Open pit mining is one of the surface mining method use in exploiting less overburden material. Open pit method is one of the special mining method use to extract competent ore with flat shape.
This method is unique with it's step wise approach use to mine the deposit in benches.
The safety requirements for the success of this type of mine is not restricted to
Open pit blasting at first may require box cut approach to create additional free face at the horizontal direction for successful throw.
Mostly if the blasting is carried out at the last bench floor or as the first blasting for face opening.
In such case where:
1. Environmental Impact has to be controlled,
2. Safety of workers, equipment and nearby things have to be presented,
3. Blast economy in place of cost of blasting material effect on overall blasting cost have to be minimize,
4. Success optimum fragmentation size with less than 5% oversize and undersize, and
5. Dilution, overbreak and back break needs to be prevented for high safety and low downstream operation cost sake...
The use of control blasting tends to be very crucial and must be practice in all mine site.
Control blasting is a special Blasting approach that involves strategic control of Explosive energy to avoid damaging intert rock wall and to avoid environmental impact of blasting.
Control blasting techniques include;
1.Presplitting technique,
2.smooth blasting,
3.cushion blasting,
4. line drill hole,
5.post splitting,
6.Air decking among many others. All control blasting techniques try to achieve the same aim but through different objective.
To control the rate of shock energy, gas energy, radial cracks, flyrock, ground vibration, airblast released during blasting.
But optimizing powder factor, drill and blast design parameters.Thou rock heterogeneity and anisotropic nature makes control blasting design varies from formation to formation.Most civil engineering construction blast involved control blasting to avoid overbreak of wall and floor. Some of the reasons why control blasting is needed:
1. To avoid overbreak: this is creation of discontinuity by the shock energy beyond the bench width during charge detonation.
2. To avoid reduction in support wall strength and competency: fracturing of support wall and toe reduce the stability of slope which prompt rock to easily fall under load redistribution during excavation.
3. To prevent high ground vibration, flyrock, airblast and other environmental Impact
4. To control blasting cost through charge weight reduction and still achieve optimum fragmentation
5. To control dilution rate in grade variation formation.
For civil engineering works, control blasting techniques are use to fragment road obstructing rocks (outcrops) during tunneling, building foundation laying and road construction.
This idea is to ensure the hindring rock is break down with competent and smooth face at LHS AND RHS.
Now to presplitting blasting technique after understanding reason why we need control blasting in mining and petroleum (casing perforation at production zone) in civil engineering.
Presplitting is one of the control Blasting techniques use to ensure good Explosive-rock interaction during Blasting. It involves splitting the production zone from the supporting rock block (unblast rock wall). This techniques is differ from post-splitting by the initiation timing as the one detonate production hole first why another detonate perimeter hole First. Presplitting had been practice earlier before the use of Explosive in construction.
In Egypt and other countries, the use of wedge wood and fire heat had be ultilize to split rock mass, likewise in Northern locations spallation effect of freezer water is used. Drill holes are filled with water, as the water frozen it expand and split rock mass.
This traditional approach was used before 1950's. The first presplitting blast wast done in Lewiston power plant as part of Niagera powder project. This was a project involve the blast of dolomite & limestone formation with shale bed. Many Presplitting method was applied including line hole drilling with three hole step charging, cushion blasting before presplitting was use with dynamite cartridge. The are various type of presplitting:
Traditional, precision and many others but all still in the optimize of Explosive load, stemming and spacing parameters.
From different scholars (Joseph Konya, cook, beckee-kietiakowsky) point of view, Explosive released energy theory were studied to ensure good control during presplitting blast. For the shock wave release, various theory like; Livingston cratering theory, Bureau of mines catering theory, shockwave spallation theory and strain wave theory among others were propanded and infused to ensure the control of micro-fracturing rate during presplitting.
And for the gas energy study for precision splitting, the following equation of state was propanded to calculate the explosion pressure released from charge at detonation; Beckee-kistiakowsky-wilson (BKW) equation of state, Cook's equation of state, Taylor's equation of state, Outoyne-Skidmore-Konya (OSK) equation of state and many more. Learning on the shock and gas energy generation rate during detonation gives the scholar great confidence to give precise and accurate approach to presplitting blast and other Control blasting techniques. If the capacity and energetic strength of each Explosive charge is understand, to control it damage rate becomes easy and accurate.
Presplitting Blast design Parameters
Presplitting make use of the following approach in order to ensure proper Explosive energy;
1. Explosive load weight: blast energy both shock and gas energy increases with exolosive load. During presplitting blast charge, decoupling of peripheral (control holes )to reduce the charge density. Decoupling is the use of small diameter Explosive cartridge in large diameter hole. The decoupling ratio is the ratio of the the Explosive cartridge diameter to the drill hole diameter. Decoupling reduce Explosive energy by creating air allowance between Explosive cartridge and rock wall, therefore the release wave interact with air (having low Sonic velocity) before interacting with the rock mass. In a bit to contain the Explosive energy within the cut line, Explosive load weight is calculated using various design rule of thumb formula including;
Dcc=(Dh^2)/28
Where;
DCC is the charge weight in pounds,
She is the Explosive cartridge diameter.
Also Joseph Konya propose presplitting Konya constant to model Explosive load weight,
K=(40579/E)^0.625
EL=7000*(S/K)^2
Where;
K is the Konya presplitting constant,
E is the rock Young modulus,
EL is the Explosive load weight.
S is the spacing
2. Spacing: spacing is the distance between two adjacent holes in thesame row. Providing wide and optimum space between hole controls the volume of rock block to be fragment by each energy release from each kilogram of edplosive. Increase in spacing reduce the efficiency of hoop stress distribution amongs the holes detonated. The spacing between presplitting holes increase the resistance offer by the rock mass against the release Explosive energy.
Some rule of thumb formula use for spacing during presplitting blast include;
S=10Dh
3. Stemming length: stemming according to konya have no effect on Explosive shock wave but affect the explosion pressure. Well confined holes generate high explosion energy, this will avoid flyrock and also support good throw and backbreak.
4. Initiation timing sequence: Initiation timing of blast charge simple explains the detonation pattern follow by each of the hole during firing. In order for presplitting blast to prevent overbreak at the design boundary between the production block and the supporting rock wall,the peripheral holes (The hole at the production and support wall boundary) are decoupled and initiated first ahead of the production holes. This is where the name Presplitting comes from. The peripheral holes split apart the production section of the ore deposit, creating a free face between the two rock block. This split point avoid further fracturing during production hole Blasting.
Summarize Theory of presplitting blast
The whole idea is to divide the drill hole into two sections.
1. Production holes: holes to be blasted for Run-off- mine crude production Or the holes place in the rich grade zone during dilution control or the hole arranged on the rock mass to be broken away from highway.
2. Peripheral holes or the control holes: These are hole placed at the boundary of the blast rock and supporting wall.
The hole powder factor or load weight is controlled base on the rock strength and the Explosive strength.
Also the hole design is done using specified rule of thumb formula.
The initiation system use in presplitting is design such that the initiating signal reach the peripheral holes first before the production holes. This is achieve through the use of delay detonators (which can be surface delay or down-The-Hole delay). In this case, the charge in peripheral holes are detonated before the production holes charge. The peripheral holes charge is light and decoupled, this ensure perfect splitting and detachment of production block from the insitu rock with smooth face. As soon as the energy from the production hole gets to the split point it's escape to the surface vertically.
Some of the special blasting accessories use for presplitting blast include: delay detonator, decoupling sleeves among others.
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