1 Introduction to slope materials and Processes
Hill slopes occupy most of the land
surface excluding terrace and plains formed by river deposits. Hill slopes
formation rate is extremely variables and affected by many influence. Generally
hill slopes subject to extreme climatic and seismic conditions and formed on
rock with closely spaced joints, which finally change the form of hill slopes.
In many engineering projects of small
magnitude, such as a quarry or single road cutting, the factors involved can be
identified, assessed, and accommodated in a design. In larger projects, such
as construction of a large open-pit mine or a motorway through and over a
mountain range, the number and complexity of interaction of factors increases
rapidly. The longer the projected life of the project the greater is the
complexity. In research projects to investigate the development of natural hill
slopes over hundreds of thousands of years the unknowns are so many that they
swamp the accessible information. Most geomorphologists recognize that
they may never achieve a complete understanding of the development of natural
hill-slopes. There is, however, a much greater chance of achieving an
understanding of current processes of change and, by recognition and
interpretation of relict deposits left by formerly active processes, of
realizing the magnitude and relative significance of past and present processes
in creating the modern form.
Rocks and soils have properties which
are the result of their origins and alteration by weathering, erosion, and
deposition. Rock is not only formed by igneous, sedimentary, and metamorphic
processes, but contains locked in stresses as strain energy. Release of that
stress, tectonism, and weathering create joints and other planes of weakness
along which weathering can act preferentially. Weathering creates new materials
either as fragments and grains of unaltered original rock or, by chemical
alteration, as new species, especially of clay minerals. Rock and soil respond
to applied stresses in ways which are controlled partly by their inherent
properties and partly by the nature, magnitude, and frequency of the stress
application. The behavior of rock and soil under stress has a major influence
upon the effects of processes of weathering and erosion.
Weathering and removal of rock and
soil on hill slopes is not a uniform process in either time or space: it is
episodic and depends upon the availability of energy and a transporting medium.
As a result hill slopes can be regarded as a system of stores, which are
periodically unlocked by processes. Very resistant stores, such as are provided
by massive hard rock outcrops, may only yield material at very infrequent
intervals. Soil slopes in a humid tropical climate may yield solutes almost
continuously, but solids by landslide processes much less frequently. Each
process, therefore, has its own magnitude and frequency of operation which is
controlled by the resistance of the hill slope rock and soil, and by the intensity
of the denudational processes. By tracing and measuring the movement of: material
from hill slopes by different processes, and by measuring the modification of hill
slope form produced by them, it is possible to evaluate short-term changes of
slope profiles. In theory this should eventually provide an understanding of
how hill slopes evolve. In most environments, hill slopes change too slowly for
the progression from long steep slopes to slopes of lesser angle to be
observed. That such changes occur is indicated from geological sections, in
which erosional surfaces have been cut across complex structures to produce
unconformities which may be preserved in the depositional record.
Slope profiles along a cliff which
had been protected from wave attack at its base for varying periods, so that a
sequential development of hill-slope forms was assumed to have been produced.
Less secure methods involve the measurement of different slope profiles in one
area and the assembly of these profiles into a sequence. Such methods are
extremely uncertain because the underlying assumption of sequential change
cannot be verified.
1.1
Energy for hill slope processes
The energy available for slope
processes is derived from three sources: solar radiation, gravity, and endogenetic
forces. Solar radiation directly promotes weathering processes but much of it
is effective by driving the circulation of water in a hydrological cycle
between the atmosphere, pedosphere, lithosphere, and ocean. Raindrops striking
the ground, water flowing, and boulders falling or rolling down slope, have
energy provided by the gravitational force that attracts them towards the
center of the Earth. Endogenetic forces are generated by radioactive decay of
natural isotopes producing heat. Geothermal heat drives volcanic activity and
creates the stresses which are released in earthquakes. Over most of the land
surface tl1e energy available from solar and gravitational sources for
geomorphic work is several thousand times greater than that available endogenetically.
Only where volcanic activity, or sudden release of seismic energy, concentrates
power can internal energy produce distinctive landforms or land-forming
processes.
1.2 Why we need knowledge of hill slope?
We need understanding of hill slope
for these purposes:
•
For making transport routes
•
For making deep cuts on slope
•
For creating artificial slope from spoil heaps
•
Erecting buildings and structures on natural and
artificial slopes
•
For secure cultivation practice
1.3 Mass of the hill slope, rock and soil
Rock and soil are the main slope components.
Rock on the slope can be found in different weathering conditions with more or
less soil cover. The problems on rock slope associate with rock and its
mineralogy, weathering, amount of joints, joint orientation, water on rock
fractures, and slope angle. Rock on the slope surface generally has extensive
fracture network which we call joints. The joint orientations play a vital role
in slope modification and failure. Rain water and seepage water percolated into
the joints also add a catalyst force in terms of pore water pressure for the
slope failure. Among all, rock type is the prime factor which directly
facilitates other factors for slope development and modification.
The veneer of soil cover above the
rock on the slope surface also has effective role in hill slope development and
modification. The major failure problems on soil slope associate with soil
type, soil depth, water in soil, vegetation cover, rainfall intensity, and
slope angle. The contact between soil and rock (permeable layer and impermeable
layer) is the key surface of problem by which most of slope modification
processes are raised.
1.4 Hill slope processes
There are four processes that work
together to modify the slope surface:
·
Weathering
·
Erosion
·
Transportation and Deposition (mass wasting)
Erosion is transportation of loose
rock material over longer distances by a moving substance, typically a fluid
(water, wind). Erosion is responsible for transporting rock material
(sediments) over long distances, ultimately - from continents to ocean floor.
Mass wasting is a down slope movement
of broken rock material due to gravity. Processes of mass wasting incorporate
both transport and deposition. Mass wasting transport loose rock and soil
materials over relatively some distances and deposit them.
In hill slope movement, material is
constantly moving down slope in response to gravity. Movement can be very slow,
barely perceptible over many years. Movement can be devastatingly rapid,
apparent within minutes. Whether or not slope movement occurs depends on slope
steepness and Materials. Due to continuous movements of materials of slopes,
the each slope has typical slope profile. Some slopes are gently rounded, while
others are extremely steep. Profiles of naturally-eroded slopes are primarily
dependent on climate and rock type
2.1 Slope Processes: mass movement and erosion
Mass movement is a sudden,
catastrophic, periodic removal of material toward down slope. It occurs due to
failure in mass of slope. It can be surficial or deep-seated. It is often
called ‘slope instability’, ‘mass wasting’. Erosion is a gradual,
semi-continuous process. It is usually superficial, not deep-seated. It occurs
because of loss of strength in the material. Erosion is generally called ‘hill slope
processes’ or ‘soil erosion’.
2.2 Hill slope evolution
Three end members have
been proposed as general models for slope evolution: slope decline (Fig 3.2, A,
B and C), slope replacement (Fig 3.2, E and F), and parallel retreat (Fig .2 D).
It is possible to relate these end members with the diffusion. Slope decline is
a solution to the diffusion equation with zero slope at the drainage divide and
a fixed elevation at the base level.
There are many
classification schemes for mass movement (landslides) proposed by different
authors like Campbell (1951), Hutchison (1968, 1969, 1977), Crozier (1973) and
Varnes (1958, 1978).
3.1 Types of Landslide/mass movement 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.
3.1.1 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 rock fall, 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.
3.1.2 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.
Table 3.1, Types of
Landslide (Varnes, 1978)
Type of movement
|
Type of material
|
||||
Engineering soils
|
Bedrock
|
||||
Predominantly fine
|
Predominantly coarse
|
||||
Falls
|
Earth
fall
|
Debris
fall
|
Rock fall
|
||
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
|
||||
3.1.3 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.
3.1.4 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.
3.1.5 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.
3.1.6 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.
3.1.7 Flows
There are several types
of flows and a short description of them is given below.
a.
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.
b.
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
c.
Debris avalanche
A debris avalanche is a
variety of very rapid to extremely rapid slide-debris flow process.
d.
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
e.
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.
3.1.8 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 rock fall, rock/debris avalanches.
The characteristic features of the types of landslides are
simply illustrated in Fig. 3.2.
A landslide has distinct parts. Recognizing 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. Factor of safety 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.
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