THREE-DIMENSIONAL SEEPAGE BACK ANALYSIS OF DAM FOUNDATION AND ABUTMENTS: A CASE STUDY OF THE KARKHEH DAM

Abstract

The foundation of the Karkheh Dam comprises a heterogeneous geological profile of highly permeable conglomerate layers interbedded with relatively impervious mudstones. Following the extreme flood events of March and April 2019, the reservoir first reached its normal water level (NWL) of 220 m above sea level and subsequently rose by approximately 5 m, peaking at 224.82 m on April 5. Although the dam crest is at 234 m, these conditions raised concerns regarding structural stability and potential seepage hazards, particularly during future floods. To mitigate these risks, strict operational measures were implemented to limit the hydraulic gradient within allowable design values, thereby reducing the likelihood of internal erosion and piping in the foundation and abutments. A three-dimensional seepage model of the foundation and abutment system was developed using FEFLOW 3D, incorporating six conglomerate layers at four reservoir levels. The model was calibrated against monitored seepage discharges by adjusting hydraulic conductivity coefficients and validated with piezometric data. The key results included the identification of the uplift pressure distribution beneath the spillway chute and pinpointing the location of maximum uplift pressure, which is critical for implementing new drainage wells. In addition, the analyses determined a threshold reservoir elevation above which the maximum average hydraulic gradient increases to levels that pose a heightened risk of internal erosion in the foundation and abutments. These findings, along with refined permeability parameters and hydraulic gradient distributions, provide deeper insight into the dam’s hydro‑geotechnical response under extreme hydrological loading and support targeted hydraulic safety management.