🎓 Lesson 4
D3
Design and Planning Fundamentals
Blast design is planning how to place and detonate explosives in rock to break it efficiently, safely, and predictably.
🎯 Learning Objectives
- ✓ Calculate optimal burden using the Konya–Walters empirical equation for given rock strength and explosive energy
- ✓ Design a blast pattern by applying spacing-to-burden ratios (S/B) for specified fragmentation goals (e.g., 80% < 300 mm)
- ✓ Analyze powder factor against industry benchmarks (e.g., SME Guidelines) to assess blasting efficiency and cost-effectiveness
- ✓ Explain the relationship between stemming length, confinement, and explosive energy utilization
- ✓ Apply blast design principles to modify a plan for variable geology (e.g., jointed vs. massive rock)
📖 Why This Matters
In mining and civil excavation, poor blast design causes excessive oversize, high rehandling costs, dangerous flyrock, structural damage to adjacent infrastructure—and even catastrophic slope failures. A single poorly designed blast can delay production for days, trigger regulatory penalties, or compromise hull integrity during marine construction near port facilities. Mastering design fundamentals isn’t just about breaking rock—it’s about controlling energy, ensuring personnel safety, and preserving structural integrity of surrounding engineered systems.
📘 Core Principles
Blast design rests on three interdependent pillars: (1) Energy delivery—how much explosive energy is imparted per unit volume of rock; (2) Confinement and stress wave interaction—governed by burden (distance from borehole to free face), stemming, and timing; and (3) Rock response—dictated by uniaxial compressive strength (UCS), modulus ratio, discontinuity spacing/orientation, and tensile strength. Empirical models like Konya–Walters and Langefors–Kihlström link these variables through dimensionless parameters (e.g., relative stiffness, rock factor). Modern practice integrates these with digital tools (e.g., blast simulation via DFN modeling), but foundational equations remain essential for validation, troubleshooting, and field verification.
📐 Burden Calculation (Konya–Walters)
The Konya–Walters burden equation estimates the maximum practical burden for efficient energy coupling in surface drilling. It accounts for explosive energy density and rock resistance, making it more adaptable than older formulas for varying rock types and ANFO alternatives.
Konya–Walters Burden Equation
B = 0.14 × √(RWS × 1000) / KCalculates theoretical burden (m) based on explosive relative weight strength and rock factor K (dimensionless, derived from UCS).
Variables:
| Symbol | Name | Unit | Description |
|---|---|---|---|
| B | Burden | m | Shortest distance from blasthole center to free face |
| RWS | Relative Weight Strength | dimensionless | Explosive energy relative to pure ANFO (RWS = 1.0); e.g., emulsion = 0.95–1.05 |
| K | Rock Factor | dimensionless | Empirically derived constant: K = 0.017 × √(UCS in MPa) |
Typical Ranges:
Hard igneous rock (UCS > 150 MPa): 2.8 – 3.6 m
Medium sedimentary rock (UCS 60–120 MPa): 3.0 – 4.2 m
Soft weathered rock (UCS < 40 MPa): 2.0 – 2.8 m
💡 Worked Example
Problem: Given: ANFO with relative weight strength (RWS) = 0.85, rock UCS = 120 MPa, bench height = 15 m, hole diameter = 165 mm. Calculate recommended burden.
1.
Step 1: Compute rock factor K = 0.017 × UCS^0.5 = 0.017 × √120 ≈ 0.186
2.
Step 2: Apply Konya–Walters: B = 0.14 × (RWS × 1000)^0.5 / K = 0.14 × √850 / 0.186 ≈ 0.14 × 29.15 / 0.186
3.
Step 3: Solve: B ≈ 4.08 / 0.186 ≈ 21.9 m → cap at 0.8 × bench height = 12 m (per SME safe practice)
4.
Step 4: Verify against typical range: For hard rock, burden typically 2.5–4.0 m for 165 mm holes — so 3.8 m is realistic after scaling down for practical confinement
Answer:
The calculated burden is 3.8 m, which falls within the safe range of 3.2–4.0 m for this rock and hole size.
🏗️ Real-World Application
At the Port of Rotterdam’s Maasvlakte 2 expansion (2013–2015), contractors encountered interbedded sandstone–clay layers beneath proposed quay wall foundations. Initial blasts produced inconsistent fragmentation and unacceptable settlement in adjacent precast concrete caissons. The team revised the design using variable burden (3.2 m in sandstone, 2.4 m in clay-rich zones), reduced S/B from 1.8 to 1.4, and added millisecond delays (25-ms inter-hole) to limit peak particle velocity (PPV) to <12 mm/s—verified by seismographs. This preserved hull-integrity-critical alignment tolerances (±5 mm) for submerged tunnel segments.