10.3: Resisting vs. Driving Forces
Resisting vs. Driving Forces
Slope movement occurs when forces pulling down-slope (mainly gravity) exceed the strength and inertia of the earth materials that compose the slope. Mass wasting can be initiated on slopes already on the verge of movement by rainfall, snowmelt, changes in water level, stream erosion, changes in groundwater, earthquakes, volcanic activity, disturbance by human activities, or any combination of these factors.
The steepest angle at which rock and soil material is stable and will not move downslope is called the angle of repose Links to an external site., which is measured relative from the horizontal. When a slope is at its angle of repose, the shear force of gravity is in equilibrium with the normal force. If the slope becomes just slightly steeper, the shear force exceeds the normal force, and the material starts to move downhill. The angle of repose varies for all material and slopes depending on many factors, such as grain size, grain composition, and water content. In general, if the total shear force acting on the earth materials exceeds the total normal force acting on the earth materials, the slope will become unstable and a landslide is likely (Figure 10.5).
Figure 10.5: Differences in the shear and normal forces of the gravitational force on slopes with differing steepness. The gravitational force is the same in all three cases. (CC-BY-SA-NC 4.0, Paul Inkenbrandt Links to an external site.; modified by Chloe Branciforte)
There are several contributing factors that increase the effects of down-slope forces and weaken the cohesion of slope materials, thereby increasing the chance of a mass wasting. Some of these contributing factors can be prevented by human mitigation, while others we are unable to change.
Geology
The overall strength and composition of the rock, orientation of bedding, or foliation planes are all important contributing factors. For example, a bed oriented in the same direction as the slope is more likely to fail than bedding going against the slope (Figure 10.6).
Figure 10.6: Relative stability of slopes as a function of the orientation of weaknesses (in this case bedding planes) relative to the slope orientations. (CC-BY 4.0, Steven Earle Links to an external site.; modified by Chloe Branciforte)
Water
Slope saturation is a primary cause of mass wasting. A small amount of water helps to hold grains of sand or soil together; for example, you can build a larger sand castle with slightly wet sand than with dry sand. Too much water, however, causes the sand to flow away quickly. Rapid snow melt or rainfall adds extra water to the soil, which increases the weight of the slope and makes sediment grains lose contact with each other, encouraging flow.
Landslides and flooding are closely allied because both are related to precipitation, runoff, and the saturation of ground by water. In addition, debris flows and mudflows usually occur in small, steep stream channels and often are mistaken for floods; in fact, these two events often occur simultaneously in the same area.
Vegetation
Tree roots, grasses, and other indigenous vegetation can help bind soil together and prevent motion. After a wildfire has ravaged a slope, it is vulnerable to mass wasting and flash flooding during heavy precipitation events.
Undercutting
The process of undercutting occurs naturally along coastlines and river bank; it involves running water or waves removing material from the base of the slope, but not at the top, thereby increasing the slope’s angle and the likelihood of slope failure. Undercutting is particularly dangerous when the underlying rock layers slope towards the area.
Ground shaking
An earthquake, volcanic eruption, or even just a truck driving by can shake unstable ground loose and cause a slide. The occurrence of earthquakes in steep landslide-prone areas greatly increases the likelihood that landslides will occur, due to ground shaking alone or liquefaction Links to an external site.. For example, during the 1994 Northridge earthquake Links to an external site., more than 11,000 landslides were triggered in the immediate area (10,000 km2). In volcanic regions, lahars Links to an external site. and general slope failures are possible. In some cases, submarine landslides Links to an external site. occur, which may induce tsunami Links to an external site. waves.
Human Impacts
Humans can contribute to mass wasting in a number of different ways. When an area is under construction, the overall slope must be considered. Construction planners need to consider drainage, loading (addition of weight), undercutting, removal of vegetation, and ground shaking. If the slope becomes compromised, the slope will need to be stabilized before moving forward with construction; however, stabilization does not always prevent future landslides.