Rescue
When the Ground Moves
In the last few years in some sections of North Carolina, it seems as if the rainfall just won’t end. At DuPont State Recreational Bob Twomey Forest in calendar year 2020, one of our rain gages measured 120 inches of rain from January 1 to December 31. The two years before were wet years, and 2021 is on track to be a wetter than normal year. Likewise, it seems as though hurricanes have lashed the southern United States severely over the last few years, as evidenced by Hurricane Fred in August of this year. One of the most devastating events from a tropical storm dumping many inches of rain, besides the damaging flooding, is that of landslides. In this article, I am going to explain what often triggers landslides, why they occur, and why they can be so dangerous and their impacts on flooding sometimes. During my career with the Natural Resources Conservation Service, especially here in the mountains of western North Carolina, I saw many slides in different settings, and witnessed their awesome power. I was able to study why slopes slid and the underlying causes of these slides, and their contributing forces related to devastating flooding. Most landslides occur on slopes, sometimes on slopes deemed flat enough that no one would expect the ground to slide. As such, we need to look at the more common reasons a slope could suddenly collapse. Gravity, slope geology — that is the rock types, depth to rock, rock strike and dip angles — soil types and consistency, the cohesiveness of the soil particles, soil fill or undisturbed soil, vegetation (or the lack thereof) upon a slope and slope cuts — all are major contributing factors in slope stability.
Gravity First, all slopes are under the
natural effects of the earth’s gravity. This is a constant. Gravity can play a very major role in whether a slope is susceptible to slide, given the other physical factors mentioned above. The steeper the slope, in general, the greater the effect of gravity upon its stability.
Slope Geology Slope geology is the presence of the type of rock and its depth, consistency, and strike/dip angles relative to the angle or steepness of the slope, be they man made slopes or natural slopes, as on a mountainside. Examples of man-made slopes include earthen dams, roads cut across a natural slope, where part of the road is “cut” material and the other part of the road is fill material. Many roads are thus partially “benched” (cut into the slope) and the other part of the road is the resultant fill material from the bench cut. Basically, a road of this type is built as 50 percent cut and 50 percent fill material, resulting in “two slopes upon a slope.” Here the cut bank generates a slope where the road was “benched into the hillside” and the fill slope — where this cut material was placed to create the other half of the roadway, creating its own slope. So, from top to bottom, you have the natural hillside slope, the cut slope, the road, the fill slope, and the rest of the natural hillside slope below the fill.Thus, you can begin to see a road that’s cut and filled on an existing slope.This is a very common way to build anything from a logging road to a major highway.
Step slope behind house slid, taking out retaining wall and uncovering house and deck footings.
on a slope tends to be more stable than soil on a slope that has been disturbed, such as through any type of construction activity. Vegetation on any slope tends to make that slope more stable, as the roots from the vegetation tend to bind soils and rocks together. Trees generally provide the greatest
stabilizing factors, but grasses or brush also help stabilize soils on a slope. So, why all of this information just discussed? Because all of these factors will have a bearing on when, where and how a slope may slide under see GROUND MOVES page 14
Soil Type Obviously, the soil type present on an existing slope has a major effect on the overall long-term stability of that slope, be it an earthen dam, a road installed across a natural slope, a “cut out area” for a house or other building, and so forth. Sandier soils lack the ability to be as cohesive as clay soil.The stickiness of sand is far less than the stickiness of a clay type soil, thus the stability of the slope can be far less, and especially so under saturated conditions. Undisturbed soil
A steep fill slope on a highway that slid under saturated soil conditions.
Carolina Fire Rescue EMS Journal
www.carolinafirejournal.com
Fall • 2021 13
