The mechanism of deformation and failure of the nearly horizontal strata landslide is studied from lithologic characters and inducing factors
and the start-up of these landslides is mainly due to sustained rainfall or intense rainfall. From the point of view of mechanics
the start-up criterion of the landslide is discussed; also critical depth of fracture water is deduced. Studies indicate that the effect of the fracture water is conditional; if the thickness of sliding mass is a constant
when the length of sliding surface exceeds a fixed value (safe length)
the depth of fracture water has little effect on the stability of the slope. For different length of sliding surface
the critical depth of fracture water is calculated for shallow
medium-bedded and thick-bedded landslides respectively.