5. The selection of the optimum bench height
depends on number of considerations
including;
1. Production requirements
Planned production of the mine will
determine the size of the bench and
the size of equipment.
6. 2. Existing equipment and the availability of
capital for new equipment purchase
The bench height is designed focusing the
existing equipment. The desired bench
height then should be selected depending
on the size and type of equipments to be
used.
The planner should look ahead to when
these equipments are to be replaced by
new ones
7. for example in open pit to reduce cost
purchasing larger and more
productive shovels and truck. If the
need arise and the capital is available
to accommodate, planner should
consider changing bench heights.
3. Regulatory requirements and other
factors
Other factors include safety and
weather conditions.
8. Bench faces are mined steeply and
result into rock fall and raveling
initiation therefore catch benches
are left in pit wall to retain rock falls
and reveling.
Minimum bench width = 4.5m
+0.2H
Berm height = 1m + 0.04H
Face angle – depends on rock
strength and structures orientation
9. The purpose of benches is to control
To control the depth of the blast
holes.
To control the slope of the pit walls.
To control dangers of highwall faces.
Provide enough space to allow
sustained and uninterrupted
production.
Allows easy transportation of mined
ore.
10.
11. In most applications, the
primary purpose of slope stability
analysis is to contribute to the
safe and economic design of
excavations. So that there are
No loss of life;
No equipment damage;
No sustained loss of production
12.
13. In the typical open pit
Increasing the slope angle means;
Decreases stripping ratio and/or increases
recoverable ore
Consequently, produces a higher benefit or
return on investment
However, increasing the slope angle;
Decreases the stability of the slope
Rate of slope failures increases
The slope failures result in operating costs
such that the expense of cleaning up failed
material, lost production and unrecovered
ore.
14. Mining at a flatter angle results in
Higher stripping costs
reduced ore recovery.
However
mining at flatter angle increase the stability of
the slope.
To balance the above, mining at optimum slope
is advised since mining at a flatter angle
results in higher stripping costs and reduced
ore recovery. Conversely, mining steeper than
the optimum results in slope instability costs
greater than the increased ore recovery.
15. In any economic open pit mine, a
variety of slope failures will be
presented at various locations in the
mine at any time. The successful
management of these failures is the
art of good open pit mining.
The absence of any failures is a sign
of over conservative, hence, inefficient
economic slope design and mine
management.
16. A slope is a vertical or inclined
boundary surface between air and
rock.
The term “slope failure” refers to any
slope instability that affects its
performance, and man's operation in
a natural, geologic environment
Slopes failures can either occur
naturally or are engineered by
humans
17.
18. The rock of a slope can be considered to
consist of the following components
Intact rock; the primary unbroken rock as
determined from a piece of core cut for
compression testing. Rock substance is also
used for unbroken rock
Fractures; the geologic structures such as
joints, bedding, foliation, and minor faults
that break the intact rock into more or less
discrete blocks.
Rock mass; the combination of intact rock
and fractures considered as a unit. Soil could
be considered as a special case of rock mass
19. Significant failure modes which
can occur in an open pit mine
are
I. Plane failure
ii. Circular failure
iii. Wedge failure
iv. Toppling failure
20. Plane failures occur when the geologic
discontinuities such as a bedding plane,
faulty, jointing or foliation strike parallel to
the face and dip into the excavation in such a
way that a block of rock is kinematically free
to move. Single discontinuity is the only
surface along which failure develops
It is one of the most dangerous failure modes
especially when it occurs along a principal
fault. Significant volumes of rock can be
moved in this case.
21.
22. Wedge failure occurs when two or more
discontinuities intersect and their line of
intersection daylights in the face.
It generally occurs on a smaller scale (1 or 2
benches), but it can cause significant damage
23. This failure occurs in a homogeneous ground of
low mechanical resistance (low cohesion).
The failure is done by shearing of the ground
along a circular line.
The grounds favorable for this failure mode are
soils, filling materials, non competent
homogenous rocks and resistant rocks but whose
level of fracture confers them weak mechanical
characteristics.
When the stability conditions are not satisfied,
very high volumes of rock can be moved.
24.
25. A steeply dipping fracture set strikes nearly
parallel to the slope face and forms “tall” slabs
that peel away from the slope.
This failure mode is much rare than the 3 other
modes.
A progressive degradation of the benches
could be observed and might lead to the
instability of the slope.
If this type of structure is present, the bench
face height should be limited to a distance
approximately equal to the bench width. This
will help catch any toppling material and
decrease the chances of impacting equipment
working on the pit floor below
26.
27. The type of rock of which or in which the
slope is made
Presence of potential failure surface in
rock(frequency of geological
discontinuities) and the steepness of its
angle of dip towards the excavation;
The geometry of the cross-section of the
slope (height, slope angle, berms);
Magnitude of externally applied loads on
the rock slope structures;
Vibrations from blasting and seismic forces
28. SF = ratio total Resisting force to the total
Driving force
