The occurrence time of landslides has a significant characteristic with periods distribution

according to landslide statistics and research results nowadays

in addition to rainfall reason

moon gravitational tide was recognized as another apparent reason. The formulas of tidal stress at the surface of earth were derivated by modifying a simplified elastic earth model proceeding from the theory of Earth tide. The formulation of simplified Fellenius method was modified taking the effect of tidal stress into account; then the mechanism of landslide triggered by tidal stress was discussed. The Xintan landslide in Zigui county

Hubei province occurred in 1985 was taken as an example

based on the modified broken lines method

to discuss the correlation between tidal stress and time

and how the safety factor of stability change at the time. The results show that

the tidal stress and the safety factor of slope stability changes with time at daily period; and range of safety factor of this slope could up to 3.4%

according to the calculation. Another conclusion is that the landslide occurs when the value of tidal stress rise to the maximum. It is also shown that the earth tidal stress is another probable triggering factor on landslide which has been in the critical failure condition

besides the rainstorm and other factor.