This document discusses considerations for rock breakage from blasting. It addresses the importance of the free face in compressional and tension wave reflection, and how observing throw can determine if burden is optimal. Blast design should consider fragmentation needs, rock quality, and site factors. Proper burden is critical for fragmentation and avoids issues like backbreak, flyrock and vibration. Bench stiffness above a ratio of 2:1 promotes better breakage through flexural failure. Row delays also impact breakage and safety factors.

1. ROCK BREAKAGE MECHANISM

2. CONSIDERATIONS FROM FRACTURE
MECHANISM
 Why free face is important :
 Compressional wave reflected back as tension wave which further breaks the radial
cracks formed by compression wave
 Observing throw will help us in deciding if burden kept for blast is optimal or not

3. BLAST DESIGN CONSIDERATIONS
 Fragmentation required : Machinery capacity
 Rock quality : Soft, hard, watery holes, Discontinuity
 Site consideration : Local Hindrance, Vibration etc
Machinery Capacity(In Cu. M) Fragment Size(m)(Considering 80%)
3.1 1.16
3.8 1.248
15 1.97
24 2.31

4. How to decide from blast if Burden is
sufficient or not ?
 If burdens are too small, rock is thrown a considerable distance
from the face, fragmentation may be excessively fine
 If burdens are too large, severe backbreak.
 Excessive burdens may also cause flyrock vertical cratering and
high levels of air blast, rock breakage coarse
 Excessive burdens cause over confinement of the blastholes, which
result in significantly higher levels of ground vibration.

5. BURDEN IMPACT

6. BENCH STIFFNESS


7.  The burden rock is more difficult to break by flexural failure when bench heights
approach the burden dimension in length. When bench heights are many times the
burden in length, the burden rock is more easily broken.
 Stiffness = Bench height/burden
 Generally stiffness ratio>2 is good for fragmentation. Best =3-4

8.  When the burden rock bulges at its center. tensile stresses result at the face and
compression results near the charge. Under this type of bending condition. the rock will
break from the face back toward the hole This mode of failure generally leads to
desirable breakage,
 This second case is undesirable. This mechanism occurs when cracks between blastholes
link before the burden is broken and is normally caused by insufficient blasthole
spacing.
 The bending mechanism or flexural failure is controlled by selecting the proper
blasthole spacing and initiation time of adjacent holes

9.  Guidelines for row-to-row initiation are as follows:
 a) Short delay times cause higher rock piles closer to the face.
 b) Short delay times cause more endbreak.
 c) Short delay times cause more violence, air blast and ground
vibration.
 d) Short delay times have more potential for flyrock.
 e) Long delay times decrease levels of ground vibration.
 f) Long delay times decrease the amount of backbreak.

ROW TO ROW DELAYS

10. TIMING DIAGRAM