In this study, the effect of implementing a cement–bentonite grout curtain at different depths on seepage control and pore water pressure reduction in the foundation of the Aqdakesh concrete dam was numerically investigated. The simulation was carried out using SEEP/W under steady-state conditions to evaluate the distribution of hydraulic head, flow lines, and uplift pressure beneath the foundation in scenarios with and without a grout curtain at depths of 5, 10, 15, and 20 m. The results showed that increasing the grout curtain depth from 0 to 10 m led to an approximately 75% reduction in seepage discharge and a decrease in uplift pressure in the downstream zone, whereas increasing the depth beyond 10 m resulted in only a limited reduction in seepage discharge (approximately 10%) and uplift pressure. This behavior is attributed to the reduction in the permeability of the clayey layers and the increase in their density and cohesion as a result of grouting, which restricts seepage paths. The selection of the cement–bentonite mixture, due to its low permeability and its ability to maintain integrity under differential settlement, improves foundation stability and prevents the occurrence of boiling. From an economic point of view, increasing the curtain depth beyond 10 m increases construction costs by approximately 45%, while yielding only about a 10% additional reduction in seepage discharge. For the Aqdakesh Dam under the current conditions, a depth of 10 m is an optimal option from both technical and economic points of view.