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 ….
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?