A landslide is the rapid, downslope movement of rock, debris, or earth under the direct influence of gravity. This phenomenon occurs when the forces pulling the material down a slope overwhelm the internal strength of the slope material itself. The frequency and destructive potential of these events are strongly correlated with the landscape’s geometry, meaning a slope’s angle directly controls its stability. Understanding why a steeper hillside is more likely to fail requires looking at the fundamental balance of forces acting within the earth material.
The Physics of Slope Failure: Shear Stress vs. Resistance
Gravity pulls the slope material straight down, but on an incline, this force resolves into two distinct components. Shear stress acts parallel to the slope surface, attempting to pull the material downhill. Normal stress acts perpendicular to the slope, providing the frictional resistance that holds the slope together.
As the angle of the slope increases, the shear stress component that drives movement grows significantly larger. Conversely, the normal stress component that resists movement simultaneously decreases. This happens because the normal stress is less effective at pinning the material in place.
Landslides are triggered when the downhill-pulling shear stress exceeds the material’s total shear resistance, causing a sudden break. When a slope becomes steeper, the driving force increases while the resisting force diminishes, tilting the balance toward failure.
The Destabilizing Role of Water Saturation
Water acts as a powerful destabilizing agent, especially on vulnerable steep slopes, by influencing both the driving and resisting forces. Heavy rainfall or snowmelt first increases the overall weight of the soil and rock mass. This added load translates into a higher shear stress, pushing the slope closer to its failure threshold.
Water filling the microscopic spaces between soil and rock particles generates pore water pressure. In a saturated slope, this pressure pushes the grains apart, reducing the effective normal stress that holds the particles in frictional contact. Since shear strength is proportional to this effective stress, increased pore water pressure causes a dramatic reduction in the slope’s internal strength.
This process effectively lubricates the potential sliding surface, making it easier for the material to move. When a steep slope is saturated, the combination of increased shear stress from the added weight and decreased shear resistance makes failure imminent. An extreme example of this strength loss occurs during liquefaction, where saturated sandy soils temporarily lose nearly all shear strength.
Geologic Factors and Material Cohesion
The likelihood of failure is determined by the inherent strength and structure of the earth materials. This strength is defined by two properties: cohesion and the angle of internal friction. Cohesion represents the internal bonding strength of the material, such as cemented bonds in rock.
The angle of internal friction measures how well particles lock together to resist sliding, a property dominant in coarse-grained materials like sand and fractured rock. Steep slopes are more stable if they are composed of solid, intact rock with high cohesion and friction. However, many steep slopes consist of weak, highly weathered rock or unconsolidated soil.
A dangerous scenario involves steep slopes where the underlying geologic structure is oriented unfavorably. If layers of rock or ancient sediment bedding planes are tilted parallel to the slope face, they create natural planes of weakness. When such a layer is weak or contains clay, it acts as a ready-made slip surface, allowing the driving shear stress to overcome the material’s limited resistance and trigger a landslide.