ROCK BLASTING – PROCEDURES

ROCK BLASTING – PROCEDURES

ROCK BLASTING

Drilling and Blasting is the controlled use of explosives and other methods such as gas pressure blasting pyrotechnics, to break rock for excavation. It is practiced most often in mining, quarrying and civil engineering such as dam or road construction. The result of rock blasting is often known as a rock cut.

Drilling and Blasting currently utilizes many different varieties of explosives with different compositions and performance properties. Higher velocity explosives are used for relatively hard rock in order to shatter and break the rock, while low velocity explosives are used in soft rocks to generate more gas pressure and a greater heaving effect. For instance, an early 20th-century blasting manual compared the effects of black powder to that of a wedge, and dynamite to that of a hammer. The most commonly used explosives in mining today are ANFO based blends due to lower cost than dynamite.

The controlled use of explosives to excavate rock has been part of construction engineering for hundreds of years. In any blasting situation, the geologic structure of the rock mass will be the most important consideration. Other considerations include the degree of scarring that would be acceptable (some areas can tolerate more blasting scars than others), cost and safety (blasting cannot be performed in close proximity to populated areas).

 EFFECT OF GEOLOGIC PROCEDURE

The first consideration when designing a blasting operation should be the local geologic conditions. Rock competency and fracture patterns can have a significant impact on the success of a blasting operation.

DISCONTINUITY SETS.

When discussing blasting, the single most important geologic factor is fracture; the spacing and orientation of any breaks or discontinuity sets in the rock in particular. The orientation of the discontinuity sets with respect to the cut slope angle will influence any slope failures that may occur along the slope face. The modes of failure can be grouped into four primary mechanisms as shown below;

• Planar failure (a)
• Wedge failure(b)
• Circular failure (c)
• Toppling failure (d)

 The four primary mechanisms of slope failure(d)

 SOME MAJOR PROCEDURES IN BLASTING

1. PRESPLIT BLASTING;

DESCRIPTION

Presplit holes are blasted before production blasts. Procedure uses small diameter holes at close spacing and lightly loaded with distributed charges.

ADVANTAGES

This Protects the final cut by producing a fracture plane along the final slope face that fractures from production blast cannot pass. Can produce steeper cuts with less maintenance issues perform well in hard competent rocks.

LIMITATIONS

The small diameter borings limit the blasting depth to 15m (50 ft). Bored holes traces are present for the entire length of boring. It does not perform well in highly fractured weak rocks.

2. SMOOTH BLASTING.

DESCRIPTION

Smooth blast holes are blasted after production blast. Procedure uses small diameter holes at close spacing and lightly loaded with distributed charges.

ADVANTAGES

It produces a cosmetically appealing stable perimeter. This can be done on slopes years after initial construction. Drill hole traces are less apparent than prespliting.

LIMITATIONS

The small boring diameter limits blasting depth to 15m (50 ft). Borehole traces are present for much of apparent than prespliting the boring length.

3. CUSHION BLASTING.

PROCEDURE

Cushion blasting is done after production blast. Larger drilled holes are used with small diameter, lightly loaded distributed loads. Spaces around the explosives are filled with crushed rock to cushion the explosive force.

DESCRIPTION

Reduces the amount of radical fracturing around the borehole and also reduces borehole traces. The large diameter holes allow blasting depths up to 30m (100 ft).