Rock Blasting

In earthworks activity, contractor may encounter hard material during carry out excavation works. If the material to be excavated is too hard (rock), normally geotechnical engineer and client will advise the contractor to carry out blasting work.

These are two common type of controlled blasting techniques to be used for highway rock excavation either Control Bulk Blasting or Presplit Blasting.

1. Controlled bulk blasting is used in situation where importance is placed on accurate removal of rock. Overbreak is reduced and the resultant faces require less scaling. Smooth or trim blasting is therefore employed towards the margins of bulk blast excavations. This involves small diameter drill holes at closer centres, reduce charges detonation delays and a more careful approach to the positioning of holes relative to the design slope.
2. Prespliting is employed to reduce further the harmful effect of blasting on the final face. This involves the drilling of closely – spaced holes along the design slope and charging these relatively lightly. The Presplit charges are detonated. Simultaneously, prior to bulk charges in the main area to be excavated. This effectively forms a fracture plane along the design slope.
   

Before blasting is carry out, contractor shall submit details of blasting proposal to Engineer or Client for approval not less than 24 hours. The proposal shall include the following data;

1. Diameter of holes
2. Spacing of holes
3. Depth of holes
4. Inclination of holes
5. Type of explosive
6. Explosive charge per hole and how it is distributed
7. Explosive charge per delay
8. Delay periods
9. Blast pattern
10. Total length of stemming in each hole
11. Type of stemming
12. Number of holes and total amount of charge to be fired in the blast
13. Initiation sequence

BLASTING SAFETY CONSIDERATION
For the prevention of ground vibrations, flyrock and air blast at the blasting site, the following safety precautions should be taken.

1. Employment of millisecond delay electrical detonators with a minimum delay of 25-millisecond delay between each shorthole.
2. Usage of minimum charge / delay during blasting.
3. Deck loading of explosives on boreholes to reduce the explosive charge action.
4. Accurate positioning of boreholes so that optimum amount of explosives will be utilised.
5. Slightly undercharging the first nows of shortholes on the free face of the trench.
6. Usage of 76mm series drill steel for drilling purposes.
7. Usage of earth for overburden etc, to cover the top of the free face of the rock to be blasted