Surface hydraulic conductivity mapping via rock mass characterization of the Chelchel dam site in Bale Zone, southeastern Ethiopia

Abstract

The Chelchel dam site, located in southeastern Ethiopia, is likely affected by seepage problems associated with the characteristics of the underlying sedimentary rocks. The area was investigated at six selected locations using direct and indirect approaches to characterize and classify the rock mass and estimate its geomechanical and hydraulic properties. In this study, the rock mass rating (RMR), Q-system and geological strength index (GSI) were evaluated. The hydraulic properties of rock masses were determined using empirical methods based on the concepts of the cubic law. Evaluation of the geomechanical and hydraulic properties provide valuable geotechnical information regarding the rock mass quality and hydraulic behavior of the dam site. The results indicate that the dam site falls under fair and good rock mass quality. The hydraulic conductivity of the dam site was calculated using the Lugeon test data and discontinuity parameters such as aperture, spacing and surface roughness. The calculated hydraulic conductivity values based on average apertures and spacing frequency vary between 2.26E-05 and 1.17E-01 m/s while values calculated considering the effect of roughness and aperture vary between 7.61E-14 and 2.20E-08 m/s. Moreover, the hydraulic conductivity from the Lugeon test in boreholes, near the surface, ranges from 3.97E-09 to 8.35E-03 m/s. The study findings indicated that discontinuity aperture and spacing have a direct control on hydraulic conductivity, leading to seepage and instability problems. Thus, the dam site has a high to very high hydraulic conductivity from medium to high permeable rock masses with open to moderately wide apertures and moderately to closely spaced discontinuities. The rock mass consists of thinly bedded shale, mudstone, sandstone and some limestone rocks. Therefore, the geological media of the dam site is considered a pervious formation where excess seepage beneath the foundation is expected unless proper remedial measures like grouting, cutoff-wall and proper drainage are applied.