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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 6 of 9

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Triggering mechanisms

A triggering mechanism is an external stimulus that initiates the movement of a landslide (Wieczorek, 1996). Examples of triggering mechanisms include precipitation, changes in ground-water levels, snowmelt, stream erosion, and earthquake shaking. Carson (1976) referred to these mechanisms as short-term triggers. Varnes (1978) observes that landslides can have only one trigger but numerous causal factors. Causal factors are defined as conditions that contribute to instability but may not initiate failure. Carson (1976) referred to causal factors as long-term triggers, and his list included slope angle, weak soil and rock units, weathering, mass-movement history, and deconsolidation. Deconsolidation is the expansion of soil or rock due to unloading or stress relief. Human activities such as excavations and loading of slopes can either be triggering mechanisms or causal factors depending on other slope conditions. For example, if a trench fails following excavation with no other external stimulus, then the excavation was the triggering mechanism. However, if the trench is open for several days and fails during a rainstorm, then the excavation contributed to the failure and was a causal factor.

The primary focus of this study was to determine landslide susceptibility by examining the causal factors. The triggering mechanisms for landslides in the Atchison study area were not determined as part of this study. Triggering mechanisms are very important in studies of the initiation of individual landslides and in determining the probability of future landslides.

Causal factors controlling slope stability

Data from recent landslides--combined with geologic maps, NRCS soil maps, and topographic maps--were used to identify the causal factors for earth slides and earth flows. Rock-fall hazards were not incorporated in this study because they were previously mapped on the landslide inventory map (Ohlmacher, 2000). Rock and soil type, slope angle, and slope line--"an imaginary line on the ground surface indicating the direction of steepest gradient at a given point, and therefore intersecting the contour lines at right angles" (Jackson, 1997, p. 599)--were the causal factors examined in this study.

Geology and landslides

Earth slides and earth flows tend to occur in fine-grained materials such as shale and unconsolidated alluvium. In the study area, the geologic units with the majority of the recent landslides were the Kanwaka Shale (57 landslides), glacial drift (30), the Oread Limestone (28), the Lawrence Formation (23), and the Tecumseh Shale (17) (table 2). However, these numbers are misleading because they do not take into account the extent of each unit's exposure in the study area. For example, glacial drift is exposed over 98.78 km2 (38.41 mi2), whereas the Lawrence Formation only is exposed over 0.39 km2 (0.15 mi2). When the 23 landslides that occurred in the Lawrence Formation are divided by the area of exposure, the number of landslides per unit area is 59 (table 2). This number is a better indicator of a geologic formation's susceptibility--Lawrence Formation (59) and glacial drift (0.3)--than the number of landslides taken alone.

Table 2. Geologic units and recent landslides in the vicinity of Atchison, Kansas.

Geologic unit Number of landslides Percentages of landslides Area of exposure (km2) Landslides per unit area of exposure (1/km2)
Quaternary alluvium 6 3.3% 38.73 0.16
loess 7 3.8% 29.40 0.24
glacial drift 30 16.3% 98.78 0.30
Atchison formation 7 3.8% 0.27 25.64
Topeka Limestone 0 0% 0.14 0
Calhoun Shale 0 0% 0.94 0
Deer Creek Limestone 1 0.5% 6.79 0.15
Tecumseh Shale 17 9.2% 10.69 1.59
Lecompton Limestone 8 4.4% 6.42 1.25
Kanwaka Shale 57 31.0% 7.68 7.42
Oread Limestone 28 15.2% 2.15 13.02
Lawrence Formation 23 12.5% 0.39 58.97
Totals 184      


The geologic unit in the study area with the most landslides per unit area of exposure is the Lawrence Formation (table 2). This finding agrees with the general perception of the Lawrence Formation as one of more landslide-prone rock units in northeast Kansas (F. Wilson, personal communication, 1997; D. Thompson, personal communication, 1999). After the Lawrence Formation, the most susceptible units are the Atchison formation, the Oread Limestone, and the Kanwaka Shale.

The distribution of slope angles within a geologic unit's outcrop area also affects the susceptibility ranking of each unit. Glacial drift is located at the tops of hills and in the western portion of the study area where gentle slopes dominate. The Lawrence Formation is located at the base of the Missouri River bluffs where steep slopes dominate. Thus, the Lawrence Formation is in an area that is more prone to landslides because of slope angle. It would have been advantageous to normalize the number of landslides (table 2) by the area of exposure on slopes greater than 5 degrees, which is the minimum slope angle for landslides in the study area. However, the required slope data are currently unavailable.

Because the Oread Limestone has two limestone members with thicknesses of 2 m (6.4 ft) or more, it should be resistant to landslides. In the study area, however, the Oread Limestone is the third most susceptible rock unit, based on both the percentage of landslides and the landslides per unit area (table 2). Some of this is due to the fact that parts of the Oread Limestone were incorporated into landslides that involved the Lawrence Formation and Kanwaka Shale. Additionally the Snyderville Shale Member is a 3.8-m-thick (12.5-ft-thick) shale unit near the base of the Oread Limestone (table 1). Small landslides have been observed in the Snyderville Shale in Douglas County. The presence of the susceptible shales surrounding and within the Oread Limestone causes this formation to be susceptible to landslides.

The Atchison formation is composed of alluvium and is the second most important unit on the landslides per unit area list (table 2). The Atchison formation is exposed primarily along the south side of White Clay Creek (table 2). The exposure area is small, but the number of landslides is significant yielding a high value for the landslides per unit area. White Clay Creek flows at the base of the bluff where the Atchison formation is exposed. Thus, the landslides in the Atchison formation are the result of undercutting by White Clay Creek, and the susceptibility of the Atchison formation may be slightly lower on slopes that are not being undercut.

No recent landslides were mapped in the Calhoun Shale and the Topeka Limestone. The exposure area of these two formations is small (0.6% of the total study area), and the slopes in the exposure area are gentle. It is unknown whether these two units are involved in recent landslides in other portions of Atchison County. All the remaining units have at least one recent landslide, and thus it seems reasonable to assume that all the rock units could fail under the right conditions.

Shale units were associated with a high percentage of the recent landslides. Twenty-six shale samples were collected from outcrops by removing the upper weathered material to a depth where the shale showed clear bedding. Nonetheless, the samples should still be considered partially weathered. Additionally, a bias exists for collecting the fine-grained portions of the shale units. Thus, the data presented are not representative of the whole unit; for example, the Kanwaka Shale contains sand layers that were not sampled as part of the study.

The shale samples were subjected to a variety of soil analyses (table 3). They were analyzed for Atterberg limits--that is, the water-content boundaries between the semiliquid and plastic states (called the liquid limit) and between the plastic and semisolid states (called the plastic limit) (Jackson, 1997). Based on their Atterberg limits, most samples were classified as clays of low plasticity (CL) and clays of high plasticity (CH), using the Unified Soil Classification System (USCS) (table 3table 3). CL and CH soils have the behavior of clays due to the minerals present but may not necessarily contain high concentrations of clay-sized particles.

Table 3. Soil analyses performed on 44 samples.

  Atterburg limits  
Rock unit Liquid limit Plastic limit Plasticity index USCS Specific gravity Activity
alluvium            
landslides 41.7-54.6 19.0-21.2 21.0-36.9 CH,CL 2.74 0.71-1.90
loess 27.0-50.2 18.3-23.2 4.2-31.1 CL, ML, CH   0.50-0.94
"Nortonville Clay" 61.7-66.2 17.2-17.9 43.7-49.0 CH   0.99-1.08
glacial drift 32.1-59.2 13.6-18.2 18.4-41.1 CH, CL, SC   0.80-1.34
Atchison formation            
Topeka Limestone            
Calhoun Shale 49.6 22.5 27.2 CL 2.73 0.74
Deer Creek Limestone 37.4-60.1 18.4-24.0 19.0-36.1 CH, CL 2.75-2.85 0.71-0.86
Tecumseh Shale 41.2 25.8 15.4 ML   0.74
Lecompton Limestone            
Kanwaka Shale 36.2-65.1 21.2-26.1 15.1-39.0 CH, CL 2.74-2.83 0.43-0.84
Oread Limestone            
Lawrence Formation 35.3-53.5 18.9-22.2 16.4-31.2 CH, CL 2.74-2.82 0.50-0.70


Table 4 shows that a high percentage of clay-sized fragments were observed in the Kanwaka Shale (up to 87% clay), the Lawrence Formation (up to 68%), and in recent landslides (up to 65%). Expansive clays were found in the clay minerals of the Kanwaka Shale. Expansive clays increase in volume as the moisture content increases. This leads to the degradation of the rock structure and the formation of soil from the shale (Ingram, 1953). Additionally, the expansive nature of the clays can weaken the soil, as the moisture content increases.

Table 4. Gradation for 44 samples of shale and unconsolidated units.

  ASTM gradation  
Rock unit Gravel Sand Silt Clay Number of samples
alluvium         0
landslides 0.4-12.6 2.2-8.0 32.4-56.0 29.0-65.0 5
loess 0 0.3-10.0 59.6-79.0 11.0-39.8 6
"Nortonville clay" 0 1.5-4.5 45.0-49.0 49.5-50.0 2
glacial drift 0-6.5 28.5-55.4 17.5-36.7 24.0-35.4 5
Atchison formation         0
Topeka Limestone         0
Calhoun Shale 0 6.0 38.0 56.0 1
Deer Creek Limestone 0 5.0-8.0 38.0-59.0 33.0-57.0 4
Tecumseh Shale 0 7.5 59.0 33.5 1
Lecompton Limestone         0
Kanwaka Shale 0-7.8 0.1-18.2 12.8-60.9 32.5-87.0 12
Oread Limestone         0
Lawrence Formation 0-4.0 1.0-8.0 29.8-58.5 39.5-68.0 8


Six samples from the Kanwaka Shale were analyzed for their clay mineralogy using X-ray diffraction (XRD), and two samples were reanalyzed by K/T Geoservices to verify the results. All the samples contained a poorly crystallized, expandable material that included mixed-layer illite/smectite. Two samples contained an expandable mixed-layer chlorite/smectite. One sample contained some Ca-montmorillonite, which is expansive but not as expansive as Na-montmorillonite. Non-expansive clay minerals in the samples include illite, kaolinite, and chlorite.

The results of the XRD analysis were extrapolated to samples from other units by examining the Atterberg limits. The position that the Atterberg limits plot on a plasticity chart can be related to the clay mineralogy of a soil (Casagrande, 1948; Holtz and Kovacs, 1981). In this study, the liquid limit and plasticity index for each sample were plotted on a plasticity chart, which contained the ranges for montmorillonite, illite, kaolinite, and chlorite (fig. 12). Montmorillonite is an expansive clay mineral in the smectite group. The plasticity chart shows that the samples are composed of a mixture of montmorillonite and illite with some samples near each clay mineral. These results are in agreement with the XRD analysis of the six samples presented above. Underwood (1967) states that shales with clay fractions containing illite and montmorillonite have lower shear strengths and higher swelling potentials and are more prone to landslide problems than shales composed of kaolinite and chlorite.

fig. 12


Fig. 12. A plasticity chart showing the relationship of the soil samples to clay mineralogy. The figure includes samples from the Lawrence Formation (blue triangles), the Kanwaka Shale (red diamonds), the Tecumseh Shale (black open diamond), the Deer Creek Limestone (green squares), the Calhoun Shale (blue open squares),glacial drift (blue circles), the "Nortonville Clay" (black pluses), loess (red open triangles), and landslides (black crosses). The points where Atterberg limits plot on plasticity charts relate to the dominant clay mineralogy of the sample (Casagrande, 1948; Holtz and Kovacs, 1981). The Atterberg limits of the Atchison samples plot in the area between montmorillonite, a smectite clay, and illite, with some samples having more illite and others more montmorillonite.



Activity values for the samples were also calculated (table 3). Activity is defined as the plasticity index divided by the percentage of clay-sized fragments in a sample (Skempton, 1953). Shales with activity values greater than 0.75 are prone to landslide problems (Underwood, 1967). With the exception of the Lawrence Formation, all the other geologic units tested had at least one sample with an activity value close to or greater than 0.75, indicating that all of these units can contribute to landslide problems.

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Kansas Geological Survey
Web version December 22, 2000
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