Study notes

Qwestrum Engineering360 · Mining & Metallurgy · Mining Methods

Drilling and Blasting

Drilling and blasting design sets burden B, spacing S, and powder factor PF = explosive mass/rock mass to fragment rock safely and economically. Delay timing controls vibration; stemming length prevents gassing-off at collar.

Exam tip: keep SI units consistent end-to-end, write the governing relation symbolically before substituting, and sanity-check magnitude and sign.

Key formulas & points

Skim these first — then read the full notes below.

Delay timing sequence controls vibration
Stemming prevents premature escape
Blast pattern burden, spacing, subdrill

Topic details

Introduction

Blast design is a core mining engineering calculation — Indian exams give hole diameter, bench height, and explosive density, then ask burden, spacing, and PF. DGMS regulates explosive storage, transport, and electric detonator timing in gassy mines.

Scope in B.Tech and GATE syllabus

Vibration scales with charge per delay, not total blast — staggered delays reduce peak particle velocity at nearby structures. Stemming with crushed stone (typically 20–30× hole diameter length) traps explosion gases.

Why this topic matters in practice

Hartman & Mutmansky relates fragmentation to specific charge and rock factor — oversize increases secondary breakage cost.

Key relations & formulas

powder factor PF = explosive \frac{mass}{rock} mass

(kg/t)

Formulas (Indian textbook notation)

$burden B \approx 0.7 \times \sqrt{D\times \frac{\rho_{exp}}{\rho_{rock}}} (simplified)$

V = B\times S\times H

(blasted volume per hole)

Notation and sign conventions

Relation 1 —

powder factor PF = explosive \frac{mass}{rock} mass

powder factor PF = explosive \frac{mass}{rock} mass

(kg/t)

Write this relation with symbols exactly as in Dharmarajan Mining Methods — Standard reference before substituting numbers. Examiners award partial marks for a correct setup even when arithmetic slips.

Relation 2 —

burden B \approx 0.7 \times √

Formulas (Indian textbook notation)

$burden B \approx 0.7 \times \sqrt{D\times \frac{\rho_{exp}}{\rho_{rock}}} (simplified)$

Relation 3 —

V = B\times S\times H

V = B\times S\times H

(blasted volume per hole)

Fundamentals and definitions

Powder factor PF = total explosive mass / rock mass blasted (kg/t or kg/m³). Lower PF means more rock per kg explosive — economical but risk of oversize. Typical PF 0.3–0.6 kg/t for hard rock open pit varies with strength and desired fragmentation.

Governing relations in practice

Burden B is shortest distance between free face and hole; spacing S between holes in row. Approximate B from empirical formulas involving hole diameter D, explosive and rock density — calibrate to field measurements. Volume per hole V ≈ B × S × H (bench height) plus subdrill (0.3–0.5 H below floor).

Design and analysis considerations

Delay sequence: inner rows fire before outer to create free face; millisecond delays within row control vibration frequency. In gassy coal mines, permitted explosives and maximum charge per delay are statutory.

Advanced theory and extensions

Stemming, primer placement, and detonation cord vs electronic detonators affect fragmentation uniformity. Flyrock and air overpressure require exclusion zone — safety distance from scaled distance equations.

Assumptions and validity limits

State assumptions explicitly before using any relation for drilling and blasting — steady state, uniform properties, linear elastic material, ideal gas, incompressible flow, etc., as applicable.

Wrong assumptions invalidate the entire solution even when the formula is correct. In Mining Methods viva and GATE descriptive questions, listing valid assumptions often earns separate marks.

Step-by-step problem approach

1. Read the question and list given data with SI units (common in Mining Methods papers).
2. Draw a neat labelled diagram where applicable — examiners in Indian universities award diagram marks even when arithmetic slips.
3. Identify which relation from this topic applies to drilling and blasting.
4. Use equation 1:

powder factor PF = explosive \frac{mass}{rock} mass

.
5. Use equation 2:

burden B \approx 0.7 \times √

.
6. Substitute values, compute, and verify units and sign (direction).
7. State conclusion in one line — e.g. safe/unsafe, stable/unstable, feasible/infeasible.

Applications & exam relevance

Drilling and Blasting appears in coal and metal mines. In Indian mining curricula this topic is tested because it connects theory to surface and underground extraction.

GATE and semester exams often combine drilling and blasting with earlier units — revise prerequisites before attempting mixed problems.

Industry interview panels sometimes ask: "Where did you use drilling and blasting?" — answer with a lab, mini-project, or plant visit example if possible.

Common mistakes in exams

• Using total blast charge instead of charge per delay for vibration assessment
• Omitting subdrill length from effective bench height in volume calculation
• PF calculated with explosive volume instead of mass
• Burden too large — blowout, toe, and flyrock; too small — crushing and high cost

Quick revision checklist

Before attempting drilling and blasting problems, confirm you can:

1. Delay timing sequence controls vibration
2. Stemming prevents premature escape
3. Blast pattern burden, spacing, subdrill

Revise the solved examples in Dharmarajan Mining Methods — Standard reference and one previous-year GATE or university paper for this unit.

Worked examples

Try the problem first — open the solution when you are ready to check.

Powder factor and blast volume

Problem

Bench H = 12 m, B = 4 m, S = 5 m, one row of 6 holes, 48 kg explosive per hole. Find blasted rock mass (ρ = 2.7 t/m³) and PF.

Solution

V_total = B × S × H × n = 4 × 5 × 12 × 6 = 1440 m³
Rock mass = 1440 × 2.7 = 3888 t
Explosive = 48 × 6 = 288 kg
PF = 288/3888 ≈ 0.074 kg/t

Conceptual check — Drilling and Blasting