ embankments are loosed and permeable and there is possibility of erosion within dam embankment. Geotechnical and seepage analyses of the earth dams were conducted to evaluate the dams’ safety against the leakages through the embankment dam.

Samples were collected at three different locations from the upstream and downstream sides of the dams, at depth of 600 mm using auger borer, and geotechnical tests were conducted on the samples, according to BS1377 of 1977 to determine the specific gravity (SG), sieve analysis, cohesion (C), angle of internal friction (?), coefficient of permeability (K), and natural moisture content (NMC). Steady- state analysis, using SEEP2D was employed to investigate the seepage flows within the dams, to simulate flow rates, pore pressure, velocity magnitude, hydraulic gradient, and seepage quantity.

Specific gravity values of the samples ranged from 2.36 to 2.79 (upstream) and 2.27- 2.75 (downstream). The particle sizes passing through sieve number 200 (0.002 mm) varied from 1.00 – 21.21% (upstream) and 0.58- 23.71% (downstream), while maximum dry densities are within the limiting values of 19- 23.5 kN/m² (upstream) and 35.5 – 39.5 kN/m² downstream. Permeability coefficients obtained varied averagely from 1.36 x 10^-5 to 8.18x 10^-4 at upstream and 9.32 x 10^-6 to 4.45x 10^-4 at downstream, and values obtained classified the soils as low- permeability, silty clay embankment materials. Natural moisture contents varied from 10.47% - 26.72% at upstream, 10.71%- 23.60% at downstream. Seepage analyses results for Igbeti, Awon and Asa dams were respectively: flow rates (3.9 x10^-7 – 7.8 x10^-7; 2.98x10^-6 – 3.32x10^-6; 6.00x10^-6 – 6.60x10^-6 m³/s); pore pressure (50000 – 16800; 60000 – 93000; 130000 – 185000 kN/m); velocity magnitude (0.0 – 0.000705; 7.93x10^-11 - 6.30x10^-6, 9.24x10^-11 – 5.85x10^-6 m/s).

The flow rates through the selected dams showed saturated embankments with the possibility of piping, and excessive leakage. Installation of internal drainage facilities such as sand filters, and toe gravel drains were recommended.