Slopes are typically categorized in two types: natural and artificially-made slopes. Natural slopes are formed due to physical processes that include plate tectonics and weathering/erosion of rock masses that result in material deposition. Artificially-made slopes are established to facilitate infrastructure projects, ex., embankments, earth dams, road cuttings etc.
The stability of a slope is of critical importance in Geotechnical Engineering applications. A slope movement (also referred as a landslide) can lead to severe issues including infrastructure damage or/and casualties. Slope stability depends on the capability of the soil mass to withstand its gravitational forces, the additional loads acting on the slope, as well as potential dynamic loads (such as that of an earthquake).
A common misconception is that landslides occur in steep and remote slopes and do not actually impact human infrastructure. However, statistics show that most of world’s regions are impacted by (at least) some types of landslide phenomena that can be triggered by several factors including erosion, precipitation, earthquakes, human activity etc.
Landslides may occur rapidly or progress steadily at a fixed rate. They are common in soils and rockmasses with poor mechanical properties (highly fractured or weathered). However, a landslide can be triggered also by deformations along discontinuity layers of strong rocks. The nature and type of landslide phenomena are complex and are further analyzed below.
Types and Examples of Slope Failure
The most common and complete classification system of landslides is that provided by Varnes (1978), who introduces a system that requires the definition of the landslide material and the type of the movement induced. The ground materials are distinguished in 5 categories:
Rock: An intact rockmass which used to be located at its initial position (i.e., it has not been eroded) before the movement occurred.
Soil: Soil mass formed or transferred due to weathering and erosion of rocks. Soils consists of solid particles and voids filled with liquid and/or air, representing a three-phase system.
Earth: The soil material in which more than 80% consist of particles smaller than 2 millimeters (upper limit of sand particles).
Mud: The soil in which more than 80% consists of particles smaller than 0.06 millimeters (upper limit of silt particles).
Debris: The soil material that has 20%-80% of particles that are bigger than 2 millimeters.
The 5 types of landslide movements that can be observed are categorized as following:
Falls
Falls are downward movements that progress rapidly and may not be preceded by initial movements or warnings. They occur when a rocky mass is detached from a slope along a discontinuity plane that can be associated with fractures, joints or bedding (Figure 1). Falls are controlled by the shear strength of the discontinuity plane which is reduced with mechanical weathering propagation and the presence of water. Once detached, a rock boulder will follow a certain trajectory which depends on its size and shape and the topography of the region. The movement type can include free-fall, bouncing, rolling or a combination of those components. Figure 1: a) Illustration of a rockfall (USGS, 2004), b) Rockfall in Central Pyrenees, Spain (Corominas, 2017)
Topples
Topples are also failures that occur in rocky materials and resemble falls. However, this type of failure is associated with a rotational movement that occurs around a certain point located in a relatively low position (Figure 2). Topples are controlled by a combined action of gravitational forces that induce a bending moment and external forces (e.g., weathering, water pressure, freeze-thaw cycles).Figure 2: a) Illustration of a topple failure (USGS, 2004), b) Topple failures in granite boulders, Mount Evans, Arapaho National Forest, Colorado, U.S.A. (Highland and Bobrowsky, 2018)
Slides
Slides refer to ground movements along a specified surface or zone of weakness. A slide occurs when the shear stress applied along a surface overcomes its shear strength. The failure may propagate progressively initiating from a local failure zone. The main body of the slide will move downwards separating the stable from the unstable ground.
There are two main types of slides: rotational and translational slides. In rotational slides, the failure surface is curved inwards and points upwards and the landslide mass is approximately rotating around an axis transverse to the slide movement and parallel to the surface of the ground. The movement is usually associated with shear failure of the ground with its 3-dimensional geometry being “spoon-shaped”. Certain features are defined to characterize a rotational slide as shown in Figure 3. The surface of rupture is the zone in which the ground material slides. The main scarp refers to a relatively steep edge at the head of the landslide that reveals the undisturbed ground and the visible component of the rupture surface. The crown is the area above the main scarp that has not moved downwards. The main body of the slide is the entire soil mass that has slipped along the failure surface. The head is the upper section of the slide between the main scarp and the displaced ground material. The toe is the most distanced part from the main scarp where landslide material has accumulated and the foot refers to the part of the failed material that has been deposited over the initial ground surface.
Figure 3: Main features of a rotational landslide (USGS, 2004)
Nevertheless, sometimes the failure surface is controlled by pre-existing weakness planes (e.g., faults, fissures, cracks or joints). In this case, an engineering assessment must recognize those features since the failure would not be entirely controlled by the material’s shearing component. A rotational slide will eventually stop propagating as a stress equilibrium is restored with the mass movement. An example of a rotational slide is given in Figure 4.
Figure 4: The rotational Holbeck Hall landslide landslide, in Scarborough North Yorkshire, England (June 1993) (photo from British Geological Survey)
Translational slides occur along a pre-defined planar surface and the ground is subjected to no or little rotation. Translational slides are mainly controlled by weakness surfaces (joints, bedding planes etc.) or by the contact of materials with different shear strength. Theoretically, a translational slide may propagate indefinitely given that the rupture surface maintains its inclination and that its shear strength resistance remains lower that the driving force.
Figure 5. a) Illustration of a translational landslide (USGS, 2004) and b) Co-seismic translational landslide triggered in Japan in 2016 (Highland and Bobrowsky, 2018 by Khang Dang and Kyoji Sassa)
Lateral Spreads
Lateral spreads are deformational phenomena caused by liquefaction, the process during which a saturated soil (usually sands) experiences loss of strength after a sudden change in its initial stress conditions. Therefore, the soil tends to behave more like a liquid than a solid. Such deformations occur on less steep slopes and are usually triggered by dynamic loads such as that of an earthquake. Lateral spreading is usually a progressive process that occurs mainly near shores, riverbanks, and ports where loose and saturated sandy soils exist. Infrastructure founded on those type of soils is prone to extensive damage (Figure 5).
Figure 5. a) An illustration of a translational landslide (USGS, 2004) and b) Lateral spreading example caused by an earthquake in Pakistan (Independent)
Flows
According to Varnes (1978), not all types of slope movements can be categorized in the aforementioned categories. There are certain landslide phenomena that take the form of slow or fast-moving flows. In rocks, there are types of slow movements that result in folding or bending. Due to the fact that these displacements resemble viscous fluids, they can be characterized as rock flows.
Regarding flows in soil materials, Varnes (1978) distinguished 5 main categories:
Debris flow: Quick soil mass movements that include less than 50% of fine material. Debris flows are usually triggered after heavy precipitation saturates and mobilizes the soil or they can be caused by another type of landslide upwards (Figure 6a). They spatially occur near steep gullies where the conditions for such flows are favorable. An example of accumulated material after a debris flow is shown in Figure 7a.
Debris avalanche: A rapid debris flow frequently triggered in steep slopes when the material’s cohesion is relatively low or/and when the water content is high (Figure 6b).
Earthflow: Flow in fine-grained soils or clayey rocks under saturated conditions. Liquefaction of the material leads to the formation of a bowl at the head of the slope and creates a unique “hourglass” effect (Figure 6c). An example of an earthflow is illustrated in Figure 7c.
Mudflow: A rapid flow consisting of at least 50% clay, silt and sand particles. They are characterized by a high-water content and sometimes they are referred to as mudslides.
Creep: A subtle and steady type of downward flow caused by shear deformation. Shear stresses are high enough to cause displacements but not adequate to result in shear failure. A common feature that reveals creeping flow is the presence of titled tree trunks (Figure 7b).
Figure 6: Illustration of a) debris flow, b) debris avalanche, c) earthflow and d) creeping flow (USGS, 2004)
Figure 7. a) Accumulated material from a debris flow in Cephalonia Island, Greece (GEER, 2020), b) Titled tree chunks as an indicator of soil creep (USRA by Tom McGuire), and c) La Conchita landslide in Ventura County, California (1995). A slump resulted in an earthflow downwards (Photo by Mark Reid, U.S. Geological Survey)
Complex Landslides
A complex landslide comprised of a combination of the deformational components discussed above. Common complex landslides include an initial rotational or translational component followed by certain type of flow. By combining the type of movement and the type of the material, Varnes (1978) developed a classification of landslide phenomena, as presented in Table 1.
Figure 8. A complex landslide. A rotational component followed by earthflow (Conforti et al., 2014)
Table 1. Classification of landslides based on material and movement type (Data from Varnes, 1978)
Causes of Slope Failures
Slope failures can be triggered by natural of human-induced causes, or a combination of the two. The natural causes of landslides include: gravitational forces that tend to destabilize the ground, water saturation, erosion, dynamic loads (e.g., earthquakes), the sudden uplift of the aquifer level, volcanic eruptions and freeze-thaw weathering cycles.
The presence of water is one of the most common factors that triggers landslides. Water saturation can be caused as a result of heavy precipitation, snow melt or changes in the ground water level. Water saturation reduces the shear strength of soils. In particular, it decreases the normal effective stress that acts between the grains and hence, the frictional resistance is reduced. The Mohr-Coulomb failure criterion suggests that the shear strength of the ground is proportional to the normal effective stress as:
Where t is the shear strength, σn is the effective stress, σt is the total stress, u is the pore-water pressure, c is the cohesion, and φ the friction angle.
Earthquakes are also triggering factors of landslides. Ground shaking imposes destabilizing horizontal and vertical loads (with the first being the most important) and can also result in liquefaction. Seismic shaking can also act as a contributing rather than a triggering factor as it may cause deterioration of the ground’s shear strength and destabilize the slope. The slope may subsequently be prone to landsliding in static conditions, for example after heavy precipitation, or when another earthquake occurs.
The human-induced causes of landslides are actions that can destabilize slopes, including: toe excavations, infrastructure loads acting on a slope, machine vibrations that apply dynamic loads, construction of weak embankments or earth dams, and deforestation that may exacerbate extensive flooding and debris/earth flows.
References
Conforti, M., Muto, F., Rago, V. and Critelli S. (2014). Landslide inventory map of north-eastern Calabria (South Italy), Journal of Maps, 10:1, 90-102, DOI: 10.1080/17445647.2013.852142
Corominas, J., Mavrouli, O. and Roger R.C. (2017). Rockfall Occurrence and Fragmentation. 75-97. 10.1007/978-3-319-59469-9_4.
GEER (2020). The September 18-20 2020 Medicane Ianos Impact on Greece - Phase I Reconnaissance Report. GEER-068, https://doi.org/10.18118/G6MT1T
Highland, L. and Bobrowsky, P. (2018). TXT-tool 0.001-2.1 Landslide Types: Descriptions, Illustrations and Photos. 10.1007/978-3-319-57774-6_1.
United States Geological Survey (2004). Landslide Types and Processes. Fact Sheet 2004-3072.
Varnes, D.J. (1978). Slope movement types and processes. In: Special Report 176: Landslides: Analysis and Control (Eds: Schuster, R. L. & Krizek, R. J.). Transportation and Road Research Board, National Academy of Science, Washington D. C., 11-33.
WG/WLI (1994). A suggested method for reporting landslide causes. Bull. Int. Assoc. Eng. Geol. 50 (1), 71e74.
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