Investigation of underground seepage problem of a clay core rockfill dam and its fast and cost-effective treatment method

Excessive foundation seepage beneath earthfill dams can lead to stability problems and significant water losses, ultimately reducing irrigation efficiency or energy production. This study investigates the unexpected foundation seepage observed at the downstream toe of an operating irrigation-purpose clay core rockfill dam founded on complex bedrock composed of pervious karstic limestone and underlying nearly impervious schist. Although a concrete blanket and shallow grouting had previously been applied as seepage control measures, persistent leakage indicated their limited effectiveness. The study addresses this problem through three main approaches: (ⅰ) identification of the seepage mechanism and flow paths using transient finite-element seepage analyses, rather than conventional steady-state analyses; (ⅱ) reliable calibration of the hydraulic conductivity of the pervious limestone foundation based on time-dependent reservoir levels and measured seepage discharges; and (ⅲ) evaluation of a cost-effective and practical remedial measure. The results demonstrated that extending the grout curtain down to the underlying impervious schist located in very shallow depth provides an efficient and economical solution. The numerical calibration achieved a high accuracy (R² = 0.9489), and the proposed shallow grouting treatment reduced foundation seepage from 194 L/s to 0.09 L/s. Following implementation, no measurable leakage was observed in the field, confirming the effectiveness of the proposed seepage control strategy.