Groundwater pipe drainage systems （GPDS） which are important in restricting groundwater level rising and in keeping slope stability are often found in debris slopes. With the example of Guan-jia landslide

through the sensitivity analysis which are related with stability coefficient

it is found that the rise of groundwater level leads to the landslide collapse and relapse. Cutting at slope toe or loading on slope surface may destroy the pipe seepage system and make groundwater level rise obviously which lead to the pore pressure ratio

head level and hydraulic gradient rise

and the pore-water pressure on a potential slip surface to rise. Also

the internal friction angle which is the most important factor in stability decreases when the level of groundwater rises. In the remedying design

the conception of descending the level of ground-water is emphasized

and the effect of it is quite obvious.