Engineering geology
slow movements

Slow Movements

Movements such as rockslides, rock avalanches, and lahars are certainly the most spectacular and catastrophic forms of mass wasting. These dangerous events deserve intensive study to enable more effective prediction, timely warnings, and better controls to save lives. However, because of their large size and spectacular nature, they give us a false impression of their importance as mass-wasting processes. Indeed, sudden movements are responsible for moving less material than the slower and far more subtle action of creep. Whereas rapid types of mass wasting are characteristic of mountains and steep hillsides, creep takes place on both steep and gentle slopes and is thus much more widespread.


Creep

Creep is a type of mass wasting that involves the gradual downhill movement of soil and regolith. One factor that contributes to creep is the alternating expansion and contraction of surface material caused by freezing and thawing or wetting and drying. As shown in Figure 1, freezing or wetting lifts particles at right angles to the slope, and thawing or drying allows the particles to fall back to a slightly lower level. Each cycle therefore moves the material a tiny distance downslope.

Figure 1 – Creep The repeated expansion and contraction of the surface material causes a net downslope migration of soil and rock particles.

Creep is aided by anything that disturbs the soil, such as raindrop impact and disturbance by plant roots and burrowing animals. Creep is also promoted when the ground becomes saturated with water. Following a heavy rain or snowmelt, a water-logged soil may lose its internal cohesion, allowing gravity to pull the material downslope. Because creep is imperceptibly slow, the process cannot be observed in action. However, the effects of creep can be observed. Creep causes fences and utility poles to tilt and retaining walls to be displaced (Figure 2).

slow movements
Figure 2 – Effects of creep Although creep is an imperceptibly slow movement, its effects are often visible. (Photos by D. Bradley/NOAA, Marli Miller, and Tim McGuire Images, Inc./Alamy)

Solifluction

When soil is saturated with water, the soggy mass may flow downslope at a rate of a few millimeters or a few centimeters per day or per year. Such a process is called solifluction (literally “soil flow”). It is a type of mass wasting that is common wherever water cannot escape from the saturated surface layer by infiltrating to deeper levels. A dense clay hardpan in soil or an impermeable bedrock layer can promote solifluction. Solifluction is also common in regions underlain by permafrost. Permafrost refers to the permanently frozen ground that occurs in association with Earth’s harsh tundra and subarctic climates. Solifluction occurs in a zone above the permafrost called the active layer, which thaws to a depth of about a meter during the brief high-latitude summer and then refreezes in winter. During the summer season, water is unable to percolate into the impervious permafrost layer below. As a result, the active layer becomes saturated and slowly flows. The process can occur on slopes as gentle as 2 to 3 degrees. Where there is a well-developed mat of vegetation, a solifluction sheet may move in a series of well-defined lobes or as a series of partially overriding folds (Figure 3).

Solifluction
Figure 3 – Solifluction lobes near the Arctic Circle in Alaska Solifluction occurs in permafrost regions when the active layer thaws in summer. Because summers are cool and very short, frozen soils generally thaw to depths of less than 1 meter (3 feet). (Photo from the James E. Patterson Collection courtesy of F. K. Lutgens)

The Sensitive Permafrost Landscape

Many of the mass-wasting disasters described in this chapter had sudden and disastrous impacts on people. When the activities of people cause ice contained in permanently frozen ground to melt, the impact is more gradual and less deadly. Nevertheless, because permafrost regions are sensitive and fragile landscapes, the scars resulting from poorly planned actions can remain for generations. Permanently frozen ground, known as permafrost, occurs where summers are too short and cool to melt more than a shallow surface layer. Deeper ground remains frozen year-round. Permafrost is extensive in the lands surrounding the Arctic Ocean (Figure 4).

Figure 4 – Distribution of permafrost in the Northern Hemisphere More than 80 percent of Alaska and about 50 percent of Canada are underlain by permafrost.

Strictly speaking, permafrost is defined only on the basis of temperature; that is, it is ground with temperatures that have remained below 0°C (32°F) continuously for 2 years or more. The degree to which ice is present in the ground strongly affects the behavior of the surface material. Knowing how much ice is present and where it is located is very important when it comes to constructing roads, buildings, and other projects in areas underlain by permafrost.
When people disturb the surface, such as by removing the insulating vegetation mat or by constructing roads and buildings, the delicate thermal balance is disturbed, and ice within the permafrost can thaw. Thawing produces unstable ground that may slide, slump, subside, and undergo severe frost heaving. When a heated structure is built directly on permafrost that contains a high proportion of ice, thawing creates soggy material into which a building can sink (Figure 5). One solution is to place buildings and other structures on piles, like stilts. Such piles allow subfreezing air to circulate between the floor of the building and the soil and thereby keep the ground frozen.

Figure 5 -When permafrost thaws This building, located south of Fairbanks, Alaska, subsided because of thawing permafrost. Notice that the right side, which was heated, settled much more than the unheated porch on the left. (Photo by Steve McCutcheon, U.S. Geological Survey)

By E. J. Tarbuck, F. K. Lutgens, Illustrated by D. Tasa

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