Modeling Slope Stability Using a Local Landslide and SLOPE/W
Skills and concepts that students must have mastered
- Have completed some factor of safety calculations
- Able to create topographic profiles from topo maps
- Familiar with Coulomb's equation, cohesion, internal friction, pore water pressure
How the activity is situated in the course
Content/concepts goals for this activity
- Observe a landslide in the field, recognize landslide features
- Gain experience entering data and modeling slope stability using a free student version of the professional software SLOPE/W
- Use historical topographic maps to estimate fill thickness
Higher order thinking skills goals for this activity
- Recognize assumptions and limitations of modeling slopes
- Gain experience with varying values of the cohesion, internal friction and pore pressure and how it affect the factor of safety
- Compare results of their model with the reality in the field - does the model represent the reality well?
Other skills goals for this activity
- Use historical topographic maps
- Use topo maps and survey data to create multiple topographic profiles
- Use a total station
- Make a topographic map of the slope and landslide (additional activity)
Description and Teaching Materials
- Students in geomorphology go on a field trip to view some local landslides, including a large one that has formed in fill from a highway project. Students walk then entire extent of the landslide and describe the features they see. We discuss what kind of landslide do they think it is and why do they think it formed here?
- If we only have one lab period available, we will set up the total station & survey a cross section through the landslide. If we have more time available, we will place flags within the slide and slope, and we do a more complete survey. Students will then use a base map from this topographic data and return and map the landslide features and create their own topo map of the slope.
- In the classroom, students find the landslide location on older topographic maps dating from 1973 (pre-fill) and 1983 (post-fill but pre-landslide). Students create drawn topographic profiles through the present-day landslide location of these 3 surfaces (1973, 1983, present day).
- Students then enter the 1973 and 1983 data into a student version of SLOPE/W.
- Students first model the slope stability of the 1973 slope by playing with the values of cohesion, internal friction and pore water pressure, and assume this pre-fill slope is stable.
- Students then add the fill layer from the 1983 profile, and select values for the cohesion, internal friction and pore water pressure that result in a factor of safety of less than one.
- Students compare their modeled slip surface with their field observations of the scarp and toe locations. Are they in the same place? If not, why? This slide actually began as a slump, but over time the toe has continued to move downhill in a more translational type of movement. This has in turn overloaded the lower slope, causing a smaller slump to form with its toe under the road. The model represents the initial movement well, but not the continued movement.
- They write a short report including their description of the landslide based on their field observations, the results of their model including the values they used for the stability calculations, a comparison between the field observations and the model results, and an explanation for why they may differ from each other.
- I can post a working model of this slide, screenshots of the model, ground and air images of the slide including lidar data, survey data, once I am back at school.
Link to SLOPE/W software http://www.geo-slope.com/downloads/2012.aspx
Teaching Notes and Tips
- There are benefits and drawbacks to using the total station during class time. While they do enjoy working with the equipment, with a large class, each student only gets to survey a couple of points before it is another student's turn.
- The student license for SLOPE/W will only let you enter 3 soil types, so keep models simple.
- I have taught the activity twice - the first time I had the students use a software tutorial provided by the software company to learn how to input the data. There was a lot of troubleshooting and a number of students got stuck setting up the scale and coordinate system of the model so it took a couple of lab periods to successfully model the slide. The next time I taught it I set up the profiles for them ahead of time, then it took very little time to model the slide - all they had to do was work with the variables, and they didn't have to worry about figuring out the software at all. I think this was a little too easy, so the next time I will set up the coordinate system, but have them figure out how to enter their profiles.
- An additional step would be to give them another slope upon which a contractor wants to add fill, and require them to model how this could be accomplished without slope failure.
- Would be interesting to compare the survey data with lidar data