Mass movement occurs when downslope driving forces exceed the strength resisting movement. Rain can add weight and weaken resistance.
Gravity pulls soil and rock downslope, creating shear stress. Friction, cohesion and roots contribute to shear strength, which resists movement. Failure becomes more likely when stress increases or strength falls. Steeper slopes, added loads and loss of support can increase instability.
A fall involves detached rock dropping, bouncing or rolling from a steep face. A slide moves along a recognisable failure surface: planar in a translational slide, curved in a rotational slide. Material remains relatively coherent compared with a flow, where particles move past one another like a fluid.
Flows include water-rich debris or earth moving downslope; some granular flows can also be dry. Creep is very slow, persistent downslope deformation. Repeated wetting and drying can help move soil, while bent tree trunks or tilted posts may provide evidence over time. Speed and water content vary within these categories.
Water can increase the weight of slope material. As water pressure in the pores rises, the particles press less firmly against one another. This reduces their effective normal stress and frictional resistance. Saturation can also remove the suction that helps hold unsaturated soil together. In a deeply weathered tropical slope, sustained rain may trigger failure after water has penetrated to a weak layer.
Earthquakes, joint orientation, weak beds and river undercutting provide natural controls. Road cutting can oversteepen slopes or remove support; buildings add load; leaking pipes raise water pressure. Vegetation clearance may remove root reinforcement, although drainage and the depth of the failure surface affect the outcome.
Step by step
Driving force
Gravity acts more strongly downslope on steep or heavily loaded slopes.
Resistance changes
Wetness, weak layers and lost roots can lower shear strength.
Movement begins
The failure surface and material determine whether it falls, slides, flows or creeps.
Worked example: A road cut after sustained rain
In an illustrative humid-tropical hillside, a road cut removes support at the toe. Rain then raises pore-water pressure along a weak clay-rich layer. Both the human alteration and water-related loss of strength explain the slide; calling rain the sole cause misses the prepared slope condition.
Watch out for this
Every movement of wet soil is a mudflow.
A saturated mass can slide along a distinct surface. Identify its movement, not only its wetness.
Check your understanding
Why can rising pore-water pressure promote slope failure?
- It increases friction between every pair of particles.
- It reduces the effective contact stress that provides frictional resistance.
- It removes gravity from the slope.