High-Level Blast Design Basics

Designing a blast involves the consideration of multiple parameters, and achieving the desired blasting outcome necessitates a thorough understanding of the interplay and interdependence of these factors to avoid unsafe, inefficient, or wasteful blasting results. Table 1 provides an outline of the relevant blast design parameters, along with their respective descriptions.

Table 1. Blast Parameter Descriptions (de Graaf, 2011)

Parameter Description
BlastholeA hole that is drilled for explosives placement either in rock or other material.
Hole Diameter (D)The diameter of a blasthole.
Free FaceA rock surface exposed to air that provides a space for the rock to expand to upon fragmentation.
Burden (B)The distance (perpendicular) between two adjacent blasthole rows, or between the nearest parallel (main) free face and a blasthole.
Spacing (S)The distance (perpendicular to the burden) between two adjacent blastholes in any row of blastholes parallel to the free face
CrestThe line connecting the highest points on the top of a free face.
ToeThe line connecting the lowest points on the bottom of a free face.
Bench Height (H)The vertical distance between the Toe and the crest of the bench.
Toe BurdenThe distance between the blasthole and the free face at the grade or floor level.
Floor/GradeThe floor level determined by the mine design and the surveyor.
Stemming (T)The inert material, such as drill chippings or crushed stone aggregate, used in the collar portion of a blasthole to confine the gaseous products of detonation.
Charge Length (L)Length of the changed explosives column measured from the floor/grade to the starting position of the stemming material.
Sub-Drill (U)The portion of the blasthole drilled below the planned floor/grade level.
Total Charge lengthThe total charge length is the portion of the blasthole combining the charge length and the Sub-Drill length.

When designing a blast, several parameters are considered, including bench height, hole diameter, rock type, and explosive type. The fundamental principles of blasting are closely tied to hole diameter and burden, forming the foundation of the initial design process. The following guide outlines the design process:

  1. Calculate the linear charge mass (the amount of explosives used, mass per meter in the hole).
  2. Determine the required burden (the distance between the blasthole and the free face of the rock).
  3. Calculate the hole spacing based on the selected burden-to-spacing ratio (either staggered or square pattern).
  4. Compute the technical powder factor (the amount of explosive mass per blasted rock mass/volume).
  5. Calculate the necessary stemming length (the material placed on top of the explosives in the blasthole).
  6. Apply sub-drill design if required (adjustments to the hole depth).
  7. Calculate the actual powder factor after design adjustments.
  8. Design the timing sequence (the pattern or sequence in which individual holes detonate/propagate).

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It’s important to note that this process may vary depending on the available information during the design phase and should not be considered an industry standard.

Figure 1 provides schematics of a simplified blast bench layout and illustrate the blast parameters listed in Table 1. Please note that these schematics are not drawn to scale, and terminology may vary between countries and industries. A high resolution copy of the below diagram can be downloaded here.

Figure 1. Simplified blast bench layout

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