Triggers and Driving Forces

External forces include: 1) increase in slope angle, 2) removal of support low on a slope, usually through ocean-wave and stream erosion, 3) adding mass high on a slope through sediment deposition, 4) removal of vegetation.

Internal forces include: 1) essentially weak material on top of the failure surface, 2) water in many roles, 3) decrease in cohesion. 4) earthquakes.

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External forces:

1) Increase in slope angle will contribute to slope failure. Processes that are involved are typically fault movements. Road cuts can also be considered as an increase in slope angle. Through human intrusion previously unexposed bedrock is exposed to elements, and the surface material that had previously been stable is now free to move at an increased rate causing the potential for hazards. 

2) Removal of support on a slope typically through ocean-wave and stream erosion will contribute to slope failure. On the cut bank of a stream, rapid erosion is taking place. Once this material has been removed there is no longer support for the surface material and slope failure can occur. The intensity of the landslide depends on the height of the surface material. More material mean larger landslide event.

3) Adding mass high on a slope usually through sediment deposition on dumping of waste material by humans can cause slope failure. The added weight could be just enough for a mass to slip along a slip surface.

4) Removal of vegetation along slopes will put the slope at higher risk for saturation causing slope failure.  Vegetation loss on a cut bank puts the slope at an even higher risk for failure. If the vegetation is not supporting the bank and the stream continues erosional processes, the rate for slope failure drastically increases unless it is quickly stabilized.  

Internal forces:

1) Essentially weak materials on top of a failure surface such as clay is formed during chemical weathering as rocks exposed at the surface decompose and form new minerals under conditions of low temperature and pressure (Abbott 2004). Clay minerals can become weak or strong depending on the physical conditions present. If  the clay material are weaker there is a better chance for gravity to take over and cause slope failure (Abbott 2004).

2) Water is second to gravity when it comes to slope failure. The more saturated a material is the more likely it will loose its equilibrium and become an hazard. Water weakens earth material by weight, interaction of clay materials, decrease in cohesion of rocks, subsurface erosion and pressure between slope materials (Abbott 2004). As long as material is absorbing water beyond it carrying capacity it puts the slope at greater risk for failure.

Here is an example of soil types and percents in the Eureka River Landslide and Dam, Peace River Lowlands, Alberta

(Miller 2002)

3) Decrease in cohesion is weakening the bond between materials. Once pressure is released on rocks and become exposed they increase opportunity for water infiltration.

4) Earthquakes can cause slope failure by ground shaking. Some slopes could be on the verge of failure and all that is needed is a little nudge from an earthquake.

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