Author's personal copy
7.14 Processes, Transport, Deposition, and Landforms: Slides
S Burns, Portland State University, Portland, OR, USA
r 2013 Elsevier Inc. All rights reserved.
7.14.1
7.14.2
7.14.2.1
7.14.2.2
7.14.2.3
7.14.2.4
7.14.3
7.14.4
7.14.5
References
Introduction
Types of Sliding
Rotational Slides
Translational Slides
Compound Slides
Complex Slides
Initiation of Slides
Reactivation of Ancient Landslides
Concluding Remarks
Glossary
Block slide Landslide movement in large intact pieces
before breaking up.
Complex slide Landslide where there is both sliding and
flowage; generally the sliding occurs first.
Compound slide Landslide where the upper part of the
failure surface is steep and then flattens with depth; there is
a scarp at the top of the slide with minor surface scarps
further down the slide where it flattens out.
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Fragipan Zone of dense silt in a soil, anywhere between
30 and 200 cm in depth.
Landslide Deposits of rock, debris, and earth that have
moved down a slope by gravity.
Slump (rotational slide) Landslide movement along a
curved surface with little internal deformation of the mass.
Translational landslide Landslide movement along a
planar surface of rupture and then slides out over the
original ground surface.
Abstract
Landslides are deposits of rock, debris, and earth that have moved down a slope by gravity. The five major types of
landslides based on movements are falls, topples, slides, spreads, and flows. This chapter describes slides, which are
landslides that move on a failure or slip surface down a slope. Two main types of slides are rotational slides where
movement occurs along a curved surface and translational slides where movement is along a planar failure surfaces.
Composite slides are a combination of the first two types where the upper part of the slide shows mainly rotation, but the
lower part is mainly translational. A complex slide is where a slide transitions into a flow. Reactivation of ancient slides
continues to be a problem as increased numbers of people continue to move onto these surfaces, and the potential of
people losing their homes due to landslides has increased.
7.14.1
Introduction
A landslide is a movement of a mass of rock, debris, or earth
down a slope (Cruden, 1991). Landslides have been classified
in many ways, but probably the most used system is that of
Cruden and Varnes (1996) that emphasizes the type of movement and the type of material. This concept first gained
popularity when Varnes (1978) published his approach of
combining three types of materials (rock, debris, and earth)
with the five types of movements (falls, topples, slides, spreads,
and flows). The kinematics of a landslide and how the movement is distributed throughout the displaced mass is one of the
principal criteria for classifying landslides (Cruden and Varnes,
1996). This chapter concentrates on the slides, whereas the
other related chapters cover the other four movements.
The first major characteristic of a slide is that it is initiated
by slippage along a well-defined planar surface at the base.
Many times, this surface is predetermined by the geology of
the site. The slip surface or failure surface is generally a change
in parent material, a paleosol, a shale, or a clay layer lying
below a more porous rock or soil or a fragipan (dense layer of
silt) in a loess soil. If one of these potential failure surfaces is
parallel to the land surface, it is called a dip slope.
7.14.2
Burns, S., 2013. Processes, Transport, Deposition and Landforms: Slides. In:
Shroder, J. (Editor in Chief), Marston, R.A., Stoffel, M. (Eds.), Treatise on
Geomorphology. Academic Press, San Diego, CA, vol. 7, Mountain and
Hillslope Geomorphology, pp. 152–157.
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Types of Sliding
Slides can be grouped into two major groups based on the
morphology of the slide. Rotational slides move along a rupture surface that is curved and concave up. A translational slide
Treatise on Geomorphology, Volume 7
http://dx.doi.org/10.1016/B978-0-12-374739-6.00159-7
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Processes, Transport, Deposition, and Landforms: Slides
displaces along a planar or undulating ground surface. Five
examples of rotational and translational slides are found in
Figure 1. Varnes (1978) emphasized that characterizing the
slide movement is important, i.e., rotational versus translational, because different stability analyses infer different failure mechanisms and choosing the wrong approach could give
wrong interpretations.
7.14.2.1
Rotational Slides
Two examples of rotational slides are shown in Figures 1(a)
and 1(b). The first is a rotational rock slide, and the second is
a rotational earth slide. Movement is along a curved surface.
Sometimes these are called slumps. This circular rupture dictates that the displaced mass moves along the failure surface
with little internal deformation. The upper part of the material
moves almost vertically down while at the same time tilting
backward the upper surface of the material (Cruden and
Varnes, 1996). A common way of describing landslides is to
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compare the ratio of the depth of the surface of rupture (D) to
the length of the surface rupture (L). Skempton and Hutchinson (1969) have informed us that most rotational slides
have a D/L ratio between 0.15 and 0.30.
The circular movement is generally controlled by the local
geology. Many times the parent material is homogeneous,
especially clay-rich soils (Cruden and Varnes, 1996). Because
homogenous materials are not really common in nature, rotational slides are not as common as translational slides. Rotational slides are common in human-constructed slopes,
which in many places are made of fill, a fairly homogeneous
material. Rotational slides are also common above a flat-lying
geological unit with low permeability, such as a shale or clay
layer. Water has a hard time passing through the lower layer,
building up the pore water pressure above it and therefore
producing the rotational failure with the bottom of the failure
plane arc intersecting the lower layer.
The surface morphology of rotational slides is characterized by a steep, almost vertical scarp and a flat surface on the
Sandstone
Shale
(d)
Bedrock
(a)
Head
Main scrap
Bluff line
Toe
en
ab
Gr
Clay
ge
Rid
(b)
Highly
disturbed
clay
(e)
Bootle
gge Cove
Stiff cla r
Clay
y Slip
surface
Sensitiv
e clay
re
ssu
Pre
(c)
Figure 1 Examples of rotational and translational slides. (a) rotational rock slide; (b) rotational earth slide; (c) translational rock slide (upper
portion is a rock block slide); (d) debris slide; and (e) translational earth block slide. Reproduced from Varnes, D.J., 1978. Slope movement
types and processes. In: Schuster, R.L., Krizek, R.J. (Eds.), Special Report 176: Landslides: Analysis and Control. Transportation Research Board,
National Research Council, Washington, DC, pp. 11–33.
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Processes, Transport, Deposition, and Landforms: Slides
upper part of the slide. A good example of the steep scarp and
flat upper surface can be seen in Figure 2 – a rotational slide in
the mountains of western Colorado. Many times almost a
stair-step of several flat surfaces occurs between the main scarp
and the flat surface as shown in Figure 1(b). These two
characteristics, together with back-tilted bedding (in cross
section) or upper surface, make rotational slides easy to recognize. Note the back-tilted geological units in the Savage Island landslide in Washington, a classic rotational landslide
(Figure 3). One can also see the back rotation of the landslide
where homes are on the surface of the rotational slide as seen
in Figure 4 in Tigard, OR. Sometimes, depressions form at the
base of the scarp and the back-tilted upper slide surface and as
water finds its way to the slide, a sag pond can develop.
Figure 2 Rotational slide in western Colorado. Note the steep scarp
and the flat upper portion of the slide below the scarp.
also called translational slides planar slides. They tend to be
shallower and much longer on the slope than rotational failures and have D/L ratios o0.1 (Skempton and Hutchinson,
1969). If the surface is highly inclined, the landslide may
continue unchecked for long distances from the original site
of movement. If the movement is not a long distance, the
sliding mass may remain fairly intact as a block (Figures 1(c)
and 1(e)) and therefore, given the name of a block slide
(Panet, 1969). As the translational sliding continues, the
displaced mass may break up more and more, especially
if its velocity and water content is higher. These particular
translational slides are sometimes called debris slides and
can be seen in Figure 1(d) (Cruden and Varnes, 1996). If
the moisture content is really high, the lower part of the
slide might even develop into a debris flow (Cruden and
Varnes, 1996).
Failure surfaces of translational slides occur along many
different types of discontinuities (Cruden and Varnes, 1996).
Sometimes the failure surface is a fault or a joint in the rock,
but most of the time it is a bedding surface, especially if the
lower bed is an impermeable like a clay or shale layer. A classic
example of the latter is the Gros Ventre landslide in Wyoming
that occurred in June 1925 where spring snow melt saturated
the sandstone lying on top of a shale layer in a dip slope, and
the material slide (38 million m3 of debris) down the slope
and dammed up the stream creating a lake (Figure 5). In some
cases the discontinuity is a paleosol, an ancient clay-rich soil
that can also act as an aquiclude, as seen in the million cubic
meter translational slide along the Wilson River Highway in
Oregon in 1991 (Figure 6; Pfeiffer et al., 1998). Sometimes
that failure surface is the contact between the bedrock and the
residual soil lying on top of it as seen in a small translational
slide in Switzerland (Figure 7). Rock slopes with pronounced
structural discontinuities, such as foliation or inclined bedding, and with prominent joint planes will tend to yield
translational slides.
One of the most studied landslides in the Pacific Northwest
of the US is the Bonneville or Bridge of the Gods translational
slide in the Columbia Gorge between Oregon and Washington
(Figure 8). The first dam on the lower Columbia River,
Figure 3 Savage Island rotational slide in eastern Washington. Note
the back-tilted formations at the right side of the slide, indicative of
rotation. Courtesy of John Allen.
Figure 4 House showing rotation. It was built on the toe of an
ancient rotational slide that reactivated. It was taken down the day
after this photo was taken in 1996.
7.14.2.2
Translational Slides
In a translational slide, the mass moves along a planar surface
of rupture and then slides out over the original ground surface
(Cruden and Varnes, 1996). Hoek and Bray (1981) have
Author's personal copy
Processes, Transport, Deposition, and Landforms: Slides
Figure 5 Gros Ventre translational slide just east of Jackson Hole,
WY. Upper portion of the slide is mainly sandstone that failed on a
layer of shale.
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Figure 8 Bonneville landslide in the Columbia Gorge between
Oregon and Washington. The last movement on this translational
slide was about 550 years ago.
Bonneville dam, buttressed up to the landslide; the U.S. Army
Corps of Engineers wanted to make sure that the landslide
did not move, so they studied it in depth. It had moved on a
clay-rich, south-dipping formation about 550 years ago, and
dammed up the Columbia River for a few years (O0 Connor
and Burns, 2009).
7.14.2.3
Figure 6 Site of a translational landslide at milepost 31 on the
Wilson River Highway in the Coast Range of western Oregon. The
failure plane was a clay-rich paleosol dipping 651 to the northwest.
The slide debris had been removed before the road could be
reopened. The slide occurred in 1991 and contained over a million
cubic meters of debris.
Sometimes the slide is intermediate between a rotational slide
and a translational slide and is called a compound slide
(Cruden and Varnes, 1996). The D/L ratios are also intermediate between the two types (Skempton and Hutchinson,
1969). Failure surfaces start with a steep scarp at the top and
then flatten with depth. These slides generally have intermediate scarps that result from both internal deformation
within the slide mass and shear along surfaces within the slide
material (Cruden and Varnes, 1996). Grabens in many cases
can result within a slide mass. In many cases this indicates
a weak layer at the boundary between weathered and
unweathered slide material. Cruden et al. (1991) found that
the width of the graben may be proportional to the depth of
the surface rupture.
7.14.2.4
Figure 7 Small translational slide that happened in Leysin,
Switzerland after a long period of rainfall in 1974. The failure plane
was the bedrock soil interface of this dip slope.
Compound Slides
Complex Slides
Sometimes slide movement involves both sliding and then
flowage. Cruden and Varnes (1996) recommended that these
landslides can be called complex slide flows. The literature is
filled with terms like mudslide and flowslide (Hutchinson,
1988), but Cruden and Varnes (1996) recommended that
these terms not be used because they are confusing, and
therefore one should use the term complex slide flow. In these
complex landslides, the displaced material first moves by slide
movements then subsequently flows. It is especially common
in fine-grained and weak soils. The most common name used
to describe these landslides is slump-earthflow, but Cruden
and Varnes (1996) recommended that we discontinue the use
of the term slump and just call these complex earth slide-earth
flows.
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7.14.3
Processes, Transport, Deposition, and Landforms: Slides
Initiation of Slides
Slides are initiated when the resisting strength of the parent
material is exceeded by the shear stress. The two main triggers
are a climatic infusion of water into the system or earthquakes
(Wieczorek, 1996). An increase in driving forces or a reduction
of resisting forces will bring the ratio of driving forces to unity.
If this ratio is 41, the slope is stable, but when it falls below 1,
the slope fails. Driving forces can be increased by steepening
the slope or adding weight to the slope through rainfall or
soil. Resisting forces can be reduced by adding water to the
soil pores.
7.14.4
Reactivation of Ancient Landslides
As the human population expands more and more, people are
building houses on ancient landslides (Burns, 2002). If
soils have moved once, they have a greater chance of moving
again. The chances of having one’s house destroyed by the
reactivation of an ancient slide has greatly increased. In recent
years we have noted an increase in landslide losses related to
reactivation of ancient slides (Burns, 2002). The Federal
Emergency Management Agency (FEMA) of the US declared its
first landslide disaster area ever when a large translational
landslide reactivated in 1998 in Kelso, WA and destroyed 60
houses and caused losses of over $25 million (Figure 9). Because landslides are not covered under normal homeowner
insurance policies, most of the people lost most of the value in
their homes. In the end, with help from FEMA and local
agencies, homeowners whose houses had been destroyed or
were on the scarp and ready to be destroyed got $0.30 on the
dollar value for their homes (Burns, 2002). Another reactivated translational landslide in Stevenson, WA occurred in
2007 and destroyed additional homes (Figure 10).
Many ancient translational and rotational slides continue
to creep each year during the wet seasons. Sometimes they
totally reactivate if the rainfall is large enough, a part of the
slope has been oversteepened, or an earthquake has triggered
movement. It becomes imperative to map the distribution of
ancient landslides and encompass them into landslide hazard
maps so planners can prevent people from building on land
that might reactivate.
7.14.5
Concluding Remarks
In nature, one can classify all slides into two main categories.
Rotational slides move along curved surfaces in parent material which is mainly homogeneous. Translational slides
occur along planar failure surfaces. To a lesser extent, one finds
other slides that show characteristics of the two main types
just mentioned. Composite slides generally show rotation at
the top of the slide and translational movement in the lower
half. Complex slides have the characteristics of the lower part
of the mass showing flow.
Figure 9 Destroyed houses on a reactivated translational landslide
in Kelso, WA in 1998.
Figure 10 Reactivated ancient translational landslide in Stevenson,
WA in 2007.
References
Burns, S.F., 2002. Reactivation of ancient landslides in the Pacific Northwest, USA
from 1996–2002. In: Ciesielczuk, J., Ostafizzuk, S. (Eds.), Landslides. Mineral
and Energy Institute, Polish Academy of Sciences, Krakow, Poland, pp. 35–47.
Cruden, D.M., 1991. A simple definition of a landslide. Bulletin of the International
Association of Engineering Geologists 43, 27–29.
Cruden, D.M., Thomson, S., Hoffman, B.A., 1991. Observations of graben geometry
in landslides. In: Chandler, R.J. (Ed.), Slope Stability Engineering –
Developments and Applications – Proceedings on the International Conference
on Slope Stability, Isle of Wight. Thomas Telford Limited, London, pp. 33–36.
Cruden, D.M., Varnes, D.J., 1996. Landslide types and processes. In: Turner, A.K.,
Schuster, R.L. (Eds.), Landslides: Investigations and Mitigation. Special Report
247. Transportation Research Board, National Research Council, National
Academy Press, Washington, DC, pp. 36–75.
Hoek, E., Bray, J.W., 1981. Rock Slope Engineering. Institution of Mining and
Metallurgy, London, 358 pp.
Hutchinson, J.N., 1988. General report: Morphological and geotechnical parameters
of landslides in relation to geology and hydrogeology. In: Bonnard, C. (Ed.),
Proceedings of the Fifth International Symposium on Landslides. A.A. Balkema,
Rotterdam, The Netherlands, 1, pp. 3–35.
O0 Connor, J.E., Burns, S.F., 2009. Cataclysms and controversy – aspects of the
geomorphology of the Columbia River Gorge. In: O0 Connor, J.E., Dorsey, R.J.,
Madin, I.P. (Eds.), Volcanoes to Vineyards: Geologic Field Trips through the
Dynamic Landscape of the Pacific Northwest. Field Trip Guide 15. Geological
Society of America, Portland, OR, pp. 237–251.
Author's personal copy
Processes, Transport, Deposition, and Landforms: Slides
Panet, M. 1969. Discussion of K.W. John’s paper (ASCE Paper 5865, March 1968).
Journal of the Soil Mechanics and Foundation Division, ASCE 95(SM2).
pp. 685–686.
Pfeiffer, T., D0 Agnese, S., Pheiffer, A., 1998. Wilson River rockslide, milepost 31,
Highway #6, Wilson River Highway, Tillamook County. In: Burns, S.F. (Ed.),
Environmental, Groundwater and Engineering Geology: Applications from
Oregon. Star Publications, Belmont, CA, pp. 249–266.
Skempton, A.W., Hutchinson, J.N., 1969. Stability of natural slopes and
embankment foundations. Proceedings of the Seventh International Conference
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on Soil Mechanics and Foundation Engineering. Sociedad Mexicana de Mecana
de Suelos, Mexico City, State of the Art Volume, pp. 291–340.
Varnes, D.J., 1978. Slope movement types and processes. In: Schuster, R.L., Krizek,
R.J. (Eds.), Special Report 176: Landslides: Analysis and Control. Transportation
Research Board, National Research Council, Washington, DC, pp. 11–33.
Wieczorek, G.F., 1996. Landslide triggering mechanisms. In: Turner, A.K., Schuster,
R.L. (Eds.), Landslides: Investigations and Mitigation. Special Report 247.
Transportation Research Board, National Research Council, National Academy
Press, Washington, DC, pp. 76–90.
Biographical Sketch
Dr. Scott Burns is a professor of geology at Portland State University where he has been teaching for 21 years.
Before coming to Portland, he taught for 20 years in Switzerland, New Zealand, Washington, Colorado, and
Louisiana. He is an engineering geologist and a geomorphologist who has been studying landslides for over 35
years. His expertize in the landslide field lies in the study of hazard mapping, urban landslides, debris flow
initiation, and reactivation of ancient slides. Other research interests lie in the study of terroir (relation of wine to
geology, soils, and climate), Missoula Floods, pedology, biogeomorphology, and environmental geology (radon
and heavy metals and trace elements). He has been chair of the engineering geology division of the Geological
Society of America, president of the Association of Environmental and Engineering Geologists and vice president
of the International Association of Engineering Geologists. He has been the chair of his science departments and
the faculty senates at three different universities. Scott holds BS and MS degrees from Stanford University and a
PhD from the University of Colorado in Boulder.