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

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

Example - rock mass data Rock mass Joint sets

Example - cave data Cave face Stresses Spalling

Example - primary results Size distribution Statistics

Example - Secondary data Input files Draw column Draw bell

Example - Secondary results Statistics Primary & secondary size distributions

Example - Batch run

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

Primary Fragmentation l Each in situ block is generated independently of other blocks In situ fragmentation Primary fragmentation Combined block

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

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

Block generation Blocks are identified from corner Potential block

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

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

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

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 Read a block: - calc block strength - calc cycle height Determine whether block splits Move its products down the column Calc fines generated See if products reached drawpoint Calculate arching effects Yes No

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

Draw rate factor l Faster draw results in larger blocks:

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

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

Compare to BCF-DOS For Parkes data set (three joint sets): BCF-dosBCFV3 %<2m Ave volume 0,300,13 Max volume 3,262,08 For PMC data set (five joint sets): BCF-dosBCFV3 %<2m 3 23%17% Ave volume 1,461,69 Max volume Stress free runs:

Runs with 5 MPa stress For Parkes data set (three joint sets): BCF-dosBCFV3 %<2m 3 51%34% Ave volume 0,882,22 Max volume 15,425,8 For the PMC data set, the results were as follows: BCF-dosBCFV3 %<2m 3 3,943,64 Ave volume 8,119,0 Max volume

Secondary test runs Using identical primary blocks: Cubic jointsParkes dataPMC data BCF-dosBCFV3BCF-dosBCFV3BCF-dosBCFV3 %<2m 3 16,0%16,22%86,7%87,8%22,13%22,85% Ave vol2,942,750,400,3651,901,79 Max vol106,5 12, BCF-dos uses BCF-dos primary blocks, BCFV3 uses BCFV3 primary blocks – Cycle height halved Cubic jointsParkes dataPMC data BCF-dosBCFV3BCF-dosBCFV3BCF-dosBCFV3 %<2m 3 16,0%28%86,7%65,6%22,13%21,7% Avevol2,942,460,400,541,902,35 Maxvol10678,212,778,

Hang-up runs at Premier Mine Ba5 cave : 75 drawpoints - Average of 4% high hang-ups, 20% low hang-ups during January Draw height was 50 to 100m BCFV3 Results (15 MPa stress in horizontal cave back): Draw ht No. Hh % tons No. Lh % tons 30m316%2030% 50m25%1722% 100m00%1118%

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