Final Wall Stability in Metal Open Pit Mines Using
Presplit Blasting
Kazem Oraee
Ali Mozafari
Arash Goodarzi
Nikzad Oraee-Mirzamani
Importance of presplit drilling and blasting in open pit mines
 slope stability study is one of the most considerable parameter for safety and economical factors of
open pit mines.
 failure of mines’ walls can potentially cause loss of lives, roads blocking, damaging to mining
machinery, temporarily or permanently halt production and in the worst case scenario closing the
mine.
Production and Mining Benefits
•
Ability to maintain safe conditions in mine
•
Increased stability of the rock wall
•
Improved stripping ratios
•
Reduction in ore dilution
•
Less broken rock to load and transport
•
Reducing the vibration level in the rock mass
•
Minimizing production schedule disturbances for mine stability
comparison with mechanical rock reinforcement
Wall failure in an open pit mine
Failure in Pit Slop in Grasberg Gold Mine in Indonesia in 2003
The Grasberg mine is the largest gold mine in the world
Failure in Pit Slop Angouran Lead Mine in Iran 2006
Wall Failure in Pit Slop Angouran Lead & Zinc Mine in
Iran 2006
25 million tons rock slide
Pit wall stability is dependent on:
• geotechnical and hydrogeological issues
• level of design safety (based on risk assessments)
• ability to secure the ground (scaling, ground
support, slope dewatering)
• detrimental effects of blasting
General Principals of Control Blasting
drilling smaller diameter blasting holes along
the final excavation boundary
 drilling straight holes
charging with lower explosive than main holes
firing holes in sequential timescale
Controlled Blasting Methods
 Line blasting
 Smooth blasting
 Cushion blasting
 Presplit blasting
Presplit Hole Design
 As a general guide for the presplit holes, spacing
can be 8 to 12 times of the hole diameter or about
one third to less than one half of the normal spacing
used in production blast holes. It also can be
determined by using the following equation:
S ≤ 2 rb × 2.54 × (Pb + T) / T
• Where S = Spacing between two presplit holes (cm)
• rb = Borehole radius ( cm)
• Pb = Borehole pressure ( Mpa)
• T = Tensile strength of rock( Mpa)
In this equation borehole detonation pressure can be
calculated by:
Pb = 1.69 × 10-3 Ye (VOD)2 (re / rb)2.6
• Where Pb = Borehole pressure in psi
• Ye = Specific gravity of explosives
• VOD = Detonation velocity of explosive charge ft/s
• re = Radius of explosive charge in inches
• rb = Radius of borehole in inches
Bore hole detonation pressure (Pa) for the full
charge hole can be calculated by:
Pd = × Ye × (VOD)2 × 106
• Where Pd = Detonation pressure (MPa)
• Ye = Density of explosive (kg/m3)
• VOD = Velocity of detonation (m/s)
Buffer Holes Design
(Buffer row is used to minimize blasting damage from production row to final wall)
 Normally the spacing and burden of about 2/3rd of
production holes are used in buffer row but if presplit
and buffer rows are of same diameter then, the burden
in front of the presplit row to buffer row should be 1.5
times the presplit spacing holes and in case of different
diameters, then diameter of buffer holes can be set to
12 to 15 times of presplit holes diameter.
 As a general rule, when the diameters of production and
buffer holes are the same, then the burden and spacing
of the buffer row should be 70 to 80% of the production
blast holes.
Hole Charge Distribution
 Powder factors will normally range from 1.6 to 4.8 kg/m3
 The ratio of the charge diameter to hole is about 0.3 – 0.4
 The specific charge recommended for presplit holes is
0.35 to 0.5 kg/m2 and generally charge factor in buffer
hole is about 75% of a production hole
 The charge density has to be reduced to 5 to 15 percent
of the charge in production holes
Presplit Charging Methods
Common charging methods in presplit holes to minimize near field blast damage
Air deck
charging
Suspended
charging
Low density
charging
Continues column
charging
Guidelines are offered by Gustafsson, 1981,
DuPont Hand Book, 1977, etc. which
recommends the charge loads and blast hole
pattern for presplitting as shown at the below
table:
Hole Diameter Charge Mass / Meter of
(mm)
Blast hole(kg)
Cartridge
Diameter(mm)
Presplit Space
(m)
89
0.65
23
1.0
102
1.0
29
1.4
115
1.0
29
1.4
152
1.8
45
1.8
Shooting the Presplit Line
 In order to make a free face to reflect shock wave
resulted from blasting in production holes, the
presplit row must be fired at least 50ms before the
main production blast.
 As a rule, if the presplit holes are to be detonated
with production blast holes, generally 200 to 350ms
(not more) of delaying interval between presplit
holes and the nearest production row or buffer row
is recommended.
 To achieve optimum presplit results, zero detonation
delay (simultaneous blasting) to be used between
presplit holes, although if the numbers of holes in
presplit row are more than usual pattern, blasting
may be done in separate groups with minimum delay
in sequence.
Presplit Drilling & Blasting Key Points
• hole spacing and charging dependent on hole size, rock strength,
explosive strength and decoupling ratio
• highly jointed rock requires closer hole spacing than massive rock
• when firing presplit row with production row, a minimum of 200 -
350 milliseconds time delay between presplit holes and nearest
production holes is recommended
• use zero delays between holes to achieve optimum presplit results
• a bottom charge is often used to assure that the toe is pulled
• straight hole drilling is a necessity
Chador Malu Iron Mine Site Profile
Chador Malu iron ore mine with 400 million tons
of ore reserve is the biggest iron concentrate
producer in Middle East, located in 180km northeast of Yazd province in central Iran.
Reconnaissance for the Chador Malu deposit was
first done in 1921 and more detailed work was
carried out in the beginning of 1960s.
Petrography studies on the mine rocks shows that
major rocks in Chador Malu mine area are
Metasomatite, Albitite, Diorite, Magnetite and
Hematite. The iron ore concentrate contains
about 68% iron and 0.045% phosphorus.
Chador Malu drilling fleet
Production & Buffer Hole Drilling: DMH, DML and DM45(Rotary system)
presplit Hole Drilling: Titon 600 (DTH system)
Main drill patterns:
Hard Rock: Burden = 6 m & Spacing = 7 m
Fractured Rock : Burden = 7m & Spacing = 8 m
* ANFO explosive with Dynamite primers are used for blasting.
Chador Malu iron ore mine
Drilling pattern for production holes at Chador Malu iron ore mine
Hole
Diameter
(mm)
Sub
drilling
(m)
Burden
(m)
Spacing
(m)
Charge
Density
(gr/m3)
Stemming
(m)
Explosive
Bottom
Charge
(m)
Hole
Depth
(m)
Bench
Height
(m)
Hole Angle
(degree)
251
2.25
6
7
1.0
7.25
ANFO
1
17.25
15
90
Drilling pattern for buffer holes at Chador Malu iron ore mine
Hole
Diameter
(mm)
Decoupling
ratio
Buffer
Burden(m)
Buffer
Spacing(m)
Charge
Density(gr/m3)
Stemming
(m)
Explosive
Hole
depth
(m)
Hole
Angle(degree)
165
0
3
4
1.0
2
ANFO
6
90
Drilling pattern for presplit holes at Chador Malu iron ore mine
Hole
Diameter
(mm)
Charge
decoupling ratio
Cartridge
Diameter
(mm)
Presplit
Spacing
(m)
Charge
Density
(gr/m3)
Burden
(m)
Time
Delay
(ms)
Stemming
&
Subdrilling
(m)
114
0.35
40
1.45
1.03
6
200
0
Explosive
Azar
Powder
Hole
Depth
(m)
Hole Angle
(degree)
15.5
75
Presplit Drilling Operation
presplit Holes
Production Holes
Buffer Holes
A schematic illustration showing drilling and blasting design in Chador Malu mine
A schematic illustration showing of drilling profile in Chador Malu mine
presplit Drill Rigs
Drilling angles
30°
30°
-Sideways: 30°/ 30
Drilling rig position for presplit drilling at
Chador Malu mine
Visual presplit evaluation
 examine the presplit face and adjust spacing or charging
with regard to:
- smoothness
of presplit surface
- percent of half-casts visible
- occurrence of crest failures
- occurrence of plane and wedge failures in final wall
The sources of drilling deviations
Drilling deviations in difficult condition
Designed hole
direction
A schematic illustration showing sources of hole drilling
deviations on mine bench
misalignment Collaring
Collaring offset
Planed hole
In-hole deviation
Incorrect depth
Deviation due to
collar error
Conventional Drilling Methods in
Surface Mines
1.Top Hammer
2.Down the Hole
3.COPROD
4.Rotary
1
2
3
4
Drilling deviations depend on drilling rigs
Double Benches Drilling
Using double benches presplit blasting to reduce wall failure and
achieve less total drilled meters per tone
Successful Presplit Blasting in an
Open Pit Mine
Aitik Copper Mine
The Aitik copper mine is situated in Sweden. It is one of Europe's largest open pit copper mines.
Associated with the copper, large quantities of gold, silver and since 2008 molybdenum have
been mined at Aitik.
Aitik Copper Mine in Sweden
Mine Specification
Production Started with 2 Mt/y in 1968 and now 36 Mt/y
Proven Reserves : 520 Mt
Grade: 0.31 % Copper ,0.2 gr/t Gold & 2.0 gr/t Silver
Mine Life: 2025
Final pit depth: 400 m
Production Holes: 311 mm
W/O: 1:1
Pit Slope Design: 46 degree
Bench height: 15 m
Presplit drilling pattern
Buffer Holes 6 1/2 in
Pre-split Holes 5 1/2 in
Production Holes 12 ¼ in
15m
15m
16m
Typical presplit Drilling Design , Aitik Copper Mine in Sweden
5 ½” Pre-split holes
15 m depth 22 degree
incline
6 ½” Buffer holes
15 m depth
4.5
m
4.5
m
5.0
m
6.0
m
10.5
m
12 ¼” Production holes
16 m depth
8.0
m
12 ¼” Production holes
17 m depth
Presplit blasting evaluation
Type of Damage
Origin
Correction
Back break around the presplit
holes caused no half-casts visible
in fracture line
Shortage of burden or spacing in
presplit line or excessive charging
in buffer or presplit holes
Modifying burden, spacing and
hole charge density
Back break visible just around
the presplit hole area
Detonation pressure in presplit
holes was more than rock
compression strength
Modifying hole charge density
and charging method
Back break visible just between
the presplit hole area
Shortage of buffer row spacing
Increasing the spacing in buffer
row and decreasing charging
density in buffer holes
Roughness and the irregularity of
final wall behind the presplit
surface
Excessive spacing or shortage of
burden in presplit row
Decreasing spacing and
hole charge density in presplit
row
Shortage of rock breaking in final
wall
Excessive burden between presplit
and buffer rows
Decreasing burden between
presplit and buffer rows
High weathering and poor
condition of final wall rock
Using guide holes between
presplit holes or drilling
presplit holes with retract bit
for fracture control
Excessive joints and cracks at
final wall crest
Half-casts visible remained after blasting of presplit holes
Wall damages due to inappropriate spacing in presplit row
Key Notes
 Several techniques are used for improvement wall stability in open
pit mines which among them, presplit blasting is the most pragmatic
and effective approach for tackling this issue in metal open pit
mines
 The influence of geology can never be completely eliminated but
certain measures can be taken to ensure acceptable blasting results
such as selecting appropriate drill rig to control minimum drilling
deviation on presplit row
 DTH and COPROD drilling systems give less deviation than the other
drilling methods thus they could be right choices for presplit drilling
 Orientation of geological structures has the great influence on the
presplit fracture, thus, survey on structural mapping and joint sets is
very important to obtain the good final wall in open pit mines
 Visual evaluation of presplit blasting results and modifying
parameters according to mentioned table is crucial to achieve
successful results.
Thanks for your kind
attention