Hydraulic conductivity evaluation for seepage analysis on stratified sedimentary rocks at Chelchel dam site in Bale Zone of Ethiopia using hydraulic tests and geostatistical techniques

A dam site with stratified sedimentary formations usually shows a spatial variability in its hydraulic conductivity causing seepage problems that demands estimating its values and spatial distribution in the current dam site. Based on field observation, borehole logging and geotechnical data, the area is classified into clayey silt, gravely sand, basalt, mudrock, sandstone and limestone layers. Data from Lugeon and falling head in-situ tests were used to calculate the hydraulic conductivity values of these rocks using empirical methods. These results indicated that the right bank has a higher hydraulic conductivity and weaker rock mass quality that can cause seepage problems. Statistical and geostatistical analyses were also performed to evaluate the distribution, trends and variation of hydraulic conductivity. Multiple regression analysis revealed that hydraulic conductivity has an exponential relationship with depth and RQD in an inverse manner. The transformed hydraulic conductivity values were used for geostatistical spatial analysis and interpolation. The spatial maps produced from sequential gaussian simulation are similar to the actual observation. The rock-fill embankment dam to be built in the study area requires the application of a combined seepage controlling measures like excavation and removal at overburden along the river channel; effective compaction at the left abutment and spillway; clay blanketing at the reservoir but the intercalated sandstone-shale-mudstone rocks from 10 to 40m with medium to higher permeability and the deeper cavernous limestone require other foundation treatments including vertical cutoff wall at shallow depth upstream of the dam axis and grouting at deeper depths within limestone cavities.