Heavy rainfall contributes to landslides primarily by saturating the soil, which makes it heavier and less stable.
Increased Weight: Water fills the pore spaces within the soil, significantly increasing its overall weight. This added load puts more stress on the slope.
Reduced Cohesion/Friction: Water acts as a lubricant, reducing the friction between soil particles. It also decreases the "effective stress" holding the soil together, essentially weakening the bonds within the soil mass.
Increased Pore Water Pressure: As water infiltrates and saturates the soil, it creates pressure within the pore spaces (pore water pressure). This upward pressure effectively reduces the normal force on the potential slide surface, making it easier for the soil mass to move downslope under gravity.
