BCF

1. BCF • A program to evaluate rock fragmentation in caving mines • The program models three processes: • Primary fragmentation - release of blocks from the cave back • Secondary fragmentation - reduction of blocks in the draw column • Hang-ups - blockages in draw bells

2. Method of operation • Input is rock mass data & cave data • Uses rules/mechanics to produce primary blocks and calculate secondary fragmentation in draw column • Uses Kear’s model (rule based) to predict hang-ups in draw bells using secondary blocks as input.

3. Example - rock mass data Rock mass Joint sets

4. Example - cave data Cave face Stresses Spalling

5. Example - primary results Size distribution Statistics

6. Example - Secondary data Input files Draw column Draw bell

7. Example - Secondary results Primary & secondary size distributions Statistics

8. Example - Batch run

9. Hang-up run results High hangups Low hangups Oversize block statistics

10. Primary Fragmentation • Each in situ block is generated independently of other blocks In situ fragmentation Primary fragmentation Combined block

11. Primary fragmentation • Stress fracturing and shearing along joints are considered Shearing along inclined joints Stress fractures seen as additional joint set Primary fragmentation more slabby owing to stress fractures Combined block

12. Block generation • Relative joint densities used to select block corner Joints generated along each intersection line

13. Block generation Blocks are identified from corner Potential block

14. Combination of blocks In compressive field stresses blocks may combine to until strength criterion is exceeded or end of joint is reached Combined Block

15. Block generation • Writes volume, aspect ratio, joints to file 1 1 Aspect ratio = 1,0 1 2 1 Aspect ratio = 1,67 1 Aspect ratio = 7,0 1 1 10 Aspect ratio = (Area x L) / (6 xVol)

16. BCF SECONDARY FRAGMENTATION • Primary blocks are broken down into smaller fragments by • repeatedly splitting in the draw column • rounding of corners • splitting when a temporary arch fails • Secondary fragmentation is based on concept that blocks with high aspect ratios will split more easilt than block with low aspect ratios • Fines generated by primary fragmentation reduce the effectiveness of the secondary fragmentation process by acting as a cushion

17. Flow chart for secondary fragmentation Read input parameters from primary file Calculate: - cave height & pressure - fines factor from rounding of corners - cushioning factor owing to primary fines Yes Read a block: - calc block strength - calc cycle height No Determine whether block splits Move its products down the column Calc fines generated See if products reached drawpoint Calculate arching effects

18. Cave pressure • Cave pressure affected by “bin effects” based on Udec model • As width:height ratio of active draw zone increases dead weight loading of base increases

19. Draw rate factor • Faster draw results in larger blocks:

20. Calculate cycle height • Cycle height is distance that block travels down column before splitting • Cycle height depends on aspect ratio of block, block strength, cave pressure, draw rate • Block strength related to intact rock strength, block volume, veinlets, whether block is combined (contains joints) and joint strength. • Progressive splitting of blocks increases their cycle height by reducing volume, aspect ratio and number of contained joints.

21. Hang-up analysis • Robin Kear’s model Upper area If 25 blocks fill 40% of an area a hang-up is registered Lower area

22. Compare to BCF-DOS Stress free runs: For Parkes data set (three joint sets): BCF-dos BCFV3 3 %<2m 97 98 Ave 0,30 0,13 volume Max 3,26 2,08 volume For PMC data set (five joint sets): BCF-dos BCFV3 3 %<2m 23% 17% Ave 1,46 1,69 volume Max 59 259 volume

23. Runs with 5 MPa stress For Parkes data set (three joint sets): BCF-dos BCFV3 3 %<2m 51% 34% Ave 0,88 2,22 volume Max 15,4 25,8 volume For the PMC data set, the results were as follows: BCF-dos BCFV3 3 %<2m 3,94 3,64 Ave 8,11 9,0 volume Max 364 785 volume

24. Secondary test runs Using identical primary blocks: Cubic joints Parkes data PMC data BCF-dos BCFV3 BCF-dos BCFV3 BCF-dos BCFV3 3 %<2m 16,0% 16,22% 86,7% 87,8% 22,13% 22,85% Ave vol 2,94 2,75 0,40 0,365 1,90 1,79 Max vol 106,5 106,5 12,73 12,73 535 174 BCF-dos uses BCF-dos primary blocks, BCFV3 uses BCFV3 primary blocks – Cycle height halved Cubic joints Parkes data PMC data BCF-dos BCFV3 BCF-dos BCFV3 BCF-dos BCFV3 3 %<2m 16,0% 28% 86,7% 65,6% 22,13% 21,7% Ave vol 2,94 2,46 0,40 0,54 1,90 2,35 Max vol 106 78,2 12,7 78,7 535 762

25. Hang-up runs at Premier Mine Ba5 cave : 75 drawpoints - Average of 4% high hang-ups, 20% low hang-ups during January 1999. Draw height was 50 to 100m BCFV3 Results (15 MPa stress in horizontal cave back): Draw ht No. Hh % tons No. Lh % tons 30m 3 16% 20 30% 50m 2 5% 17 22% 100m 0 0% 11 18%

26. Future development • More data needed for validation • Model method • base on tons rather than blocks • allow mixing in draw column • Output • present range of possible results • Output as production rate/delays