The Free Face
1. A free face is where the rock and air meet – the surface of the bench is not a free face.
2. Free faces enable the explosives’ energy to perform the greatest amount of work on the rock mass.
3. A blast will be more efficient if it has two free faces rather than one.
Open joints are also free faces.
Two free faces
Blast Patterns
Two basic blast patterns are used:
1. Box pattern, also known as a Square pattern, and
2. Staggered pattern.
Box (Square) Pattern
Staggered Pattern
Chevron Patterns
1. When only one free face exists, you can use the blast design to create more free faces.
2. A chevron design initiates the first hole in the middle of the face.
3. Subsequent holes then have two free faces to blast into.
Chevron – Staggered V
Effects of Blast Patterns
1. Fragmentation is enhanced
2. Muckpile profile is enhanced
3. Overbreak is minimised
4. Vibrations are minimised
Controlling Movement
1. The Blast Pattern is used to control where the muckpile will land.
2. This makes clearing of the muckpile more efficient.
Controlling Movement
Individual Hole Initiation
1. Blasts are not initiated one row at a time.
2. Much better results are achieved using patterned blasts with each hole firing individually.
3. This gives each hole at least two free faces to blast in to.
4. Smartdets and Shocktubes make this easy to achieve.
Individual Hole Initiation
MAJOR FACTORS INFLUENCING BLAST EFFICIENCY
1. ATTITUDE
a. PAYING ATTENTION TO DETAILS
b. EACH OPERATION MUST BE COMPLETED
c. AS PRECISELY AS POSSIBLE
d. TOTAL QUALITY MANAGEMENT (T.Q.M)
e. GROUP EFFORT
2. COMMUNICATION
a. SAFE BLASTING PRACTICES REQUIRE
b. GOOD COMMUNICATION.
c. COMMUNICATION BETWEEN MEMBERS OF
d. SAME GROUP AND BETWEEN GROUPS.
e. OPTIMUM BLAST DESIGNS DEPEND ON
f. INPUT FROM EACH GROUP.
3. BLAST DESIGN
KEYS TO EFFICIENT BLAST DESIGN
a. UNIFORM ENERGY DISTRIBUTION
b. APPROPRIATE ENERGY CONFINEMENT
c. PROPER ENERGY LEVEL
d. ADJUSMENT OF DESIGN TO MEET
e. EXISTING CONDITIONS
4. GEOLOGICAL EFFECTS
Blasting results are influenced more by rock properties than explosive properties.
a. Rock properties:
b. Compressive strength >> Tensile strength
c. Rock Structure:
*) Rock fragmentation is primarily controlled by bedding, jointing, and faulting.
*) Smaller drill pattern minimize the adverse effects of bedding and fractures but increase drill and blast costs.
*) Explosives with high gas production (ANFO) are appropriate for highly jointed or fractured rock.
*) The orientation of the free face to the joints sets is also a key consideration for fragmentation and wall control.
d. W a t e r
*) Static water
*) Dynamic water
*) Multiple priming is advised in wet blast hole
