2. Slope stability
Slope stability is the potential of soil covered slopes to
withstand and undergo movement.
Stability is determined by the balance of shear stress and shear
strength.
A stable slope may be initially affected by preparatory factors,
making the slope conditionally unstable.
The field of slope stability encompasses static and dynamic
stability of slopes of earth and rock-fill dams.
3. failure of rocks
1. rock falls
2. rock slides
both rotational and translational.
Rock falls occur on very steep slopes when rock blocks get
detached from the joints without sliding.
Rock slide occur due to geologic slope failure that
include slumps, slides, falls, and flows.
4. A rock fall is a fragment of rock (a block) detached by sliding,
toppling, or falling, that falls along a vertical or sub-vertical
cliff, proceeds down slope by bouncing and flying along
ballistic trajectories or by rolling on talus or debris slopes.
A rockslide is a type of landslide caused by rock failure in
which part of the bedding plane of failure passes through intact
rock and material collapses in a group and not in individual
blocks.
7. In translational slides the mass displaces along a planar or
undulating surface of rupture, sliding out over the original
ground surface.
Rotational slides move along a surface of rupture that is curved
and concave
9. Plane failure
A rock slope undergoes this mode of failure when
combinations of discontinuities in the rock mass form blocks or
wedges within the rock which are free to move.
A planar failure of rock slope occurs when a mass of rock in a
slope slides down along a relatively planar failure surface.
10. Typical view of Plane failure (A = Sliding plane, B = Slope face)
11. Wedge Failure
Wedge failure of rock slope results when rock mass slides
along two intersecting discontinuities, both of which dip out of
the cut slope at an oblique angle to the cut face, thus forming a
wedge-shaped block.
13. Circular failure
Circular failure is generally observed in slope of soil, mine
dump, weak rock and highly jointed rock mass.
It is very important to identify the position of most critical
circle in analysis of such failure
15. Toppling failure
Toppling failures occur when columns of rock, formed by
steeply dipping discontinuities in the rock rotates about an
essentially fixed point at or near the base of the slope followed
by slippage between the layers.
Jointed rock mass closely spaced and steeply dipping
discontinuity sets that dip away from the slope surface are
necessary prerequisites for toppling failure.
17. Objectives of rock slope stability analysis
To identify the lethological characteristics and their disposition.
To find out the geological structure viz folds, fault and joints.
18. Stability analysis of slope
Most conventional stability analyses of slopes have been made
by assuming that the curve of potential sliding is an arc of a
circle.
The procedures of stability analysis may be divided into two
major categories.
1. Mass procedure
2. Methods of slices
19. CULMANN METHOD
A technique for the calculation of slope stability based upon the
assumption of a plane surface of failure through the toe of the
slope has been proposed by Cullmann.
it has been found that plane surfaces of sliding are observed
only with very steep slopes, and for relatively flat slopes the
surfaces of sliding are almost always curved.
20. Culmann Approach to Slope Stability
By above equation s, find the c’ and C’m
,
Factor of safety with respect to strength
21. Failure analysis by slip circle method
The shear strength is acting
along the length of the
sliding arc at moment arm
length r. the slope is stable
when
W1l1 ≤ W2l2 +sLr
The factor of safety is
𝐹𝑂𝑆= W2l2 +sLr W1l1
Swedish slip circle method
22. Ordinary slip circle method
In cases where the effective angle of shearing resistance is not
constant over the failure surface.
In this method, slip surface is divided into a number of vertical
strips or slices.
The forces between slices are neglected and each slice is
assumed to act independently as a column of soil of unit
thickness and width.
24. Bishop method of slices
the analysis is carried out in terms of stresses instead of forces
which were used with the Ordinary Method of Slices.
The major difference between the Bishop Method and the
Ordinary Method of Slices is that resolution of forces takes
place in the vertical direction instead of a direction normal to
the arc.
27. Numerical methods of analysis
Numerical modelling techniques provide an approximate solution
to problems which otherwise cannot be solved by conventional
methods.
Example- Complex geometry, material anisotropy, non-linear
behavior, in situ stresses etc.
Continuum modelling
Discontinuum modelling
Hybrid/coupled modelling
28. Continuum modelling
Modelling of the continuum is suitable for the analysis of soil
slopes, massive intact rock or heavily jointed rock masses.
This approach includes the finite-difference and finite
element methods that discretize the whole mass to finite
number of elements with the help of generated mesh
29. Discontinuum modelling
Discontinuum approach is useful for rock slopes controlled by
discontinuity behavior.
. Rock mass is considered as an aggregation of distinct,
interacting blocks subjected to external loads and assumed to
undergo motion with time.
30. Hybrid/coupled modelling
Hybrid codes involve the coupling of various methologies to
maximize their key advantages.
Hybrid techniques allows investigation of piping slope failures
and the influence of high groundwater pressures on the failure
of weak rock slope.
