This study numerically evaluates the stability of RS walls with select, marginal, and fly ash backfills under rainfall infiltration. A transient seepage, global stability, and lateral deformation were analyzed considering different rainfall intensities. As infiltration flux increases from 10 mm/h (low rainfall) to 80 mm/h (very heavy rainfall), wall stability decreases significantly due to the excessive buildup and inadequate dissipation of pore water pressure. Pore water pressure increases considerably due to infiltration. Fly ash fill exhibits approximately 125% higher pore water pressure than select fill. To allow for the dissipation of pore water pressure, the effect of chimney drains of various thicknesses was analyzed on the stability of the RS wall. It was observed that the stability of the wall increased with increasing thickness of the chimney drain.