Landslide Triggering Mechanisms
A large landslide damages State Highway 4 near Stella, Washington.

Your job on this assignment is to learn about landslide triggering mechanisms well enough that you can diagnose several different landslide problems. After you have studied this page thoroughtly you are to go to the Landslide Study Question Conference and participate in a discussion of six different landslides.

The study questions that pertain to this assignment are titled Slide 1, Slide 2, etc. You should post a detailed message for at least one of these slides. If you are the first person to post a message for a slide you should state the triggering mechanism and how the slide might have been caused by natural or human actions. If someone else has already posted a message for that slide you should review what has been posted and post your own message that either agrees or disagrees.

Be sure that you look at all six slides and come to an understanding of them. Your quiz on this lesson will contain similar slides for you to diagnose on your own.

 

Landslide Triggering Mechanisms

The soils on most hillsides are in a stable state. If they were unstable a landslide would have taken them to the bottom of that hillside a long time ago. The landslides that we see today occur when natural processes or human activities disturb a stable slope.

The processes and activities that cause landslides are frequently referred to as "triggering" conditions. Although people trigger a few landslides most are triggered by natural processes. A summary of landslide triggering conditions is given below.

The columns of this table separate the two landslide triggering categories (natural and human), the rows of the table are the different landslide triggering processes, and the cells within the table give examples of situations that can caused landslides.


Natural Triggers Human Triggers
Removal of
Support		 erosion at the base of a slope by streams, waves, glaciers excavation at the base of a slope or excavation on a hillside
Removal of Vegetation forest fires timbering
Addition of Moisture rainfall or snowmelt sewage or runoff disposal, broken water pipes, improper grading
Addition of Weight heavy snowfall, volcanic ash, landslides placement of fill
Oversteepening (this term is sometimes used in the same context as "removal of support") placing fill at a gradient that exceeds the angle of repose
Vibrations earthquakes, thunder blasting, operation of heavy equipment


Removal of Support:

Excavation or erosion at the base of a slope (or on a slope) can cause an unstable situation. The removed material was frequently supporting the soil directly upslope from the disturbed area. This loss of support can immediately or eventually result in landsliding.


Removal of Vegetation:

Vegetation does two important things to reduce landslide incidence. 1) the vegetation removes water from the soil; and, 2) the root systems support the soil and provide a stabilizing effect. Areas that experience forest fire or clear-cut timbering are subject to landsliding for many years after the vegetation is removed.


Addition of Moisture:

Many soils, especially clays, are hard when dry but transform into soft muds when a small amount of moisture is added. The addition of water reduces the shear strength of the soil and can result in landsliding. Also, water pressure within the porosity of a soil has the effect of "inflating" the pore spaces and reducing the frictional forces between soil particles. This loss of friction can result in a rapid landslide. The addition of moisture is the most common source of landslide problems.


Addition of Weight:

Adding weight to the top of a slope has the same effect as increasing the force of gravity. If the added weight exceeds the shear strength or increases the pore pressure of the soils below landsliding can result.


Oversteepening:

The angle of repose is the maximum angle that a material can be stacked and remain stable. If soil is piled at an angle that exceeds the angle of repose, landsliding can result. This term is sometimes used to describe situations in which erosion, excavation, or grading results in slopes that exceed the angle of repose.


Vibrations:

Sudden movements can cause the particles in a soil to briefly lose contact with one another. When this happens, the frictional forces that enable that material to remain on a slope are lost and landsliding can be triggered. Vibrations from earthquakes, blasting, heavy equipment, and loud noises have been known to trigger landslides.


The landslide photo at the top of this page was taken by R. L. Schuster of the United States Geological Survey. It is part of the USGS Landslide Photo Collection.