causes and mechanism of failure
causes of failure
landslides can be triggered by both natural and man-made changes in the environment conditions. the geologic history of an area, as well as activities associated with human occupation, directly determines, or contributes to the conditions that lead to slope failure. the causes of landslide can be inherent, such as weaknesses in the composition or structure of the rock or soil; variable, such as heavy rain, snowmelt, and changes in ground-water level; transient, such as seismic or volcanic activities; or due to new environmental conditions, such as those imposed by construction activities (Varnes and the IAEG, 1984). among these factors, rainfall, earthquake and human activities are important trigger factors.
monsoon rainstorm
the Himalayas are affected by the monsoon, as are other parts of South Asia in general. due to the recurring of the Summer Monsoon near the Bay of Bengal towards northwest, there is a general decrease in rainfall from East to West. thus while Eastern Himalayas (Assam) have about eight months of rainy season (March-October), the Central Himalayas (Bhutan, Sikkim, Nepal and Kumaon) have only four months of rainy season (June-September) and in the Western Himalayas (Kashmir), the Summer monsoon is active only for two months (July-August) (Chalise, 1994).
Monsoon rainstorms initiate many landslide each year in the Himalaya region. during heavy rainstorm, loose/unconsolidated deposits, and strongly weathered and fractured sedimentary and metamorphic rocks become saturated and with an increase of precipitation and raise of ground water level. as a result, these materials are especially prone to sliding when slopes are steep. Rainstorms, therefore, are recognized as important landslide triggers in the Hindu Kush-Himalayan region.
the relationship between rainfall and incidence of landslide has been studied by many scientists in China, India and Nepal. (Li and Li, 1985, Dhital et al, 1993, Joshi, 1997). the studies carried out in China show that
if cumulative precipitation amounts to 50 mm to 100 mm in one day, and daily precipitation is more than 50 mm, somewhat small-scale and shallow debris-landslide will occur;
when cumulative precipitation, within two days amounts to 150 to 200 mm, and daily precipitation is about 100 mm, the number of landslides has a tendency to increase with precipitation; and
when cumulative precipitation exceeds 250 mm in two days, and has an average intensity of more than 8 mm per hour in one day, the number of large and vast landslides increases abruptly.
studying the relation between rainfall and landslides in China has also showed that under the same rainfall conditions, the landslides triggered have many differences in their quantity, size and density due to the different geological and topographical conditions. therefore, the landslides have obvious regional characteristics. for a given region, the conditions of geology and topography are the decisive factors under which a landslide can be induced.
the principal geological factors impacting on landslide process are the type of bedrock, crack and structure, soft band, the thickness of weathering zone, the thickness and the grain composition of the soil (the surface deposits). the impact of topographical conditions are represented in two respects. on the one hand, there are the regional cut depth, cut density and the erosion basis plane. on the other hand, there are impacts of the gradient and the form of slope and water convergent area on the upper part of the slope.
earthquakes
the Himalaya mountain belt represents a type example of an orogen formed due to collision of two continents viz., the Asia and the India. the mountain lies in a major global seismic belt where earthquakes of magnitude 4.5 to 5.5 occur every year. in the region of Himalaya bounded by latitude 22°N to 38°N and longitude 72°E to 98°E, over 600 earthquakes of magnitude 5 and above have occurred during the period of 1950 to 1990. till date four very major (great) earthquakes of magnitude more than 8 have been recorded in the Himalaya or adjacent regions. these are the Great Assam earthquake of 1897, Kangra earthquake of 1905, Bihar-Nepal earthquake of 1933 and the Assam earthquake of 1950 (Thakur et. al. 1999).
earthquakes not only trigger landslides, but over time, the tectonic activity causing them, can create steep and potentially unstable slopes. it is recognized that significant numbers of landslides occur only when earthquake magnitudes are greater than 6. in the mountain areas, large-scale landslide triggered by earthquakes can block rivers and form lakes (box 1).
apart from the characteristics of earthquakes themselves (i.e., seismic accelerations, continuous time of shock, focal depths, and angle and direction of the approach of seismic waves etc.), environmental factors, such as geology, landform and drainage, play an important role in the formation of landslide induced by earthquakes.
the influence of geology is reflected in both geologic structure and lithologic character. The landslides triggered by the Songpan earthquake (Aug. 16, 1976, M = 7.2), in northwestern Sichuan Province, can be taken as an example. the earthquake induced more than 170 slumps, slides, and falls, which occurred predominantly along the active tectonic faults in the strong seismic region (fig. 1). on slopes consisting of loosened limestone and igneous rocks, the falls occurred readily, but on the slopes consisting of claystone, shale, and phyllite, the falls were few in number.
surface water
erosion, or soaking of surface to cause shallow sliding.
effects of water infiltrating from surface. causes shallow failures.
various surface treatments, according to material type. grass planting with or without the combination of jute netting and mulch for soils. revetments for steep toe slopes in soil and soft rock. surface renderings for rock slopes without noticeable ground water presence.
groundwater
ground water causes increased pore water pressure at depth.
failure plane is deeper than in surface water failure.
Ideally, remove ground water by drainage.
weathering
rock shear strength is reduced by weathering.
rock strength is reduced as constituent minerals are broken down into weathering products and clay minerals. physical bonds between rock constituents are weakened or broken. the rock can fail along weakened fracture planes or through its body.
progressive. cyclic failure possible. difficult to stabilise.
undercutting
slope is undercut by a flowing stream or by the opening up of a road cutting.
incision (down cutting) or lateral scour by streams is a major cause of slope failure. The initial failure can work rapidly up slope.
stream bank and stream bed protection required. may be too late to save slope from progressive failure up slope.
addition of weight
weight added usually by the dumping of spoil or landslide debris.
remove extra material and re-vegetate slope.
erosion
removal of particles from the surface by flowing water.
an arbitrary depth limit of 25 mm has been adopted for erosion. this depth refers only to the initial removal of particles and is used to distinguish erosion from mass movements. if particles are continually washed away, the surface will be progressively lowered, giving rise to the forms of erosion described in 'a' to 'c'3 below. for example, a gully 2 m deep can be developed by the steady removal of particles from its sides to a depth of no more than 25 mm at a time. the process, which causes this, is still erosion.
sheet erosion
water flows over surface in an even film, not in channels.
vegetation stabilisation should be adequate.
rill erosion and gully erosion to less than 2 m depth
scour by water flow in channels.
gullies begin as very shallow, narrow incisions in the slope (rills). an arbitrary depth limit of 2 m has been set for gullies as erosion features. if a gully is deeper than 2 m, its sides fail in ways similar to a normal hill slope. hill slope protection measures are then appropriate.
check dams to stabilise gully floor. vegetation to stabilise gully head.
piping
removal of fines along an underground channel.
percolating ground water in permeable fine soils of low plasticity can remove fines along fissure to a point where an underground stream is formed. the roof of this stream cavern can enlarge upwards towards the surface and eventually collapse to create an open, elongated chasm or pit.
difficult to stabilise unless underground waterways are exposed and treated as gullies. even this will not stop piping in lateral channels. a deep interceptor drain can be considered.
slide within soil or along soil/rock interface
any mass movement of soil or debris down slope.
includes translational slides of soils or debris, rotational slumps, and flows. the plane of failure can be:
- within a soil or debris mass;
- along the interface between soil and weathered rock;
- the uppermost layer of weathered rock itself (in which case the failure
plane would be in rock;
- between soil and a rock plane in unweathered rock.
translational slides are the most common form of slide in Nepal. in these a 'slab' of material of more or less uniform thickness slides off the surface. translational slides are typically rectangular in plan, with a straight head scar and straight sides running parallel down slope. they are frequently quite shallow, i.e. one metre deep or less. they can be caused by ground water pore pressure along a slide plane or by weathering or undercutting of the slope. they can be shallow or deep, according to the structure of the superficial layers.
a slump is a rotational movement of material, forming a spoon-shaped scar on the hillside, which is roughly circular in plan. the debris forms a bulge near the toe. slumps are commonly caused by high ground water pore pressures deep in the hillside, and the slip circle usually goes several metres deep.
in practice in Nepal, deciding if there is a rotational or a translational mode of failure is usually extremely difficult. many slides are a compound of the two types, in which a rotational component at the head degenerates into a translational component below. this is because coarse, non-plastic debris masses cannot sustain a circular slip plane except at the crown. deciding which mode is dominant is useful because rotational failures indicate a deep failure plane and may therefore be more difficult to stabilise than a translational slide.
flows are caused by liquefaction of material, usually by the action of heavy rainfall upon a permeable soil surface. the soil literally flows down the slope. the failure plane is usually shallow, sometimes only a few centimetres deep. however, the fluid mass is very difficult to control or stop. deep flows, which can travel a long way, are very destructive and potentially pose a high risk to life and property.
for slides less than 100 mm deep, vegetation and/or bolsters should hold slope. fences may become undercut by liquefaction.
plane failure in rock
any mass movement whose failure plane or planes is controlled principally by fracture planes in rock, and whose debris consists chiefly of rock fragments.
the weathering grade of the rock is 1 - 4 (the rock rings when struck with a hammer). failure types commonly include plane failure, wedge failure, and toppling (rockfall).
standard rock mechanics solutions.
disintegration
tensile failure occurs in very soft rock or consolidated soil.
a special type of rock failure, found in massive or sparsely
very difficult to stabilise. cut back to a stable angle, which is determined by shear strength of, saturated, weathered material.
fall
weathering of rock layers whose susceptibility to weathering is strongly contrasting.
this failure occurs typically in alternating thin beds of hard and soft rock e.g. sandstone and mudstone or siltstone. these formations are characteristic of the Middle Siwalik rocks of Nepal. the cause is a combination of weathering of the soft rock layers and plane failure of the hard rock layers. the soft rocks weather back from the face to leave the hard rocks sticking out. eventually the hard rocks overhang so far that they break off along vertical fractures. The process then starts again and the whole face retreats. this mechanism is very common in Nepal.
field study of signs of instability
before we undertake any remedial work on an unstable slope we need to try to understand what is happening on the site. this experience provides an opportunity for you to apply your work on cause and mechanisms of slope failure, components of an unstable slope and the severity of instability.
look particularly for the following features and identify them on your drawing.
· different types of parent materials describe them briefly in your note book;
· the location of the boundaries;
· presence of water;
· cracks;
· causes and mechanism of failure; and
· other features.
Sketch of the landslide of Km 75+950
description of site features at Km 75+950
(the site numbers described below are indicated
in the above figure.)
1 fields at top of site
this area consists of gentle slopes on a gritty, sandy soil, which is apparently derived principally from weathered gneiss. The gentle slopes are levelled into bari risers.
the soil is very permeable and readily takes in rainwater. the ploughed surface of the fields prevents run-off and allows the water to enter even more readily. this area is at least two hectares, which is a large surface area to collect water. the water sinks into the sandy alluvium beneath.
there are cracks along the edge of the crown behind the vertical sand cliff. this suggests that the cliff is retreating by periodic falls from the front face.
2 yellow cliff
the sand cliff is composed of sandy alluvium. it stands about 8 m high and is either vertical or overhanging.
it is damp, even late in the dry season. the damp areas appear darker and moisture-loving plants are present. the water comes from within the alluvium, percolating down from the bari fields above.
the sand cliff is failing by the mechanism of shear failure and the cause is ground water. tall flakes of sand are drying and splitting off and will fall from the face. there is nothing to stop this process continuing for the foreseeable future.
3 trenches
below the sand cliff is a deep, elongated hollow, accompanied by two more hollows a little further down the slope. these hollows are parallel to the sand cliff and are developed in sandy and cobbly alluvium.
water accumulates in the hollows and, since it cannot escape, it will percolate into the ground. the sandy texture of the soil will encourage this.
the trenches and the sand cliff above are a sign that major slope disruption has taken place, probably caused by deep shear failure.
4 temporary spring
the dampness of this hollow can be felt. Also, ferns and mosses are present. they are plants that like to grow in damp places. this is a point where water comes out of the slope as surface water, having travelled underground either from the bari fields above or one of the trenches in location 3.
there is no instability associated with this feature, although it lies at the head of a steep-sided gully. the very steep slopes around this head area are almost certainly caused by ground water emerging into the water channel, causing localised slumping. however, active movement seems to have ceased, as the slopes are all stable now.
5 cracks and hollows
there are two deep, wide cracks at the base of a face of alluvium with cobbles exposed. their large size suggests that the movement could be deep. any water running down the face of the alluvium will go into the cracks and find its way deep into the slope.
the freshness of the cracks indicates that the ground is under tension and actively sliding. this will probably continue and very likely get worse in the near future.
6 irregular ground
this slope is still on alluvium, although there no very clear evidence of this can be seen. the whole of the lower part of the slope, especially between here and location 5, is full of hollows. many of these feel damp to the touch, or contain the heads of small watercourses, or show signs of having flowed with water. the grass, muse kharuki, which likes to grow in damp places, is found everywhere.
this type of irregular surface is typical of ground, which is full of minor springs in the rainy season. the slope is fed by ground water that has percolated down within the alluvium.
the springs wet the surface soil and cause it to move down the slope either by slow sliding or locally by flowing. the instability seen on this slope looks as though it has gone on for many years and will continue to do so.
7 rain-fed terraces
terraces are normally shaped with a gentle fall to the downhill side. on this slope the surface of the terraces falls back towards the hill. this is probably caused by rotational failure. the exposed vertical face a little way above the terraces shows that the slope is still formed on alluvium.
the ploughed surface is very open-textured, and this and the local slope direction will encourage rainwater to percolate into the slope. any water entering the slope will go deeper into the ground.
there is no surface instability associated with these fields, but there is the deeper rotational failure.
8 cracked wall
this wall is located at the base of the alluvial terrace, at the point where the alluvium sits on the underlying phyllite.
although there is no obvious sign of ground water emerging from below the wall, its location at the base of the alluvium and the presence of springs further round at the same level (see locations 9 and 10) strongly suggest that ground water will be the cause of any ground movement.
cracks in the wall show that it is subsiding and that some form of slow mass movement is under way. careful examination of the stone pitching above shows that it also appears to be disturbed. there is a 'ripple' in its surface running diagonally upwards to the left from the top of the crack in the wall.
9 spring line
the material immediately above the path is alluvium and that immediately below it is weathered phyllite. the line of the path is on the junction between the two.
water moving down within the alluvium encounters the less permeable phyllite and moves sideways, emerging at the surface as a line of springs. in the dry season most of the springs are only damp hollows but in the rains they will start to flow.
when the springs flow the ground around them becomes very wet and starts to slide slowly downhill. however, this is noticeable only at the left hand end of the location.
10 active spring
this is a permanent spring and the most active point of water emergence on the spring line. the ground around it is very wet. the slope above it is not supported and the slope below is constantly moist and very soft.
11 left-hand grey slope
the material is highly weathered phyllite.
it is wetted, mostly during the rainy season, by water coming from the springs above it. this means that it becomes very wet and does not get a chance to dry out. the slope is affected by both surface water and ground water. spring water emerges from the base of the wall.
the slope has rills on the surface but also shows some local signs of having flowed. movement of this type will probably continue every year.
12 colluvium slope
the slope at first looks as though it is formed on alluvium, but the evidence of the phyllite immediately to the left and right shows that phyllite is the natural rock. this means that the alluvial material must have come down from above and now lies on the surface as a colluvial layer.
the slope is wetted by water emerging from the springs above.
although there are some small hollows and irregularities in the surface, the slope generally looks stable at the moment.
13 right-hand grey slope
this is also part of the weathered phyllite that is exposed at location 11.
there is a spring at the foot of the gabion check dam and there is clear evidence of surface movement of water.
the slope exhibits classic signs of erosion by surface water. the slope is cut into rills about 0.4 m deep, but the inter-rill areas are obviously regularly washed clear of debris to leave a weathered rock surface. this is a sign of sheet erosion.
Summary of main movement of water on site
|
|
water entry to slope |
water exit from slope |
| 1 |
fields at top of site |
surface |
ground |
| 2 |
yellow cliff |
ground |
ground |
| 3 |
trenches |
ground + surface |
ground |
| 4 |
temporary spring |
ground |
surface |
| 5 |
cracks and hollows |
surface |
ground |
| 6 |
Irregular ground |
ground + surface |
surface + ground |
| 7 |
Rain-fed terraces |
surface + ground |
ground |
| 8 |
cracked wall |
ground |
ground |
| 9 |
spring line |
ground |
surface |
| 10 |
active spring |
ground |
surface |
| 11 |
left-hand grey slope |
surface + ground |
ground + surface |
| 12 |
colluvium slope |
surface |
ground |
| 13 |
right-hand grey slope |
surface |
ground + surface |
|
|
|
