Technical

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AutoStem vs Commercial Explosives

High Explosives

Commercial explosives are classified as a “High Explosive” due to the rate of decomposition being supersonic.

Characteristics

High explosives are marked by detonation, which is an extremely rapid decomposition reaction. This means that they break down chemically at extremely high speeds.

Velocity of Detonation (VoD)

The VoD for high explosives used in rock-breaking ranges between 343 and 7000 meters per second, which is supersonic.

Effects

Detonation shatter, pulverize and splinter nearby materials with fragments propelled away at an extremely high speed.

Reaction Process

High explosives are marked by detonation, which is an extremely rapid decomposition reaction. This means that they break down chemically at extremely high speeds.

Effect on Rock

Because detonation occurs so rapidly, surrounding materials are shattered, pulverized, and split, with fragments ejected at high speeds. The instantaneous energy release does not allow time for pressures to dissipate naturally, resulting in localized damage.

Low Explosives

Autostem Cartridges are classified as a “Low Explosive” due to the rate of decomposition being subsonic.

Characteristics

Low explosives do not detonate but rather burn through deflagration, which is a quick decomposition reaction that is still significantly slower than a detonation.

Velocity of Detonation (VoD)

The velocity of deflagration is below 343 meters per second, which is subsonic, below the speed of sound.

Effects

Deflagration push, shove, and heave, often with extremely limited shattering and little production of secondary missiles (Flyrock).

Reaction Process

This thermal process involves combustion, radiating outwards from the ignition source, consuming the fuel.

Effect on Rock

The slower pressure buildup of deflagration means that structures have more time to respond to the developing forces. The maximum pressure is often limited by the structural integrity of the surrounding environment, reducing the risk of catastrophic failure compared to the effects of a high explosive.

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Powder Factor Considerations

Definition of Powder Factor: Powder factor is a measure used in blasting operations to evaluate the efficiency of the explosive used. It is defined as the amount of explosive (in kilograms or pounds) required to break a unit volume of rock (usually in cubic meters or cubic yards).

Comparison to Commercial Explosives: Powder factors are lower when using AutoStem rock breaking cartridges compared to when using traditional commercial explosives. This combined with lower VOD’s compared to commercial explosives reaps great benefits in the control of shock waves (ppv’s) and the distances it takes to dissipate. The table below compares powder factors when using AutoStem rock breaking cartridges to when using different types of commercial explosives.

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Powder Factor Comparison

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ANFO

ANFO’s powder factor with its much lower relative density compares the best with AutoStem, however this example excludes the use of a pentolite booster which is recommended when using ANFO. Consideration must also be given to ANFO’s volatility to water and wet holes whereas with AutoStem, all the Generation III cartridges are all water resistant.

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Packaged Explosives – 32 x 200mm Commercial Cartridge

Packaged explosives in cartridge form, address both the water constraint as well as the booster requirement, however the high relative density of the explosive increases the powder factor gap when compared to AutoStem.

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Pumpable explosives

Emulsions also address the water constraints but the use of a booster with emulsion is recommended. Relative densities of emulsions vary from 0.8 to 1.25g/cc depending on the client’s requirements however 1.1g/cc is the norm in the South African underground mining industries. Another slight increase in powder factors occur as pumpable explosives fill the entire diameter of the hole where there are gaps between the cartridges and sidewalls of the blasthole when using cartridges.

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Peak Particle Velocity (PPV)

Peak Particle Velocity (PPV) is a critical measurement in blasting operations, particularly when assessing the impact of ground vibrations on nearby structures.

Lower Powder Factors

AutoStem rock breaking cartridges can achieve the desired fragmentation with lower powder factors due to the smaller weight of cartridges required for the same length of blasthole compared to commercial explosives.

Instantaneous Ignition

Standard AutoStem cartridges contain an igniter that enables instantaneous ignition of all connected cartridges, like when using detonating cord with commercial explosives. This results in a higher weight of explosives per delay, which will increase PPV’s.

Sequential Initiation

Sequential (det ready) AutoStem cartridges allow for single-hole initiation, analogous to using detonator assemblies or electronic detonators with commercial explosives. This capability is significant in managing ground vibrations effectively and can be beneficial when blasting near sensitive structures.

  • K: Rock Factor (with “500” being typical for hard rock).
  • B: A constant relating to the characteristics of the rock.
  • R: Distance from the charge to the point of interest (usually in meters).
  • Q: Explosive charge mass per delay (in kilograms).

The table below shows the minimum distance from a structure that the various options described above can be used without ground vibrations exceeding a selected 50 mm/s for illustration purposes.

This formula is often used to estimate the safe distance from a blast to a structure based on a target PPV, in our case, 50 mm/s for illustration purposes. Using this formula helps determine the minimum distance required to avoid exceeding the selected PPV threshold and thus prevent damage to nearby structures.