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 ….
1. Bench parameters (geometry):
a. height
b. width
c. slope angle
Types of Benches
1. Working bench
…... 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…
a. properties & size of equipment defines the width
b. same plus selectivity defines height
c. geotechnics define slope angle
2. Catch bench
a. remnant bench left to:
* catch the material/rock falling down the slope
* facilitate access to the face
Working Bench, Catch Bench
Catch Bench Geometry
H&K p. 252 & following
Bench facts:
1. face angle varies from 55o –80o
2. most important design component for
a. optimisation
b. equipment selection
c. production planning
3. types
a. working bench
b. safety bench
c. double bench
Working Bench
Typical Berm Design
Double benching ?
Triple benching ?
Bench height vs. slope angle
Bench Cleaning ?
Pit Design: Benches
Higher and wider benches yield :
1. Less selectivity (dilution, recovery)
2. Fewer working places thus less flexibility
3. Flatter slopes: large machines need wide benches
4. Fewer equipment relocations & set-ups
5. Higher productivity at lower unit cost
6. Improved control and supervision
How High Should a Bench Be ?
1. Deposit character and geology: selectivity
2. Production strategy: ore/waste ratios, blending requirements, no. of working faces, operating/capital costs, etc.
3. Slope stability considerations
4. Equipment set / equipment specific optimum geometry
Is this slide a problem ?
Lots of added work ($$$$….)here
Would you drive there?
Reduced Bench Heights
Advantages
1. ramp volume reduced (fill ramps)
2. contour areas easier to drill & blast
3. ore grading and selective mining is easier
4. multi-row blasts easier to blast
5. single pass drilling easier
Disadvantages
1. poor bench grade control by shovel is accentuated
2. reduced drilling yield and increased drill and blast cost
3. sub grade costs more (relatively larger)
4. reduced shovel productivity
Bench Height - Optimization
Are bench dimensions always important ?
Bench Height and Dilution
Watch Your Blasting !
Slope Design : Summary
Slope Design: Conclusions
1. Maximize the height of the benches
* at the expense of mining selectivity?
2. Minimize the width of working benches
* at the expense of productivity?
3. Minimize the number & width of catch benches
* at the expense of safety?
4. Minimize the width & number of haul roads
* at the expense of productivity & safety?