2. Importance of the subject
• Open pit mining is a very cost-effective mining method allowing a
high grade of mechanization and large production volumes. It is
therefore possible to mine mineral deposits of a very low grade which
could not be mined economically using underground methods.
• Mining depths in open pits have increased steadily during the last
decades, and open pits with mining depths of up to and exceeding 500
meters are not unusual.
• Design of open pit slope angles is becoming more and more
important as the mining depths of open pits continuously increase.
Small changes in the overall pit slope angle have large consequences
on the overall economy of the mining operation.
3. • The actual slope angles used in the mine depend upon
(i) the presence of haulage roads, or ramps, necessary
for the transportation of the blasted ore from the pit
(ii)possible blast damage
(iii) ore grades and
(iv)economical constraints
4. SLOPE STABILITY
It is one of the greatest problem which any open cast mine encounters. Scale of
this problem is divided in:
• Gross stability problem: It refers to the overall problem of stability of major
parts of slope due to large shear failure and it generally occurs in deeply
weathered rock.
• Local stability problem: It refers to problem which is much lower in scale and
it generally doesn’t affect more than a couple of benches at one time. It
mainly occurs due to shear plane joints or slope erosion due to surface
drainage.
5. FACTORS AFFECTING SLOPE STABILITY
Following factors affect slope stability:
• Slope Geometry
• Geology and geological structure
• Ground water
• Lithology
• Dynamic forces
• Method of mining and equipment used
• Angle of internal friction
• Cohesion
6. Slope Geometry –
Height, overall slope angle and area of failure surface are the elementary
geometrical slope design parameters. Slope stability decreases sharply if the
height of slope increases. The chances of development of failure
increases on increasing the overall slope angle.
Geological Structure –
The main geological structure affecting the stability of the slopes are:
• Amount and direction of the dip
• Joints and discontinuities
• Faults
Lithology –
The rock materials forming a pit slope determines the rock mass strength
7. Ground water –
The presence of ground water causes following altercations is the strata:
• It alters the friction and cohesion parameters.
• It reduces the normal effective stress.
Dynamic Forces-
Due to vibration caused by blasting process shear stress increased, it maximizes the
dynamic acceleration Of material which causes instability in the slope plane.
Due to blast damage and back break, these factors also affect the failure in rock
mass i.e. the bench face angle, vibrations from blasting.
8. TYPES OF ROCK SLOPE FAILURE –
Following are the different types of failure that a slope can undergo:
• Planar failure
• Wedge failure
• Circular failure
• Toppling failure
9. Planar failure – Failure where a block of rock slide on a single plan dipping out of the
face.
10. Conditions for plane failure –
(a) The plane on which sliding occurs must strike parallel or nearly parallel
(within approximately ±20◦) to the slope face.
(b) The sliding plane must “daylight” in the slope face, which means that
the dip of the plane must be less than the dip of the slope face, that is,
ψp < ψf .
11. Wedge Failure – This failure happens when discontinuity plane striking obliquely
to the slope face where sliding takes place along the line of intersection of two such
planes.
12. Conditions –
• The dip of the line of intersection is smaller than dip of slope face.
• The two sets of discontinuities must intersect each other.
• The sliding will take place along the dip of intersection of two discontinuities
13. Circular failure –
The circular failure occurs when the joint sets are not very well defined. When the
Material of the spoil dump slopes are weak such as soil, heavily jointed or broken
rock mass, the failure is defined by a single discontinuity surface but will tend to
follow a circular path.
Toppling failure-
It occurs in slopes having near vertical joint sets, very often the stability depends
on the stability of one or two key blocks, when they collapse, failure occurs.
This type of failure involves rotation of blocks of rocks about some fixed base.
15. Slope Stability Assessment-
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.
Two major analytical approaches for stability analysis and design of open pit slope:
1. Limit Equilibrium Method
2. Numerical Modelling
16. Numerical modelling
• These are computer programs that represent the mechanical
response of a rock mass subjected to a set of initial conditions
such as in situ stresses and water levels, boundary conditions.
• Numerical models divide the rock mass into zones. Each zone
is assigned a material model and properties
• Numerical modelling techniques have been widely used to
solve complex slope problems, which otherwise, could not
have been possible using conventional techniques. These
models are used to simulate rock slope as well soil slope with
complex conditions.
