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

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.

Example - rock mass dataRock 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 & secondarysize distributions

Example - Batch run

Hang-up run results

High hangups

Low hangups

Oversize blockstatistics

Primary Fragmentation

Each in situ block is generated independently of other blocks

In situ fragmentation

Primary fragmentationCombined block

Primary fragmentation

Shearing along inclined joints

Primary fragmentation more slabby owing to stress fractures

Combined block

Stress fracturing and shearing along joints are considered

Stress fractures seen as additional joint set

Block generation

Relative joint densities used to select block corner

Block corner

Joints generated along each intersection line

Block generation

Block corner

Blocks are identified from corner

Potential block

Combination of blocks

Block corner

In compressive field stressesblocks may combine to

until strengthcriterion is exceeded or

end of jointis reached

Combined Block

Block generation

Writes volume, aspect ratio, joints to file

1

1

11

1

12

10

1

Aspect ratio = 1,0

Aspect ratio = 1,67

Aspect ratio = 7,0

Aspect ratio = (Area x L) / (6 xVol)

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

Flow chart for secondary fragmentation

Read input parametersfrom 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 splitsMove its products down the columnCalc fines generatedSee if products reached drawpointCalculate arching effects

Yes

No

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

Width toheight ratioof activelydrawncolumn

Percentageof cavedweightacting onbottom ofcolumn

1 : 1 44%

1 : 2 30%

1 : 3 23%

1 : 4 20%

1 : 5 18%

Draw rate factor

Faster draw results in larger blocks:

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

0 20 40 60 80 1 00

Dr aw ra te (cm /da y)

Dra

w r

ate

fa

cto

r

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.

Hang-up analysis

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-dos BCFV3%<2m 3 97 98Ave

volume0,30 0,13

Maxvolume

3,26 2,08

For PMC data set (five joint sets):

BCF-dos BCFV3%<2m 3 23% 17%Ave

volume1,46 1,69

Maxvolume

59 259

Stress free runs:

Runs with 5 MPa stress

For Parkes data set (three joint sets):

BCF-dos BCFV3%<2m3 51% 34%Avevolume

0,88 2,22

Maxvolume

15,4 25,8

For the PMC data set, the results were as follows:

BCF-dos BCFV3%<2m3 3,94 3,64Avevolume

8,11 9,0

Maxvolume

364 785

Secondary test runs

Using identical primary blocks:Cubic joints Parkes data PMC data

BCF-dos BCFV3 BCF-dos BCFV3 BCF-dos BCFV3%<2m3 16,0% 16,22% 86,7% 87,8% 22,13% 22,85%Ave vol 2,94 2,75 0,40 0,365 1,90 1,79Max 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 dataBCF-dos BCFV3 BCF-dos BCFV3 BCF-dos BCFV3

%<2m3 16,0% 28% 86,7% 65,6% 22,13% 21,7%Ave vol 2,94 2,46 0,40 0,54 1,90 2,35Max vol 106 78,2 12,7 78,7 535 762

Hang-up runs at Premier Mine

Ba5 cave : 75 drawpoints - Average of 4% high hang-ups, 20% low hang-ups during January 1999. Draw heightwas 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%

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