π Lesson 8
D5
Real-World Project Walkthrough
Blast design is the process of planning how to safely and efficiently break rock using explosives, like choosing where to drill holes and how much explosive to use.
π― Learning Objectives
- β Calculate optimal burden and spacing for a given rock type and bench height
- β Design a blast pattern using standard spacing ratios (S/B) and stemming length guidelines
- β Analyze powder factor to assess blast efficiency and compare against industry benchmarks
- β Explain how delay timing affects ground vibration and muck pile distribution
- β Apply the Kuz-Ram model to predict fragment size distribution
π Why This Matters
In mining and civil construction, 70β80% of excavation costs are tied to drilling and blasting β yet poor blast design causes excessive oversize, high re-handling costs, damaging ground vibration, and unsafe flyrock. A single optimized blast can save $250k+ annually in a mid-sized open-pit operation. This lesson bridges theory to real-world decisions made daily by blasting engineers on site.
π Core Principles
Blast design rests on four interdependent pillars: (1) Rock mass characterization β strength, discontinuity spacing, and weathering dictate energy coupling; (2) Explosive energy delivery β measured via relative weight strength (RWS) and bulk density; (3) Pattern geometry β burden (B) governs confinement and energy transfer, spacing (S) controls fracture coalescence, and subdrill ensures toe breakage; (4) Initiation strategy β millisecond delays allow stress wave interaction and reduce peak particle velocity (PPV). Modern practice treats blast design as a feedback loop: geological model β pattern design β simulation (e.g., DFN or SABRE) β field validation β refinement.
π Burden Calculation (LangeforsβKihlstrom)
The LangeforsβKihlstrom formula estimates initial burden based on rock strength and explosive energy. It balances confinement and throw, serving as the foundational input for spacing and powder factor calculations.
Langefors Burden
B = K Γ β(d Γ Οβ Γ RWS)Empirical estimate of burden based on rock strength, explosive properties, and hole diameter.
Variables:
| Symbol | Name | Unit | Description |
|---|---|---|---|
| B | Burden | m | Shortest distance from free face to first row of holes |
| K | Rock Factor | dimensionless | Function of UCS: K = 0.29 Γ UCSβ°Β·β΅ (UCS in MPa) |
| d | Hole Diameter | cm | Drill hole diameter converted to centimeters |
| Οβ | Explosive Density | g/cmΒ³ | Bulk density of loaded explosive |
| RWS | Relative Weight Strength | dimensionless | Energy output relative to pure TNT (TNT = 1.0) |
Typical Ranges:
Hard rock (UCS > 100 MPa): 7.5 β 10.5 m
Medium rock (UCS 50β100 MPa): 5.5 β 8.0 m
Soft rock/overburden: 3.0 β 5.0 m
π‘ Worked Example
Problem: Given: ANFO with RWS = 0.82, rock uniaxial compressive strength (UCS) = 120 MPa, specific gravity = 2.65 g/cmΒ³, hole diameter = 115 mm.
1.
Step 1: Compute rock factor K = 0.29 Γ UCSβ°Β·β΅ = 0.29 Γ β120 β 0.29 Γ 10.95 = 3.18
2.
Step 2: Calculate burden B = K Γ β(d Γ Οβ Γ RWS), where d = hole diameter in cm (11.5 cm), Οβ = explosive density (0.85 g/cmΒ³ for ANFO)
3.
Step 3: B = 3.18 Γ β(11.5 Γ 0.85 Γ 0.82) = 3.18 Γ β(7.99) β 3.18 Γ 2.83 = 9.0 m
Answer:
The calculated burden is 9.0 m, which falls within the safe range of 7.5β10.5 m for hard rock benches β₯12 m high.
ποΈ Real-World Application
At Newmontβs Boddington Mine (Western Australia), engineers redesigned the main pit production blast after observing >18% oversize (>76 cm) and PPV exceeding 12 mm/s near infrastructure. Using updated Q-slope data and seismic tomography, they reduced burden from 9.2 m to 8.4 m, increased spacing ratio from 1.15 to 1.32, and introduced 42-ms electronic delays. Post-blast imaging confirmed 92% fragmentation <63 cm, PPV reduced to 6.8 mm/s, and shovel productivity increased by 14% β validating the revised design per ISEE Blastersβ Handbook (2022).
βοΈ Design Exercise
You are tasked with designing a production blast for a limestone quarry (UCS = 85 MPa, SG = 2.55). Bench height = 10 m, hole diameter = 102 mm, using emulsion explosive (Οβ = 1.20 g/cmΒ³, RWS = 1.05). Assume stemming = 0.3 Γ burden. Calculate: (a) Langefors burden, (b) recommended spacing using S/B = 1.25, (c) powder factor (kg/mΒ³) if charge weight per hole = 42 kg and burden Γ spacing Γ bench height defines burden volume.
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