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Kansas Geological Survey, Current Research in Earth Sciences, Bulletin 244, part 3
The Relationship Between Geology and Landslide Hazards of Atchison, Kansas, and Vicinity--page 7 of 9

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Soils and landslides

The relationship between NRCS soil series and landslides is given in table 5. The greatest number of landslides, 79, occurred in Gosport soils, which develop on moderate to steep slopes underlain by bedrock. On the other hand, Martin and Vinland soils, which develop on bedrock, were involved in only one landslide each. Armster soils had 25 landslides and developed on moderate slopes underlain by glacial drift. Shelby soils, which also developed on glacial drift, had six landslides. Knox-Gosport and Knox soils had 15 and seven landslides, respectively. Knox soils developed on moderate slopes underlain by loess, and Knox-Gosport soils are a mixture of the Knox and Gosport soil series. Sharpsburg soils, which also developed on loess, have one landslide. Kennebec and Haynie soils, which develop on alluvium, have five and four landslides, respectively. None of the other NRCS soil series in the study area had any landslides.

Table 5. Soil units and landslides.

Soil unit Parent material Number of landslides Percentage of landslides
Gosport bedrock 79 54.5%
Armster till 25 17.2%
Knox-Gosport bedrock & loess 15 10.3%
Knox loess 7 4.8%
Shelby till 6 4.1%
Kennebec alluvium 5 3.5%
Haynie alluvium 4 2.8%
Martin bedrock 1 0.7%
Sharpsburg loess 1 0.7%
Vinland bedrock 1 0.7%
Aquents alluvium 0 0%
Chase terrace 0 0%
Grundy loess 0 0%
Judson alluvium 0 0%
Onawa alluvium 0 0%
Pawnee till 0 0%
Reading terrace 0 0%
Wabash alluvium 0 0%
unknown   1 0.7%
Totals   145  


The data presented in table 5 show that soils developed on bedrock are the most susceptible to landslides. This agrees with the geologic data that showed a high percentage of landslides in bedrock (table 2). The next highest percentage of landslides is in soils developed on glacial drift, followed by soils developed on loess and alluvium. The NRCS soil maps include the Atchison formation with soils developed on the glacial drift. Thus, the landslides in the Atchison formation are included with the landslides in glacial drift in table 5.

Slope angles and landslides

The slope angle for each of the 145 recent landslides was determined from the U.S. Geological Survey topographic maps. The slope angle is the angle between the horizontal and the ground surface. Figure 13 relates the frequency of landslides to the slope angle. The average slope angle for landslides is 22.2 degrees with 75% of the landslides on slopes greater than 15 degrees. One landslide is on a slope of only 5.7 degrees.

fig. 13


Fig. 13. The frequency of recent landslides for a given slope angle. The frequencies were determined by counting the number of landslides for 5-degree intervals of slope angle. The average slope angle for recent landslides is 22.2 degrees.



Landslides on slopes with low slope angles highlight the role clay layers within the shale formations play in slope stability. Above the Clay Creek Limestone Member of the Kanwaka Shale is a 20-cm (8-in) clay layer, with a high percentage of clay-sized particles and expansive clay minerals. In the Jackson Park area, several recent landslides were mapped above the Clay Creek Limestone, placing the failure planes of the landslides in the 20-cm clay layer (fig. 14). As fig. 14 shows, the portion of the failure surface that runs through the clay layer has a very low dip. This and other thin clay layers may be controlling some of the recent landslides, including those on gentle slopes (5 degrees to 15 degrees).

fig. 14


Fig. 14. Diagram of the failure plane of a landslide in the Kanwaka Shale. The clay layer above the Clay Creek Limestone Member is weak, and the failure plane of the landslide would be located within this layer. Earth slides and earth flows on gentle slopes (5 degrees to 15 degrees) may be the result of thin, weak clay layers.



Figure 13 shows a decrease in frequency of recent landslides on slopes greater than 22 degrees. This decrease may result from the fact that steep slopes tend to have thinner soils and that the percentage of the area with steep slopes is relatively small. The steepest slopes are in limited areas along the bluffs of the Missouri River and its tributaries. Normalization of the data to the percentage of the area with the various ranges of slope angle may provide a more realistic assessment of the role of slope angle in landsliding, but the slope data required for doing the normalization were not available when this report was written.

Slope line and landslides

According to conventional wisdom, landslides are more likely to occur on slopes with slope lines to the north (north-facing slopes). The hypothesis is that less direct sunlight and slightly cooler temperatures lead to less evaporation and more moisture in the soils on north-facing slopes. Slope lines for the recent landslides are plotted in fig. 15. Two maxima are observed in figure 15. The first ranges from the northeast to southeast and coincides with the slope lines for the Missouri River bluffs (the yellow area in fig. 15). The second maximum is to the northwest. This is partially due to the fact that Deer, White Clay, and Walnut creeks tend to flow along the southern side of their valleys, eroding the toe of the northwest-facing slopes. Ground-water flow may also contribute to landslides on slopes with slope lines to the northwest. Precipitation that infiltrates at the top of a hill migrates down until it reaches an impermeable layer and then flows above the impermeable layer to the face of the hill. An example of an impermeable layer is the clay layer above the Clay Creek Limestone Member (fig. 14). Bedrock in the study area dips very gently to the northwest and west (fig. 7). When ground water reaches the impermeable layer, it would flow to the northwest and exit on slopes with slope lines to the northwest, west, and north, increasing the susceptibility of these slopes to landslides.

fig. 15


Fig. 15. Rose diagram showing the slope lines for the recent landslides. Two maxima are observed in the data. The first is from northeast to southeast and is related to the bluffs of the Missouri River (yellow area in figure). The second is to the northwest and may be related to ground-water flow.



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Kansas Geological Survey
Web version December 22, 2000
http://www.kgs.ku.edu/Current/2000/ohlmacher/ohlmacher7.html
email:lbrosius@kgs.ku.edu