Chapter 15 Summary

The topics covered in this chapter can be summarized as follows:

15.1 Factors That Control Stability on Slopes

Slope stability is controlled by the slope angle and the strength of the material on the slope. Slopes are a product of tectonic uplift, and their strength is determined by the type of material on the slope and its water content. Rock strength varies widely and is determined by internal planes of weakness and their orientation with respect to the slope. In general, the more water contained by the slope material, the greater the likelihood of failure. This is especially true for unconsolidated sediments, where excess water pushes against the grains. Addition of water is the most common trigger of mass wasting and can come from storms or rapid snow melt.

15.2 Classification of Mass Wasting

The key criteria for classifying mass wasting are the nature of the movement that takes place, the type of material, and the speed of the material movement. Mass wasting events can be a precipitous fall of rock through the air, material sliding as a solid mass along either a plane or a curved surface, or internal flow of material as a viscous fluid. The type of material influences the mass movement, specifically whether it is solid rock or unconsolidated sediments. Slope failures can have translational (planar) or rotational (curved) rupture surfaces. The important types of mass wasting are creep, slump, slide, fall, and debris flow or mudflow.

15.3 Preventing, Delaying, and Mitigating Mass Wasting

We cannot prevent mass wasting, but we can delay it through efforts to strengthen the materials on slopes. Strategies include adding mechanical devices such as rock bolts or ensuring that water in the slope materials can easily drain away. Such measures are never permanent but may be effective for decades or even centuries. We can also avoid practices that make matters worse, such as cutting into steep slopes or impeding proper drainage. In some situations, the best approach is to mitigate the risks associated with mass wasting by constructing shelters or diversionary channels. In other cases, where slope failure is inevitable, we should simply avoid building in that location.

Questions for Review

gravitational force on the unconsolidated sediment
Source: Steven Earle (2015) CC BY 4.0 View source

1. In the scenario shown here, the gravitational force on the unconsolidated sediment overlying the point marked with an X is depicted by the black arrow. The red arrow in the diagram depicts the shear strength of the sediment.

  1. Draw in the two arrows that show how this force can be resolved into the shear force (along the slope) and the normal force (perpendicular to the slope).
  2. Assuming that the relative lengths of the shear force arrow (which you drew in question 1), and the shear strength arrow are indicative of the likelihood of failure, predict whether this material is likely to fail or not.
  3. After several days of steady rain, the sediment becomes saturated with water and its shear strength is reduced by 25%. What are the likely implications for the stability of this slope?
  4. Did you consider the affect of the additional weight of the water on the gravitational force acting on the slope in your answer to (c)? Does this change your answer?

2. In the diagrams shown here, a road cut is constructed in sedimentary rock with well-developed bedding. On the left, draw in the orientation of the bedding that would represent the greatest likelihood of slope failure. On the right, show the orientation that would represent the least likelihood of slope failure.

a road cut a road cut

Source: Steven Earle (2015) CC BY 4.0 View source

3. Explain why moist sand is typically stronger than either dry sand or saturated sand.

4. In the context of mass wasting, how does a flow differ from a slide?

5. If a large rock slide starts moving at a rate of several metres per second, what is likely to happen to the rock, and what would the resulting failure be called?

6. In what ways does a debris flow differ from a mudflow?

7. In the situation described in the chapter regarding lahar warnings at Mt. Rainier, the residents of the affected regions have to assume some responsibility and take precautions for their own safety. What sort of preparation should the residents make to ensure that they can respond appropriately when they hear lahar warnings? What other considerations do officials have to make in their emergency plan, other than just sounding a warning?

8. What factors are likely to be important when considering the construction of a house near the crest of a slope that is underlain by glacial sediments?