In 2015, the southeast pit wall of the Mae Moh mine in Thailand experienced a large slope failure, attributed to the hydromechanical (HM) coupling process influenced by rising subsurface water levels due to rainfall. This study presents a back-analysis of the pit wall displacement and factor of safety (FS) by integrating an available slope monitoring data with a 3-dimensional distinct element model to investigate the influence of hydromechanical coupling on discontinuities in the southeast pit wall. Three numerical modeling scenarios were considered: (1) a completely dry slope, (2) a slope with a water table without HM calculation, and (3) a slope with a water table and HM coupling calculation, where only scenarios (2) and (3) considered the water table in discontinuities. Comparative analyses between these scenarios revealed differences in FS. The dry slope case yielded the highest FS value, while the HM calculation case resulted in the lowest FS value. The numerical model result was consistent with slope monitoring data. As a result of the process in which pore pressure changes, displacement occurs, and displacement causes water pressure to change concurrently. Based on these findings, a design criterion for pit walls, both in short-term and long-term mining scenarios, should be given significant importance on the hydromechanical coupling processes associated with rainy seasons and rising of subsurface water levels.