🎓 Lesson 4
D3
Design and Planning Fundamentals
Blast design is planning how to place and detonate explosives to break rock efficiently, safely, and predictably.
🎯 Learning Objectives
- ✓ Calculate optimal burden and spacing for a given bench height and rock competency using empirical relationships
- ✓ Design a drill pattern layout (including hole depth, stemming, and subdrilling) that satisfies fragmentation and throw requirements
- ✓ Analyze and adjust powder factor to meet target fragmentation (Kuz-Ram model) while staying within safe vibration limits (USBM or DIN 4150)
- ✓ Explain the relationship between delay timing, stress wave interaction, and resulting fragment size distribution
- ✓ Apply blast design checklists aligned with ISEE Blasters’ Handbook and OSHA 1926.900 standards
📖 Why This Matters
Poor blast design causes flyrock, excessive ground vibration, oversized boulders, and wasted energy — leading to costly secondary breaking, equipment damage, safety incidents, and regulatory penalties. In hydraulic system engineering contexts (e.g., dam foundation excavation, tunnel portal development, or pump station caverns), precise blast design ensures stable rock surfaces for concrete placement, minimizes water ingress pathways, and preserves structural integrity of adjacent infrastructure. Getting it right the first time saves weeks in schedule and millions in rework.
📘 Core Principles
Blast design rests on three interdependent pillars: (1) Energy transfer — how explosive energy couples into rock via confinement, stemming, and borehole pressure; (2) Fracture mechanics — propagation of radial and tangential cracks governed by rock strength, discontinuity orientation, and in-situ stress; and (3) Dynamic interaction — timing-induced stress wave superposition that enhances breakage between holes (‘stress wave merging’). Modern practice moves beyond empirical rules-of-thumb toward integrated models combining Kuz-Ram fragmentation prediction, SAPS vibration modeling, and discrete element simulations — yet foundational geometric relationships remain essential for rapid field validation and troubleshooting.
📐 Burden–Spacing Relationship
The burden (B) is the shortest distance from a blasthole to a free face; spacing (S) is the distance between holes in a row. Their ratio (S/B) controls confinement and fracture coalescence. A well-balanced ratio ensures uniform fragmentation without excessive crushing or channeling. The ‘Konya–Lindholm’ empirical ratio is widely applied for production drilling in hard rock.
💡 Worked Example
Problem: Given: granite bench height = 15 m, rock uniaxial compressive strength (UCS) = 180 MPa, explosive: ANFO (density = 0.85 g/cm³, VOD = 4,000 m/s). Use Konya–Lindholm method to determine S/B ratio and calculate burden if spacing is set to 4.2 m.
1.
Step 1: Determine rock class — UCS = 180 MPa → Class 'Very Hard Rock' (per ISEE Rock Classification Table)
2.
Step 2: From Konya–Lindholm Table, S/B = 1.15 for Very Hard Rock with ANFO
3.
Step 3: Solve B = S / 1.15 = 4.2 / 1.15 = 3.65 m → round to 3.6 m for practical drill rig compatibility
Answer:
The calculated burden is 3.6 m, which falls within the typical range of 3.0–4.5 m for 15-m benches in hard rock.
🏗️ Real-World Application
At the Glen Canyon Dam spillway repair project (2021), engineers faced tight tolerances for excavation adjacent to operational concrete structures. Using a modified V-cut pattern with 3.2-m burden, 3.7-m spacing, 0.4-m subdrill, and 25-ms inter-hole delays, they achieved >95% material within 0.6-m fragment size (per ASTM D5733 sieve analysis) while limiting peak particle velocity (PPV) to 12 mm/s — below the 25 mm/s limit specified by USBM for historic concrete. Post-blast LiDAR scanning confirmed <25 mm overbreak — critical for subsequent grouting and liner installation.