The Effect Of Delay Time On Fragmentation Distribution Through Small And Medium Scale Testing And Analysis

The Robert M. Buchan Department of Mining

The effect of delay time on fragmentation distribution through small and medium scale testing and analysis

by P. Katsabanis and O. Omidi

1

Recent small scale set up •

Material: Commercial grout – UCS approximately 50 MPa – P wave velocity: 4000 m/s – Density: 2.34 g/cm3

•

Geometry – – – –

•

Block Dimensions: 60 cm x 40 cm x 25 cm Borehole diameter: 12 mm Burden: 7.5 cm Spacing: 10.5 cm

Boreholes charged with: – Detonating cord (2 strands of 50 grain/ft) – Water as a coupling medium – Some tests had copper liners

•

Delays between holes

– up to 200 ms: detonating cord – larger than 200 ms: using sub millisecond electronic detonators (Orica)

New work

Recent data (Omidi, 2015) 90 80 70

x50, mm

60 50

x50

40

fit x50 x50 lined

30

fit x50 lined

20 10

0 0

0.5

1 1.5 Delay Time, ms

2

2.5

Slope at average size 0.03

Slope at x50

0.025 0.02 0.015

Swebrec

0.01

Rosin Rammler

0.005

0 0

500

1000

1500

Delay time, mus

2000

2500

Average slope through x80, x20

Slope of the x80-x20 secant 2.5

2 1.5 Swebrec

1

Experiment Rosin Rammler

0.5

0 0

500

1000

1500

Delay time, mus

2000

2500

Effect of delay on 50% and 80% passing sizes 180 160 140 120 Size, mm

100 x50, mm

80

x80, mm

60 40 20

0 0

20

40

60 Time, mus

80

100

120

Effect of delay on 10%, 50% and 80% passing sizes 180 160 140

Size, mm

120 100

x10, mm

80

x50, mm

60

x80, mm

40 20

0 0

500

1000

1500

2000 2500 Time, mus

3000

3500

4000

4500

Data from Johansson and Petropoulos 80.0 70.0

x50, mm

60.0 50.0 40.0

x50

30.0

fit

20.0 10.0

0.0 0

0.05

0.1

0.15 0.2 Delay Time, ms

0.25

0.3

0.35

Uniformity vs. delay time Slope through x20, x80, 100*mm^(-1)

2.5

2 Grout

1.5

Fit grout Cu lined

1

Johansson, r1

Johansson, r2 0.5

Granodiorite

0 0

0.5

1

1.5

2 2.5 Delay Time, ms

3

3.5

4

x80 and x20 vs. delay time 160

140

Particle Size, mm

120

100 x80

80

fit x80 x20

60

fit x20

40

20

0 0

0.5

1

1.5 Delay Time, ms

2

2.5

Normalizing the old USBM data 40

Normalized average size

35 30 25 20

reduced scale, x80 reduced scale x50

15 10 5

0 0

20

40

60 80 Time, ms/m of burden

100

120

140

Summary • Simultaneous initiation or very short delays – coarse fragmentation • Development of the small sizes appears to be in time less than 200 ms. • X50 does not provide clear information. • Larger sizes, such as the x80 show an optimum fragmentation at some intermediate delay time. • In the present (small scale) tests the critical time was 0.6-0.8 ms

Time of detachment of burden, Tmin • 0.6 ms is well past stress wave interactions • Stagg and Rholl have suggested a mechanism of accumulated damage. • For this to occur the burden must not be detached before a subsequent hole detonates. • Hence the arrival and opening of cracks at the face of the blast signify the termination of the above process.

Assuming this breakage

Key issues • Time for stress wave interaction • Time of detachment of burden • Action of gases

Important processes • Interaction between stress waves – before 200 ms • Fragmentation optimization: 0.6 ms • Time for radial cracks to reach free face: – 0.3 – 0.5 ms using Zhang’s crack velocities of 210400 m/s – 0.4 ms using Stagg and Rholl’s formula

X50 data (Omidi, 2015) 90 80 70

x50, mm

60 50

x50

40

fit x50 x50 lined

30

fit x50 lined

20 10

0 0

0.5

1 1.5 Delay Time, ms

2

2.5

Small bench • • • • • •

Hole diameter: 12mm Hole depth: 60cm Burden: 20 cm Spacing: 30 cm Collar: 20 cm Explosive: Detonating Cord (80g/m)

2 ms delay between holes Frame: 6ms after initiation of 1st hole

0.5 ms delay between holes Frame: 6ms after initiation of 1st hole

Conclusion • Small scale tests suggest there is influence of delay time on the larger sizes of blast induced fragmentation as well as on the uniformity of the fragmentation. • The larger sizes of the blast, such as the x80, show a clear optimum at a delay time • The smaller sizes do not show a clear relationship with delay. • Uniformity of the fragmentation, defined as the slope between the 80% and 20% passing sizes, shows a clear optimum value. • It appears that the optimum delay time is related to the time at which the previous hole extends radial cracks to the free face. • Tests using copper tubes as liners in the boreholes, to inhibit gas penetration, showed that x80 increased with delay time, while fragmentation was similar in the short delay times.