Processes
This rural state road near the Little Juniata River, Barree, PA is sandwiched between a stream and steep hillside covered in vegetation in a mountainous area. The hillside was blasted out to fit the road, removing confining pressure and exposing a steep rock face. We would expect a very high slide or rock fall risk here. (Photo: C. Thomas.) Many factors cause surficial processes. We will first list the factors covered in the textbook, Section 10.3 and try to relate these to what we already know about soils. Then we will discuss how humans have altered these factors for worse and for better. Remember that slope failures are a natural process, so preventing them is purely to protect human life and property if these natural processes should become natural hazards.
Natural Factors
Firstly we have to remember what the driving force is of slope movement. Materials on earth, like water, are constantly moving to their lowest energy state. This means that the driving force of slope movement is gravity: gravity is constantly pulling earth materials downward. To understand the natural factors that cause slopes to fail, we have to think of factors that resist gravity or give our slope some resisting forces: what are the characteristics of a slope material that will help it to resist the driving force of gravity? Slopes fail when the resisting forces are overrun by the driving force of gravity.
Surficial processes happen naturally, so we expect several factors within nature lead to these events. You’ll notice many of these factors are the same factors that control the development of soil (…and surface water and ground water. Remember this table?) It’s usually not a single factor that causes a landslide and other processes, but a combination of several factors. Let’s list of these as they are explained in the textbook and try to relate it to what we already know.
Type of Earth Material (pp 334 – 336): The underlying resisting forces of slopes are going to be the inherent strength of the slope materials like soil strength and rock strength. So, again, just like with Earthquakes and Volcanoes, we have to consider the strength of our rocks. In general, unconsolidated rock/sediment will be more likely to move than consolidated, well-cemented rock/sediment. However, different Earth materials have different strengths and resistance to movement or breaking apart. Some rocks fracture apart easily (e.g., slate, shale), some sediment moves apart easily (e.g., sand), some rocks dissolve easily in water (i.e., ionically bonded mineral dominated rocks like limestone and sandstone), and some sediment moves apart in water (e.g., clay and it’s high shrink-swell potential). In Central Indiana, clays that shrink and swell as they get wet and dry can cause subsidence problems. (Something to think about: What type of Earth materials were cited as a cause of the Vaiont Dam failure?)
Slope (pp 336 – 338): The angle the rock sits within the Earth, or the slope of the surface of the Earth can both create conditions favorable for slope failures. When we discuss slopes, we often discuss them in percentages: Indiana has an average slope of less than 5%. This means that the average rise in elevation for every mile (5280 feet) you go in any direction isn’t going to increase by more than 250 feet. (Rise divided by run multiplied by 100 or 250 / 5280 x 100 = 4.7) To look at this in terms of an angle of the slope, average Indiana slopes are less than 3 degrees. (Something to think about: What role did slope play in the Vaiont Dam failure?)
Different slope angles. This shows an area of Yosemite National Park with several different slopes. The two that are highlighted show two steep slopes. This is a modified version of Figure 10.4 of your textbook. (Image Credit: Pearson Education Inc., 2011 and modified by D. Lani Pascual IUPUI / Earth Science)