Effect of geological conditions on the degree of

4.  Degree of rock fragmentation is an index used to assess the efficiency of rock blast.
 Fragmentation , which is the resulting size distribution , is a key issue in this process.
 Fragments size produced by blasting depends on two parameters:
 Uncontrollable parameters(Geology of Mine Site)
 Controllable parameters(Mine Design)
 Blast should be designed based on the geological conditions of the mine site since it
affects the distribution of explosive energy in rock mass.
 The geology of the rock frequently affects the fragmentation more than the explosive
used in the blast.

5.  Jointing
 Joints aperture
 Joints filling
 Joint orientation
 Voids
 Discontinuities
 Bedding / foliation
 Weathering of rock

6.  Jointing can have a pronounced effect on both fragmentation and stability of
the perimeter of excavation.
 Close jointing usually results in good fragmentation.
 Widely spaced jointing, usually results in a very blocky muck pile.
 Because the joint planes tend to isolate large blocks in places. where the
fragmentation is unacceptable.
 The best solution is to use smaller blast holes with smaller blast pattern
dimensions.

7. Figure 1.—Effect of jointing on the stability of an excavation (plan
view).

8. Figure 2.—Tight and open corners caused by jointing (plan
view).

9.  Increasing joint surface separation severely decreases the quality of the excavation profile as a result of
increased cratering of joints.
 Open joints arrest or cause branching of cracks being propagated between two perimeter holes.
 Joint opening was increased, the angle of cratering was also increased.
 When the joints are tight and cemented, they have no significant effect on the overbreak.
 If the aperture of the joint is small with strong and stable filling material, the overbreak will depend upon
the orientation of joints with respect to blast holes.
 Joint aperture of (3mm) acts as a free face thereby reflecting back explosive energy without producing
a split.

10.  The filling material within a joint changes the wave transmission characteristics of the joint.
 The smaller the width of the filling material and the closer the impedance of the filling material to that of
medium, the better the strain energy transmission through the would be.
 With increase in the joint width, the energy loss into the joint will be considerable particularly when the
joint is closer to face.
 The presence of clay material in joints, its swelling potential and thickness contribute to poor rock mass
quality, thus resulting in excessive overbreak and underbreak.

11.  Bedding has a pronounced effect on both fragmentation and stability of the excavation perimeter.
 Open bedding planes, open joints, or beds of weak materials should be treated as the zones of
weakness.
 Their presence of in a rock mass will lower its strength and allow for easier fragmentation.
 The number of bedding planes in a rock mass increases, the ability for those bedding planes to control
the maximum fragmentation size also increases.
 Bedding at the bottom of a bench allows for easier movement and better fragmentation.
 Stemming, rather than explosive should be loaded into bore hole at the location of these zones.
 Beds can create toe problem because the toe rock tends to break along the bedding or foliation.

12. Figure 3—Stemming through weak material and open
beds.

13.  Voids and zones of weakness such as solution of cavities, mud seams and shears are serious problem
in the blasting.
 Explosive energy always seeks the path of least resistance.
 The rock burden composed of alternate zones of hard material, weakzones, or voids, the explosive
energy will be vented through weak zones and voids, resulting in the poor fragmentation.
 When blasthole intersects to voids or particularly care must be taken in loading the charge or voids
loaded with heavy concentration explosive resulting in excessive air-blast and fly-rock.
 A higher powder factor seldom will correct the this problem.

14.  Weathering of the rock mass can also affect fragmentation by creating zones of rock with strengths that
differ from the rock below or surrounding a contact zone.
 Weathering which results in weakened rock can cause confinement problem.
 Weathering that exposes a hard layer that ends up being the surface can result in cap rock problems.

15.  The discontinuities are weakest links in the rock mass.
 The spacing, orientation, and aperture of the discontinuities have a major influence on the initiation and
propagation of blast damage in a rock mass.
 The discontinuities create imbalance in the distribution of explosive energy causing poor fragmentation
and blast damage.