Bench parameters and their elements
Bench parameters and their elements
Now, let’s learn the terminology associated with the pits. Using this figure, we can define key elements.
Bench floor: Horizontal surface of the bench
Bench face: Vertical surface of the bench
Face angle (α): The average angle the face makes with the
horizontal
Bench height (H): Vertical distance between upper and lower
bench surface
Toe: Interior vertex formed by the intersection of the bench
face and bench floor
Crest: Exterior vertex formed by the intersection of the
bench floor and bench face
Bench width: The distance between the crest and the toe measured
along the upper surface
Bank width: The horizontal projection of the bench face
There are two types of benches in each open pit mine:
working benches and safety (catch) benches.
Working Benches
Working benches are where unit operations (drilling, blasting,
loading, and hauling) are performed. Working benches are wider than safety
benches to provide enough room for the mining equipment to perform their tasks.
Safety (Catch) Benches
Safety benches are designed to collect the sliding material
from the benches above and stop the downward progress of large rock pieces or
boulders.
Bench height
Bench height is one of the first parameters to be determined
during the mine design process. Can you guess the parameters that should be
taken into account when determining proper bench height?
Bench height depends on several parameters:
Overburden and Ore Properties
We discussed before that ultimate pit slope depends on
material (ore and waste) properties. These properties include rock strength and
the number and direction of rock discontinuities. Benches are just like smaller
pit walls with a much lower height (usually 20 to 50 feet). Therefore, benches
should also be designed in such a way that no risk of instability is created
due to excessive height of benches. There is a direct relationship between rock
strength and safe values for the bench face angle and bench height.
Production Rate
Production rate of a mine is a parameter that is usually
determined by market demand and selling capacity. When the required production
rate is determined based on the market situation, then the annual mine
production rate that includes ore and waste extraction is determined in such a
way that enough product for the sale purposes is guaranteed. Higher benches are
normally required to achieve higher production rates.
Degree of Selectivity
We discussed selectivity in the last lesson, and the
definition of the parameter is unchanged. However, insofar as bench height is
concerned, there is a selectivity issue, which is illustrated with these three
figures, below. These figures (are attempting to) illustrate local variations
in grade, and the effect of different bench sizes on the selectivity.
Size and Type of Equipment to Meet the Production Requirements
After the annual production rate of the mine is determined,
the required daily and even hourly production rates will be calculated based on
assumptions about the number of working days per year and the number of shifts
per day. Then mining equipment is selected so that these production
requirements can be met. Loading equipment is usually the first to be selected;
next, appropriately sized trucks are chosen to match with the loader, and then
the complete fleet of loading and hauling equipment will be specified. The
maximum reach of the loading equipment should be roughly the same as the bench
height.
Regulatory Limits
Federal and state regulatory agencies have regulations that
limit bench height for specific mining operations, depending on the commodity,
to ensure the safety of mine workers and equipment inside the pit. The
engineering calculations may indicate that a greater bench height is safe, but
it is unlikely that the regulatory agency would allow an exception to their
rule.
Bench Face Angle
Bench face angle is ultimately limited by geotechnical
considerations, as we generally want to have as steep of an angle as we can
sustain without an unacceptable risk of a slope failure. Despite excellent
engineering and slope monitoring, failures can occur. This photo was taken in
2013 shortly after the slope failure at the Bingham Canyon Mine. Look at the
shops and warehouses in the picture to appreciate the size of this failure. It
is truly a miracle that no one was killed – the potential was there for massive
loss of equipment and multiple fatalities.
Safety Berm
A safety berm is a pile of broken material constructed along
the crest to form a “guard rail” to prevent trucks or other equipment from
inadvertently driving over the edge of the haul road. It also serves to catch
falling rocks. The height of the safety berm should be greater than or equal to
the tire radius of the largest truck. These berms are required by federal
regulation, but even with berms, accidents can happen. The picture on the left
shows a haul truck, or what remains of it, after going over the highwall; and
the one on the right shows a piece of rock that slid off the highwall and
landed on a truck below. The former most certainly resulted in a fatality,
whereas the latter probably did not, in this instance.
Haul Roads
These are added to the pit walls to provide access between
benches and the plant next to the pit. Open pit haul roads have a gradient up
to 15%. The width of the haul road is designed based on the width of the
largest truck that travels in the pit and the number of the lanes that are
required to provide proper access for haul trucks, auxiliary equipment such as
explosives trucks and water trucks, and smaller vehicles, such as pickup
trucks, while minimizing congestion.
Now, let’s review the bench geometry concepts and open pit
elements by looking at two annotated photographs.
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