Hydraulic Analysis of Rock Fill Dam under Karst Environment (Weyib River)

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In this paper, the result on research of hydraulic analysis of rock fill dam under Karst foundation is presented. Karst foundation is known for its rock fill dam’s water tightness problems resulted from soluble nature of lime stone and progressive leakage. The hydraulic flow feature in this foundation has heterogeneity characterized by temporal and spatial variation. Hydraulic problems related to Karst foundation are possible causes of dams’ failures. It is well known that the key factors in the analysis of rock fill dams are direct methods of foundation investigations. But, under Karst environment, the data obtained through the process has point data nature and the foundation has heterogeneity. Therefore, such methods will result better if supported by alternative ways. One of the alternative ways of estimating the hydraulic and material properties of the dam and its foundation was estimating the average saturated hydraulic conductivities of the aquifer/ foundation and relating obtained result with Karst hydraulic and material parameteric values of similar Karst features of other places. To this regard the results obtained through published other researches were used for estimation, comparison and verification proposes. The location of the research area is between two river gauge stations that represent Karst aquifer/foundation. The river is Weyib which is found in south east part of Ethiopia. The river gauge stations were Alemkerem and Sofumar found at close distance and the area in between them was selected by taking the advantages of their spatial and temporal flow difference and its prominent Karst formation. In estimating of average saturated hydraulic conductivities of the aquifer/ foundation, average base flow estimation was conducted by three methods. The methods were USGS programme software, Excel hydrograph separation and Chpmans’ algorithm using excel. Stream flow measured at Sofumar and Alemkerem were used for base flow estimations. As per the result obtained through different methods, USGS result was selected after comparisons of the methods. During comparision, USGS software programme was selected for it has the smallest values of standard deviation of base flow to the average value.Average base flow result obtained were 8.67cumec and 18.49cumec for Alemkerem and Sofumar respectively. Average base flow difference obtained was 9.82cumec which was used for estimation of average saturated hydraulic conductivies of the aquifer and foundation. Again, based on the two contributing sides area sizes of the river, base flow results obtained were 1.89cumec and 7.91cumec for the left and right side respectively. During estimation of average saturated hydraulic conductivities, unconfined well hydraulics formula was used. Based on the result, estimated values were 6.90E-04 m/sec and 1.5E04m/sec for the left and right side of the river. Once the average values and the average saturation level/river bed were obtained, the respective values of different levels of the aquifer/ foundation were calculated with respect to it. In doing so, empirical formula was used and the results of the two methods were compared with scientific out puts of local and international researches and found sufficient and comparable. The ranges of values obtained by British geological society were 1.14E-03 m/sec and 1.14E-07m/sec and the ranges of values in this research were found almost similar. During the analysis of the dam, the aquifer was considered as foundation and the average values were required to choose dam type having similar properties with the foundation especially under saturated condition. According to different references, rock fill dam materials under compaction could have hydraulic conductivities values of 1E-05 m/sec to 1E07m/sec. Hence, in this research, homogeneous rocks fill dam, averagely addressing the hydrogeological variation was hydraulically analyzed. For the sake of convenience, the location in between the two gauge stations with its geo reference data was taken into account and the cross section of dam and the foundation was divided in two different layers with a total of 250meters above and below the river bed level. The base of karstification level was the water table/river bed level and a total of 500m depth was considered. The layering or zoning were for every 10m to 20m depth and karstification level findings with their respective values found similar with other extensively conducted researches results. The dam under consideration was with and without upstream water proof material. The “without” condition was for estimating leakage magnitude through foundation and dam body where as the “with” condition is required for estimating the water proof material properties parametric values. Geo-Studio, 2012 software was used in calculating seepage magnitudes.The results obtained by software were compared with quantitative analysis done by means of excel spreadsheet. Safe slope values obtained considering equivalent dynamic loading conditions at downstream face and the dam and its foundation as equivalent porous media were 1.87 and 2.55 for the “with’ and the “without” condition. Correspondingly, seepage values obtained were 1.04E04 cum/sec and 2.87E-07 cum/sec respectively. As per the analysis, rock fill dam was found impossible without abutment and dam water proof treatment works. Dam, foundation and abutment treatment for water proof works resulted as 10 3 to 10 5 times the foundation and dam materials saturated hydraulic conductivities parameteric values and these values were similar with other researches results. By using spreed sheet quantitative analysis, obtained values for normal effective stress were 49.99 Kpa and 99.79KPa for 10mts and 20mts heights respectively. Correspondingly, these values were comparable with Geo-Studio software outputs which were 50kKPa and 100KPa. Observed and measured seepage data were taken from dams under operation found in different parts of the world. These data were systematically referred and arranged to predict the maximum safe dam heights with manageable seepage magnitude. No rock fill dam without foundation, abutment and dam water proof treatment work, dam height up to 70mts with all parts water proof treatment and above 70mts higher seepage related risks that require proper management were concluding remarks of this hydraulic analysis of rock fill dam under Karst environment. The research had resulted in a better understanding of the threat posed by Karstic physical nature considering dams. This had led to analyze the hydraulic properties, material strength and safe dam height and type under Karst environment. Hence, it can be taken as a base for hydraulic analysis of rock fill dam in other similar areas and for those who have keen interest in further research.



Rock Fill Dam, Karst Environment, Weyib River, Hydraulic Analysis