Mass wasting is a natural geological process defined as the large-scale movement of rock, soil, and sediment down a slope due to the direct force of gravity. It differs from other forms of erosion because the material is not carried away by a moving medium like water, wind, or ice. This process shapes the landscape by transferring material from higher elevations to lower ones, where it can then be transported by other agents.
Fundamental Causes of Slope Instability
The driving force behind all mass wasting is gravity, which constantly pulls material toward the center of the Earth. On a sloped surface, this gravitational force can be separated into two components: one that presses the material into the slope and one that acts tangentially, pulling the material downslope. Downslope movement occurs when the shear stress—the tangential force pulling the material down—overcomes the shear strength—the internal resistance of the material to movement.
Water significantly reduces a slope’s shear strength and increases shear stress. When water fills the pore spaces in soil or rock, it adds considerable weight to the slope. More importantly, water pressure within the material pushes the grains apart, reducing the friction and cohesion that hold the slope together. This lubrication effect destabilizes the slope, which is why intense rainfall or rapid snowmelt often trigger mass wasting events.
The steepness of a slope is a major factor, as the shear stress component of gravity increases with the angle of the slope. For loose, granular materials, there is a maximum angle, called the angle of repose, at which the material can rest without sliding. Exceeding this angle, typically between 30 and 37 degrees for dry materials, creates an inherently unstable slope. The presence of vegetation also plays a significant role, as root systems anchor the soil and rock, increasing the material’s shear strength.
Categorizing Mass Wasting Events
Mass wasting events are classified based on two primary characteristics: the type of material involved (e.g., rock, debris, or earth) and the mechanism of movement (fall, slide, or flow). The speed of the movement can range from imperceptibly slow to dangerously rapid, which differentiates the categories.
Falls represent the most rapid form of mass wasting, involving the free-fall of rock or sediment from a steep cliff or slope. These events, such as rockfalls, occur when material detaches along existing fractures or bedding planes. The movement is vertical or near-vertical and often includes bouncing and rolling as the material descends.
Slides involve the movement of a coherent mass of material along one or more distinct surfaces of weakness. A translational slide moves along a relatively flat or planar surface, such as a bedding plane or a fault. In contrast, a rotational slide, also known as a slump, involves movement along a concave, curved rupture surface. Slumps are common in thick deposits of unconsolidated sediment, resulting in a backward rotation of the material.
Flows occur when the material behaves like a viscous fluid, with significant internal mixing and deformation during movement. The speed and consistency of a flow depend highly on its water content. Debris flows contain a mixture of coarse material, rock fragments, and soil mixed with abundant water, resembling wet cement and moving rapidly. Earthflows are slower movements that involve primarily fine-grained, clay-rich material and often develop at the lower end of a slope.
Creep is the slowest and most widespread type of mass wasting, involving the gradual, imperceptible downslope movement of soil and loose material. This movement is often aided by cycles of freezing and thawing or wetting and drying, which cause soil particles to be lifted and then settle vertically downward. The cumulative effect of creep over long periods can be observed in tilted utility poles, retaining walls, and curved tree trunks.
Managing the Risk of Mass Wasting
Human activities frequently increase the risk of mass wasting by altering the natural stability of slopes. Road construction is a common anthropogenic trigger, often creating unstable, over-steepened cut banks or weak, poorly compacted fill banks. Altering natural drainage patterns through construction can also concentrate water, leading to oversaturation and a reduction in soil strength.
Engineers employ several techniques to mitigate mass wasting in vulnerable areas. One approach is to construct physical barriers like retaining walls at the base of slopes to support the toe and prevent movement. For rock slopes, rock bolts or soil nails are drilled deep into the hillside and anchored in stable bedrock to secure unstable rock masses.
Effective drainage control is a primary mitigation strategy, as removing water reduces weight and increases shear strength. This can involve diverting surface water away from the slope or installing perforated pipes and drains to remove subsurface water. Additionally, slope reduction, such as terracing, can decrease the slope angle, lowering the shear stress and increasing the stability of the hillside.