A landslide spreads as it goes down a slope. What makes a slope stable or unstable: fs =safety factor; fs= shear strength (internal resistance to movement) / shear stress (force causing movement parallel to slope, increasing with slope angle). On a slope 2 components to gravity: slope increases, the gp (the gravity pulling the material straight down) decreases while. Gs (the gravity that is pulling the material down the slope) increases. When gs > gp, the material gives away and slides down. Nature of slope material loss of strength fluid-like behaviour lubrification and liquefaction. Anything with water = weak: water saturated materials: a) b) c) Clay, mud, slit: clays swells with water present and shrinks when no water present (smectite, sensitive clays: held together by salt when dry (crystallised structure) when wet, structure rearranges and liquifies (parallel structure) Sedimentary rocks have bedding planes (layered structure) These planes are parallel: prone to slide on top of each other.