7 slopes and slope stability

Mass Wasting
 Mass Movement is defined as the down slope movement of rock and
regolith near the Earth's surface mainly due to the force of gravity.
 Mass movements are an important part of the erosional process, as it moves
material from higher elevations to lower elevations where transporting
agents like streams and glaciers can then pick up the material and move it
to even lower elevations.
 Mass movement processes are occurring continuously on all slopes; some
act very slowly, others occur very suddenly, often with disastrous results.
 Any perceptible down slope movement of rock or regolith is often referred
to in general terms as a landslide.
 Landslides, however, can be classified in a much more detailed way that
reflects the mechanisms responsible for the movement and the velocity at
which the movement occurs.R. R. Gadgil, Dept. of Earth Science, Goa University
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Factors influencing slope stability
 Basically mass movements occur whenever the downward pull of gravity
overcomes the forces-usually frictional-resisting it.
 The downslope pull tending to cause mass movements, called the shearing
stress, is related to the mass of material and slope angle.
 Counteracting the shearing stress is friction, or, in a coherent solid, shear
strength.
1. Effects of slope and materials:
The steeper the slope, greater the shearing stress and therefore, greater the
likelihood of failure.
For dry, unconsolidated material, the angle of repose is the maximum slope
angle at which the material is stable.
Solid rock can be perfectly stable even at a vertical slope but may lose its
strength if it is broken up by weathering or fracturing.
R. R. Gadgil, Dept. of Earth Science, Goa University
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1. Effects of slope and materials:
Also in sedimentary rocks, there may be weaknesses along bedding planes, some
units may themselves be weak or even slippery. Such are potential planes.
Slopes may be steepened to unstable angles by natural erosion of water and ice as it
can undercut the soil removing the support beneath.
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gravity: 2 factors in balance
1) gravity--pulls object to center of Earth
• component perpendicular (normal) to surface
(contributes to friction)
• component parallel (shear) to surface
(contributes to sliding)
2) friction--resists block sliding downslope
• depends on angle of slope; slipperiness of slope;
and magnitude of normal component of gravity
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This California Cliff holds its
near vertical cliff only
temporarily, eventually the
weakly cohesive sediment
collapses
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2. Effects of fluid:
Addition of some moisture to dry soils may increase adhesion. However,
saturation of unconsolidated materials reduces the friction between particles that
otherwise provide cohesion and strength and reduced friction destabilizes the
slope.
It is also effective in promoting sliding for rocks under stress due to gravity.
The very mass of water in saturated soil adds extra weight, and thus extra
downward pull.
It can also seep along bedding planes in layered rock, reducing friction.
Frost wedging is also responsible rock falls and landslides.
Clays might absorb as much amount of water and form slippery sticky substance
and can destabilize the slope.R. R. Gadgil, Dept. of Earth Science, Goa University
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Filling a reservoir behind a
dam may also raise the water
table enough to cause slides
around the edges of the
reservoir (8-12). In some cases,
the rising water fills fractures
in surrounding sedimentary
layers sloping toward the
reservoir, causing massive
sliding into the reservoir (A
Coherent Translational Slide
Triggered by Filling a
Reservoir—The Vaiont
Landslide). R. R. Gadgil, Dept. of Earth Science, Goa University
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Vaiont Reservoir Disaster, October 1963, Italy
On night of October 9, a rock
slide 2km long, 1.6km wide and
over 150m thick moved suddenly
and filled the 270m-deep
reservoir for 2km upstream.
The movement took less than a
minute that water from the
reservoir was ejected 260m up
and propelled in great waves
both upstream and downstream.
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1. Adverse geologic conditions, including weak rocks and limestone with open fractures,
sinkholes and clay partings inclined towards the reservoir.
2. Increased water pressure in the valley rocks due to impounded water.
3. Heavy rains from september until the day of the disaster further increased the weight of
the slope materials, raised the water pressure in the rocks and produced runoff.
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3. Effects of vegetation:
Plant roots can provide a strong interlocking network to hold
unconsolidated materials together and prevent flow.
In addition, vegetation takes up moisture from the upper layers
of soil, and can thus reduce the overall moisture content of the
mass, increasing its shear strength.
Commonly, vegetation tends to increase slope stability.
However, the plants also add weight to the slope. If the added
weight is large and the root network of limited extent, the
vegetation may have a destabilizing effect instead.
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4. Earthquakes:
Landslides are a common consequence
of earthquake in hilly terrain.
Seismic waves release stress and
fracture the rocks.
THE NEVADOS HUASCARAN
DEBRIS AVALANCHE, PERU, 1970
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5. Quick Clays:
Water saturated muds in marine bays, estuaries and old saline lakebeds are called
quickclays because they are especially prone to collapse and flow when
disturbed.
Quick clays are common in northern polar latitudes.
The grinding and pulverizing action of massive glaciers can produce a rock flour of
clay sized particles, <0.02mm diameter.
When this extremely fine material is deposited in a marine environment, and the
sediment is later uplifted due to tectonic movements, it contains salty pore
water.
The NaCl in pore water acts as a glue, holding the clay particles together.
Fresh water subsequently infiltrating the clay washes out the salts, leaving a delicate
structure of particles.
Seismic wave vibrations break the structure apart, reducing the strength of quick
clay by as much as 20-30times that is prone to sliding.
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Classification
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Classification
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Classification
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Classification
• One of the most common landslide types is a rotational slide, or slump (8-
22).
• Homogeneous, cohesive, soft materials, those that lack a planar surface that
guides landslide movement, commonly slide on a curving slip surface concave to
the sky.
• The surface curves because at the top of the moving mass, gravity pulls it straight
down; that vertical part of the slip surface is the headscarp.
• Farther downslope, the mass is also pushing outward, toward the open air where
less load pushes down.
• The combination of the two forces rotates more and more outward toward the
slope ( 8-23 and 8-24A).
• The curvature of the slip surface rotates the slide mass as it moves, so the upper
end of the slide block tilts backward into the original slope while it moves.
• The lower part of the mass moves outward from the slope, leading finally
towards the lowest end, toe.
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Classification
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Classification
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Classification
• Translational slides move on preexisting weak surfaces that lie more or less parallel to a slope.
These may be planes along inherently weak layers, such as shale, old fault or slide surfaces, or
fractures. Some involve soil sliding off underlying bedrock. Compared to a rotational slide, a
translational slide is shallow, which is demonstrated by the fact that trees slip down the surface
and remain vertical rather than rotating with the sliding surface ( 8-24B ).
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Slump at Fargo, North DakotaR. R. Gadgil, Dept. of Earth Science, Goa University
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Slides and Slumps
Rock Slide/Block Glide: it is the
most simplest form. The movement is
relatively rapid and most commonly
ocurs where steeply dipping bedded
strata or sheeting nearly parallels the
surface slope.
Rockslides are generally shallow.
The dip of strata is an obvious
factor along with inherent strength,
presence, spacing and orientation of
joints and other fractures.R. R. Gadgil, Dept. of Earth Science, Goa University
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Slides and Slumps
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A rockfall triggered by blasting, Frank Slide, Alberta,
April 29, 1903
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Creep
Creep is imperceptible and
nonaccelerating downslope
movement.
The cumulative result become
obvious over a period of years.
Most stonewalls and
pavements on hillside show
downslope motion by tension
cracks, downslope tilt or
visible displacement.R. R. Gadgil, Dept. of Earth Science, Goa University
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Creep
All this is because of essentially
planar nature of soil creep.
Each layer of soil is carried downhill
by the motion of the layer beneath it,
and the effect is cumulative, with the
maximum rate at the surface
exponentially decreasing to zero with
depth.
Soil creep is aided by expansion and
contraction of soil by heating and
cooling, freezing and thawing or
wetting and drying.
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Creep
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Creep in North Dakota
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Creep in Nevada
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Marathon Basin, W. Texas
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Garnet schist, Black Hills, South Dakota
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Flows
Incohrent rock debris may be mobilized sufficiently so that it
flows like viscous fluid.
In a flow, the material moves in a more disorganized fashion, with
mixing of particles within the flowing mass, as a fluid flows.
Solifluction: If soil or regolith is saturated with water, the soggy
mass may move downhill a few mm or few cms per day or per
year.
It is a form of mass wasting common wherever water cannot
escape from a saturated surface layer of regolith by percolation
into deeper levels.R. R. Gadgil, Dept. of Earth Science, Goa University
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Slope stabilization
If a slope is too steep to be stable under the load it carries, any of the
following steps will reduce slide potential
1. reduce the slope angle/modifying the slope geometry
2. Drainage
3. place additional supporting material at the foot of the slope to prevent a
slide or flow at the base of the slope or by inserting resistant structural
elements into the slope, or
4. Reduce the load (weight, shearing stress) on the slope by removing some
of the rock or soil (or artificial structures) high on the slope.
If earthmoving equipment is being used to remove soil at the top of a
slope, for example, the added weight of the equipment and vibrations
from it could possibly trigger a landslide.
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Modifying the
slope geometry
1. Excavation of the head of the slope
2. Increasing weight at the slope toe,
even though this solution means
occupying a large area at the base of
the slope where available space is
usually scarce.
3. A wall with proper foundations can be
constructed
4. Put rip-rap at the slope toe
5. Slope stepping: construction of
benches and berms.
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Modifying the
slope geometry
2. Increasing weight at the slope toe, even
though this solution means occupying a
large area at the base of the slope where
available space is usually scarce.
constructed
5. Slope stepping: construction of benches
and berms.
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Modifying the
slope geometry
constructed
and berms.
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Modifying the
slope geometry
constructed
and berms.
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Drainage methods
1. Because water is often the main cause of slope instability, drainage is
normally the most effective measure.
2. Drainage measures can be at ground level, with drainage ditches and
channels, or at depth, using horizontal or “Californian” drains, wells
or vertical drains, drainage adits and drainage wwalls.
3. Surface drainage measures prevent runoff water from infiltrating the
slope or penetrating discontinuities. They also prevent the erosive
effect of water
4. “Californian” drains are subhorizontal boreholes with a diameter
ranging from 100-150mm and a maximum length of 30-40mR. R. Gadgil, Dept. of Earth Science, Goa University
52

 Decreasing the water content might be done by
covering the surface completely with an
impermeable material and diverting surface runoff
above the slope.
 Alternatively, subsurface drainage might be
undertaken.
 Systems of underground boreholes can be drilled
to increase drainage, and pipelines installed to
carry the water out of the slide area.
 All such moisture-reducing techniques naturally
have the greatest impact where rocks or soils are
relatively permeable.
 Where the rock or soil is fine-grained and drains
only slowly, hot air may be blown through
boreholes to help dry out the ground. Such
moisture reduction reduces pore pressure and
increases frictional resistance to sliding.
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Drainage methods
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Drainage methods
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Resistant structural elements
1. Pile walls: are alignment of piles,
arranged at intervals to form a more or
less continuous structure crossing the
sliding mass and embedded in stable
ground. Diameters vary from 0.65 to 2m
and they are often shored up with beams
at the surface.
2. Micropile walls: have a similar function
but are smaller 12-15cm in diameter and
are upto 15-20m long. They are
reinforced with a steel tube filled with
injected concrete.R. R. Gadgil, Dept. of Earth Science, Goa University
57

Resistant structural
elements
1. Pile walls: are alignment of piles,
arranged at intervals to form a more or
less continuous structure crossing the
sliding mass and embedded in stable
ground. Diameters vary from 0.65 to 2m
and they are often shored up with beams
at the surface.
2. Micropile walls: have a similar
function but are smaller 12-15cm in
diameter and are upto 15-20m long.
They are reinforced with a steel tube
filled with injected concrete.R. R. Gadgil, Dept. of Earth Science, Goa University
58

elements
3. Jet-grouting columns: are normally
used to stabilize slopes in granular soils
and even cohesive ground by cutting into
the sliding surface and creating areas
with greater shear strength. The ground
is penetrated, generally with boreholes
0.4-1m in diameter and cement is
injected at high pressure (at 30-60MPa)
through a high-speed rotating drill which
penetrates and breaks up the surrounding
ground. The result is a high strength
column made of the ground itself and the
injected material.R. R. Gadgil, Dept. of Earth Science, Goa University
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elements
4. Anchors are elements consisting of
steel cables or bars anchored in stable
areas of the rock mass. They work by
traction and exert a force opposed to the
movement and an increment of normal
stress on the failure surface. Anchors are
usually 15-40m long, with a load
capacity of about 60-120 tons per anchor.
The anchor heads may be joined together
at the surface with concrete beams so
that they work together.
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elements
4. Anchors are elements consisting of
steel cables or bars anchored in stable
areas of the rock mass. They work by
traction and exert a force opposed to the
movement and an increment of normal
stress on the failure surface. Anchors are
usually 15-40m long, with a load
capacity of about 60-120 tons per anchor.
The anchor heads may be joined together
at the surface with concrete beams so
that they work together.
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Resistant structural elements
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Anchored pile walls
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Anchors
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elements
5. The use of rock bolts to stabilize rocky
slopes and, occasionally, rockslides has had
greater success. Rock bolts have long been
used in tunneling and mining to stabilize
rock walls. It is sometimes also possible to
anchor a rockslide with giant steel bolts
driven into stable rocks below the slip plane.
Again, this works best on thin slide blocks
of very coherent rocks on low-angle slopes.
They are usually 3-6m long and 25-40mm in
diameter
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Walls and Retaining elements
1. Walls are built at the toe of a slope to reinforce it
and prevent deterioration in this area. The
disadvantages of retaining walls are that the slope
toe must be excavated before they are built, which
in itself favors instability and that they do not
prevent sliding from occurring on failure surfaces
above or below the wall.
1. Flexible gabion walls consist of fragments of
rock, or riprap enclosed in steel-mesh; they work by
gravity and may be constructed in stepped
formation facing towards or away from the slope.
Advantage: water can flow through them.
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Gabion walls
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2. Diaphragm walls are
made of reinforced concrete
sections constructed insitu in
slots mechanically excavated
below the ground surface;
their stabilizing action is
similar to that of pile walls,
although in contrast to these,
diaphragm walls are
continuous structures.
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2. Diaphragm walls are
made of reinforced concrete
diaphragm walls are
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2. Diaphragm walls are made
of reinforced concrete
diaphragm walls are
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3. Reinforced
earth walls have
an outer facing
made of
prefabricated
concrete or metal
sheets and a soil
infill, reinforced by
strips or bands of
metal or synthetic
material anchored to
the facing and to the
slope. R. R. Gadgil, Dept. of Earth Science, Goa University
73

Reinforced earth walls
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Reinforced earth walls
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Surface protection measures
 These measures aim to
1. Eliminate rockfall problems
2. Increase slope safety by preventing surface failure
3. Prevent or reduce erosion and weathering on the slope face
4. Prevent infiltration of runoff water
 The most common measures taken are
1. Installing wire meshes
2. Shotcreting the slopes
3. Building revetment walls at the slope toe
4. Laying geotextiles
5. Implementing water infiltration control
6. Using plant species to help reinforce the ground surface of slopes excavated in soils
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Installing wire mesh
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Shotcreting
This process consists of covering the slope surface
with a mixture of shotcrete or gunite (cement, water
and aggregate), projected pneumatically with a hose
and nozzle.
The slope is normally treated with several layers, with a total
thickness of 5-8cm.
It can be reinforced by fixing a metal mesh to the slope and
spraying the mixture onto it.
Holes are drilled through the shotcrete layer to facilitate
drainage.
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Shotcreting
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Shotcreting
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Shotcreting
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Revetment walls
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Revetment walls
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Laying geotextiles: Geotextiles are permeable fabrics which,
when used in association with soil, have the ability to separate,
filter, reinforce, protect, or drain.
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Laying geotextiles: Coir mesh
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Laying geotextiles: Jute
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7 slopes and slope stability

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