Landslides

The term “landslide” refers to a wide range of mass wasting events that result in rock, soil, or fill moving downhill under the influence of gravity (Figure 10.8). Landslides may be triggered by high rainfall, earthquakes, erosion, deforestation, groundwater pumping, or volcanic eruptions. They may occur rapidly, such as in some mud and debris flows, or they can be as slow as soil creep: slow land movement that usually does not cause loss of life, but can still destroy roads and buildings.

Landslides and slumps are common problems in parts of the South Central that have a wetter climate and/or the presence of steep slopes, such as west Texas, the Central Lowland, and the Interior Highlands, but they can also occur in areas with low relief (Figures 10.9 and 10.10). Heavy rain, snowmelt, groundwater percolation, and water level changes along coastlines, earthen dams, and the banks of water bodies are conditions under which landslides can occur. These flood-related conditions are associated with precipitation, runoff, and saturation of the ground. Hazards occur from mudflows themselves, but also from backwater flooding, dam failure, and debris that rushes downstream and causes further erosion.

Figure 10.8: Common types of landslides.

In northern Arkansas, there is a risk of potential landslides associated with earthquakes in the New Madrid seismic zone. Steep slopes in this area increase the likelihood that landslides will occur when the ground shakes or when water rapidly infiltrates the soil during an earthquake, though, as mentioned earlier, steep slopes are not always necessary for a landslide to occur. In low-lying areas of the Coastal Plain, saturated soils and heavy rains can combine to cause soil liquefaction, which can result in laterally moving mudslides. This can be triggered by storm runoff or by rapid earth movement during an earthquake.

Figure 10.9: A landslide on the steep walls of Palo Duro Canyon in West Texas.

Figure 10.10: Landslide incidence and risk in the South Central US.

Damage to life and property can be reduced by avoiding landslide hazard areas or by restricting access to known landslide zones. Hazard reduction is possible by avoiding construction on steep slopes or by stabilizing the slopes. There are two main ways to accomplish stabilization: 1) preventing water from entering the landslide zone through runoff, flooding, or irrigation and 2) stabilizing the slope by placing natural or manmade materials at the toe (bottom) of the landslide zone or by removing mass from the top of the slope.

Expansive Soils

Soils that weather from shale, volcanic ash, or bentonite are rich in clay, which may contain minerals that can absorb water and swell up to 1.5 to 2 times their original volume. That amount of expansion can exert enough force to cause damage, such as cracked foundations, floors, and basement walls (Figure 10.11). An estimated nine billion dollars of damage to infrastructure built on expansive clays occurs each year in the United States.

Figure 10.11: Expansive soils caused cracks to form in the wall of this house in Austin, Texas.

Soil creep is a slow kind of landslide that occurs when certain types of clay in the soil on a hillside absorb water, expanding and causing the soil to swell. As the clay dries and contracts, the particles settle slightly in the downhill direction. This process can cause fences and telephone poles to lean downhill, while trees adjust by bending uphill (Figures 10.12 and 10.13).

Figure 10.12: Some influences of soil creep on surface topography.

Figure 10.13: Soil creep affects telephone poles and fence posts on a hillside in Toronto, Kansas.

While soils can swell by absorbing water, they will also shrink when they dry out, resulting in subsidence that damages landforms and infrastructure. Fissures may develop in the soil, allowing for the deep penetration of water when floods or runoff occurs. This produces a cycle of shrinkage and swelling that puts repeated stress on rock layers and human structures. While expansive soils can be found all over the US, every state in the South Central has bedrock units or soil layers that are possible sources, with Louisiana’s coastal plain and Oklahoma’s Cretaceous shales being the most susceptible (Figure 10.14).

Figure 10.14: Approximate distribution of expansive soils in the South Central US. This map is based on the distribution of types of bedrock, which are the origin of soils produced in place. (Where substantial fractions of the soil have been transported by wind, water, or ice, the map will not be as accurate.)

Significant or repeated changes in moisture, which can occur in concert with other geologic hazards such as earthquakes, floods, or landslides, greatly increase the hazard potential of expansive soils. The key to reducing this hazard is to keep the water content of the soil constant. There are also chemical stabilizers, including lime, potassium, and ionic agents, that can reduce the potential for soil volume changes by increasing the clay’s structural stability.