Mine Design: Benching
Mine Design: Benching
Who does what and how?
Elements Of Pit Slope Design
What Is a Bench?
… a ledge that, in open-pit mines and quarries, forms a single level of operation above which minerals or waste materials are excavated from a contiguous bank or bench face ….
Bench parameters (geometry):
- slope angle
Types of Benches
The mineral or waste is removed in successive layers, each of which is a bench, several of which may be in operation simultaneously in different parts of , and at different elevations in, an open pit mine or a quarry…
- properties & size of equipment defines the width
- same plus selectivity defines height
- geotechnics define slope angle
remnant bench left to:
- catch the material/rock falling down the slope
- facilitate access to the face
Working Bench, Catch Bench
Catch Bench Geometry
1. face angle varies from 55o –80o
2. most important design component for
- equipment selection
- production planning
- working bench
- safety bench
- double bench
SB= Safety bench ( In the beginning it is approx. 2/3 of bench height. At the end of mine life reduced to approx. 1/3 of bench height)
WB= Working bench
- Double benches at final pit limits.
- Avoiding intermediate catch benches facilitates steepening the overall pit slope angle
Typical Berm Design
- Berm height is usually greater than or equal to 1/2 tire radius.
- Berm slope is normally 35o
Note: Berm in this context is strictly a safety berm. Many mine design software uses the term “berm” as safety bench or catch bench
Double benching? Triple benching?
Bench height vs. slope angle
Wall, bank, and slope stability.
Mining methods shall be used that will maintain wall, bank, and slope stability in places where persons work or travel in performing their assigned tasks. When benching is necessary, the width and height shall be based on the type of equipment used for cleaning of. benches or for scaling of walls, banks, and slopes.
SCALING AND SUPPORT
Correction of hazardous conditions.
Ground conditions that create a hazard to persons shall be taken down or supported before other work or travel is permitted in the affected area. Until corrective work is completed, the area shall be posted with a warning against entry and, when left unattended, a barrier shall be installed to impede unauthorized entry.
Pit Design: Benches
Higher and wider benches yield:
- Less selectivity (dilution, recovery)
- Fewer working places thus less flexibility
- Flatter slopes: large machines need wide benches
- Fewer equipment relocations & set-ups
- Higher productivity at lower unit cost
- Improved control and supervision
How High Should a Bench Be?
- Deposit character and geology: selectivity
- Production strategy: ore/waste ratios, blending requirements, no. of working faces, operating/capital costs, etc.
- Slope stability considerations
- Equipment set / equipment specific optimum geometry
Is this slide a problem?
Lots of added work ($$$$….) here
Would you drive there?
Reduced Bench Heights
- ramp volume reduced (fill ramps)
- contour areas easier to drill & blast
- ore grading and selective mining is easier
- multi-row blasts easier to blast
- single pass drilling easier
- poor bench grade control by shovel is accentuated
- reduced drilling yield and increased drill and blast cost
- subgrade costs more (relatively larger)
- reduced shovel productivity
Bench Height – Optimization
Are bench dimensions always important?
Bench Height and Dilution
Watch Your Blasting!
Slope Design: Summary
Slope Design: Conclusions
- Maximize the height of the benches
at the expense of mining selectivity?
- Minimize the width of working benches
at the expense of productivity?
- Minimize the number & width of catch benches
at the expense of safety?
- Minimize the width & number of haul roads
at the expense of productivity & safety?
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- image of typical mine bench
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