Hydrologic characteristics of a large rockfill dam: Implications for water leakage

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The Unmun Dam, a rockfill dam with a clay core, filter zone, and sandy gravel shell, is located in the Nakdong river basin near Cheongdo-gun in Korea. In the dam area, the Cretaceous Gyeongsang supergroup andesite lies over intrusive granite. These form the intrusive contact near the main dam and cofferdam areas. The stratigraphic units in upward sequence consist of Cretaceous granite and andesite, weathered soft rock, and a soil layer of silty clay with gravel. The reservoir was first filled in August 1994 and was full by April 1998. Right after complete filling, three sinkholes were found in the dam crest near stations 2, 12, and 10 when the reservoir level was at around 150 m. The amount of leakage increased with rise of the reservoir level mainly caused by rainfall events. As a countermeasure for the abnormal leakage and dam rehabilitation, comprehensive surveys were performed on the dam including observational drillings, grouting, geophysical surveys, and tracer tests by Korea Water Resources Corporation (KOWACO). There were also further investigations for hydrologic characterization of the dam to identify water leakages. The groundwater levels at the 26 observation wells installed on the crest and dam toe were measured to evaluate the relationship between the reservoir level and groundwater level, and the influences on the dam leakage. Groundwater level at the left side of the embankment was high compared with a normal phreatic line; it was only 4 m lower than the reservoir level while 6.65 m lower than that at its right side. Fluctuation of the groundwater levels in the crest wells coincided well with the reservoir level without a significant lag time, which reflected a good hydraulic connection between the reservoir and the dam interior. Hydraulic conductivity estimated from 149 hydraulic tests at 9 monitoring wells constructed in the dam core showed a wide range between 1.42 × 10 −4 and 4.35 × 10 −8 cm/s with a geometric mean of 3.13 × 10 −6 cm/s. Furthermore, water quality at the monitoring wells were much different from that of the reservoir. High levels of turbidity and electrical conductivity were observed at the monitoring wells in the left side of the bank. All these hydrologic characteristics reflected possible occurrence of the leakage on the dam, especially in the left embankment. This paper presents a summary of several investigation studies on the hydrologic characteristics of the rockfill dam, which implied abnormal water leakage.