© 2021, The Author(s), under exclusive licence to Springer-Verlag GmbH, DE part of Springer Nature.Prediction of spatial distribution of pore-water pressure and its control are the main issues in open pit mines, since excess pore-water pressure is one of the main causes of slope instability. Horizontal drains are commonly used to relieve the excess pore-water pressure as a method of slope stabilization. This paper presents the efficacy of horizontal drains along two cross-sections in an open pit mine located in Eastern Turkey by analyzing the re-distribution of pore-water pressure as a consequence of groundwater inflow rates before and after application of horizontal drains. SEEP/W software was used to simulate the flow in saturated and unsaturated zones along North–South and East–West cross-sections. The accuracy of the input data was verified by calibrating the models under steady-state condition against the field data obtained from vibrating wire pressure transducers installed at different depths and locations. This was followed by a transient analysis to confirm the storage parameters. The results of the transient simulation run showed that steady-state model results can be used as initial conditions for transient groundwater inflow predictions. Subsequently, expansion and deepening of the open pit for the year 2019 are modeled in stage-wise for both cross-sections to calculate the groundwater inflow rates into the mine. As a last step, horizontal drains are implemented to the models. The horizontal drains increased groundwater inflow rates by only 15 and 37% for the East–West and North–South cross-sections, respectively. Additionally, the water content was found to be almost close to fully saturation state in the wall rock behind the slope faces even after the use of drains. The findings of this study were verified at the site. The mining company has reported that attempts to drain the slope walls by horizontal drains have proved to be ineffective. In conclusion, it was demonstrated that prevention of seepage from surface channels by either diversion or lining was adopted as opposed to horizontal drill holes as the most effective means of controlling groundwater influence on slope stability.