Special Blasting Technique

DRILL AND BLAST SECTION MINING SERVICES DEPARTMENT PT

KALTIM PRIMA COAL

Special Blasting TechniqueTrim and Pre Split Blasting

Reasons for Pit Wall Damage

Gas DrivenCrackExtension

Compressive and Tensile Failure

Shear Failure

Load Release

Block Heaving

No Free Face Prevents Burden Movement

Subdrill Damages Crests

Areas Concern in Wall Control Blasting

• Selection of best wall control blast method• Design blast based on geology and past

history to minimize wall damage• Development of blast pad area prior to

drilling to provide suitable blast area• Optimal implementation of chosen design

to minimize variation from design• Fragmentation and muck pile looseness

Wall Control Blasting Options

KPC has two main tools for controlling the strength and stability of interim and final pit walls:

• Trim Blasts• Pre split Blasts

Trim Blasts

The aim is to• Break rock mass to the pre-split or final wall

line

• Not to break through the line!

• Clearly, a good pre-split with a poor trim blast will not work

• A poor pre-split with a good trim blast also will not work!

• If pre-splitting is not appropriate, trim blasting may still provide a good final wall

There are some ‘Rules of Thumb’ to try in a new situation -

• Stand-off from pre-split should be about half the hole burden (i.e. 4m, KPC uses 3m)

• Last row spacing should be half the main spacing (OK)

• Burden between last (buffer) row and 2nd last row should be 3/4 the main burden (5.2m, KPC uses 6m or 6.6m)

Trim Blasts

Trim Blasts

Optimum burden relief for a detonating hole requires -

• Sufficient number of neighbouring holes to detonate

• Free face be close enough to the next hole to fire

• Enough time before this hole fires to create a free face it can use

These requirements can be investigated using JKSimBlast

Trim Blasts

Adequate burden relief is important in any production blast and absolutely critical in trim blasts.

• If any part of the blast is over-confined, material will not move properly

• This will over-confine the blastholes behind - it gets worse!

• In a small blast like a trim blast, this will not correct itself - damage will penetrate behind the pre-split or final wall line.

Trim Blasts

Issues to think about in dynamic burden relief 1. Drill Pad Preparation

Free faces – this means• shovel digs back to hard material• pad preparation dozing does not push loose material

over the free face

Accurate hole locations

2. Correct design implementation Accurate crest burdens Blastholes located to follow crest line Drilling accuracy

3. Correct tie-up

Pit AB Phase 2 Trim & Presplit Design, 20m Benches

Revised design using vertical presplit

Production Holes

Presplit Holes

40m

Trim Holes

Standoff 1m

10m

70o

17m 6.6m

6m

8m

6m

8m

3m

20m

20m

20m



40m

Standoff 1m

10m

70o

17m 6.6m

6m

8m

20m

20m

20m

6m

8m

3m

6m

8m

3m

This will damage crest and cause early failures



40m

Standoff 1m

10m

70o

17m 6.6m

6m

8m

15m

15m

15m

6m

8m

3m

6m

8m

3m


6m 8m3m Upper Bench

Production rows:1. Max 4 rows2. Burden = 7 m3. Spacing = 8 m4. Depth = 11.5m5. Spacing direction must be parallel to presplit

row6. Blast area must have a free face – without

broken material in front – parallel to presplit row


6m 8m3m Upper Bench

Trim row:1. Burden = 3.0m2. Spacing = 4.6m3. Depth = 11.5m4. Charge is as

shown in diagram

3.5m Stemming

2.0m Air Deck

4.0m Charge

2.0m Air Deck

Blast Plan Showing Tie-in for Upper Bench

Presplit Row Trim Row

Production Rows


8mLower Bench

3.5m Stemming

1.75m Air Deck

2.75m Charge

1.0m Air Deck

First Trim row:1. Burden = 6.0m2. Spacing = 4.6m3. Depth = 9.0m4. Charge is as

shown in diagram

6m3m


8mLower Bench

Second Trim row:

1. Burden = 3.0m2. Spacing = 4.6m3. Depth = 9.0m4. Charge is as

shown in diagram

6m3m

3.5m Stemming

1.75m Air Deck

2.75m Charge

1.0m Air Deck

Blast Plan Showing Tie-in for Lower Bench

Presplit Row

Trim Rows

Production Rows

Presplit Blast

• A line of closely spaced blast holes drilled at the limits line.

• Often smaller diameter than production blastholes.• Much lower charge density – laterally or vertically

decoupled.• Fired before the main blast; usually with no delay

between holes.

Why Pre-split?

• Increases wall stability.• Defines wall position, ensures consistent front row

burdens in strip mining.• Channels blast gases away from rock mass, reducing

back-break and damage.• Others (eg. limits dilution, aids equipment).

Proposed Mechanism – Tensile Failure from Compressive Shock

BlastholeBlasthole

Tensile Stress Zone

Compressive ShockCompressive Shock

Resultant Tensile Stress

Well Developed Presplit Line

Presplit Does Not Perform in Non-competent or Fractured Rock

Borehole

Adverse Jointing makes Presplit Impossible

How do we start?

The engineering variables (really variable)

• Hole diameter.• Hole spacing.• Total energy (explosive mass / product).• Energy distribution (charge positions,

decoupling).

What Else?

Rock mass variables (not so variable!)• Compressive strength.• Tensile strength.• Discontinuities (a.k.a. Joints).• Must not under-estimate joints!

A successful pre-split blast hole will…

• Generate sufficient pressure to cause tensile failure (the ‘split’)

• NOT generate too much pressure so that collateral crushing is avoided (ideally pressure is less than the compressive strength)

• Be close enough to its neighbour to form a continuous split

KPC Presplit Calculations

Explosive DynoSplitVOD (m/s) 4300

Density (g/cc) 1.1

Cartridge dia (mm) 32

Borehole dia (mm) 200

Borehole length (m) 20

Detonation pressure (GPa) 5.1

Explosion pressure (GPa) 2.5

Borehole pressure (MPa) 72

Presplit Charge One String DynoSplit 32mm diameter

Parameter ValueHole Spacing (m) 3Bench Height (m) 20

Explosive Specific Charge (kg/m) 0.88Uncharged Hole Length (m) 1

Doubled Over Length (m) 2No of Charge Strings per Hole 1

Pre-Split Powder Factor (kg/m2) 0.31

Orientation

Typical Presplit Design

75% Normal Burden 50% Normal

Spacing

50% Normal Burden

Presplit Row

Alternative Presplit Charges

Airdeck & stem or leave open

Usually Airdeck & stem

Multiple small deck

charges

Multiple decoupled charges

Continuous low

density charge

Continuous

decoupled charge

Single small

charge

When it Works!PreSplit sandstone wall at Panel 8, Hatari, 1999.

When it Works!

Unfavourable Structure

Unfavourable Material

Special Blasting Technique

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