Landslide Types and Processes

Landslide Types and Processes andslides in the United States occur in all 50 States. The primary regions of landslide occurrence and potential are the coastal and mountainous areas of California, Oregon, Land Washington, the States comprising the intermountain west, and the mountainous and hilly regions of the Eastern United States. Alaska and Hawaii also experience all types of landslides. Landslides in the United States cause approximately $3.5 billion (year 2001 dollars) in dam- age, and kill between 25 and 50 people annually. Casualties in the United States are primarily caused by rockfalls, rock slides, and debris flows. Worldwide, landslides occur and cause thousands of casualties and billions in monetary losses annually. The information in this publication provides an introductory primer on understanding basic scientific facts about landslides—the different types of landslides, how they are initiated, and some basic information about how they can begin to be managed as a hazard. TYPES OF LANDSLIDES porate additional variables, such as the rate of movement and the water, air, or ice content of The term “landslide” describes a wide variety the landslide material. of processes that result in the downward and outward movement of slope-forming materials Although landslides are primarily associ- including rock, soil, artificial fill, or a com- ated with mountainous regions, they can bination of these. The materials may move also occur in areas of generally low relief. In by falling, toppling, sliding, spreading, or low-relief areas, landslides occur as cut-and- La Conchita, coastal area of southern Califor- flowing. Figure 1 shows a graphic illustration fill failures (roadway and building excava- nia. This landslide and earthflow occurred in of a landslide, with the commonly accepted tions), river bluff failures, lateral spreading the spring of 1995. People were evacuated and terminology describing its features. landslides, collapse of mine-waste piles the houses nearest the slide were completely The various types of landslides can be dif- (especially coal), and a wide variety of slope destroyed. This is a typical type of landslide. ferentiated by the kinds of material involved failures associated with quarries and open-pit Photo by R.L. Schuster, U.S. Geological Survey. and the mode of movement. A classification mines. The most common types of landslides system based on these parameters is shown in are described as follows and are illustrated in figure 2. Other classification systems incor- figure 3. SLIDES: Although many types of mass movements are included in the general term Crown cracks “landslide,” the more restrictive use of the term refers only to mass movements, where Crown there is a distinct zone of weakness that separates the slide material from more stable underlying material. The two major types of Minor scarp Main scarp slides are rotational slides and translational slides. Head Transverse cracks Rotational slide: This is a slide in which the surface of rupture is curved concavely upward and the slide movement is roughly rotational Transverse ridges about an axis that is parallel to the ground surface and transverse across the slide (fig. 3A). Translational slide: In this type of slide, the Radial landslide mass moves along a roughly planar cracks surface with little rotation or backward tilting (fig. 3B). A block slide is a translational slide Surface of rupture in which the moving mass consists of a single Toe unit or a few closely related units that move Main body downslope as a relatively coherent mass (fig. 3C). Foot Toe of surface of rupture FALLS: Falls are abrupt movements of Surface of separation masses of geologic materials, such as rocks and boulders, that become detached Figure 1. An idealized slump-earth flow showing commonly used nomenclature for labeling the from steep slopes or cliffs (fig. 3D). parts of a landslide. U.S. Department of the Interior Fact Sheet 2004-3072 U.S. Geological Survey Printed on recycled paper July 2004 TYPE OF MATERIAL TYPE OF MOVEMENT ENGINEERING SOILS BEDROCK Predominantly coarse Predominantly fine FALLS Rock fall Debris fall Earth fall TOPPLES Rock topple Debris topple Earth topple ROTATIONAL SLIDES Rock slide Debris slide Earth slide TRANSLATIONAL LATERAL SPREADS Rock spread Debris spread Earth spread Rock flow Debris flow Earth flow FLOWS (deep creep) (soil creep) COMPLEX Combination of two or more principal types of movement Figure 2. Types of landslides. Abbreviated version of Varnes’ classification of slope movements (Varnes, 1978). Separation occurs along discontinuities such and under saturated conditions. However, dry als on shallow slopes is usually progressive. as fractures, joints, and bedding planes, and flows of granular material are also possible. The failure starts suddenly in a small area movement occurs by free-fall, bouncing, and d. Mudflow: A mudflow is an earthflow con- and spreads rapidly. Often the initial failure rolling. Falls are strongly influenced by grav- sisting of material that is wet enough to flow is a slump, but in some materials movement ity, mechanical weathering, and the presence rapidly and that contains at least 50 percent occurs for no apparent reason. Combination of interstitial water. sand-, silt-, and clay-sized particles. In some of two or more of the above types is known as instances, for example in many newspaper a complex landslide. TOPPLES: Toppling failures are distin- reports, mudflows and debris flows are com- guished by the forward rotation of a unit or monly referred to as “mudslides.” LANDSLIDE CAUSES units about some pivotal point, below or low e. Creep: Creep is the imperceptibly slow, in the unit, under the actions of gravity and steady, downward movement of slope-form- 1. Geological causes forces exerted by adjacent units or by fluids in ing soil or rock. Movement is caused by a. Weak or sensitive materials cracks (fig. 3E). shear stress sufficient to produce permanent b. Weathered materials deformation, but too small to produce shear c. Sheared, jointed, or fissured materials FLOWS: There are five basic categories of failure. There are generally three types of d. Adversely oriented discontinuity flows that differ from one another in funda- creep: (1) seasonal, where movement is (bedding, schistosity, fault, mental ways. within the depth of soil affected by seasonal unconformity, contact, and so forth) a. Debris flow: A debris flow is a form of changes in soil moisture and soil temperature; e. Contrast in permeability and/or rapid mass movement in which a combina- (2) continuous, where shear stress continu- stiffness of materials tion of loose soil, rock, organic matter, air, ously exceeds the strength of the material; and water mobilize as a slurry that flows and (3) progressive, where slopes are reaching 2. Morphological causes downslope (fig. 3F). Debris flows include the point of failure as other types of mass a. Tectonic or volcanic uplift <50% fines. Debris flows are commonly movements. Creep is indicated by curved b. Glacial rebound caused by intense surface-water flow, due to tree trunks, bent fences or retaining walls, c. Fluvial, wave, or glacial erosion of heavy precipitation or rapid snowmelt, that tilted poles or fences, and small soil ripples or slope toe or lateral margins erodes and mobilizes loose soil or rock on ridges (fig. 3I). d. Subterranean erosion (solution, steep slopes. Debris flows also commonly piping) mobilize from other types of landslides that LATERAL SPREADS: Lateral spreads are e. Deposition loading slope or its crest occur on steep slopes, are nearly saturated, distinctive because they usually occur on f. Vegetation removal (by fire, drought) and consist of a large proportion of silt- and very gentle slopes or flat terrain (fig. 3J). g. Thawing sand-sized material. Debris-flow source areas The dominant mode of movement is lateral h. Freeze-and-thaw weathering are often associated with steep gullies, and extension accompanied by shear or tensile i. Shrink-and-swell weathering debris-flow deposits are usually indicated by fractures. The failure is caused by liquefac- the presence of debris fans at the mouths of tion, the process whereby saturated, loose, 3. Human causes gullies. Fires that denude slopes of vegetation cohesionless sediments (usually sands and a. Excavation of slope or its toe intensify the susceptibility of slopes to debris silts) are transformed from a solid into a b. Loading of slope or its crest flows. liquefied state. Failure is usually triggered by c. Drawdown (of reservoirs) b. Debris avalanche: This is a variety of very rapid ground motion, such as that experi- d. Deforestation rapid to extremely rapid debris flow (fig. 3G). enced during an earthquake, but can also be e. Irrigation c. Earthflow: Earthflows have a characteristic artificially induced. When coherent material, f. Mining “hourglass” shape (fig. 3H). The slope mate- either bedrock or soil, rests on materials that g. Artificial vibration rial liquefies and runs out, forming a bowl or liquefy, the upper units may undergo fractur- h. Water leakage from utilities depression at the head. The flow itself is elon- ing and extension and may then subside, gate and usually occurs in fine-grained materi- translate, rotate, disintegrate, or liquefy and als or clay-bearing rocks on moderate slopes flow. Lateral spreading in fine-grained materi- A B C Surface rupture Surface of rupture Toe Rotational landslide Translational landslide Block slide D E F Speed Limit 65 Rockfall Topple Debris flow G H I Source area Curved tree trunks Tilted pole Main track Depositional area Soil ripples Fence out of alignment Debris avalanche Earthflow Creep J Firm clay Soft clay with water-bearing silt Bedrock and sand layers Lateral spread Figure 3. These schematics illustrate the major types of landslide movement that are described in the previous pages. For additional information on these processes and where to find photos, please see “Where to Go For More Information” at the end of this fact sheet.

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