there are many classification schemes for landslides proposed by different authors like Campbell (1951), Hutchison (1968, 1969, 1977), Crozier (1973) and Varnes (1958, 1978).
Hutchinson’s classification considers movement criteria including depth, direction and sequence of movement with respect to the initial failure. (Varnes 1978) classification is based on nature of source material and the type of movement involved.
types of landslide according to Varnes
the types of landslide proposed by Varnes (1978) is the most commonly used in the world. it was also adopted by Landslide Committee, Highway Research Board, Washington, D.C. it divides landslides into falls, topples, slides, lateral spreads and flows. wherever two or more types of movements are involved, the slides are termed as complex. Varnes (1978) has divided the material prone to landslides into classes, e.g. rock and soil. The soil is again divided into debris and earth (table 1 and fig. 1).
table 1: types of landslide (Varnes, 1978)
| type of movement |
type of material |
| engineering soils |
bedrock |
| predominantly fine |
predominantly coarse |
| falls |
earth fall |
debris fall |
rockfall |
| topples |
earth topple |
debris topple |
rock topple |
| slides |
rotational |
few units |
earth slump |
debris slump |
rock slump |
| translational |
few units
many units |
earth block slide
earth slide |
debris block slide
debris slide |
rock block slide
rock slide |
| lateral spreads |
earth spread |
debris spread |
rock spread |
| flows |
earth flow |
debris flow |
rock flow |
| (soil creep) |
(deep creep) |
| complex |
combination of two or more principal types of movement |
1. falls
falls are abrupt movements of the slope material that becomes detached from steep slopes or cliffs. movement occurs by free-fall, bouncing, and rolling. depending on the type of materials involved, the result is a rockfall, soil fall, debris fall, earth fall, boulder fall, and so on. typical slope angle of occurrence of falls is from 45-90 degrees and all types of falls are promoted by undercutting, differential weathering, excavation, or stream erosion.
2. topples
a topple is a block or serial of block that tilts or rotates forward on a pivot or hinge point and then separates from the main mass, falling to the slope below, and subsequently bouncing or rolling down the slope.
3. slides
although many types of slope movement are included in the general term “landslide”, the more restrictive use of the term refers to movements of soil or rock along a distinct surface of rupture, which separates the slide material from more stable underlying material. the two major types of landslides are rotational slides and translational slides.
rotational slides
these slides refer to a failure, which involves sliding movement on a circular or near circular surface of failure. they generally occur on slopes of homogeneous clay, deep weathered and fractured rocks and soil. the movement is more or less rotational about an axis that is parallel to the contour of the slope. such slides are characterised by a scarp at the head, which may be nearly vertical. these slides may be single rotational, multiple rotational or successive rotational types, accordingly they may have a single surface of rupture, multiple surface of rupture. a “slump” is an example of a small rotational slide.
translational slides
these are non-rotational block slides involving mass movements on more or less planar surfaces. the translational slides are controlled by weak surface such as beddings, joints, foliations, faults and shear zones. the slides material involved may range from unconsolidated soils to extensive slabs of the rock and debris. block slides are transitional slides in which the sliding mass consists of a single unit or a few closely related units of rock block that moves down slope. translational slide may progress over great distance if conditions are right.
4. lateral spreads
lateral spreads are a result of the nearly horizontal movement of unconsolidated materials and are distinctive because they usually occur on very gentle slopes. the failure is caused by liquefaction, the process whereby saturated, loose, cohesionless sediments (usually sands and silts) are transformed from a solid into a liquefied state, or plastic flow of subjacent material. failure is usually triggered by rapid ground motion such as that experienced during an earthquake, or by slow chemical changes in the pore water and mineral constituents.
5. flows
there are several types of flows and a short description of them is given below.
creep
creep is the imperceptibly slow, steady downward movement of slope-forming soil or rock. creep is indicated by curved tree trunks, bent fences or retaining walls, tilted poles or fences, and small ripples or terracettes.
debris flow
a debris flow is a form of rapid mass movement in which loose soils, rocks, and organic matter combine with entrained air and water to form a slurry that then flows downslope.
debris flow areas are usually associated with steep ravines where there are some active landslides. individual debris flow areas can usually be identified by the presence of debris fans at the termini of the drainage basins. in general, the following conditions are important for formation of a debris flow:
· slopes with 20-45 degrees
· saturated loose rock and soil materials with high content of clay minerals
· high intensity and duration of rainfall
debris avalanche
a debris avalanche is a variety of very rapid to extremely rapid slide-debris flow process.
earth flow
earth flow has a characteristic “hourglass” shape. a bowl or depression forms at the head where the unstable material collects and flows out. the central area is narrow and usually becomes wider as it reaches the valley floor. earth flows generally occur in fine-grained materials or clay-bearing rock on moderate slopes and with saturated conditions. however, dry flows of granular material are also possible
mudflow
a mudflow is an earth flow that consists of material that is wet enough to flow rapidly and that contains at least 50 per cent sand-, silt- and clay-sized particles.
6. complex movements
a complex movement is a combination of two or more types of movements mentioned above. generally huge-scale movements are complex, such as rockfall, rock/debris avalanches.
the characteristic features of the types of landslides are simply illustrated in fig. 4.
fig. 1. types of slope movement (after Varnes 1918)
component parts of a mass movement
landslide zones
a landslide has distinct parts. recognising and assessing these individually helps us understand the character of the landslide, in particular, its severity.
a landslide has four zones:
- zone of cracking
above the slide and sometimes around its sides;
- zone of failure,
the head scar (crown) and failure surface which may occupy only
a relatively small area at the top of the slide;
- zone of transport,
a damaged slope, scarred by the passage of debris on its way down
slope, this part of the slope may be stable, and may recover on its own;
- debris pile,
the detached, mobile material.
we describe the stability of a slope in terms of the factor of safety. a factor of safety of 1 means that the slope is at the dividing line between being stable or unstable. if the factor of safety is more than 1 the slope is stable. if it falls below 1 it will be unstable.
material forming the original slope
debris is not rock, it includes soil and terrace material.
soft rock may be rock that is naturally soft e.g. mudstone, or hard rock that has become soft through weathering.
hard rock has internal strength, which is much greater than the frictional strength along its fracture planes.
| type of material |
cause of failure |
mechanism of failure |
| debris |
erosion |
shear failure |
| soft rock |
weathering |
plane or shear failure or disintegration |
| hard rock |
weathering |
plane failure |
| alternating hard and soft rock |
weathering plus plane failure |
differential weathering |
landslides mapping techniques
this procedure will help you map an unstable site and observe all its significant features. the procedure is given in logical order but you do not have to follow this order in every case. an advantage of observing the site in a methodical way is that there will be less risk of missing an important feature. the column on the right suggests the action you should take.
the basis of the site record is a drawing of the site. a simple sketch will do. it does not have to be to scale. its purpose is to help you to understand the geometric relationships between features of the landslide. it also enables you to record concisely your measurements and where you took them from. any notes you make can also go on the drawing, but if they are lengthy, or if you wish to describe some detail of the slide by additional drawings and notes, these are best recorded separately in your notebook. it is good practice to make all your drawings and notes in one notebook. in this way pages do not get lost and records are kept in sequence.
| steps in a suggested procedure |
draw, measure or describe |
| step 1 |
geomorphic situation
look at the general locality and situation of the site:
- make a note of the exact location so that you
can direct others to the site if necessary;
- see if it is in a part of the landscape where
instability would be expected;
- see if the orientation of the rocks, outcropping
on the hillside around the site, indicate that the
cause of the failure may be due to rock
structure, either as planes of weakness or
movement of water along fractures;
- look at other sites in the area, they may have
a similar geomorphic situation and a similar life
progression. |
draw |
| step 2 |
sketch the site from the road or other good observation point:
- concentrate on getting the general proportions
correct;
- estimate the length from top to bottom.
record this on the drawing;
- estimate the width across the base. record this. |
draw |
| step 3 |
look for the landslide zones:
- scar;
- transport;
- debris.
note that you cannot yet see whether there is a zone of cracking above the scar. you do not have to record these zones on the drawing, but the completed drawing should be sufficiently well illustrated and labelled to let another person recognise which zones are present and where they are. |
draw |
| step 4 |
examine the material forming the original hill slope:
- debris;
- soft rock;
- hard rock;
- alternating hard and soft rocks.
all of these could be present on one landslide. the drawing should show where they are. |
describe, draw |
| step 5 |
sketch a slope profile of the site from top to bottom. angles do not have to be real, but should indicate relative steepness.
this can be augmented with more detail (e.g. with slope measurements) as you walk up the slide.
note that slopes >35° tend to be unstable unless of solid rock. |
draw |
| step 6 |
sketch the surface water drainage:
- streams;
- any springs that may be visible from where
you are standing. |
draw |
| step 7 |
sketch areas of rock outcrop
|
draw |
| step 8 |
landmarks:
- note any obvious landmarks on the site, such
as prominent trees. This will help you to keep
your bearings as you walk over and around
the site. |
draw |
| step 9 |
walkover survey
walk up the centre of the slide to the crown (head of scar). measure the angles of major slope units.
if the slope is too steep or dangerous walk around the edge, looking into the scar. |
measure |
| step 10 |
rock
visit each rock outcrop. measure any relevant rock planes or observe how the planes relate to the slope and failure planes.
make sure that the rocks observed are true outcrops (attached to solid rock beneath) and not simply large boulders partly buried on the slope.
note:
- uniformity or layering (bedding) of the rock
units;
- degree of weathering (hardness) of the rocks;
- degree of fracturing, especially any open
fractures;
- signs of water movement along fractures. |
measure describe |
| step 11 |
debris and slope
indicate the area of the slide that is occupied by debris:
- location and extent of landslide debris;
- composition of debris;
- wetness of debris;
- depth of debris / depth of failure plane;
- location, orientation and size of any cracks
in the debris or on the slope;
- any back-tilted slopes, where water may collect.
(The presence of these indicates a );
- tilted trees. These can indicate tilted ground;
- disrupted engineering structures, e.g. masonry
surface drains;
- points of ground water seepage. |
describe, draw |
| step 12 |
margins and top
look for:
- cracks in the ground. cracks are most frequent
above the head of a slide, but they often occur also
around the sides. the presence of cracks shows
that the ground is under tension and that it will
probably fail, and soon. note the location,
dimensions and orientation of the cracks. this
information tells you where, and in which direction,
the ground is under tension. the area of cracking
tells you the area over which failure is about to
take place;
- streams, springs, irrigation canals or drainage
structures, especially masonry drainage ditches.
these features may be sending water into the slide.
they may either have caused it in the first place,
or they may be contributing to further failure.
irrigation canals and masonry drainage ditches
should be inspected closely for any signs of
cracking and leakage;
- irregular topography, not due to rock outcrops.
this may indicate the presence of an old landslide,
in which case you will have to survey the whole
of this, too.
continue up the slope above the landslide until there is no further evidence of instability. this may mean walking at least fifty metres higher than the landslide scar, and much further if necessary. |
draw |
| step 13 |
base of the slide describe the features and ground conditions at the base. |
describe |
| step 14 |
causes and mechanisms of instability |
describe |
| step 15 |
history and life progression of slide |
describe |
| step 16 |
severity of instability
fill in the scores on the score sheet for assessing severity of slope instability. |
|
severity of instability and repair prioritisation
bioengineering works should be prioritised on the basis of its scope. if it is beyond its capacity, it is worth full not to consider the job. for this purpose the landslide should be classified whether it belongs to:
1 vegetation plus some 'light' engineering to protect the young plants, or
2 normal civil engineering methods, supplemented with vegetation, or
3 civil engineering methods are possible but very expensive, the risk
of failure may be high, or
4 impossible to stabilise with resources available, or not worth it
from the above classification it is understood that bioengineering is applicable only for the first and second classes of slides. like wise depth of the failure is also another criteria which should be considered for the prioritisation. for this purpose it can be grouped as following:
depth to failure plane
a less than 25 mm
b 25 - 100 mm
c 100 - 250 mm
d 250 - 1000 mm
e more than 1000 mm
history of a landslide
a not moved within the last 5 years
b moved within the last 5 years but not this year
c moves every year
d moves every year
e moved this year for the first time
life progression of a landslide
a stable slope formed, or will stabilise naturally
b further movement expected, by less serious mechanism
(post-slide adjustment)
c repeated movement expected, by initial mechanism or another
equally serious
procedure for setting priorities for landslides repair
by bio-engineering
step 1: severity rating
| repair is considered feasible by vegetation, or vegetation in conjunction with civil engineering methods. (severity rating = 1 or 2). |
go to step 2 |
|
|
| repair is feasible only by medium-scale or large-scale standard civil engineering methods. (severity rating = 3 or 4). no bioengineering requirement at this stage. wait until works are complete and then re-assess. |
end |
step 2: history and life progression of slide
history >
life progression |
no movement in last 5 years |
moved this year for the first time |
moved within last 5 years but not this one |
moves every year by initial mechanism - diminishing |
moves every year by initial mechanism - constant or worsening |
| stable slope formed, or will stabilise naturally |
priority 4
|
priority 4
|
priority 4
|
priority 4
|
- |
| repeating by less serious mechanism |
priority 2
|
priority 3
|
priority 2
|
priority 1
|
- |
| repeating by same or worse mechanism |
- |
go to step 3 |
go to step 3 |
go to step 3 |
civil engineering only |
step 3: depth of failure
|
less than 25 mm Erosion
|
priority 1 |
| 25 - 100 mm shallow flow |
priority 1
|
| 100 - 250 mm deep flow or shallow slide |
priority 2 |
| plus civil engineering works |
| 250 - 1.000 mm medium depth slide |
not applicable |
| civil engineering works alone. re-assess when complete. |
| more than 1.000 mm. |
not applicable |
| civil engineering works alone. re-assess when complete. |
check list for severity analysis
Road: _____________ Chainage: _______ Observer: _______ Date: _______
|
1 location of slide
- along the road
- above road - any distance
- below road - any distance
- between roads, i.e. above one road and below another
- through road (slide is above and below road)
|
score
1
2
3
4
5
|
2 type of slope affected
- road cutting but not hill slope
- hill slope but not road cutting
- road cutting plus hill slope
- embankment, fill or spoil slope
|
1
2
2
3
|
3 slope conditions above slide (above road, if road is at top of slide)
- crest of ridge, or gentle slope (less than 35*)
- stable, undisturbed hill slope
- unstable hill slope. cracked ground, another landslide
or topography that collects water
- cut-off drain or take-out drain
- irrigation canal
|
1
3
5
3
4
|
4 slope conditions below slide (or below road, if road is at base)
- stable, undisturbed hill slope
- intact road at base of slide. (road may be buried, but if it is
disturbed, road is not at base)
- unstable hill slope. cracks, landslide or topography
collecting water.
- stream
|
1
1
5
5
|
5 general type of failure
- sheet erosion, rilling or gullying up to 2 m deep
- gully more than 2 m deep
- mass movement (slide, flow or fall)
|
1
3
4
|
6 material forming original (failed) slope
- debris, colluvium or alluvium
- soft rock (weathering grade 5 or equivalent)
- hard rock (weathering grades 1 - 4)
- alternating hard and soft rocks
|
2
3
1
2
|
7 failure mechanism
- erosion (sheet, rill, pipe)
- shear failure (slide, slump, flow)
- plane failure in rock (slide, fall)
- collapse (fall with disintegration)
- undermining
|
1
4
2
5
2
|
8 cause of failure
- surface water. erosion, or soaking of surface: shallow
slide/flow
- ground water, causing increased pore water pressure at depth
- weathering
- undercutting of slope by stream or road cutting
- addition of spoil or landslide debris
|
1
4
5
5
2
|
9 depth of failure
- less than 25 mm erosion
25 - 100mm }
100 - 250mm } slide, slump, 3
250 - 1000mm } flow or fall
- more than 1000 mm }
|
1
2
4
5
|
10 length of failure (top to bottom)
- up to 15 m
15 - 75 m
75 - 150 m
- more than 150 m
|
1
2
3
4
|
11 history of slide
- not moved within the last 5 years
- moved this year for the first time
- moved within the last 5 years but not this year
- moves every year by initial mechanism
- moves every year by initial mechanism
|
1
3
2
4
5
|
12 life progression of slide
- stable slope formed, or will stabilise naturally
- further movement expected, by less serious mechanism
(post-slide adjustment)
- repeated movement expected, by initial mechanism
or another equally serious
|
1
3
5
|
total rating (minimum 12, maximum 54) |
|
in this fieldwork, you will map a landslide considering all aspects given to you in the previous pages. you should concentrate mainly on the following points and see landslides mapping techniques
1 map the whole landslide. mention its length and width.
2 divide the various parts of the landslide.
3 find out the landslide type.
4 identify the material type.
5 write down the causes and mechanisms of failure.
6 mention the history and life progression of the landslide.
apart from the above points, you should mark the various signs of instability due to water movement on the slope. carry out group discussions for final presentation.
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