Re: Erosion and Deposition

Area: Earth Sciences
Posted By: Richard Ted Jeo, Bio Sci Tech, USDA-ARS University of Minnesota, St. Paul, MN
Date: Fri Oct 25 11:21:52 1996
Message ID: 845849371.Es

Megan,
About your question on friction and mass movements:

In terms of mass movements (such as landslides, rock falls, mud flows, and soil creep) friction is the resistance of the material to move. When looking at mass movements, there are two types of forces involved on the slope:

1. Shear stress- this is the force that wants to make the material move and is mostly a factor of steepness of slope. Simple put, the steeper the slope, the more shear stress.

2. Shear strength- this is the force that wants to hold the material in place and it incorporates the friction component you are asking about. Shear strength takes into account such things as frictional resistance between particles or material in the slope. For example, let's look at two types of gravel. The first is angular with sharp edges on each piece. The second type is round and smooth like what you might find in a stream bed. If you were to pour each type of gravel into a large piles you would find that the angular gravel would form a steeper slope than the smooth gravel. This is because the angular gravel has more resistance because its sharp edges do not glide past each other like in the smooth gravel. The angular gravel slope has more shear strength.

When looking at a slope, you might find other things that increase the shear strength, like the ability for particles of a soil to adhere to one another (like a clay) or the presence of plants roots which tend to keep soil in place. These things also increase shear strength.

In order to get a mass movement, such as a landslide, the shear stress only needs to overcome the shear strength of an instant, and then gravity takes over and the landslide occurs.

There are a number of factors that can trigger a mass movement by increasing the shear stress or decreasing the shear strength. These include:

Water- water that is either in the soil or that is ground water can influence the balance between the two forces. Water could act as a lubricant and reduce the friction part of shear strength. Water can also add weight to a soil on a slope reducing the shear strength.

Undercutting- either by natural (such as the changing of a stream bed) or man made (such as a road cut through a hill). Material is removed from the base of a slope, thereby creating an oversteep slope and increasing the shear stress.

Frost- when water freezes it expands. This expansion has the ability to move material down slope, slowly, but then as the steepness of the slope changes, so the shear stress and shear strength changes. Mass movement by frost may not be a "quick catastrophic event" like a landslide; it may move very, very, slowly over time, making the slope move downhill slow enough that it takes sensitive instruments to measure the movement.

Seismic type waves- these include mainly earthquakes, but also originate from heavy equipment, explosions or even loud sounds. Essentially what happens is that the seismic wave causes particles in the slope to shift, move or rotate. This particle movement can cause a change in the shear strength (making it weaker) or increase the shear stress. This is one reason why there are major landslides in areas that are prone to earthquakes.

To see some pictures of landslides, click here

Hope this answers your question.
Ted

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