Optimisation of drilling and blasting focussing on fly rock

2. OPTIMISATION OF DRILLING
AND BLASTING FOCUSSING ON
FLY ROCK

3. INTRODUCTION
Blasting is an essential component of surface mining.
It serves as a leading role in fragmenting the overburden.
Although blasting presents numerous hazards, the mining
industry consider blasting as indispensible element of rock
excavation.
Danger and damage from fly rock and rock blasting has been
serious problem ever since the introduction of blasting.
Fly Rock:- Any mismatch between the distribution of the
explosive energy and geo-mechanical strength of rock mass
and confinement creates potential for fly rock. Fly rock
originates from the vertical highwall face and bench top.

4. Accidents:

5. 2
9
2
21
8
4
17
Weightage of fatal accidents in non-coal mines due to
explosives during 1996-2011
Solid Blasting Projectiles - 3%
Deep Hole Blasting Projectiles - 14%
Secondary Blasting Projectiles - 3%
Other Projectiles - 33%
Misfires/Sockets(While Drilling into) - 13%
Misfires/Sockets(Other than Drilling into) -
7%
Other Explosive accidents - 27%
21

6. FLYROCK
Flyrock is the propelling rock from the blast area.
Leading cause of fatalities and equipment damage

7. OBJECTIVE
Blasting generally has two purposes, rock fragmentation and
displacement of rock.
The movement of blasted rock (also known as muck pile)
depends on shot design parameters, geological condition and
mining constraints.
A clearance zone is to be designed and that no persons and are
machines are within this zone during blasting.
Fragmented rock is not expected to travel beyond the limits of
the blast area.
Blasting pattern is designed taking into consideration the
surrounding conditions like location of infrastructure.

8. SCOPE
In India two types of mines are operated, opencast and
underground . Around 90 % of mines are opencast mines.
Share of coal production from surface mines was increased
from less than 20% in 1973-74 to a level of more than 85%.
Number of opencast mines is increasing day by day. Most of
the mines are operated near the villages. If proper care in not
taken in respect of fly rock then accidents can takes place. For
increasing number of mines it is necessary to control the
generation of fly rock. Studies are carried in this field to
facilitate mining operation with minimum risk.

9. LITERATURE REVIEW
FLYROCKS OCCUR WHERE ENERGY IS VENTED INTO
ATMOSPHERE AND PROPELS ROCKS
WHERE BURDEN AND STEMMING IS TOO SMALL OR
HOLES ARE INITIATED OUT OF SEQUENCE
DRILLING INACCURACY
OVERCHARGING
GEOLOGICAL CONDITIONS
SYMPATHETIC DETONATION

10. CHARGING TOO NEAR TOP OF THE FACE IN
HIGH BENCH OR HIGHLY INCLINED FACE

11. LARGE BURDEN CAUSES FLYROCK

12. GEOLOGICAL CONDITIONS
Open joints,
mud seams and
cavities may
result in escape
of gases and
also blasting
nuisances.

13. DRILLING INACCURACY CAUSES INSUFFICIENT
BURDEN RESULTING IN FLYROCK

15. FAULTY DELAY TIMING AND
INITIATION SEQUENCE

18. FLYROCK PREDICTION
MODELS
LUNDBORG
GENERAL TRAJECTION THEORY
WORKMAN & SCALED BURDEN

19. MODEL 1 - LUNDBORG
 For a specific charge (powder factor)
L = 143 d (q-0.2 )
 Optimum rock size
ɸ = 0.1 d 2/3
 Maximum Throw
L max = 260 d2/3
L = flyrock throw (m)
Lmax = maximum throw (m)
d = hole diameter (ins)
q = specific charge (kg/m3)
f = boulder diameter (m)

20. Flyrock distances beyond L max are possible.
Maximum throw is the distance beyond which the
probability of flyrock landing in a square meter is
less than the probability of being killed by lightning
in 10 years ie. Less than 1 in 10 million.
Model 1 - Lund Borg

21. L =
V2
o sin 2qo
g
Maximum throw is when qo = 45°
L = horizontal throw (m)
Vo = launch velocity (m/s)
qo = launch angle (degrees)
g = gravitational constant (9.8 m/s/s)
Model 2 - General Trajectory Theory
Lmax =
g
V2
o

22. Throw is a function of face velocity and scaled
burden (burden divided the square root of the
explosives charge/m)
velocityfaceV
constantak
m/masschargem
(m)burdenB
B
m
kV
o
3.1
o













where
Model 3 - Workman and Scaled Burden Approach

23. 6.2
2
max
B
m
8.9
k
L 








Equations 5 and 6 can be combined to give
From recent flyrock investigations with accurate
burden, stemming height, loading and throw distances;
k = 27 (granite quarry)
k = 13.5 (coal overburden blast)
K= 140 (Limestone quarry)
Stemming height may substituted for burden for
cratering and gun-barrelling situations.
Model 3 - Workman and Scaled Burden Approach

24. Equation permits the production of graphs such as
Note : Lundborg’s “Safe” distance is at the part of the curves where
minor variations to burden greatly effect the maximum throw.

25. LABORATORY GEOTECHNICAL STUDY
Clearance distance design
Clearance area design
Design of clearance area for man and
machines
Minimum confinement design

26. Step 1. Determine Lmax
a) In front of face - Face Burst
6.2
2
max
B
m
g
k
L 








Burden conditions
usually control
flyrock distances in
front of the face and
Lmax is determined
from.
CLEARANCE DISTANCE DESIGN

27. Step 1. Determine L max
6.2
2
max
SH
m
g
k
L 








If the stemming height
to hole diameter ratio
is too small, flyrock
can be projected in
any direction from a
crater at the hole
collar a distance
determined from;
a) Behind Face - CRATERING
CLEARANCE DISTANCE DESIGN

28. Step 1. Determine L max
oSinq
6.2
2
max
SH
m
g
k
L 








 
 
 
 
constantnalgravitatiog
mheightstemmingSH
mburdenB
mkg/mmetrepermasschargem
mthrowmaximumL
angleholedrillθ






If the stemming length is adequate to prevent cratering, flyrock can
result from gunbarrelling - the ejection of stemming material and loose
rocks in the collar require a distance determined from;
a) Behind Face - GUN BARRELLING
CLEARANCE DISTANCE DESIGN

29. CLEARANCE AREA DESIGN
Flyrock from a
face is most
likely to be
projected
perpendicular to
the face and
least likely to be
projected
parallel to the
face. Flyrock is
most likely to be
projected into
the quadrant
shown.

30. If the stemming
height is
sufficient to
prevent cratering
(ie. Greater than
about 20 hole
diameters), the
clearance
distance behind
the face is based
on the gun-
barrelling
distance.

31. A simple construction gives the following shape

32. Apply safety (uncertainty) factors to the calculated
maximum throw. The following are recommended;
The clearance
distance for
plant and
equipment is
double the
maximum
throw.
The clearance
distance for
personnel is
four times the
maximum
throw.

33. The methodology can also be used to
determine minimum confinement
specifications to be achieved during loading.
For example:
A blast is to be 100 m (330 feet) from a
screen house in a quarry and uses 102 mm
(4”) diameter blast holes.
MINIMUM CONFINEMENT DESIGN
Maximum Throw = 100/2 = 50 m

34. FIELD DATA
The limestone quarry selected for this study was ACC
Limited, Jhinkpani producing about twenty million tons per
annum.
Quarry was operating at more than 500 m distance from the
nearest village.
The density of limestone varied between 2.5 to 2.68 ton/m3
and the limestone is medium to hard.
Three sets of joints were present and the most prominent one
was the bedding plane.
Blast hole diameter, burden, stemming length and were
recorded for 14 blasts at limestone quarry along with the
maximum distance travelled by the fragments (Table 1).

35. Drill cuttings were used as stemming material and blasting
mats were not used to control fly rock.
Other parameters at these quarries were:
Bench height: 6±10 m
Spacing: (1±1.5) Burden
Number of holes per blast: 10±40
Number of rows: 1±3
Number of inert decks per hole: 1±3
Explosives used: ANFO with 15±25% of cap
sensitive explosive
Maximum acceptable size of fragments: 1.0 m.

36. ANALYSIS OF DATA
S.No. D B S.H S.H/
B
B/D S.H/
D
Q F.LO F.LC
1. 0.100 2.7 2.4 0.89 27 24 0.45 40 40.13
2. 2.5 2.4 0.96 25 24 0.45 40 40.13
3. 2.4 3.6 1.50 24 36 0.45 25 40.13
4. 2.5 3.2 1.28 25 32 0.45 35 36.08
5. 0.115 2.6 3.1 1.20 23 27 0.45 35 32.60
6. 2.5 1.5 0.60 22 13 0.45 55 136.20
7. 2.5 2.5 1.00 22 22 0.45 35 36.08
8. 4.0 2.0 0.50 35 17 0.85 150 147.37
9. 3.0 2.2 0.73 26 19 0.43 40 47.43
10. 3.0 2.0 0.67 26 17 0.63 150 99.84
11. 3.5 2.0 0.57 30 17 0.44 60 62.60
12. 4.0 1.8 0.45 35 16 0.34 50 58.90
13. 3.0 2.0 0.67 26 17 0.49 50 72.10
14. 3.0 2.1 0.70 26 18 0.70 90 100.85

37. Where, D is diameter of hole
B is burden
S.H is stemming height
F.Lo is observed throw of fly rock
F.Lc is calculated throw of fly rock
K for limestone is 104

38. MEASURES TO CONTROL FLYROCK DAMAGE
1. Proper blast design
2. Bench height, borehole diameter, hole
inclination, burden and spacing, charge
distribution in holes, stemming, initiation
sequence and timing
3. Site control during blasting
4. More experienced drilling/blasting crew
5. More effective communication
6. Covering
7. Miscellaneous measures loose stone, toe holes

39. COVERING WITH WIRE NET

40. PROPER STEMMING TO CONTROL FLYROCK

41. CONCLUSION
The maximum fly rock distance observed at limestone quarry
is 150 m and the safe distance of 500 m is reasonable. Unless
the existing practices at limestone quarries are improved, it
may be difficult to extract limestone deposits which are
located close to dwellings.
Though blast hole diameter, burden and powder factor are
common causes of fly rock, poor stemming performance is the
major cause of fly rock at these limestone quarries.
Fly rock can be controlled by introducing better initiation
techniques.
All these factors are related to blast design and thus the fly
rock can be significantly controlled by improving the existing
blasting practices at limestone quarries.

42. REFERENCES
Lundberg, N. 1973.The calculation of maximum throw during
blasting. SveDeFo Report DS 1973:4
Workman, J L and Calder, P N, 1994. Fly rock prediction and
control in surface mine blasting, in Proceedings 20th
Conference ISEE, Austin, Texas, pp 59-74.
Persson, P, Holmberg, R and Lee, J, 1993. Rock Blasting and
Explosives Engineering , chapter 12 (CRC Press).
http://www.ntded.org/EducationResources/CommunityCollege
/Materials/.htm
http://www.khup.com/view/0_keyword-rock-blast
design/chapter-8-blast-design.html