Landslides

Landslides, though not a common problem in Kansas, are natural phenomena that occurred in the state long before human occupation. The downhill movement of masses of soil and rock come in many forms and are potentially serious.

Rock falls occur along cliffs and outcrops where blocks of rock break off and fall down the slope. 

Block slides and slumps occur where blocks or masses of intact soil or rock move downslope along a failure surface. Block slides have straight failure surfaces, and the motion is analogous to a box (the landslide mass) sliding down a ramp (the failure surface). Slumps have concave failure surfaces. As the landslide mass moves along this curved surface, it rotates and tilts trees and other objects so that they point uphill. 

Earth flows are landslides in soil in which the landslide mass breaks apart instead of remaining relatively intact as in a slump or block slide. The motion in an earth flow is analogous to a thick mixture of soil and water oozing down the slope. 

Creep is the slow, imperceptible downslope movement of soil and rock. Creep—often detected because of tilted trees, telephone poles, or walls—is widespread on hillsides throughout Kansas.

The basic ingredients for landslides are gravity, susceptible soil or rock, sloping ground, and water.

Shales, among the most common rocks found in Kansas, are most often associated with landslides. When shale— composed of clay- and silt-sized grains—is near the ground surface where the water content fluctuates, it weathers into a clayey soil that can be landslide prone. Block slides, slumps, and earth flows commonly occur in shales and the soils developed on shales.

Two other common Kansas rocks, limestones and sandstones, can pose a risk for rock fall when they are exposed in cliffs or roadcuts, especially when they overlie shales.

Landslides require hilly terrain. In general, as the slope angle increases, the potential for landslides also increases. Anything that increases the slope angle can trigger a landslide. For example, a stream that is actively eroding a hill increases the slope angle at the base. Construction practices that increase slope angle can also cause landslides. Water acts as a lubricant in soil and rock. As water content increases, the strength of the soil and rock decreases. This can lead to landslides.