Landslide occurs when the slope changes from a stable to an unstable condition. The landslide is part of the rock and soil along the pre-existing geological interface or the newly formed shear failure surface sliding down. In the slope failure, the landslides have the most serious destructive.The instability of the slope is related to the destruction of the soil under shear stress. The shear stress and soil shear strength affect the stability of the slope such as the geologic properties and the changes of soil mass in the slope, the relationship between discontinuities stratum and inclination surface of the slope.
The change of slope size and shape and erosion or excavation to the edge of the slope with a natural or artificial load on the slope, the change of groundwater level in the slope and the transient vibration caused by the earthquake and blasting will all influenced by the stability of the slope (Hales and Miniat, 2016).The type of strengths relies on the loading condition; time elapsed after the excavation and type of soil. The total stress strength is commonly used for the short-term conditions in the clayey soils and the effective stress strength is commonly used in the long-term conditions in all types of soils. The second definition is based on the force equilibrium, and the third definition is based on the movement equilibrium.
The second and third definition can be confusing sometimes whether the moment or force is a contribution on driving or resisting sides. For example, the force components along the sliding surface are considered on the resisting sides as the positive contribution as it increases the capacity of resistance against the movement. On the other hand, the component can be considered on the driving side as the negative contribution as it diminishes the driving tendency. The slices weights are situated at the tow and considered either on the driving side as the negative contribution or on the resisting side as the positive contribution.