Geology Reference
In-Depth Information
2. Structural properties of earth materials. The ori-
entation of layering of some rocks and sediments
relative to slope directions, as well as the extent of
fracturing of the materials, will affect landslide
potential.
3. Steepness of slopes. Landslides are more common
on steeper slopes.
4. Water. Landslides are generally more frequent in
areas of seasonally high rainfall because the addition
of water to earth materials commonly decreases their
resistance to sliding by decreasing internal friction
between particles (such as soil or sand grains) and by
decreasing the cohesive forces that bind clay miner-
als together. Water also lubricates surfaces along
which failure may occur; adds weight to the material;
reacts with some clay minerals, causing volume
changes in the material; and mixes with fine-grained
unconsolidated materials to produce wet, unstable
slurries.
5. Ground shaking. Strong shaking, for example dur-
ing earthquakes, can jar and loosen earth materials
making them less stable.
6. Type of vegetation present. Trees with deep, pene-
trating roots tend to hold bedrock and surficial deposits
together, thereby increasing ground stability. Removing
such trees can increase the likelihood of landslides.
Trees add weight, however, and roots can break up rock.
7. Proximity to areas undergoing active erosion. Rapid
undercutting and downcutting along stream courses
and shorelines makes the resulting slopes particularly
susceptible to landsliding. (Nilsen & Brabb, 1972)
The parts of a landslide are shown in Figure 8.3.
The key nomenclature follows (from Nilsen and Brabb,
1972):
PART A. RECOGNIZING LANDSLIDES
ON TOPOGRAPHIC MAPS, AERIAL
PHOTOGRAPHS, AND LIDAR IMAGES
According to Nilson and Brabb (1972), landslide
deposits are commonly characterized by one or more
of the following features:
1. Small, isolated ponds, lakes, or other closed depressions
2. Abundant natural springs
Main scarp
The steep surface between the slide and the undisturbed ground. The projection of the scarp
surface under the slide material is the rupture surface.
Minor scarp
A steep surface on the displaced material produced by differential movements within the slide mass.
Head
The upper part of the slide material along the contact between the displaced material and the main
scarp.
Crown
The material that is still in place, undisturbed, adjacent to the highest parts of the main scarp.
Toe
The margin of displaced material most distant from the main scarp.
FIGURE 8.3 Parts of a complex (slump-earth flow) landslide (Nilsen and Brabb, 1972).
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