2. 1. What is the main force
responsible for landslide processes?
=> The main forces responsible for Landslide
process are
a) Gravitational force
b) Force due to seepage water
c) Erosion of the surface of slopes due to flowing
water
d) The sudden lowering of water adjacent to a slope
e) Forces due to earthquakes
3. How is this force affected by slope
angle?
This force is highly affected by slope angle. The vulnerability
increases with the increase in cut slope . This is because of the
weakening of shear strength of the mass due to gravitational
pull. The seepage force tends to create extra pore water
pressure leading the mass to be susceptible to landslide. The
sudden lowering of the water level will make quick increment of
void spaces. Such a instantaneous increment or decrement of
void spaces will make the soil fragile and lead to failure causing
landslide. Erosion of the surface will also lead to landslide due
to infiltration of flowing water in the soil mass below.
Earthquake is sudden and disastrous that will have higher
energy impact and it thus make stable slope to be unstable due
to ground movement and causes landslide.
4. 2. What is the factor of safety and how
does it apply to slope stability?
=>Factor of safety is the ratio of Shear strength of
the soil to the shearing stress along the assumed
failure surface. Factors of safety is a term
describing the load carrying capacity of a system
beyond the expected or actual loads. The factor of
safety is how much stronger the system is than it
usually needs to be for an intended load.
FS =resisting forces/driving forces
=friction+ cohesion/Fp
= (W xcosθxμ+ c) / (W x sinθ)
The slope fails if FS is less than “1”
5.
6. Ans contd
In slope stability, triggering factors of a slope failure
can be climatic events which can then make a slope
actively unstable, leading to mass movements. Mass
movements can be caused by increase in shear stress,
such as loading, lateral pressure, and transient forces.
Shear strength may be decreased by weathering,
changes in pore water pressure, and organic material.
Slope stability is calculated by balancing the shear
stress and strength of the soil in various possible
conditions. A slope previously stable may be unstable
due to the change in site conditions.
7. 3. In what ways does water added to a
slope affect its stability?
The water added to a slope will
a) Increase the pore water pressure
b) Dissolve the constituents of the slope mass
c) Add the weight of the slope mass
d) Too much water in the voids spaces reduces or eliminates
the cohesion force holding the grains together
e) Slopes with high clay contents are more unstable when
wet.
f) Buoys up (keeps afloat) the grains which reduces the friction
force
In the above mentioned ways The water added will affect the
slope stability.
8. 4. What kinds of rock structures can
reduce the stability of slopes?
=>Following structures can reduce the slope stability
a) Bedding Planes -. Since they are planar and since they
may have a dip down-slope, they can form surfaces upon
which sliding occurs, particularly if water can enter along
the bedding plane to reduce cohesion.
b)Weak Layers – Clay minerals generally tend to have a low
shear strength. If a weak rock or soil occurs between
stronger rocks or soils, the weak layer will be the most likely
place for failure to occur, especially if the layer dips in a
down-slope direction .
Similarly, loose unconsolidated sand has no cohesive
strength. A layer of such sand then becomes a weak layer
in the slope.
9. Ans contd
c)Joints & Fractures - Joints are regularly spaced fractures or
cracks in rocks that show no offset across the fracture
Joints form as a result of expansion due to cooling, or relief of
pressure as overlying rocks are removed by erosion. Joints
form free space in rock by which water, animals, or plants can
enter to reduce the cohesion of the rock. If the joints are
parallel to the slope they may become a sliding surface then it
will greatly reduce the strength and stability.
d)Foliation Planes - Because the sheet silicates can break
easily parallel to their sheet structure, the foliation or
schistosity may become a slip surface, particularly if it it dips
in the down-slope direction.
10. 5. What are the major triggering
events for landslide processes?
=> Major triggering events for landslide processes are
divided into two headings
Natural causes include:
1. elevation of pore water pressure by saturation of slope
material from either intense or prolonged rainfall and
seepage
2. vibrations caused by earthquakes
3. undercutting of cliffs and banks by waves or river
erosion
4. volcanic eruptions.
11. Human causes include:
5. removal of vegetation
6. interference with, or changes to, natural drainage
7. leaking pipes such as water and sewer
reticulation
8. modification of slopes by construction of roads,
railways, buildings, etc
9. overloading slopes
10. mining and quarrying activities
11. vibrations from heavy traffic, blasting, etc
12. excavation or displacement of rocks.
12. 6. Is it possible to determine whether or
not a slope has stability problems? If so,
how is this done?
=>Yes , Its possible to determine whether or not a slope
has stability problems. This can be done by analyzing
various parameters. Testing of samples to determine the
cohesion and angle of internal friction can be done.
Assuming a slip surface by various models described ,
shear parameters ( stress and strength) of the slope can
be calculated. The critical parameters such as clay mineral
content, water content, cut slope angle, materials of the
slope mass, organic content, overburden , resisting and
driving force can be determined to determine the stability
of a slope.
13. Ans contd
Slope stability analysis is performed to assess the safe
design of a human-made or natural slopes (e.g.
embankments, road cuts, open-pit mining, excavations,
landfills etc.) and the equilibrium conditions. The factor
of safety is calculated to predict how much stronger the
system is than it usually needs to be for an intended load.
FS =resisting forces/driving forces
=friction+ cohesion/Fp
= (W xcosθxμ+ c) / (W x sinθ)
The slope fails if FS is less than “1”.
