Remember reading my Riverside Avenue Topography page? Well, this is a part two that dives more into the analysis of the slope itself and why it failed. Slopes can fail for a number of reasons due to differing controlling factors, and here we're going to take a look at a steep slope made of unconsolidated fill that lacked vegetation and failed because of heavy rainfall.

Cohesion is an important factor that influences the probability of a landslide occurring. In order to understand cohesion and Mohr/Coulomb failure before diving into the Riverside Ave slope failure, we looked at two hypothetical materials A and B below, which have different cohesive properties. The shear strength was plotted given several normal force values; all values were measured in pascals. This data defines the shear strength values given the normal force. Material A has a y-intercept value close to zero, while material B has a y-intercept value close to 4000, which suggests that Material B has a high shear stress value when the normal force value is zero. A shear strength value that greatly exceeds the normal force value indicates stronger resisting forces, meaning that Material B is more cohesive than Material A.

We can use the equations generated by these graphs to determine what the shear strength values of each material will be if the normal force value is 4000 pascals. Substituting 4000 for 'x' in the equation y = 0.696x + 59.054 for Material A and y = 0.7171x + 3947.8 for Material B, we get shear strength values of 2843 and 6816 for materials A and B respectively. These results are consistent with the data.

The 2019 landslide can be classified as a planar or translational slide, meaning that a shallow slab of unstable material moved downhill while leaving more stable material behind. Given the history of the area (see Riverside Avenue Topography) A sensitivity analysis test was conducted for the area using the conditions of the 2019 landslide. When the landslide occurred, we know that there were 3-4 inches of rain over the course of 6 hours. Runoff from an adjacent parking lot may have contributed additional rain to help saturate the ground. Today, the toe of this landslide is visible just upslope from the toe of the 1955 slide. A mixture of sand and artificial material including cement blocks and car parts composed the slope that failed in 2019. The slope was approximately 30˚ and had little vegetation to help anchor the soil. The area had a length of approximately 20 meters, and it was estimated that the thickness of the failed slab was a few feet thick. We know that a combination of heavy rainfall, the steep slope made of unconsolidated material, and the lack of vegetation contributed to the failure of the slope, but did one of these factors have a greater influence?

To test the factors controlling slope failure, I used a range of values for the slab thickness, failure plane angle, cohesion, and saturation depth. At the site of the landslide, there was not a huge toe, which suggested that the slab thickness was fairly shallow. I estimated it to be between 1-1.5 meters thick, with a saturation of 0.5-1 meters to represent it being mostly/fully saturated. The failure plane was estimated to be between 28-32˚, which is about what we observed in the field. Finally, the sandy soil is not very cohesive, especially with cement blocks mixed in. This was estimated to be between 0-3000 pascals.

Even when I used the ranges above to create the most stable slope I could, the factor of safety had a value of exactly 1.0, which still has potential to fail. However, as different combinations of values were tested, the saturation of the ground appeared to have the most influence on the factor of safety. Working with the averages for each range, it was clear that changing the saturation of the slope impacted the factor of safety more than any other parameters. This was not surprising, because water is the most important erosional agent and a rainfall event with that magnitude had a large potential to do damage. If the slope had a higher cohesion or more vegetation, the rainfall may not have caused as much damage. Higher cohesion would have held the slope together more, and the roots from any vegetation would have done the same, or potentially absorbed some of the water.

Given these results, I believe that my model results are consistent with land use changes over time, including dumping, deforestation, paving, and run-off. Over the past century, Riverside Ave has been filled with unstable materials or materials have been dumped there illegally. Both of these actions resulted in landslides over the course of this time period. The specific area of the slope we were looking at had been deforested as well, and a parking area from above contributed more water as run-off. The air photos that document this change are consistent with the model as well. As the conditions in the model were changed to fit the changes in land use in that area, the slope became less stable and the factor of safety continued to decrease. The loss of vegetation was especially unhelpful. Today, there are trees on both sides of the slope that failed, and while they do exhibit soil creep, they appear stable for now. The downward forces of gravity combined with the increasing driving stress finally led the slope to fail. And again, using what we know about hydrology, it is most likely that in the end, it was the heavy rains that caused the slope to fail. Looking at the rainfall recurrence data, the value for a 100-year 6-hour storm in Burlington is 2.8 inches, which is close to what we received last year. That reasoning combined with the fact that water is the most important erosional agent strongly suggests that intense precipitation caused the 2019 landslide on Riverside Ave.