To explore the suitability of soft rock residual soil as backfill material for geogrid-reinforced embankments under the humid and rainy climate region

this study investigated the drainage performance of geogrid-reinforced improved soft rock residual soil structures under different rainfall conditions. A large-scale rainfall infiltration model test device was designed to study the rainfall infiltration patterns

infiltration rates

and drainage efficiency of different drainage design schemes under conditions of short-term intense rainfall and short-term extreme rainfall and long-duration rainfall. The results show that: (1) The rainfall infiltration can be divided into three stages: unpressurized infiltration

pressurized infiltration

and saturated infiltration. These structures exhibit low infiltration capacity and poor drainage performance. The saturated hydraulic conductivity stabilizes around 0.28 mm/min. (2) Graded gravel drainage layers significantly improved the drainage performance of the soft shale residual soil. Medium gravel drainage layers perform best

followed by fine gravel

while coarse gravel perform relatively worst. The performance is closely related to the particle composition of the fill material. (3) At rainfall intensities of 20

30 mm/h

and 40 mm/h

the graded gravel drainage layers increase drainage volume by 21.7%

36.7%

and 34.6%

respectively. Replacing unidirectional geogrids with composite drainage geogrids increases drainage by 7.4%

10.1%

and 4.89%

respectively. The drainage duration is extended by 1.5

2.5 h

and 4.5 h. Both graded gravel drainage layers and composite drainage geogrids effectively improve the drainage performance of geogrid-reinforced soft rock residual soil structures.