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Underground Blast Design (part-1)

Introduction

Underground blast design refers to the careful planning and execution of blasting operations in underground mining environments. It involves determining the appropriate explosive materials, initiation sequence, drill patterns, and other parameters to achieve desired outcomes such as controlled rock fragmentation, safe excavation, and efficient ore or coal recovery.

The primary objectives of underground blast design are to

-ensure worker safety,

-minimize environmental impact,

-optimize rock breakage, and

-maintain stability in the mining area.

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Priming in underground mines refers to the process of preparing the explosive charge for initiation within blastholes.

Detonators are inserted into the explosive cartridges (primer). Detonators are designed to initiate the main explosive charge.

The number and type of detonators used depend on the specific blasting requirements and safety regulations.

If more than 1 cartridges is used for blasting, then based on the position of the primer the initiation system can be divided into two types: -

-direct initiation

-indirect initiation

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Direct initiation

-The placing of the detonator in the cartridge(primer) to be inserted in the blasthole with the active end (business end) of the detonator pointing inward or towards the hole. This position tends to minimize the risk of gas ignition.

-It gives proper fragmentation and energy utilisation compared to indirect initiation.

Direct initiation is commonly used in situations where:

-The explosive charge has good confinement within the blast hole.

-The rock mass is expected to be easily fragmented and requires maximum energy transfer for efficient excavation.

-The blast design aims to achieve a high degree of fragmentation and throw for efficient ore or coal recovery.

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Direct initiation in a blasthole

stemming

Normal cartridge or Explosive

Primary Explosive or cartridge

detonator

Blasthole

Detonating wire

Business end

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Indirect initiation/inverse initiation

The placing of the detonator at the back of the blasthole or the business end is facing outside of the hole. This is the usual practice when using delay detonators to minimize the danger of cutoff holes.

In solid blasting and sump development where, free face is not available, indirect initiation is done.

Indirect initiation is employed in situations where:

The explosive charge requires better confinement and more controlled energy release to minimize the risk of fly rock or excessive ground vibrations.
The rock mass is harder and less easily fragmented, requiring a more controlled initiation to achieve the desired fragmentation pattern.
The blast design aims to optimize safety by reducing the potential for overpressure, ground displacement, or damage to nearby structures.

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Indirect initiation in a blasthole