# Rules of Thumb for Blast Design

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## Introduction

The blaster-in-charge may not always have the luxury of planning the blasting program on a particular job without some restrictions.

The drilling, digging and hauling equipment may have previously been designated.

Government regulations may also restrict the maximum explosive weight per delay, total weight in a blast or even the diameter of the borehole.

Blasting procedures and results must be compatible with all requirements. If all the restrictions can be considered in the initial design then many of the problems that would otherwise develop can usually be eliminated.

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## Design Principles

Blast designs should be flexible so that they can be easily modified to account for variables. Some examples of variables are:

1. The amount of water in the borehole
2. The ease or difficulty of fragmenting various types of rock
3. The geology of the rock

> **No matter what guidelines are followed, the most important rule is SAFETY. Safety is the primary concern in blasting design and must NEVER be COMPROMISED.**

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## Burden Calculation

The burden is determined by the diameter of the explosive charge. Before applying these Rules of Thumb, the charge diameter must be known or selected.

### Formula

**B = 20 to 40 (De)**

Where:
- **B** = Burden distance
- **De** = Explosive charge diameter
- **20 to 40** = Rock hardness factor

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## Spacing Calculation

If an explosive charge can break a 6 ft. burden, it should also break an equal distance on each side. Therefore, spacing is usually equal to or greater than the burden.

### Formula

**S = 1 to 1.8 (B)**

Where:
- **S** = Spacing distance
- **B** = Burden distance
- **1 to 1.8** = Spacing ratio factor

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## Subdrilling Calculations

A sinking cut with a free face with excessive toe burden that does not have a natural contact bedding plane as a floor would require a maximum of subdrill. Lesser subdrilling is required for most conditions to keep a level floor.

### Formula

**J = 0.2 to 0.5 (B)**

Where:
- **J** = Subdrill distance
- **B** = Burden distance
- **0.2 to 0.5** = Rock hardness factor

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## Collar Stemming Calculation

The correct amount of collar stemming reduces flyrock potential and noise. The amount and type is important.

The top rock must be well broken while still maintaining control of the upward movement of a blast.

### Formula

**Tc = 0.7 to 1.3 (B)**

Where:
- **Tc** = Collar stemming length
- **B** = Burden distance
- **0.7 to 1.3** = Stemming ratio factor

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## Bottom Charge Calculation

Most blasters know that the rock at the toe of a borehole is more difficult to break and requires more energy. Along with the correct amount of subdrilling, a high energy bottom charge (25% greater RBS than the column charge) will help to pull grade and keep a level floor.

### Formula

**Lb = 0.3 to 0.5 (B) + J**

Where:
- **Lb** = Bottom charge length
- **B** = Burden distance
- **J** = Subdrill distance

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## Deck Stemming Calculation

The Rules of Thumb for length of a stemming deck between two explosive charges within the borehole is used to keep those charges from propagating or precompressing.

> **Note:** The Rule of Thumb for wet holes is twice the length of dry holes.

### Formula

**Td (dry) = 12 (D)**

Where:
- **Td** = Deck stemming length
- **D** = Diameter of borehole

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## Minimum Bench Calculation

A minimum face height is necessary to balance the burden and the spacing with the explosive charge diameter.

Too short of a bench will create craters without the proper forward movement of the bench and spread of the muckpile.

### Formula

**H = 1.5 to 3 (B)**

Where:
- **H** = Minimum bench height
- **B** = Burden distance
- **1.5 to 3** = Bench ratio factor

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## Quick Reference Summary

| Parameter | Formula | Variables |
|-----------|---------|-----------|
| Burden | B = 20-40 (De) | De = explosive diameter |
| Spacing | S = 1-1.8 (B) | B = burden |
| Subdrill | J = 0.2-0.5 (B) | B = burden |
| Collar Stemming | Tc = 0.7-1.3 (B) | B = burden |
| Bottom Charge | Lb = 0.3-0.5 (B) + J | B = burden, J = subdrill |
| Deck Stemming (dry) | Td = 12 (D) | D = borehole diameter |
| Minimum Bench | H = 1.5-3 (B) | B = burden |

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*This document is the property of Maritime Blasting Services Ltd., Moncton, NB*