Blasting Practices at Open Cast Mines

One can easily say that the most important part in extraction from an open cast mine is blasting and by following some simple standard practices, a high degree of productivity can be attained.

Open cast blasting

Following are some important facts related to blasting practices in open cast mines:-

1. Drilling: There are two major types of drilling techniques being used

• Down the Hole (DTH): It is used when the blast hole diameter varies from 100mm to 380mm and the depth of blast hole is 12-15m.
• Rotary: When the blast hole diameter > 400mm and the depth is about 20m, rotary drilling is used.

2. Bench Height: This depends on the excavator employed in the operation i.e.

• Shovel (4.6m3, 6m3, 8m3): 10-12m
• Dragline (24m3/ 96m): 30m

3. Blast Hole Diameter: Up to bench height of 8m, 100mm,, 150mm, 163mm of blast hole diameter is preferred but for bench height > 8m, a minimum of 250mm diameter of blast hole is efficient.

4. Spacing and Burden: In coal measure rocks spacing varies from 3-8m whereas burden is selected about 0.8 times the spacing.

5. Depth of sub-grade drilling: In coal mines the length of sub-grade drilling for a bench height 8m is generally 0.3m.

6. Explosives: In Indian coal mines nitroglycerine based explosives, slurries, Ammonium Nitrate and Fuel Oil Mixture(ANFO) and Liquid oxygen type Explosive(LOX) are the main explosives which are used.

Slurries are mostly used in Open Cast mines because of it is

• water resistant
• has high strength
• has high coupling characterstics
• has good fragmentation

Metalized and Aluminium slurries are used. Also, Site Mixed Slurry(SMS) has been introduced.

ANFO is not generally used because of  its

• low density
• low pressure generation
• poor water resistance

 Limitation of the use of LOX is due to

• limited availability of oxygen making reservoir and soaking plant near mine site
• difficulty in preparation of LOX cartridge
• availability of other cheaper, safer and efficient explosives.

For coal mines special Oxygen based explosive LOXITE-C is used.

 For Iron ore mines:-
1. Bench Height and Hole Diameter: If the bench height is 10-12m, then blast hole diameter should be 150mm, if the bench height <6m then blast hole diameter should be 100mm. In rare cases, 250mm blast holes are selected.

2. Spacing and Burden: In case of 150-165mm blast holes spacing and burden varies from 3-7m whereas height of the bench varies from 8-12m.

3. Stemming Height: It should be 30-40% of the hole depth.

4. Sub-grade drilling: 10-20% of hole depth.

5. Explosives: LOX, Open Cast gelignite, GN-1, slurry explosives, ANFO, metalized and Aluminium slurry, Site Mixed Slurry.

For Limestone mines:-
1. Bench Height: 9-12m

2. Hole Diameter vs Spacing and Burden:

S. No.	Hole Diameter (mm)	Spacing(m)	Burden(m)
1	100	3-5	2.5-4
2	150-160	5-7	4.5-6

3. Sub-grade drilling = 10-20% of hole depth

4. Stemming Height = 20-40% of hole depth

5. Explosive: OCG, ANFO, GN-1, Slurry.

For Coal Mines (Overburden bench), an example
1. height of bench = 13-14m

2. width of bench= 20-25m

3. Avg. Burden= 4m

4. Avg. Spacing= 5m

5. Diameter of blast hole= 250mm

6. No. of blast hole/ blast = 20-35

7. Blasting Pattern is staggered/ rectangular

8. Depth of Blast hole = 13-14m

9. length of face = 40 -100m

10. explosives: Site Mixed Slurry ( VOD = 4500m/s, Relative Weight Strength = 97)

11. Avg. Charge/hole = 200-220kg

12. Charge Length = 4.5m

13. Specific Charge = 0.55 - 0.65 kg/m3 of blasted rock

14. Equipments used: Shovel - 4.6 m3, Dumper - 35T.


In the above articles, there are a few terms that are used without definition. Please ask for any definition that needs explaining by commenting on the article.