Landslides induced by rainfall

After rainfall, groundwater pressure is built up and this elevates ground water table. Water inside the pores of soil reducesthe effective stress of soils. As shear strength of soils is represented by the subsequent relations:

Shear strength = cohesion + effective stress x tan where Φ is the friction angle of soils

Therefore the presence of water causes a decrease of shear strength of soils and this may lead to landslide. Instead rainfall creates immediate instability by causing erosion of slop surface and results in shallow slope failure by infiltration. Additionally the rain may penetrate slope surface openings and forms flow paths. Consequently this may weaken the ground.