Subsurface drainage is a common technique used to stabilize slopes and maintain sustainable water levels. This paper discusses the use of subsurface drainage to stabilize a backslope of a retaining wall located at an arterial highway in Sri Lanka that had undergone failure due to excess water pressure. The sustained loading from the failed debris on the structurally conceded wall made the rectification work more challenging. Therefore, it was necessary to adopt a cost-effective solution as a temporary measure to hold the failed debris until permanent measures were implemented. The study used field measurements and numerical modelling to analyze the effects of infiltration and groundwater flow on the slope before and after subsurface drains were employed. Introducing subsurface drains and controlled infiltration improved the slope’s stability and significantly avoided forming a perched water table. With the successful completion of this project, using non-structural measures accompanied by an accurate seepage analysis, design, and a monitoring plan proved to be a cost-effective solution for stabilizing highly saturated unstable slopes.