+1 862 207 3288 On March 22nd 2014, a large landslide occurred near Oso, Washington. As of July 23rd, 2014, all remains had been recovered and the death toll stood at 43 people. Lots of information about the landslide can be found on the American Geophysical Union’s Landslide blog.
Read about the landslide here (don’t worry, each entry is quite short):
Oso landslide in Washington State: three people killed and the river is blocked
The Oso (Steelhead) landslide: mechanisms of movement and the challenges of recovering the victims
The Oso (Steelhead) landslide in Washington State – could it have been foreseen?
Answer the following questions:
1) Describe the factors that led to this landslide: What type of material was involved- how cohesive/prone to failure is it? Was the cause primarily due to a change in slope, a change in friction/cohesion, or addition of mass? What was this cause?
2) Was the cause of this slide natural, man-made, or a combination of both?
3) Discuss the hazard assessment/mitigation efforts in effect before the slide. What evidence in the surrounding geology/geography suggests an existing landslide hazard? Was anything being to done to reduce the risk of a damaging landslide?
For questions 4 & 5, use the photo of the Oso Landslide below:
4) What type of slide do you think this is (rotational or translational)? What visual evidence in the photo above supports your choice?
5) On the image above and using diagrams from the lecture and your textbook, label the different parts of the slide. Terms you can include, but are not limited to, are: scarp, original surface, toe, head, foot.
6) When the failed material entered the river, it created another type of mass movement; what is this mass movement and why did it make the slide more damaging?
Part 2: A little physics (it is a science class after all)
We discussed in class how whether or not a slope will fail is based on the balance of gravitational vs. frictional forces using the following diagram and equations:
For simplicity, we will ignore FR, the force of the base of the slope supporting the upper slope. In the case shown above, for the slope to be stable, the frictional resistance force, Ff, must be larger than the gravitational force acting down the slope, Fll:
Fll < Ff
7) For a slope with angle θ = 30o and coefficient of friction μ = 0.6, is the slope stable? Please show your work, partial credit will be given. Please put a box around your answer.
8) For a slope with θ = 15o, for what values of μ will the slope be unstable? In other words, at what value of μ does, Fll = Ff, such that any decrease in μ will result in a slope failure? Please show your work, partial credit will be given. Please put a box around your answer.
9) For a slope where the cohesion of the vegetation and soil leads to a coefficient of friction of μ = 0.75, above what slope angle θ will the slope fail? Note: please answer in degrees, not radians. Please show your work, partial credit will be given. Please put a box around your answer.
10) Describe why the mass of a potential slide, in the slope force balance used above, does not affect whether or not the slope will fail.
