# Chapter 10: Blast Design

## Learning Objectives

- Explain the basics of blast design, including evaluating uncontrollable variables and adjusting controllable variables.
- List the blast performance factors that help control the release of explosive energy.
- Describe the significance of even distribution of energy within the rock.
- Demonstrate how to calculate the stiffness ratio.
- Explain how explosive efficiency affects the blast design.
- Describe confinement of energy and its role in efficient blasting.
- Explain the importance of pattern design and timing in blast design.
- Describe burden and spacing.
- Using burden and spacing calculations, explain how to determine blast patterns.
- Discuss the relationship between stiffness ratio and blast effects (air blast, fly rock, etc.).
- Explain the role of hole diameter and depth in the blast design.
- Describe subdrilling and how to calculate adequate subdrilling distance under different conditions.
- Explain overburden and how to minimize material fallback.
- Determine the correct collar length to prevent fly rock.
- Explain stemming and how it is used to help contain explosive gases.
- Describe the key elements, and importance, of selecting correct blast timing.
- Discuss overbreak and how to control it.
- Calculate the powder factor for a blast hole and for an entire blast.

---

## Overview

Blasting activities must, above all, be conducted safely. However, blasters are also expected to blast as effectively as possible. Blast design is the systematic process of planning and considering the many factors that contribute to the safety and desired outcome of a blast.

This chapter provides an overview of blast design, including the following elements:

- Blast design basics
- Blast performance factors
- Key factors in blast design
- Determining the blast pattern
- Selecting the blast timing
- Calculating the powder factor

---

## Blast Design Basics

Designing a blast is part science and part art. Although basic general rules apply to the blast design process, adjustments are to be expected in order to best fit specific site conditions and situations.

Each blast presents the blaster with controllable and uncontrollable variables. Variables are factors that can vary or change:

- **Controllable variables** - Those which the blaster can change or adjust, such as the drill-hole diameter or the stemming length.
- **Uncontrollable variables** - Those which the blaster has little to no ability to change, such as regulations and the properties of the rock to be blasted.

Designing and planning the blast requires the blaster to evaluate the following:

- The desired results
- Information from previous blasts
- The uncontrollable variables of the project

The blaster then adjusts the controllable variables (such as loading procedures, blast patterns, and delay intervals) to meet the various project criteria.

By adjusting these variables, the blaster has the ability to control the amount of explosive energy released during the blast. This is key to achieving optimum blast performance. When the release of explosive energy is not controlled, the blast can result in poor rock breakage and even cause fly rock and high vibration levels.

---

## Blast Performance Factors

The following blast performance factors help control the release of explosive energy:

- Even distribution of energy
- Explosive efficiency
- Confinement of energy

### Even Distribution of Energy

Even distribution of energy within the rock leads to consistent breakage. The blaster can control factors leading to even distribution of energy. These factors include:

- The stiffness ratio
- Appropriate burden and spacing
- Appropriate blast-hole diameter for the bench height
- Accurate drilling of the pattern and hole depth

> **Note:** The powder factor of a blast does not consider the distribution of energy in a blast. This makes the powder factor a poor tool for determining blast performance. The even distribution of energy is significantly more important to blast performance than the powder factor.

The distribution of explosive energy is usually better achieved by using many holes of a smaller diameter rather than by using fewer, larger-diameter holes. A test shot is always the best practice on new sites or unfamiliar rock.

The stiffness ratio, which compares the bench height to the burden distance, can be used to determine factors such as the following:

- The spacing required between blast holes
- The amount of vibration and overbreak that will result
- How the rock will break up and where it will go (i.e., the shape of the muck pile)

#### Stiffness Ratio Formula

```
SR = L ÷ B

Where:
  SR = Stiffness ratio (unitless)
  L = Bench height (ft. or m)
  B = Burden (ft. or m)
```

A stiffness ratio greater than 3 and less than 4.5 leads to appropriate and even distribution of energy and good blast performance.

### Explosive Efficiency

The explosive efficiency is the total amount of work energy from the explosive actually captured in a blast. The major work energy produced by an explosive is in the gas pressure inside of the blast hole. The explosive efficiency should be as close to 100% as possible.

### Confinement of Energy

Another key factor in efficient blasting is the ability to confine the energy of the explosives.

**Insufficient confinement** occurs when the energy is not confined long enough to break and displace the rock or when there is insufficient burden. Insufficient confinement can lead to excessive flying material (fly rock) and air blast, as well as poor rock breakage.

**Excessive confinement** (e.g., excessive toe, heavy burden, or improper timing) makes it hard to excavate or crush the resulting blast material. In some cases, the blast may freeze (i.e., it does not move or break the rock) and excessive ground vibration can occur.

The blaster can control factors to ensure appropriate confinement of the explosive energy and achieve optimal blast performance:

- Appropriate burden and spacing
- Accurate drilling of the pattern and hole depth
- Accurate collar length and appropriate selection of stemming material
- Appropriate placement of explosives (e.g., not loading explosives into cracks, voids, or seams in the rock)
- Appropriate delay intervals

---

## Key Factors in Blast Design

As a blaster, you will need to evaluate the site to determine conditions and make the decisions required to create a good blast design. Designing a blast requires careful consideration of the following key factors:

- Blast pattern design
- Burden
- Spacing
- Stiffness ratio
- Hole diameter
- Hole depth
- Subdrilling (subgrade drilling)
- Overburden
- Collar length
- Stemming
- Blast timing
- Overbreak (backbreak and endbreak)
- Hole-to-hole delays
- Row-to-row delays
- 8 millisecond rule
- Timing on low benches or short holes
- Timing effects on ground vibration
- Other considerations (geology, water conditions, equipment)
- Powder factor

---

## Determining the Blast Pattern

The process of blast design begins by determining the blast pattern. To determine this pattern, the burden and spacing must be calculated.

### Burden

The burden is the distance from the free face to the first row of blast holes, as well as the distance between rows of blast holes. The burden should always be measured from the toe of the blast (or largest portion of the burden).

Selecting the proper burden is one of the most important decisions made in any blast design. Of all the design dimensions in blasting, it is the most critical and has the least allowable error.

#### Effects of Incorrect Burden

**Burdens that are too small:**
- Rock is thrown further from the face
- Air blast levels are higher
- Fragmentation may be too fine

> **Note:** If burden is too small in sections of the blast hole (for example, if the free face is uneven), smaller-diameter explosives should be loaded in those sections. Alternatively, decking should be used to prevent fly rock and air blast.

**Burdens that are too large:**
- Increased ground vibration (up to five times the normal level)
- Severe backbreak and back shattering on the back wall
- Creation of boulders and high bottom or toe problems
- Overconfinement, which can "freeze" a shot
- Blast holes may "rifle," throwing fly rock for considerable distances
- Vertical cratering of the rock and high levels of air blast

#### Burden Calculation (Detailed Method)

When you have detailed information on the geology of a bench:

```
B = K × De × √(2SGe/SGr + 1.5)

Where:
  B = Burden (ft. or m)
  K = Unit constant (1 for imperial, 0.012 for metric)
  De = Explosive diameter (in. or mm)
  SGe = Specific gravity of explosives (explosive density)
  SGr = Specific gravity of rock
```

**Example:** If the explosive diameter is 2 in., the specific gravity of the explosives is 1.10, and the specific gravity of the rock is 2.8:

```
B = 1 × 2 × √(2 × 1.10/2.8 + 1.5) = 4.57 ft.
```

#### Specific Gravity Tables

**Specific Gravity of Rock Types:**

| Rock Type | Specific Gravity |
|-----------|-----------------|
| Basalt | 2.8-3.0 |
| Diabase | 2.6-3.0 |
| Diorite | 2.8-3.0 |
| Dolomite | 2.8-2.9 |
| Gneiss | 2.6-2.9 |
| Granite | 2.6-2.9 |
| Hematite | 4.5-5.3 |
| Limestone | 2.4-2.9 |
| Marble | 2.1-2.9 |
| Mica schist | 2.5-2.9 |
| Quartzite | 2.0-2.8 |
| Sandstone | 2.0-2.8 |
| Shale | 2.4-2.8 |
| Slate | 2.5-2.8 |
| Trap rock | 2.6-3.0 |

**Specific Gravity of Explosives:**

| Explosive Type | Specific Gravity |
|---------------|-----------------|
| Poured ANFO/WR | 0.80-0.90 |
| Pneumatically loaded ANFO | 0.80-1.00 |
| Boosters | 1.25-1.30 |
| Dynamite (NG) | 1.34-1.51 |
| Packaged emulsion | 1.10-1.20 |

#### Burden Calculation (Simplified Method)

When you don't have detailed information on the geology of the bench, burden ranges between 25 and 35 times the diameter of the explosive charge, depending on rock type:

| Rock Type | Rock Type Factor |
|-----------|-----------------|
| Hard | 25 |
| Medium | 30 |
| Soft | 35 |

```
B = (25 to 35 × De) ÷ 12

Where:
  B = Burden (ft.)
  25 to 35 = Rock type factor
  De = Explosive diameter (inches)
  12 = A constant to provide measurement in feet
```

**Example:** The burden for hard rock drilled with a 3-inch bit:
```
B = (25 × 3) ÷ 12 = 75 ÷ 12 = 6.25 ft. (rounded to 6 ft.)
```

### Spacing

Spacing is the distance between blast holes in a row. The optimum spacing of blast holes will range from 1.0 to 2.0 times the burden distance depending on the rock, the hole-to-hole delay, and the stiffness ratio of the bench.

The ratio of spacing to burden is known as the **spacing ratio**. Harder rock will require a lower spacing ratio (leading to a smaller pattern). Softer rock will require a higher spacing ratio (leading to a larger pattern).

**If spacing is too small:**
- Splitting action can occur between blast holes instead of proper breakage
- Ejection of stemming material
- Creation of boulders (oversize material)
- Breakage beyond the last row, toe, or high bottoms
- Higher air blast (overpressure)

**If spacing is too large:**
- Poor fragmentation (oversize material)
- Uneven grade

#### Spacing Formula

```
S = B × 1.0 to 2.0

Where:
  S = Spacing (ft. or m)
  B = Burden (ft. or m)
```

The multiplier of 1.0 to 2.0 includes the rock type factor, the timing factor, and the stiffness ratio.

### Stiffness Ratio

In general, the larger the explosive diameter, the more problems (such as fragmentation, air blast, fly rock, and ground vibration) are possible. Larger explosive diameters require increased burdens and spacings, even if the powder factor is held constant.

```
SR = L ÷ B

Where:
  SR = Stiffness ratio (unitless)
  L = Bench height (ft. or m)
  B = Burden (ft. or m)
```

#### Effects by Stiffness Ratio

| Effects | SR = 1 | SR = 2 | SR = 3 | SR = 4 |
|---------|--------|--------|--------|--------|
| Fragmentation | Poor | Fair | Good | Excellent |
| Air blast | Severe | Fair | Good | Excellent |
| Fly rock | Severe | Fair | Good | Excellent |
| Ground vibration | Severe | Fair | Good | Excellent |
| Comments | Severe overbreak and toe problems. Do not shoot. Redesign! | Redesign if possible. | Good control and fragmentation. | No increased benefit from increasing stiffness ratio above 4.5. |

**For construction blasting, the general rule is to aim for a stiffness ratio between 3 and 4.5. A stiffness ratio below 3 is not recommended.**

### Hole Diameter and Depth

The blaster should choose the proper drill-hole diameter for each job. The combination of hole diameter and depth plays a key role in achieving a controlled blast.

Benches less than 15 ft. (4.5 m) in length should be blasted with an explosive diameter equal to or less than 1.5 in. (3.8 cm) to achieve a stiffness ratio of about 3 and have good blast results.

### Subdrilling

Subdrilling (or subgrade drilling) means drilling a blast hole below the desired elevation. Subdrilling ensures that the blast breaks to grade and achieves proper elevation.

#### Subdrilling Formula

```
J = B × 0 to 0.5

Where:
  J = Subdrilling distance (ft. or m)
  B = Burden (ft. or m)
  0 to 0.5 = Rock type factor
```

- Harder rock requires more subgrade (e.g., factor of 0.5)
- Softer rock requires less subgrade (e.g., factor of 0.1)
- Rock with horizontal bedding requires little to no subgrade drilling (factor of 0)
- Starting with a subdrill of 0.3 is a good starting point for the first test blast on site

**Example:**
```
J = 5 ft. burden × 0.5 = 2.5 ft. subdrill in hard rock
```

> **Important:** Too much subdrilling increases ground vibration. Too little subdrilling can lead to high bottom and boulders that may require additional blasting.

### Overburden

Overburden is any material (soil, gravel, clay, etc.) that is drilled through before hitting bedrock. The driller must note the depth of the overburden and report it to the blaster. The depth of the overburden adds to the collar length and increases the amount of stemming required.

**To minimize fallback:**
- Remove overburden before drilling
- "Mud the collar" - use water to create a paste-like material to establish a clean collar
- Block the collar with a hole plug after drilling

### Collar Length

Collar refers to the top portion of the blast hole above the explosives or powder column. The collar is filled with stemming to confine the explosive gases.

#### Collar Length Formula

```
T ≥ B

Where:
  T = Collar length (ft. or m)
  ≥ = Greater than or equal to
  B = Burden (ft. or m)
```

**Example:** On a construction blast site using a blast pattern with 5 ft. of burden:
```
T ≥ 5 ft. collar
```

> **Note:** Collar length is usually rounded up to the nearest foot and does not include the depth of any overburden.

### Stemming

Stemming is the material used in the collar of the blast hole to help contain the explosive gases.

**Stemming material requirements:**
- Average diameter of about 0.05 times the diameter of the blast hole
- Should be angular to work properly
- Generally should be crushed stone
- In small-diameter holes, bird's eye gravel (tiny pebbles) should be used

> **Warning:** Drill cuttings make poor stemming material. The lack of confinement causes excessive fly rock and air blast, as well as poor rock breakage. Drill cuttings would not be acceptable in any blasting operation that is not extremely remote.

---

## Selecting the Blast Timing

Selecting the correct blast timing is crucial for a safe, effective blast. Many blasting problems (e.g., air blast, fly rock, excessive vibration, and poor fragmentation) are directly related to timing.

### Controlling Overbreak (Backbreak and Endbreak)

- **Overbreak** - Breakage beyond the excavation limits (i.e., the rock cut)
- **Backbreak** - The area of breakage occurring behind the last row of blast holes
- **Endbreak** - The area of breakage occurring at the ends of a blast pattern

In blasting operations, it's common practice to give the last row, and sometimes the end holes in a row, more time before they fire to allow earlier-firing rows to move out of the way. This reduces the resistance on those holes and reduces the pressure on the back walls, resulting in cleaner breaks with less endbreak and backbreak.

### Hole-to-Hole Delays

Hole-to-hole delays in the same row are normally calculated at 2 to 5 milliseconds per foot of burden. The general rule of thumb is 3 ms/ft.

```
HHD = B × 3

Where:
  HHD = Hole-to-hole delay (ms)
  B = Burden (ft.)
```

### Row-to-Row Delays

Row-to-row delays should be 2 to 3 times the hole-to-hole delay.

```
RRD = HHD × 2 to 3

Where:
  RRD = Row-to-row delay (ms)
  HHD = Hole-to-hole delay (ms)
```

### The 8 Millisecond Rule

The 8 millisecond rule relates to timing that affects ground vibration. The rule states that a minimum 8 ms separation is required between charges to obtain the benefit of reduced vibration levels from the use of delays.

### Timing on Low Benches or Short Holes

On benches with a stiffness ratio of less than 3, the timing is critical for controlling fly rock, air blast, and overbreak. For low-stiffness-ratio benches, the ideal hole-to-hole delay is shorter than for tall benches.

### Timing Effects on Ground Vibration

Use longer row delays to spread the vibration peaks over a longer time. Do not design a blast that has multiple holes firing at the same time; this will lead to increased ground vibration.

---

## Calculating the Powder Factor

The powder factor (PF) is the amount of explosive (in pounds or kilograms) required to blast a given volume or weight of rock. This value is useful for tracking explosive consumption or estimating the amount of explosives required to blast a given amount of material. However, use the powder factor as a guideline only. It does not tell you about the distribution of the explosive energy.

### Powder Factor Formula (Volume)

```
PF = W ÷ V

Where:
  PF = Powder factor (lb./yd.³ or kg/m³)
  W = Weight of explosives (lb. or kg)
  V = Volume of rock (yd.³ or m³)
```

### Powder Factor Formula (Weight)

```
PF = W ÷ Wr

Where:
  PF = Powder factor (lb./ton or kg/tonne)
  W = Weight of explosives (lb. or kg)
  Wr = Weight of rock (tons or tonnes)
```

### Calculating Volume of Rock

```
V = B × S × L

Where:
  V = Volume per blast hole (ft.³)
  B = Burden (ft.)
  S = Spacing (ft.)
  L = Bench height (ft.)

To convert to cubic yards: V (yd.³) = V (ft.³) ÷ 27
```

---

## Conversions

### Imperial to Metric

| Imperial | Metric |
|----------|--------|
| 1 inch | 25.4 mm |
| 1 foot | 0.3048 m |
| 1 yard | 0.9144 m |
| 1 pound | 0.4536 kg |
| 1 short ton (2000 lb.) | 0.9072 tonnes |
| 1 cubic foot | 0.0283 m³ |
| 1 cubic yard | 0.7646 m³ |

### Metric to Imperial

| Metric | Imperial |
|--------|----------|
| 1 mm | 0.03937 inches |
| 1 m | 3.281 feet |
| 1 m | 1.094 yards |
| 1 kg | 2.205 pounds |
| 1 tonne | 1.102 short tons |
| 1 m³ | 35.31 cubic feet |
| 1 m³ | 1.308 cubic yards |