Presentation on theme: "Final Wall Stability in Metal Open Pit Mines Using Presplit Blasting"— Presentation transcript:
1 Final Wall Stability in Metal Open Pit Mines Using Presplit Blasting Kazem OraeeAli MozafariArash GoodarziNikzad 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 BenefitsAbility to maintain safe conditions in mineIncreased stability of the rock wallImproved stripping ratiosReduction in ore dilutionLess broken rock to load and transportReducing the vibration level in the rock massMinimizing production schedule disturbances for mine stability comparison with mechanical rock reinforcement
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 boundarydrilling straight holescharging with lower explosive than main holesfiring holes in sequential timescale
9 Controlled Blasting Methods Line blastingSmooth blastingCushion blastingPresplit blasting
10 Presplit Hole DesignAs 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) / TWhere 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)2.6Where Pb = Borehole pressure in psiYe = Specific gravity of explosivesVOD = Detonation velocity of explosive charge ft/sre = Radius of explosive charge in inchesrb = Radius of borehole in inches
12 Bore hole detonation pressure (Pa) for the full charge hole can be calculated by: Pd = × Ye × (VOD)2 × 106Where 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/m3The ratio of the charge diameter to hole is about 0.3 – 0.4The 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 holeThe 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 damageAir deck chargingSuspended chargingLow density chargingContinues 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 ofBlast hole(kg)Cartridge Diameter(mm)Presplit Space(m)890.65231.0102291.41151521.845
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 mFractured 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 mineHoleDiameter(mm)Sub drilling(m)BurdenSpacingCharge Density(gr/m3)StemmingExplosiveBottom ChargeHole DepthBench HeightHole Angle(degree)2512.25671.07.25ANFO117.251590Drilling pattern for buffer holes at Chador Malu iron ore mineHole Diameter(mm)Decoupling ratioBufferBurden(m)Buffer Spacing(m)Charge Density(gr/m3)Stemming(m)ExplosiveHole depthHole Angle(degree)165341.02ANFO690Drilling pattern for presplit holes at Chador Malu iron ore mineHole Diameter(mm)Charge decoupling ratioCartridge DiameterPresplit Spacing(m)Charge Density(gr/m3)BurdenTime Delay(ms)Stemming&SubdrillingExplosiveHole DepthHole Angle(degree)1140.35401.451.036200Azar Powder15.575
27 Drilling rig position for presplit drilling at Chador Malu mine
28 Visual presplit evaluation examine the presplit face and adjust spacing or chargingwith regard to:- smoothness of presplit surface- percent of half-casts visible- occurrence of crest failuresoccurrence of plane and wedge failures in final wall
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 MineThe 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 SpecificationProduction Started with 2 Mt/y in 1968 and now 36 Mt/yProven Reserves : 520 MtGrade: % Copper ,0.2 gr/t Gold & 2.0 gr/t SilverMine Life: 2025Final pit depth: 400 mProduction Holes: 311 mmW/O: 1:1Pit Slope Design: 46 degreeBench height: 15 m
39 Typical presplit Drilling Design , Aitik Copper Mine in Sweden wall5 ½” Pre-split holes15 m depth 22 degreeincline4.5 m4.5 m5.0 m6 ½” Buffer holes15 m depth6.0 m10.5 m12 ¼” Production holes16 m depth8.0 m12 ¼” Production holes17 m depth
40 Presplit blasting evaluation Type of DamageOriginCorrectionBack break around the presplit holes caused no half-casts visible in fracture lineShortage of burden or spacing in presplit line or excessive charging in buffer or presplit holesModifying burden, spacing andhole charge densityBack break visible just around the presplit hole areaDetonation pressure in presplit holes was more than rock compression strengthModifying hole charge density and charging methodBack break visible just between the presplit hole areaShortage of buffer row spacingIncreasing the spacing in buffer row and decreasing charging density in buffer holesRoughness and the irregularity of final wall behind the presplit surfaceExcessive spacing or shortage of burden in presplit rowDecreasing spacing andhole charge density in presplit rowShortage of rock breaking in final wallExcessive burden between presplit and buffer rowsDecreasing burden between presplit and buffer rowsExcessive joints and cracks at final wall crestHigh weathering and poor condition of final wall rockUsing guide holes between presplit holes or drilling presplit holes with retract bitfor fracture control
41 Half-casts visible remained after blasting of presplit holes
42 Wall damages due to inappropriate spacing in presplit row
43 Key NotesSeveral 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 minesThe 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 rowDTH and COPROD drilling systems give less deviation than the other drilling methods thus they could be right choices for presplit drillingOrientation 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 minesVisual evaluation of presplit blasting results and modifying parameters according to mentioned table is crucial to achieve successful results.