by Philip S. Prince
The two images below show the same model landslide at different points during its evolution. The lower image, of course, looks much uglier, with a broken-up slide block and huge, looming headscarp. Notably, the thick toe labeled in the upper image is conspicuously missing…(spoiler: you can watch this model evolve at 1:04 in the video link at the end).
So, how did the slide progress from a not-too-impressive feature to a mangled mess? The answer relates to that toe, or rather its absence in the lower image, with a few details of the slide’s shape thrown in (more on that in a bit). Toe removal, however, is the main idea, and removing a landslide’s toe invites trouble in the form of probable continued movement. A slide like the one shown above (that is, not a rapid, flow-type slide) can move until the toe becomes large enough to resist the driving load that led to slide movement, assuming that friction on the sliding surface remains low. Removing the toe allows driving load to overwhelm resistance again if friction on the slide plane is sufficiently low, leading to renewed movement of the slide. The GIF below illustrates this idea in basic terms. I-26 drivers in western North Carolina might appreciate the pink excavator…
A well-developed landslide toe can therefore be regarded as the “prop” that supports the rest of the upslope slide mass atop the weak failure surface. Altering a toe is often a fast track to instability and slide reactivation. The simple model shown in the GIF below demonstrates toe development, removal, and reactivation in a slide that behaves much like the one drawn above. When toe material is removed, the slide advances until enough supporting toe is restored. Note that this model is much “neater” than the model in the first images!
Continued toe removal leads to continued motion, particularly in the model above, as the slide mass does not break apart and continues to exert a strong driving load on its base. Once the slide mass is broken along its edges (lateral scarps), it no longer has the small amount of support provided by the cohesion (“stickiness”) of the model material. Real rock and many soils have some cohesive strength as well. It is never very much, but it does provide a component of resistance to sliding. Once the material is broken, however, this tiny bit of resistance is lost, and ongoing movement of the slide due to toe removal is that much easier. In a model like the one above, the entire slide block can be consumed by removing the initial toe and continuing to remove material pushed out into the new toe.
Weakness and potential instability associated with a landslide can persist for a long time after the slide becomes dormant and is not obvious in the landscape. Nearly “invisible” old slides that aren’t currently moving can be reactivated long after they stopped moving by disturbing the slide toe. Using lidar to identify such old slides is a key part of pre-construction site assessment.
Destabilization of landslides by toe removal is a very real thing, with actual scenarios–both large and small–arising from construction and natural stream or wave erosion. Many are slow and manageable, but they can be ongoing, hard to stop, and quite expensive over time. A famous example I am fond of is shown below. This is the “Galloping Highway” slide in Giles County, Virginia, along US 460. The toe of an older, dormant slide was removed for the highway cut, initiating literally decades of reactivated–and slowly ongoing–movement. This slide is, in effect, a real-world example of the previous GIF where an “invisible” slide came back to life. The slide block is nearly 1,000 feet across along US 460.
The slides shown below are in McDowell County, North Carolina. Both are situated for possible reactivation due to the road cut that has impacted their toes as well as steady stream erosion of the toes, which would pre-date the road cut. I imagine both have a reactivation history due to stream erosion; the smaller slide on the right appears to have reactivated more recently due to the road cut.
The next impressive example is along a railroad grade in western North Carolina. Lumpy areas just above the tracks look very much like recent reactivation, and a faint toe seems to be pushing out of the base of the cut. Two logging roads at the right of the slide are offset by scarps, indicating recent slide movement. The relative impact of the railroad cut and possible removal of material from the slide toe as it slowly pushes outward can’t be discerned from lidar, but the offset roads and ugly toe indicate this one definitely deserves engineering attention.
The slide above looks quite different from the first few examples and provides a nice way to circle back to the very first model in this post. The shape of a landslide’s failure (sliding) surface strongly impact what happens to the sliding mass as it moves downhill. A slide on a failure plane with constant slope can move intact, but a failure plane with changing slope requires the slide mass to constantly change its shape as it moves. If the failure plane becomes less steep, the top of the slide has to stretch to match its shape, forcing cracks or new internal scarps to form. The uppermost part of the slide can actually tilt backward until a depression forms (see above and below). The more the slide moves, the more exaggerated the cracking, internal breakup, and back-rotation of the upper part of the slide become. The GIF below illustrates the initial phases of the concept.
Obviously the gap below the slide block can’t exist…the collapse and stretching of the block are constantly developing as it moves. Physical models offer a nice way to show this process in action. Note how the cracking accompanies the backwards tilting of the upper part of the slide. This model is the last one shown in the video link.
So what happens to these early cracks if the toe is removed and movement continues? With the right failure surface shape, something like the very first model develops. This is where the video comes in. Linked below are more reactivated landslide models, including all those shown above. Some really fall apart with toe removal and continued movement; in others, a smaller and smaller block continues downslope with little or no internal break-up. I assure you that this is the ONLY place you will see this many model landslides in one place, whatever that may mean! The two styles of slide described in this post are represented. They can be distinguished by how much the slides break apart as they continue to move after toe removal. I think it’s cool to watch their motion all at once. It beats trying to piece together months or years of episodic movement!