1. Final Wall Stability in Metal Open Pit Mines Using Presplit Blasting
Kazem Oraee
Ali Mozafari
Arash Goodarzi
Nikzad Oraee-Mirzamani

2. 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

3. Wall failure in an open pit mine

4. Failure in Pit Slop in Grasberg Gold Mine in Indonesia in 2003
The Grasberg mine is the largest gold mine in the world

5. Failure in Pit Slop Angouran Lead Mine in Iran 2006

6. Wall Failure in Pit Slop Angouran Lead & Zinc Mine in Iran 2006 25 million tons rock slide

7. 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

8. 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

9. Controlled Blasting Methods
Line blasting
Smooth blasting
Cushion blasting
Presplit blasting

10. 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)

11. 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

12. 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)

13. 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.

14. 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

15. 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

16. Cartridge Diameter(mm)
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
(mm)
Charge Mass / Meter of
Blast hole(kg)
Cartridge Diameter(mm)
Presplit Space
(m) 89
0.65 23
1.0
102 29
1.4
115
152
1.8 45

17. 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.

18. 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 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

19. 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 north-east 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.

20. 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.

21. Chador Malu iron ore mine
Drilling pattern for production holes at Chador Malu iron ore mine
Hole
Diameter
(mm)
Sub drilling
(m)
Burden
Spacing
Charge Density
(gr/m3)
Stemming
Explosive
Bottom Charge
Hole Depth
Bench Height
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
Hole Angle(degree)
165 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
Presplit Spacing
(m)
Charge Density
(gr/m3)
Burden
Time Delay
(ms)
Stemming
&
Subdrilling
Explosive
Hole Depth
Hole Angle
(degree)
114
0.35 40
1.45
1.03 6
200
Azar Powder
15.5 75

22. Presplit Drilling Operation
presplit Holes
Production Holes
Buffer Holes

23. A schematic illustration showing drilling and blasting design in Chador Malu mine

24. A schematic illustration showing of drilling profile in Chador Malu mine

25. presplit Drill Rigs

26. Drilling angles
30°
30°
-Sideways: 30°/ 30°

27. Drilling rig position for presplit drilling at Chador Malu mine

28. 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

29. The sources of drilling deviations

30. Drilling deviations in difficult condition
Designed hole direction

31. A schematic illustration showing sources of hole drilling deviations on mine bench
Collaring misalignment
Collaring offset
Planed hole
In-hole deviation
Incorrect depth
Deviation due to
collar error

32. Conventional Drilling Methods in Surface Mines
Top Hammer
Down the Hole
COPROD
Rotary 1 2 3 4

33. Drilling deviations depend on drilling rigs

34. Double Benches Drilling
Using double benches presplit blasting to reduce wall failure and achieve less total drilled meters per tone

35. Successful Presplit Blasting in an Open Pit Mine

36. 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.

37. 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: % 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

38. Presplit drilling pattern
Buffer Holes 6 1/2 in
Pre-split Holes 5 1/2 in
Production Holes 12 ¼ in
5.0 m
10.5 m
15m
15m
16m

39. Typical presplit Drilling Design , Aitik Copper Mine in Sweden
wall
5 ½” Pre-split holes
15 m depth 22 degree
incline
4.5 m
4.5 m
5.0 m
6 ½” Buffer holes
15 m depth
6.0 m
10.5 m
12 ¼” Production holes
16 m depth
8.0 m
12 ¼” Production holes
17 m depth

40. 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
Excessive joints and cracks at final wall crest
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

41. Half-casts visible remained after blasting of presplit holes

42. Wall damages due to inappropriate spacing in presplit row

43. 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.

44. Thanks for your kind attention