# Chapter 18: Drilling

The primary purpose of any borehole is to allow the placement of a specific explosive charge in a specific location to achieve a specific result. This requires accuracy in borehole placement, alignment, depth, and diameter. Any deviation from these design parameters will negatively affect the quality of the blast. Quality drilling results are challenged by mechanical, human and geologic factors. Understanding these factors is critical to overcoming the challenges they present.

The blast designer must also consider these factors and allow for normal, acceptable deviations when developing a drill pattern. These acceptable margins of error and the potential impact of any deviation from design must be clearly communicated to the two individuals responsible for implementing the design (1) drilling supervisor (excluding crews) and (2) blaster-in-charge (including the blasting crews).

---

## Driller Communication

> *The quality and content of communications among the designer, the driller and blaster-in-charge are critical and cannot be overemphasized.*

### DRILLER COMMUNICATION

The level of communication required is often perceived to depend on the complexity of the blast being prepared. This can be seriously misleading and result in delays to correct drilling errors, or even lost in the necessity to carry out blasts or recognized but accepted higher risk levels.

#### Verbal Communication

For drilling and blasting crews that have extensive experience working together, verbal communication of basic pattern information or job sequencing may be acceptable, but should always be supported with written communication.

A classic (and common) example illuminates the potential for error when depending on verbal communication alone. In a secondary boulder blasting operation the blaster-in-charge might direct the driller to "drill all boreholes half-way through the boulders" without reference to borehole directions and minimum acceptable burden to any open boulder surface exposed toward nearby equipment or personnel. This "half-way through" verbal instruction can also be understood to mean the minimum depth required to break the boulder, but no one knows until the blast is detonated.

This example shows that simple verbal communication can lead to miscommunication and fail to convey all the necessary information. Whenever possible some form of written communication should govern any blaster drilling crews (even if it is as simple as a hand-sketched diagram). This gives the driller the ability to request and record clarifications, which can then be passed on to other drillers.

#### Written Communication

Most production blasting operations have standard blast reports to record the information such as the following six items (1) total boreholes to be blasted, (2) average borehole depth, (3) total explosives, (4) total powder factor, (5) average collar depth, and (6) maximum explosives charge per delay (see Chapter 27 for a discussion of blast record requirements).

Often ground conditions, drilling deviations, or anomalies in the pattern are recorded on a hand-sketched blast diagram that is maintained by the blaster-in-charge as loading progresses. The accuracy of this process depends on the amount of free time that the blaster-in-charge has on the day of the blast (usually very little).

The drilling crew should record most of this important information concerning the pattern and ground conditions as the drilling progresses. This record is most accurate when the driller records data on the drill log on a borehole-by-borehole basis. This log should then be submitted to the blast designer on a timely manner to permit any required changes well in advance of beginning the loading operations. Rock conditions like those shown in figure 18.1 are to be reported in the drill log as indicated in figure 18.2.

This assures proper design and implementation of the blast. Minimum recommended drill log information is found in table 18.1.

![Figure 18.1— Complex geologic structures reported in the drill log. (Courtesy B. Wingfield)](images/364.png)

### Figure 18.2 — Sample drill log. (Courtesy: J. Stiles)

---

## Minimum Recommended Drill Log Information

| ✓ | Information |
|---|-------------|
| ✓ | Date drilled |
| ✓ | Driller name |
| ✓ | Rig identifier |
| ✓ | Pattern dimensions or identification |
| ✓ | Angle and azimuth of borehole |
| ✓ | Required subdrill length |
| ✓ | Minimum accepted pattern deviations |
| ✓ | Borehole numbers entry (stakes) |
| ✓ | Large comments section for each borehole |
| ✓ | Pattern map with reference points (side, etc) |

*Table 18.1 — Minimum recommended drill log information.*

The comments section contains the most important information for blasting crews. Thus, the comments section should be quite large. Typical information included in the comments section is given in table 18.2.

### Drill Log Comment Section Information

| ✓ | Information |
|---|-------------|
| ✓ | Bit diameter |
| ✓ | Rig condition |
| ✓ | Offset from design location |
| ✓ | Angle and its direction |
| ✓ | Depth |
| ✓ | Deviation—known or suspected (See chapter 32) |
| ✓ | Cable condition |
| ✓ | Geologic information, depth, and thickness encountered: |
|   | - hard, medium, or soft zones |
|   | - Cracks |
|   | - Voids |
|   | - Water, dry, HC |
| ✓ | Location of additional and/or lost boreholes |
| ✓ | Additional information the driller deems important for the blaster-in-charge |

*Table 18.2 — Drill log comment section information.*

Adoption of standard abbreviations or symbols for these notions that space and time are saved while drilling.

The drilling crews are normally provided with guidelines to determine what depth to drill the boreholes on a pattern. These guidelines can vary from quite crude to quite sophisticated. The blaster should be informed of what level of control has been implemented for controlling drill depth when checking the boreholes for depth during the preblast check. The less precise the guidelines the more overbilling will occur to compensate for possible error in borehole depth calculations.

---

## PATTERN LAYOUT CONTROLS

Incorporating good survey and layout tools is a very beneficial investment. Drilling excessive depths wastes money and loading overfilled depths wastes more money. Some common methods to control drill depth are provided in table 18.3.

### Suggested Drilling Crew Tools For Defining Borehole Depth

| Method |
|--------|
| Paint marks on ground at specific elevations |
| String lines at blast perimeters |
| Contour map of blast area |
| Borehole markers |

*Table 18.3—Suggested drilling crew tools for defining borehole depth.*

Requires driller to use a hand level
Requires driller to use a hand level
Shows borehole numbers and elevations at the markers
Requires driller to calculate borehole depth

A common complaint with drill logs and blast reports is that they are easily lost or damaged. A properly designed and prepared drill log can be used as the template for a loading guide, or it should be provided inside a weatherproof, tamper-proof container at blast site. In addition, the driller should be issued waterproof containers for filling out the drill logs and they should place, or for any work they have done. Borehole markers (See figure 18.3) are an effective method to provide specific information to the driller.

![Figure 18.3 — Example of a borehole marker (flag) with driller information (depth and borehole number). (Courtesy B. Wingfield)](images/367.png)

Accurate information in the drill report depends on the driller's initiative and an accurate means of obtaining information. A suggested list of driller tools is provided in table 18.4. Each drill crew should be issued these tools so their proper use will help ensure drilling is done to design specifications.

On many blasting projects, surveyors set stakes (See figure 18.4) giving grade and station information to the excavation crew (e.g. cut or fill information relative to the final grade depth). They use symbols to communicate this information and "grade" may represent final excavation grade or the top of pavement as required in highway or plant construction.

Since drillers also use this information at project startup the driller should review grade stake information with the surveyors to determine if it includes or does not include subdrill.

![Figure 18.4 — Example of a surveyor grade stake. (Courtesy: D. Rensari)](images/367.png)

> **Caution**
>
> Drillers must know if they are to add subdrill to the surveyor's grade depth. A misunderstanding here can result in hard bottom or loss of grade.

A common problem in the surface coal mines is when multiple drills work on the same bench (day and night shifts). It is very difficult and time consuming for blasters to orient 5 to 6 drill logs for the same blast and correctly match boreholes on the bench with boreholes on the drill log. Borehole markers like that shown in figure 18.3 in combination with drill logs help the blaster. Critical information on the borehole marker should match information on the drill log. A list of suggested driller tools is given in table 18.4. Proper use of these tools helps assure drilling is done to design specifications.

### Suggested Driller Tools

| Tool | Description or use |
|------|-------------------|
| Pattern map (that includes): | Cuts |
| | Maximum acceptable deviation from pattern |
| Set of tables | Angle vs. depth |
| | Angle vs. offset |
| Abney level | Measure borehole angle |
| Compass | Determine borehole azimuth |
| Inclinometer/Clinometer | Instruments to adjust drill depth and/or angle |
| Tape measure | Minimum length at least 3 borehole spacings |
| Weighted tape | At least twice the expected borehole depth in length |
| Loading poles | They must be straight to check borehole alignment |
| Mirror | Use sunlight to check borehole alignment |
| Markers to record/Borehole | Borehole # |
| | Drilled depth |
| Spray paint (in an alternative color) | Mark cut contours on ground, or alignment or back or rib of underground headings, muck lookings |
| Spray paint (in alternative color) | Clearly mark drilling stake |
| Chalk line or shovel | Remove loose rubble from collar area before drilling |
| Grout fixer | Insert in borehole ½H ↓ |
| Drill log | See table 18.1 |
| Your head GPS | Verify location of borehole |
| Cell phone | Exchange telephone numbers with the blaster-in-charge and USE it. |

*Table 18.4 — Suggested driller tools.*

---

## WHEN THE BLASTER HAS NO CONTROL OVER THE DRILLING

All rock is blasted for a customer, whether that customer is a fill location on a highway or a high production crushing operation. Before there, however, comes a much more important customer – the front end loader or shovel operator that has to dig and load out the broken rock. All year of three customers win the products – the "job smither" one we are interested in at this point.

The most important thing to keep in mind is that once the blast has been detonated, all evidence of drilling quality is gone – forever – except for preplift lines. It is the blaster-in-charge's responsibility to: (1) (like) evaluate, (2) quantify, and (3) attempt to overcome the results of poor drilling before leaving the blast design.

In open pit mines and quarries, drilling patterns and depths are normally pre-determined by a planning group or the blasting supervisor. Problems with the quality of drilling can thus be addressed directly without the customer and are simply a step closer to remediation.

On some projects, however, the blaster-in-charge can be challenged with drilling quality that is marginal or even unacceptable, and may have no direct way to address the concern with the drilling contractor.

At this point the blaster-in-charge may have only two options (1) accept the pattern as drilled and load according to the original design, or (2) attempt to overcome the drilling errors with varying column loads and timing within the blast. In either case, the blaster-in-charge is faced with the possibility of an unacceptable blast result.

> **Caution**
>
> Blasting results from a poorly drilled pattern cannot be corrected with good blasting practice.
> At the very best, the results can only be made "acceptable"

### Identify Drill Quality Deviations

Table 18.5 lists some sample methods of identifying downtable problems.

> **Caution**
>
> The blaster-in-charge should document in detail all drilling deviations from the blast design.

### Methods For the Blaster-In-Charge To Identify Poor Drilling Quality

| Method | Result |
|--------|--------|
| Examine a cross section of the drill cuttings. | Provide a rough idea of the rock strata when drill reports lack information, or variable ground conditions, are expected. |
| Read the drill reports and compare to the as-drilled pattern. | Verify borehole diameter—undersize or oversize. |
| | Generous downhole. Manage chip sample together and test for rock-fly expected. |
| Conventional loading: Frequently check column rise vs. calculated value. | Borehole liners often do indicate getting wider cut rate in the hole-plan. |
| | Check that the borehole will accommodate the expected column of explosives. |
| Borehole camera | Mark problem boreholes as observed using a camera flag, say red. |

*Table 18.5 — Methods for the blaster-in-charge to identify poor drilling quality.*

### Essential Blaster Tools To Quantify and Document Drilling Quality

| ✓ | Tool | Use |
|---|------|-----|
| ✓ | Pattern map | Definition of: |
| | | Reference points |
| | | Design stake or, |
| | | Maximum acceptable deviation from pattern |
| ✓ | Set of tables | Angle vs. depth |
| | | Angle vs. offset |
| ✓ | Angle indicator | Measure borehole angle |
| ✓ | Compass | Determine borehole azimuth |
| ✓ | Tape measure | Minimum length at at least 3 borehole spacings |
| ✓ | Weighted tape | At least twice the expected borehole depth & length |
| ✓ | Mirror | Use sunlight to examine borehole (See chapter 19) |
| ✓ | Spray stake, spray paint, etc. | Mark problem boreholes: Different stake or paint colors could be used to denote the specific problem. |
| | | Mark problem boreholes as substandard using category |
| ✓ | Chalk line or shovel | Clear loose material away from borehole collars. |
| ✓ | Capacitying lamp | Insert to borehole of holes |
| | | Looking (See chapter 25) |
| ✓ | Blast log | Borehole numbers |
| | | Required pattern |
| | | Actual depths |
| | | Required subdrill |
| | | Borehole load |
| | | Stemming height vs. pattern (See table 18...) |
| ✓ | Cell phone | Exchange telephone numbers with the driller. Use it. |

*Table 18.6 — Essential blaster tools to quantify and record borehole quality.*

A competent and responsible blaster is expected to acquire the training necessary to perform a proper assessment of drilling quality (See tables 18.5 and 18.6). When the blaster-in-charge recognizes questionable drilling quality the first and most important blaster-in-charge action is to check all boreholes and note all deviations from the design before beginning loading operations.

> **Caution**
>
> Check all boreholes and note all deviations from the design before beginning loading operations.

Measuring borehole deviation is discussed and illustrated in chapter 32.

### Strategies To Overcome Poor Drilling

Some very basic steps that can be taken by the blaster-in-charge to reduce the effect of poor drilling quality are noted below. Since deviations from the blast design can produce unexpected results, additional discussions relevant to decisions to make adjustments are offered in other chapters (See table 18.7) of this handbook and this should be referenced before applying these measures. The blaster-in-charge is encouraged to understand the potential consequences of field decisions made to overcome poor drilling results. Suggested methods for the blaster-in-charge to compensate for borehole deviations are listed in table 18.8.

### Additional 18th Edition Information When Making Decisions for Poor Drilling Performance

| Chapter No. | Topic |
|-------------|-------|
| 5 | Project Communication |
| 6 | Blast Design Process |
| 7 | Drilling and Blasting For Downstream Benefit |
| 9 | Fragmentation Process |
| 14 | Blast Design Principles |
| 11 | Explosives Properties and Performance Characteristics |
| 15 | Flyrock |
| 16 | Misfires |
| 17 | Blaster Checklists |
| 26 | Vibration |
| 27 | Record Keeping |
| 29 | Risk Management |
| 31 | Blast Planning |
| 33 | Surface Blasting, Construction |

*Table 18.7—Additional 18th Edition information when making decisions for poor drilling performance.*

### Suggested Blaster Methods to Compensate For Borehole Deviations

| Borehole deviation | Method |
|--------------------|--------|
| Extra burden | Use lower ton loads |
| | Use higher density, high energy explosive to an adjusted collar (stemming length) |
| | Increase the delay time available for burden movement |
| | Use best quality of stemming material available |
| Insufficient burden | Use packaged explosives in face boreholes to lower the energy level |
| | Use lighter ton loads |
| | Use best quality stemming in face boreholes are lowest in face boreholes |
| | Deck load in main production |
| | Use lower energy explosives in main production boreholes |
| Short boreholes | Coat bore of boreholes immediately around short boreholes with higher density, higher energy explosives. |
| Boreholes too deep | Back fill with crushed stone (preferably) to sub-column of design depth. |
| Undercut boreholes | Tamp smaller diameter cartridges as required to achieve desired column cut |
| Oversized boreholes | Use larger explosive canisters or make adjustments to boreholes |
| | Deck load in main production boreholes |
| | Re-deck with single column charge in main production boreholes |

*Table 18.8 — Suggested blaster methods to compensate for borehole deviations.*

Up to this point, all responsibility has been placed on the blaster to identify, quantify and attempt to overcome the results of poor drilling control. None of that, however, will solve the real problem – that must be the responsibility of the blasting contractor's management team.

Before the real problem can be clearly defined and addressed, the drilling and blasting management team will have to implement some if not all of the following recommendations in table 18.9.

### Drilling and Blasting Management Team Responsibilities

| Responsibility |
|----------------|
| Establish a procedure for informing drilling supervisor of deficiencies. |
| Establish lines of communication between drilling and blasting companies. |
| Assign blasting personnel to the drilling rig at startup. |
| Insist that drilling crew(s) prepare detailed drilling reports, and submit copies to the blasting crew before loading begins. |
| Require all drillers to record and maintain detailed blast logs. |
| Direct blasting personnel to check all boreholes before loading and document deficiencies |
| Have drilling supervisor or pattern scale the borehole on a checklist. |
| Include those drilling quality report sheets in all blast reports. |
| Communicate and address identified deficiencies. |
| Ensure good bench preparation (See Bench Preparation section of this chapter) |

*Table 18.9 — Drilling and blasting management team responsibilities.*

If these steps do not improve the situation, the blasting contractor may be forced to call back the drill crew in extreme cases and/or change payment terms for contract drilling — in short, no pay for deficient work.

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## FACTORS INFLUENCING DRILL CHOICE

When specifying the type of drill system to use on a project, the first question to answer is "what is required of the drill?" At this point, the drilling contractor must often decide whether to accept lower productivity from a poorly suited drill rig, or insist on full performance from a properly matched one. In general, the drill types as table 18.10 are better suited to the application factors listed.

### Factors Influencing Drill Choice By Drill Type

| Factor | Rotary | Rotary Percussion (Top Hammer) | Down-the-hole Hammer (DTH) |
|--------|--------|-------------------------------|---------------------------|
| Geology | Soil-soft limestone | Soft medium-hard | Medium-hard to hard |
| | Soft to medium sandstone | | Very hard |
| Hole Diameter | 150mm (6") to 500mm (20") | <150mm (6") | 75mm (3") to 300mm (12") |
| Hole Depth | Rotary optimum >15m | <20m (65') | 3m (10') to 65m (200') + |
| Bench Height | ≥9m (30') | <9m (30') | 3m (10') to 60m (200') |

*Table 18.10 — Factors influencing drill choice by drill type.*

The next step is to determine suitability of the drill system and carrier to the application at hand. Obviously, hand drills are of little use on a 15 meter (50 foot) deep cut, and it would be equally inappropriate to drill 2 meter (6 foot) deep service trenches with a heavy rotary drill.

> **Caution**
>
> Inappropriate drill substitutions are commonly made in coal initiation blasts.

The choice of carriers for surface operations can also be driven by the slope or width of drill access points. Two situations are (1) steep terrain on alpine road building projects and (2) narrow spaces between buildings on a demolition project. In these and similar situations, the contractor must often make a choice based on two factors: (1) obtaining a carrier that will reach the work and (2) efficiency of the drilling system.

### Planning Drilling Capacity

A common, and inefficient, practice is to use several lighter to medium weight top hammer drill rigs for large projects that have relatively deep cuts. The argument in favor of the practice is that minimal site preparation is required for the smaller, track mounted drills.

There is an equally compelling argument to use the lighter drills for bench preparation and pioneering, to be followed by larger drill rigs on production blasting.

These smaller units are designed to quickly drill smaller diameter boreholes to relatively short depths. This is an ideal combination for leveling large areas in preparation for the larger production drills. The larger units, working on a prepared bench, quickly bang the benefits of larger patterns, higher daily production rates and the lower costs of bulk explosives to the prepared portion of the project while the lighter units are freed up to extend the bench preparation.

In the majority of civil engineering projects, at least two drill types are required to cover the full range of drilling requirements. This combination can be as simple as a light track drill for pioneer work followed by a light rotary or DTH rig for production blasts, or it may require a machine in the final excavation limits. Whichever combination is chosen, it must be sequenced to meet advance requirements in the various drilling applications in such a manner that no one stage of drilling suffers inaction. A common practice to ensure that no delays are incurred on the following work is to have drill capacity approximately 20% more than excavating capacity.

In open pit blasting operations, the main focus is done with large rotary drills, but even these operations require smaller units to handle the occasional oversize boulder or high bottom on the pit floors. In addition, more and more mines and quarries are electing to pre-shear the final walls to maximize recovery and improve safety. The mine operator should be able to specify and obtain a medium drill rig that would satisfy oversize, high bottom and pre-shear requirements.

For underground drilling operations, access plays a major role in defining the type of drill that will be used. Jacked drills are still the machine of choice in narrow drifts or box passes simply because the cost of developing accesses to permit the use of more sophisticated drill rigs is prohibitive. Large, competent ore bodies are more conducive to the larger headings and advances required for jumbo and ring drill operations.

### Anticipating Drilling Energy-Infrastructure Requirements

The investment in drilling equipment and the energy infrastructure to operate this equipment should match the actual production requirements. Energy costs are becoming more important in underground drilling operations, and for that reason it may be more economical to use hydraulic rigs, even for relatively low production requirements, than pneumatic units. This is more obvious in high altitude operations where compressed air becomes quite expensive as a power transfer medium. Electric-hydraulic jumbo rigs will definitely drill much faster than simple pneumatic ones, but come at a much higher capital cost. In some instances, a simpler pneumatic drill, operated at a constant production level, will be more economical than a powerful hydraulic unit that will only be operated to 50% of its capacity. This is a decision best made at the Mine Planning level, as future operation needs may play a role in the choice of drill.

---

## FACTORS AFFECTING DRILL PERFORMANCE

Many blasting crews have limited exposure to the drilling operations, and as such are only aware of problems when a loading operation is stopped due to delays in the drilling. The blaster-in-charge should take the time to determine the causes of the drilling delays, as most of these causes will have a direct impact on the quality of the drilling. The quality of the drilling, in turn, has a direct impact on the level of control the blaster-in-charge will have on the results of the blast.

A section on the proper choice of drill type for specific applications or ranges of applications has been presented earlier in this same chapter. This discussion of drill performance makes two assumptions: (1) The drill is in good operating condition, and (2) A suitable driller can be found, or trained, to operate the drill(s) in question.

This leaves two main contributing factors affecting drill performance: (1) Suitability of site preparation for efficient drilling, and (2) The suitability of the drill tools available to the job at hand.

### Underground Site Factors

As noted above, the use of high production rig will require larger accesses that in turn require more ground support than smaller section drifts. Tight curves and steep inclines will also be important factors in rig access to headings and stopes.

A general trend over the years has been to use larger and more powerful hydraulic drillers, primarily with the intent of maximizing drill meters (feet) per shift. Care should be exercised in the use of these larger machines, since the bit sizes normally used increase with the hammer size. Narrow ore bodies, incompetent hanging and footwalls, and heavily jointed formations are prone to overbreak and dilution when explosives concentrations associated with these large bits sizes are used. Reduced drilling costs must be balanced against the increase in ground stabilization, dilution and downgrade potential.

### Surface Bench Preparation

First and foremost, no drill can be operated efficiently in poorly prepared terrain. This is, in fact, the most common complaint from drillers. It is not uncommon for a driller to spend more time setting up on a borehole position than will be spent actually drilling the borehole. Further compounding the problems will be access to the drill rig for fuel and water trucks, or for mechanics rigs if repairs or maintenance are required.

Sometimes the terrain presents drill setup challenges as shown in figure 18.5. In these situations great care should be taken to ensure safety for the driller to drill the required borehole.

![Figure 18.5 — Drill setup in difficult terrain. (Courtesy: D. Rensari)](images/375.png)

What does this mean to blasters-in-charge? The main concern is time—most blasting operations are carried out in advance of excavation crews. Delays in drilling will impact excavation schedules and possibly payment clauses in the general contract. Blast efficiency can also be affected due to improperly completed boreholes. The blaster-in-charge should realize that if it is difficult to get the drill close to the borehole then the explosives truck won't get to it at all. This means blasters that work in difficult terrain will have to either drag loading hose or carry the charges for these problem boreholes.

It follows that proper bench preparation is not only critical to efficient drilling; it is also of tremendous importance to the blasting operation. The five bench site preparation conditions in table 18.11 are some common factors related to low drill productivity:

### Site Conditions Contributing To Low Drill Productivity

| Condition |
|-----------|
| Inconsistent or poor quality stripping. |
| Overburden quality |
| Insufficiently dewatered surface |
| Uneven rock surface |
| Steep rock surface |

*Table 18.11 — Site conditions contributing to low drill productivity.*

#### Inconsistent or Poor Quality Stripping

Inconsistent or poor quality stripping, leaving ditches and windrows that the drill must be maneuvered around, will complicate the driller's job.

#### Poor Overburden Quality

Good quality overburden soil can be used on an as sits blast site. High clay overburden material is extremely easy to drill through and keep open. Excessive overburden depths put the boreholes at risk of collapse. Most cobbles and boulders encountered in clay are hard, rounded, and well anchored and will immediately "kick" the bit off line.

Even shallow depths of overburden material that is soft, wet or sandy create problems for the driller and blaster-in-charge.

#### Insufficiently Dewatered Surface

All surfaces where water should be drained or pumped off the drill pattern, and the residual mud removed.

#### Uneven Rock Surface

If stripping all overburden results in an uneven rock surface small track mounted top hammer drills do quite well, but larger units are forced to maneuver very slowly under these conditions to avoid tipping over. Rig stability and uniformity of the pattern are often compromised.

#### Steep Rock Surface

Exposed steep rock surfaces create poor traction conditions. This condition is worsened when snow or ice is present and is particularly dangerous when the slope is toward an open face. Again, under these conditions rig stability and pattern uniformity are often compromised.

---

## Rotary Drills

Investment into rotary drill rigs for limited bit diameter ranges. These ranges are based on the pulldown, rotational torque, and air compressor capacity of the rig, and are calculated for the most common drilling conditions that the rig is expected to operate in. Bit manufacturers give a range of bit options speeds relative to the pulldown on the bit, depending on the formation type of rock being drilled. Bit manufacturers and drill manufacturers also provide guidance on drilling parameters.

All predictive equations depend on one critical condition—a massive, consistent rock type. This is rarely encountered in real-world drilling conditions, so the driller, the site conditions and the drilling tools being used will be the most influential variables in maximizing and predicting drilling rates.

In summary, the simplest method to optimize (NOT maximize) rotary drill penetration rates is to determine how much pulldown is required for the bit to advance one insert length revolution, then adjust everything else on the drill to maintain maximum rotary vibration occurs. At this point the drill release (below the RPM) until the string stabilizes again. Continue until minimum advance/revolution is possible. The resulting drill rate will be the most predictable rate for the given conditions.

---

## Downhole Hammers

Downhole hammers are simple impact mechanisms in which the impacting piston strikes directly on the top of a rock bit. Their efficiency is primarily based on two relationships: (1) piston diameter to bit diameter, and (2) impact blow energy to rock strength.

The relationship between piston diameter and bit diameter determines the efficiency of the transfer of energy from the piston to the bit. The more closely the piston diameter approaches the bit diameter, the more efficient the energy transfer. The relationship between impact energy and rock strength determines the efficiency of the use of the impact energy in breaking rock ahead of the drill.

Downhole hammers are quite efficient in harder rock formations, but are problematic in soft or smeary material, especially when water is present at the borehole. In these conditions, it is quite common to "bury" the bit in soft smeary so to become "muddied in" by rings of compacted cuttings building up in the borehole behind the hammer.

Downhole hammers are less prone to borehole deviation than top hammer drills, but are not immune. Collaring errors, excessive pulldown and high rotation speeds will result in borehole deviation, which may not be evident in the collar zone.

### Top Hammers

Top hammer drills lose from 5% to 10% of their impact energy as steels are added to the drill string; so they are primarily used for small diameter, relatively short boreholes. Impact energy is usually very high for the bit diameters used, so they have fairly high penetration rates, and for this reason they have fairly high drill string costs. Deviation is a major problem with top hammer drills due to the flexibility of the relatively small diameter drill string following the bit. High penetration rates make it more difficult for the driller to react in time to compensate for formation changes or cracks, which can desensitize penetration.

---

## DRILLING ACCURACY

It is important to refer to the very first sentence of this chapter—The primary purpose of any borehole is to allow the placement of a specific explosive charge in a specific location to achieve a specific result. In its simplest sense, this is what drilling efficiency is all about. In a more general sense, this is what blasting efficiency is all about.

The best indicator of drilling efficiency is the level of accuracy achieved by the driller. A variety of studies has been completed over the years to determine borehole deviation as a fraction of its depth. Recent studies have concentrated more on the cause of drilling deviation in an attempt to quantify and predict the effects of the most obvious variables – diameter, drill type, depth, borehole angle, geology, etc.

These studies may have missed a factor that is obvious to most drillers: The single biggest contributor to drill deviation is the push for increased drill production. The simplest solution for most operators is to emphasize accuracy over meters (feet) per shift. A common experience in the blasting industry has been that with increased accuracy, patterns can be expanded, sometimes by as much as 20%. Simple mathematics shows that 20% less drilled meters (feet) per shift for the same yield in tonnes (tons) broken produces lower drilling and blasting costs.

Techniques for effective borehole drilling are listed in table 18.12 and are discussed below. All are offered with the assumption that they are of limited value unless the effort to achieve overall borehole accuracy takes precedence over the speed at which it is drilled.

### Techniques For Effective Borehole Drilling

| Technique |
|-----------|
| Collaring |
| Casing |
| Bit wear and borehole diameter |
| Vertical vs. angle boreholes |
| Maintaining borehole accuracy and depth |

*Table 18.12 — Techniques for effective drilling.*

### Collaring

The driller rarely encounters perfect conditions for collaring a new borehole, and should be allowed to move the collar location to a better position to ensure a good start for the borehole. Figure 18.6 shows a well collared borehole in solid rock. The maximum distance the driller is allowed to "skid" the borehole should be clearly defined and any boreholes within this radius must be drilled parallel to the original direction per the blast design. Any boreholes drilled outside this radius must be drilled at an angle that ensures that the borehole will bottom out in the design position.

![Figure 18.6 — Properly collared borehole in solid rock. (Courtesy: D. Rensari)](images/378.png)

For top hammer drills, collaring should be done at half throttle, and possibly even less, and this energy level maintained until the bit is well established into solid rock. If it is evident that the bit has "kicked off", the bit should be lifted off the bottom of the borehole, and the feed moved to align directly over the socket. The feed should then be securely anchored in position, adjusted to be parallel with the original design direction and drilling recommenced. A good general practice for top hammer drills in underground drilling operations is to maintain half throttle until the first complete coupling is into the solid section of the borehole.

Most DTH drillers will advance that bit carefully until at least half the length of the hammer and then attempt to push faster, sometimes causing serious deviations below this depth. Downhole hammer drills should be run at reduced power until the entire length of the hammer is into solid rock, especially if they have been fitted with a guide sub.

Rotary rigs are a special case, since the driller often cannot see the borehole during the collaring process, and the rigs are too large to easily reposition when a borehole kicks off. For rotary drills, a stiff lead pipe with tight table bearings is often all the driller has to maintain alignment. As with the other two drilling systems, full throttle and full feed are essential for smooth bit rotation only after the bit is stabilized into massive rock directly behind the bit. Stabilizers directly behind the bit can be used to maintain this effect.

When the driller sees indications of a poorly aligned borehole (table brushing forced strongly to one side, visible bow in the drill pipe) the drill should be immediately stopped and the drill repositioned to start a new borehole. Rotary rigs are a special case, since the driller often cannot pull a borehole back onto line – so only guarantee that the borehole continues on the deviation established by the bit.

### Casing

In some cases, mostly submersive drilling, casing is a pre-requisite to keeping boreholes open and clean for loading. Often the casing is set into a socket in the rock created with an oversized bit, then the actual borehole completed from inside the casing. Rigidity of the casing and casing supports are critical for these operations, especially in strong currents or tidal waters. Figure 18.7 shows a properly installed casing tube.

![Figure 18.7 — Properly cased borehole collar. (Courtesy: D. Rensari)](images/379.png)

Casing on surface operations is normally placed after the borehole has been drilled, so has little effect on the accuracy of the borehole. However, it can be essential to maintain the borehole depth by preventing loose rock falling back. Therefore, it should be placed immediately after completing the borehole, seated tightly into solid rock and extending high enough to be easily seen by the loading crew. Surface water is normally not a concern for capped holes, mud or cuttings from around the borehole must be prevented from falling in.

### Bit Wear and Borehole Diameter

The bit is literally grinding or in the focal point of all the energy that a drill rig generates. Thus, it is understandable that wear will be highest at this contact. In the end there is a lot more rock than bit, the bit will eventually wear out. The more question is, "at what point has it reached its effective life?"

Several factors come into play in defining the minimum acceptable wear, including the type of bit. For very small diameter boreholes, such as those drilled with hand-drills or underground development rigs, this can be as little as 3 millimeters (⅛ inch). For top hammer drills, the acceptable limit may be 5 millimeters (¼ inch), and for downhole hammer rigs it may be as much as 7 millimeters (¼ inch).

The final call is based on what the blast design can tolerate, and this will also include such variables as cartridge size, borehole depth, explosive characteristics, type of initiator, etc.

Another concern is the integrity of the bit itself. Often, the steel of the bit body can wash away faster than the carbide inserts wear, introducing the risk of a loose carbide insert in the borehole. This results in the complete destruction of the bit and possibly the entire series of the borehole.

Excessive feed force and high rotation speeds will rapidly wear the gauge buttons on the bits, resulting in undersized boreholes, especially in the bottom. Since the buttons that maintain accuracy, torque, and fed force concentration is required, drillers should be encouraged to maintain careful control on feed and rotation rates.

Rotary bit wear is normally not rated on diameter loss, but on the degree of wear evident in the cones or the bearings. These can be assessed only by visual methods so the driller should check for bit wear on a visual basis and retain worn bits after wear is noted.

### Vertical vs. Angled Boreholes

In underground drilling operations, very few boreholes are drilled purely vertical, so underground applications will not be addressed in this section. The vast majority of surface blasting operations, however, do utilize vertical boreholes, with angled boreholes reserved for special cases such as pre-sheared final walls or "helpers" on the front row to pull excessive toe burden (See figure 18.8).

The primary advantage for designs using vertical boreholes is simplicity. Patterns can be laid out quickly and easily, borehole depths are solved on a two dimensional basis, and deviations are immediately apparent. There is no need for the drillers to set exact angles and bearings (azimuth) on each borehole, and generally speaking collaring is faster when the bit enters the rock at a perpendicular contact. Figure 18.9 shows three rows of vertical boreholes. The toe burden and spacing can be visualized at the collars.

![Figure 18.8 — Bench face suggesting the need for angled borehole drilling. (Courtesy: J. Brulia)](images/380.png)

![Figure 18.9 – Three rows of vertical boreholes. (Courtesy: D Rensari)](images/381.png)

From time to time an extreme toe burden distance occurs as shown in figure 18.10 where rows of boreholes of varying angles are required for toe burden control to transition to vertical boreholes for ongoing development.

![Figure 18.10 — Excess toe burden. The two rows of angled boreholes used to transition to vertical boreholes is shown in figure 18.12. (Courtesy: D. Rensari)](images/381.png)

In figure 18.11 the casings are left to illustrate the borehole angles to transition to vertical boreholes. Notice the difference in borehole collar burden distance to produce uniform toe burden. This is done to control the burden at the borehole bottom blasting a face like that shown in figure 18.8.

![Figure 18.11 — Two rows of angled boreholes to transition to a vertical face. (Courtesy: D. Rensari)](images/382.png)

The reasons for considering angled boreholes are sufficient for testing a few blasts using this technique. The most often cited advantages to angled boreholes are listed in table 18.13.

### Advantages Of Angled Drilling

| Advantage |
|-----------|
| Reduces backbreak |
| Potential for improved fragmentation |
| Creates higher post-blast headroom (floor) drilled |
| Reduces or eliminates high bottom and toe problems |
| Potential increases in powder load |

*Table 18.13— Advantages of angle drilling.*

Before attempting to design a pattern using angled boreholes the blaster-in-charge should consider the guidelines listed in table 18.14.

### Guidelines When Angle Drilling

| Guideline |
|-----------|
| Maximum angle from the vertical should not exceed 30° |
| Burden is the perpendicular distance between the boreholes at depth and the evident burden on the rock surface. The evident burden can vary widely depending on the dip of the rock surface, and the pattern will not have the traditional alignment of vertical patterns |
| Subdrill length will not need to be increased beyond the standard subdrill used for an equivalent vertical pattern |
| Drilling sequence along the lines, as opposed to along the rows is helpful in maintaining the correct bearing on the boreholes |
| Driller needs: |
| - Carpenter helper |
| - Handheld angle measuring tools. |
| Have the borehole locations surveyed in wherever possible |
| Driller must take extra care collaring the borehole to avoid having the bit "walk" away |
| Adjust collars to the same depth "below ground surface" that vertical boreholes would have |

*Table 18.14 — Guidelines when angle drilling.*

### Maintaining Accuracy and Depth

The two steps to take to maintain drilling accuracy and depth are: (1) to collar the borehole into the rock properly (several suggestions for collaring have been covered in an earlier portion of this chapter) and simpler yet (2) don't force the bit any faster than it can efficiently advance the borehole. The two most common causes for borehole deviation, after poor collaring are: (1) excessive rotation speeds and (2) excessive thrust on the bit.

Drill and drill string manufacturers have concentrated heavily on developing drilling systems and tools to minimize deviations while maintaining maximum penetration rates. These systems are costly so the reader will need to justify their benefit. These tools alone cannot guarantee a straight borehole since poor drilling practices will override these factory installed control systems.

Percussive bits will form a relatively small crater directly beneath the insert at each impact of the piston. The bit needs only be advanced enough for the insert to rest on solid rock before the next impact arrives. Adjustments to rotation speed are done to match the advance of the gauge buttons along the circumference of the borehole with the optimum distance between impacts, so keep bits must turn more slowly than smaller ones.

Excessive rotation speeds will rapidly wear a bit, causing undersized boreholes, and will pull off line almost instantaneously when a crack or a distant change in the rock formation is encountered.

The bit should slide across the bottom of the borehole between impacts without losing contact with the rock, with just enough feed force to overcome the recoil energy of the impact. Excess force will cause the drill string to flex, which will slightly change the direction of the bit. This initial deflection will quickly increase as the drill steel flexes even more to follow a curved borehole.

Rotary drills are normally leveled on jacks before cutting commences, and if good collaring techniques are used the borehole can be started on mark and on line. Heavy feed pressures and high rotation speeds, however, will also cause a rotary bit to drift especially if the pull up is considerably smaller than the bit. Stabilizers directly behind the bit can be used to minimize this effect. The simple matching of pulldown and rotation to the rock will be more effective in the long run.

Most drillers will drill an extra half to full meter (yard) beyond the design depth to compensate for cuttings that either aren't cleared from the borehole during drilling, or fall back in as the bit is withdrawn.

> **Caution**
>
> Extra drilling is not normally done when blasting coal or coal overburden.

A simple technique to alleviate both these problems is to slow the drill down during the last one or two meters (yards) of the borehole. This will allow the flushing air to clear any suspended cuttings and reduces the quantity of loose cuttings around the collar. With this technique, the air can often be shut off entirely while withdrawing the bit. This further reduces the disturbance of loose material trapped in cracks in the borehole wall or on the inside of the cuttings pile at the collar of the borehole.

---

## INDICATORS OF POOR DRILLING

The first suspicion of poor drilling practice frequently doesn't arise until after a poor blast has occurred.

### Best Indicator Of Effective Drill Performance

> *The best indicator of a proper balance between drill energy, bit type, flushing air volume and ground conditions is a clean, consistent cutting pile at the collar of the borehole.*

At this point the drilling is only one of four possible causes, and must be weighed against: (1) blast design, (2) blast loading and tie-in, (3) explosives performance, and (4) the principal cause of the poor results. This generally results in heated debates by the various blasting stakeholders (i.e. blast designers, blasters-in-charge, explosives suppliers, and drillers). In the end the usual result is to blame the geology. Some immediate indicators of poor drilling practice (and/or poor design, poor loading, poor tie-in and/or poor product performance) after a blast are listed in table 18.16.

### Poor Blasting Result Indicators

| Indicator |
|-----------|
| Variable fragmentation |
| High Suction |
| Excessive backbreak |
| High ground vibration |
| Excessive (unexpected) |
| Tight digging |
| Excessive blast fumes |

*Table 18.16 — Poor blasting result indicators.*

The first section of this chapter clearly states how to avoid problems with drilling quality and if properly implemented can eliminate drilling practice as a possible cause of poor blast results. The following guidelines are therefore suggested as proactive measures to identify poor drilling practice.

Indicators for the loading crew are given in table 18.17.

### Preblast Indicators Of Poor Drilling

| Indicator | Possible causes |
|-----------|-----------------|
| Boreholes out of pattern alignment | Something hit a unconvertible. Check to verify if angled properly to compensate. |
| Borehole deviates from the design angle | If collared on the mark, this results in problems at depth. Check to verify if the borehole was deliberately angled to compensate for being off the mark. |
| Shallow boreholes | Very verify if cuttings or debris have built up below the borehole. |
| Too deep borehole | Not marked to the rod specification. |
| Fine powdery cuttings at the collar indicate one or more of the factors listed | Insufficient flushing air. |
| | Operating with excessive rotation. |
| | Worn or undersized bits. |
| Cratering around the collar | Operating with excessive pulldown, especially during the collaring. |
| A circular ridge or annulus (good drilling practice) minimizes the effect. | |
| Excessive cutting indicates one or more of the factors listed | Insufficient flushing air |
| | Excessive rotation |
| | Worn or undersized bit |
| | Excessive pulldown and feed that has to bored, or stick steels or drill pipe | Incorrect pulldown and/or feed settings. |
| | Borehole or string drag in the borehole |

*Table 18.17 - Preblast indicators of poor drilling.*

The borehole loading method can provide indications of drilling quality as summarized in table 18.18.

### Indicators of Poor Drilling During Loading—Column Rise By Loading Method

| Loading Method | Comment |
|----------------|---------|
| Bulk loading - column rise is consistently faster or slower than anticipated | Check the bottom condition to verify the comment |
| | Verify the proper kit was used |
| Conventional loading - Cartridges jamming in hole, or charge not consistent with cartridge count | USIG proper package diameters are verified for the bit diameter chosen, then check the borehole condition. |

*Table 18.18 - Indicators of poor drilling during loading—column rise.*

Poor drilling practice results in high drilling costs. Therefore it is to the drilling contractor's advantage to work closely with the blasting crews to eliminate drilling problems. The two most obvious indicators that controls are required are: (1) high repair and maintenance costs on drill rigs and (2) high drill string costs. These directly affect the profitability of the current project, but will also follow into the next project. Some common indicators for the drilling supervisor of poor drilling practice are listed below in table 18.19.

### Common Drilling Quality Indicators For The Drilling Supervisor By Drill Type

| Drill Type | Indicator |
|------------|-----------|
| Downhole | Indicator of worn gauge |
| | Button wear on rib or outside-wear buttons |
| | Indications of button contact on the bit face (for thinned) that |
| | Cracked or chipped carbide |
| | Coupling failure |
| | Thread damage or prematurely worn free threads |
| | *Handle - use of sub |
| DTH rigs | Bellows wear |
| | Bit wear, or stack bit (may hit pin or inner) |
| Rotary | Bearing wear |
| | Cone chip wear |
| | Change teeth broken |
| | Shirttail damaged (used as stabilizer) |
| | Roller cone bearing failure |

*Table 18.19 — Common drilling quality indicators for the drilling supervisor by drill type.*

In summary, both the blasting crew and the drilling contractor will be negatively affected by poor drilling practice. Every effort should be made by both parties to mutually identify problems areas in the drilling practice, and correct them immediately.

---

## FACTORS AFFECTING DRILL OPERATION ON THE BLAST SITE

For the purpose of this section the blast site is defined as a previously or partially drilled pattern in which explosives loading operations have commenced. The delineation can be extended to include any drill pattern in which boreholes are confirmed or suspected. The latter situation is extremely hazardous, and warrants special attention on the part of the drilling and blasting personnel.

A common practice is to keep a drill in reserve for redrill holes throughout or to expand the loading operations. Redrilling on a site during loading, introduces risk factors that might not be acceptable on a regulatory level. The methods outlined in table 18.20 help identify hazards in drilling operations.

### Methods To Mitigate Hazards On Drilling Sites

| Method |
|--------|
| Complete an operations review with all the drillers and blasters involved and document all hazards and the procedures to eliminate or control them. |
| All relevant local, state and federal regulations pertaining to the drilling operation must be included in the development of these procedures. |
| Formulate, from all the requirements that have been assembled, the minimum acceptable distance between new boreholes and any loaded boreholes that they must be drilled close to. |
| All communications should be targeted to arrive at a new borehole for within three (3) times the maximum deviation as previously determined. |

*Table 18.20 — Methods to mitigate hazards on drilling sites.*

Essential steps to prepare the blast site for a safe and successful drilling operation are given in table 18.21.

### Steps To Prepare The Blast Site For Successful Drilling

| Step | Pattern preparation |
|------|---------------------|
| 1 | Gather information |
| 2 | Before drill move | Prepare to advise undetonated and preferably straight line access for the drill to required borehole locations. |
| 3 | | Clearly mark all blocked boreholes to ensure visibility to driller. |
| | | Mark radius of all blocked boreholes in a different manner from unloaded boreholes. |
| 4 | | Clearly demarcate loaded section (e.g. stakes, tape) |
| 5 | Explosives and accessories | Remove from marked drilling area and access routes. |

*Table 18.21 – Steps to prepare the blast site for successful drilling.*

### Drilling Area Responsibility

> *The responsibility for who will control the drilling operations should be clearly defined.*

Normally this responsibility belongs to the blaster-in-charge, but in some regulatory jurisdictions a competent individual can be delegated. This individual should direct the driller when moving within the loaded portion of the pattern, and as to the location, direction and angles of the boreholes.

Drilling activity causes on the blast site are summarized in table 18.22 along with actions to take. The blaster-in-charge is responsible for the elimination of such hazards.

### Issues Affecting Safety And Effective Drilling On The Drill Site and Methods To Improve Them

| Issue | Method |
|-------|--------|
| Multiple drills inside the blast area | Keep apart and under separate control |
| Dust and noise | Always use suppression practices on the drill to... |
| | Shut off drill temporarily when loading/priming to minimize distracting the driller and the person directing the drilling |
| Drill movement with mast cranked | Prevent contact with bulk loading and stemming equipment |
| Electric drills | Route trailing cables from damaging |
| | Allow additional time in-area drills for an electrical storm |
| Drill breakdown | Communications to the intent is at drill breaks down or catches fire |

*Table 18.22 — Issues affecting safety and effective drilling on the drill site and methods to improve them.*

The material on dust and noise control can be tabulated as indicated in tables 18.26 and 18.27.

### Dust Control Features For Drill Operation

| Feature | Person Affected |
|---------|-----------------|
| Cabin with forced air ventilation | Driller |
| Water injection dust collector | Driller |

*Table 26.18– Dust control features for drill operation.*

### Noise Abatement Features For Drill Operation

| Feature | Operating component |
|---------|---------------------|
| Muffler | Top hammer, Pneumatic |
| Insulated engine compartment housing | Engine noise |
| Blasting protection | Noise from steel striking rock |

*Table 18.27 — Noise abatement features for drill operation.*

---

## DRILLING FOR SURFACE BLASTING OPERATIONS

Surface borehole drilling is the most complex and critical, and can be most hazardous, of all applications addressed in this chapter. Whether close for underground construction or mining, the problems and solutions are always in three dimensions and are often "invisible" until after the blast has been initiated. In addition, the blasts are normally highly confined and drill setup is complicated by restricted accesses and generally difficult working conditions. The majority of underground drillers take great pride in the quality of their work.

There are three principal drilling applications for underground blasting operations: (1) Development blasting—done to gain access to production zones (2) Production blasting—done to remove the highest yield of rock or mineral at lowest cost, and (3) Contour control blasting—done to minimize overbreak in areas that will be required to stand open for long periods of time.

These applications can be either independent of each other, or they can be combined in the normal drilling and blasting sequence. Chapter 35 discusses and illustrates the various underground blasting methods.

### Development Blast Drilling

This section discusses the common cuts of: (1) burn, (2) drift, (3) raise, and (4) shaft advances that are applicable to headings that are blasted "full face." In large tunnel, shaft, or slope developments involving double or multiple pass techniques the initial pass will probably fall under this section, with subsequent passes using production techniques as presented below.

A general guideline for drilling control is that the maximum acceptable deviation is 10% of the burden. This applies to a drift round or tunnel heading as shown in table 18.28.

### Maximum Acceptable Drilling Deviation In Drift or Tunnel Rounds

| Maximum acceptable drilling deviation in drift or tunnel rounds. Advance: 4 meters (13 feet). |
|------|
| Advance 4 Section (13 feet) |

*Table 18.28 — Maximum acceptable drilling deviation in drift or tunnel rounds. Advance: 4 meters (13 feet).*

The first example in table 18.28 represents the production or "stoping" boreholes that break the main volume of the blast. The second example applies to the cut or "opening" boreholes in the same blast. Dimensions are taken for a common parallel (sometimes called "burn") cut used in drift and tunnel advance.

It should be quite obvious where the most stringent drilling control must be maintained. The design distance is the cut between centers of the opening (relief borehole(s)) and the nearest charged borehole is normally between 1.5 and 1.75 times the diameter of the relief borehole(s). Any deviation outside these values puts the cut at risk of failure—which in turn will guarantee a poorly performing round.

Demanding a drilling accuracy of 1.5 centimeters (0.6 inch) in a 4 meter (13 foot) deep borehole under the normal conditions encountered in development drilling is impractical. To compensate for this most cuts are ever designed, ensuring up to 50% more relief than is theoretically required for the advance.

### Look-out

Another critical control that must be maintained by the driller is the "look-out" on the perimeter boreholes of the round. Look-out is the distance the borehole is directed outward from the collaring point to ensure a clean break to slightly outside the pay line at the back of the round. This extra width and height is to assure that on the following round the driller (drill) will be able to complete the perimeter boreholes without becoming ledged against the ribs (walls) or back (roof).

Control of look-out is especially important in trenching operations to minimize excess concrete costs, but can be equally important in mine accesses where ore dilution or ventilation are concerns. The driller must exercise the same control on the lifter (floor) boreholes as on the other perimeter boreholes. This is commonly overlooked and can result in excessive burden on the back of the lifters, which will produce a shorter advance on the floor and leave a tighter muckpile.

### Face curvature

As headings advance the face can take on a curvature, with the forward part of the curve in the center of the face and the following arm(s) on the ribs, back, or floor. A simple method to monitor and correct this condition is to snap two string lines across the face at the horizontal and vertical centers of the heading. It will quickly become obvious how much the depths of the boreholes in the next round will have to be adjusted to bring the heading back to "square".

One of the more serious risks in any drilling and blasting sequence is to drill into explosives left from the previous blast. In development work, by definition every single drilling cycle will be faced with this risk. The risk is exacerbated by the sheer number of boreholes that are drilled in a normal round—a surface operation might require one or at most two boreholes to liberate 30 meters³ (39 yards³) of rock, a tunnel heading may require thirty or forty. Under no circumstances should drilling commence until the face has been thoroughly cleaned and all sockets (sometimes called "bootlegs") from the previous round identified and marked.

A common practice in drift and tunnel drilling is to have all stoping boreholes looking slightly up to facilitate draining of the flushing water and to leave a clean dry borehole for the loading crew. If standardized, this practice can also help minimize the risk of collaring a borehole into a bootleg. Most governmental jurisdictions stipulate a minimum distance that must be maintained between a bootleg and a new borehole. It is suggested that this distance be incorporated into the drilling standards in such a way that boreholes are collared "on the muck side" of the target spot and "the muck" drilled away at a proper angle – no one ever suggested that bootlegs should be collared to the inside end.

### Production Blast Drilling

Many modern production operations are essentially a sequence of initial development of an opening that is then expanded using the same drilling equipment and bits. These would include cut and fill, shrinkage, drift mining, smaller room and pillar applications, and large tunnel projects where point burnuts are driven then expanded using benching techniques.

Patterns are normally the same as the larger patterns used in the development blasts, and boreholes are drilled parallel to and to the same depth as the original development boreholes. Drilling thus are essentially the same as has been mentioned during the drilling of the development blast.

The remainder of this section will concentrate on what are called long-hole production drilling applications. These are defined as production drilling in which more than one extension drill steel is used to achieve the required depth, and which is done using methods other than development drilling.

Drilling accuracy in the shorter boreholes is less problematic due to the larger patterns used, but becomes extremely important as the borehole depth increases.

The targets for long-hole drilling accuracy are usually set as a percentage of the borehole depth, with 2% being the maximum acceptable and 1% assumed to be the minimum consistent deviation achievable. Borehole depths will run from 5 to 50 meters, and sometimes even more. To put the acceptable deviation in perspective with production drilling requirements table 18.29 shows what percentage of the burden the possible deviation for a range depth.

### Borehole Deviation for Long-Hole Drilling (Metric units)

| Borehole Depth | Target Deviation % |
|----------------|-------------------|
| | 1% | 2% |
| 5m | 0.05m | 0.10m |
| 10m | 0.10m | 0.20m |
| 15m | 0.15m | 0.30m |
| 20m | 0.20m | 0.40m |
| 30m | 0.30m | 0.60m |
| 50m | 0.50m | 1.00m |

*Table 18.29a — Borehole deviation for long-hole drilling (Metric units).*

### Borehole Deviation for Long-Hole Drilling (U.S. units)

| Borehole Depth | Target Deviation % |
|----------------|-------------------|
| | 1% | 2% |
| 15' | 0.15' | 0.30' |
| 30' | 0.30' | 0.60' |
| 50' | 0.50' | 1.00' |
| 65' | 0.65' | 1.30' |
| 100' | 1.00' | 2.00' |
| 165' | 1.65' | 3.30' |

*Table 18.29b — Borehole deviation for long-hole drilling (U.S. units).*

As discussed in the *Development Blasting* section of this chapter, borehole deviation should be controlled to 10% or less of the design burden. Tables 18.29a and 18.29b clearly show that a long-hole's design depth is restricted by the drilling accuracy.

### Borehole Length Restriction

> *A borehole's designed depth is restricted by the drilling accuracy.*

The primary rule is to drill on the mark and on line. In underground operations this can be a serious challenge and for that reason, some of the basic methods for improving drilling accuracy that were discussed earlier in this chapter will be expanded on here.

The majority of long-hole drilling is done with the drill shell "floating" (unsupported) and this can cause the driller some serious problems when drilling under high backs or in wide or poorly prepared headings. An advance bar often overlooked is the ability of the driller to reach the drill shell to confirm whether the angle and direction are correct. Under these conditions drillers sometimes rely on a visual assessment and adjust the borehole position and angle accordingly. Long-hole drilling solutions are three dimensional, often involving deep boreholes and requiring precise control. The driller should not hesitate to call in survey control when working outside the normal parameters of drilling conditions. The cost of a failed blast easily justifies a few hours of downtime.

All feed shells on the be prone to movement due to worn feed guide shims, centalizers, pivot pins and bushing. The driller should take a few minutes to check the condition of these components and insist on having any deficiencies corrected as soon as possible.

A variety of guide tools are available to help the driller maintain borehole alignment, including special bits, couplings, and drill steels. These will be of no use if the borehole is at art started on line. Incline angle indicators are also becoming more common, but again these tell the driller the direction of the feed shell, and not of the borehole. They must be calibrated on a regular, and the drillers should be given angle indicators to check the feed shell angle from tie-in to turn as a test against the accuracy of the electronic units.

Most long-hole drilling is done with very top hammer drills, and as a result the driller is working with a relatively flexible drill string. The drill steel is often handled manually leading to a choice to use the lightest possible series of drill string, and the boreholes are finished with water, which is not always in good supply.

For these reasons it is recommended to choose bit sizes that are from 10 millimeters to 13 millimeters (½ inch) larger than the outside diameter of the couplings. Larger bits will introduce more potential for deviation and can affect the ability of the flushing water to clear the boreholes. Smaller bits will not leave enough room for cuttings to efficiently pass the couplings during flushing, and in extreme cases could wear on the exterior that the couplings won't enter the borehole.

The hanging and foot walls normally define drilling limits in at least one direction in long-hole drilling. In some cases, this limit may be a backfilled stope or a pillar blasting may be an open slope. The driller should reduce the penetration rates when approaching the inspected contact with these limits. This will give the driller a better chance to react quickly when contact is confirmed thus reducing ore dilution. It can also help avoid the somewhat embarrassing event of losing a drill string in a breakthrough borehole.

---

## Contour Blast Drilling

The basic common applications of contour control in underground applications are: (1) stope backs, (2) gallery or chamber ribs and backs, (3) tunnel and shaft circumference, and (4) long term hangings. On occasion contour control will be required in multiple access situations where extra ground support is considered too costly.

Contour blasting is expensive on a per unit yield blasted basis. This is primarily due to additional: (1) drilling, (2) name boxes, and (3) priming costs. The true economics of contour control is seen in the ground support costs required and additional ore recovered when contour blasting is properly implemented. The quality of the final contour directly affects the amount of work to be done that has been defined as critical enough to require exceptional ground stability.

The first step in assuring this level of quality in the drilling. Table 18.30 summarizes basic guidelines to improve the quality of contour drilling.

### Basic Guidelines To Improve The Quality Of Contour Blasting

| Guideline |
|-----------|
| Accurately lay out contour boreholes with a direction line painted on the back or ribs in the immediate area of the boreholes. |
| Carefully control set up and penetration rates to ensure collaring accuracy in the first meter of penetration. |
| Place a loading pole or similar device in each borehole as it is drilled to determine if the borehole has remained in alignment and on grade. |
| Mark and record any boreholes suspected of excessive deviation. It may be necessary to redrill them after consultation with the Blaster-in-charge. |
| Take extra care in bad ground conditions. Any change in ground condition should be reported immediately to the person responsible for the existing drilling plan. |
| A rock core control should be enforced on the use of boreholes made for this purpose and then the borehole shall not be used for the contour. |
| Mark and list all anomalies encountered. |
| Take extra care to thoroughly flush clean all boreholes as they are completed. |
| Patience is required when drilling the patterns |

*Table 18.30 — Basic guidelines to improve the quality of contour blasting.*

---

## Drilling for Surface Blasting Operations

### Flushing the Borehole

In the vast majority of surface drilling operations, the borehole flushing medium is air. The volume of air exiting the bit must be high enough to provide a flow velocity that will carry away the cuttings the bit produces. The velocity required is affected by the: (1) Particle density and mean size of the chips, and (2) the total volume of chips being produced. In general the required velocities will be close to the values listed in table 18.21.

### Required Air Velocity To Remove Particles During Drilling

| Air velocity (uphole) | Metric system | US system |
|-----------------------|---------------|-----------|
| Minimum | 900 | 15.0 |
| Normal | 1,800 | 30.0 |
| Normal large | 3,500 | 59.0 |
| Maximum (abrasion risk) | 4,900 | 81.0 |

*Table 18.21 - Required air velocity to remove particles during drilling.*

Insufficient air velocity results in the bit having to grind the cuttings smaller before they can be blown clear. Often these cuttings will remain in suspension in the borehole until the air flow is stopped. Both of these conditions result in slower penetration rates and accelerated bit wear.

Equations 18.1 and 18.2 show how to calculate the drill's air bailing velocity based on the volume of air.

$$v_b = 12,760 \times \left(\frac{V}{d_{bit}^2 - d_{out}^2}\right)$$ <!-- VERIFIED -->

**Equation 18.1**

Where:
- $v_b$ = Bailing Velocity (meters/minute)
- $V$ = Volume of air delivered (meters³/minute)
- $d_{bit}$ = Bit diameter (centimeters)
- $d_{out}$ = Drill steel or pipe diameter (centimeters)

### Example 18.1

Calculate the change in air bailing velocity by increasing the bit diameter from 89 millimeters to 114 millimeters on a 4.5 centimeter diameter steel with an air volume of 9.91 meters³/minute.

**Step 1**

Calculate the air bailing velocity using the 89 millimeter diameter bit.

Convert the bit diameter 89 millimeters to 8.9 centimeters. Then use equation 18.1 to determine the bailing velocity of the 89 millimeter diameter bit

$$v_{89} = 12,760 \times \left(\frac{9.91}{8.9^2 - 4.5^2}\right)$$

$v_{89} = 2,136$ The bailing velocity is 2,136 meters/minute.

**Step 2**

Calculate the air bailing velocity using the 114 millimeter diameter bit. Convert the bit diameter 114 millimeters to 11.4 centimeters. Then use equation 18.1 to determine the bailing velocity of the 89 millimeter diameter bit.

$$v_{114} = 12,760 \times \left(\frac{9.91}{11.4^2 - 4.5^2}\right)$$

$v_{114} = 1,147.2$

The new bailing velocity is 1,147.2 meters/minute.

**Step 3**

Calculate the change in air bailing velocity.

$v_{89} - v_{114} = (2,136 - 1,147)$

$v_{89} - v_{114} = 989$

The bailing velocity was decreased by 989 meters/minute, or 46%.

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$$v_b = 183.4 \times \left(\frac{V}{d_{bit}^2 - d_{out}^2}\right)$$ <!-- VERIFIED -->

**Equation 18.2**

Where:
- $v_b$ = Bailing Velocity (feet/minute)
- $V$ = Volume of air delivered (feet³/minute)
- $d_{bit}$ = Bit diameter (inches)
- $d_{out}$ = Drill steel or pipe diameter (inches)

### Example 18.2

Calculate the change in air bailing velocity by increasing the bit diameter from 3.5 inches to 4.5 inches on a 1.75 inch diameter drill steel with an air volume of 350 feet³/minute.

**Step 1**

Use equation 18.2 to determine the bailing velocity of the 3.5 inch diameter bit

$$v_{3.5} = 183.4 \times \left(\frac{350}{3.5^2 - 1.75^2}\right)$$

$v_{3.5} = 6,983$

The air bailing velocity is 6,983 feet/minute

**Step 2**

Calculate the air bailing velocity using the 4.5 inch diameter bit.

$$v_{4.5} = 183.4 \times \left(\frac{350}{4.5^2 - 1.75^2}\right)$$

$v_{4.5} = 3,733$

The air bailing velocity is 3,733 feet/minute

**Step 3**

Calculate the change in air bailing velocity.

$v_{3.5} - v_{4.5} = (6,983 - 3,733)$

$v_{3.5} - v_{4.5} = 3,250$

The air bailing velocity is decreased by 3,250 feet/minute, or 46%.

Cuttings that are held in suspension will fall to the bottom of the borehole when the driller turns off the air. A common practice to overcome this problem is to (1) maintain flushing air and rotation as the bit is retracted and (2) resume air flow and rotation as pipe/steel are removed from the string. This can compound the problem by dislodging any loose material on the wall of the borehole and allowing it to drop to the bottom of the borehole. The driller will normally overdrill the borehole to compensate for the anticipated loss in depth.

An alternative practice that should be attempted is to blow the borehole clean from the bottom, lift the bit a short distance off the bottom of the borehole and then stop the airflow. The bit can often be retracted with minimal rotation, leaving a clean, unobstructed borehole when the bit comes out.

### Borehole Quality

> *The borehole must be in good enough condition for the blaster to place the required explosive charge at the required depth in the correct location.*

It is often necessary to drill on consecutive benches in mines, quarries and on larger civil engineering projects. In bench mining there is inevitably a shattered rock zone in the original subdrill elevation of the previous blast. Rock in this subdrill zone is subject to being cratered out by the airflow when collaring new boreholes. One technique to prevent broken material from falling back into the borehole is to use heavy water injection when drilling through the broken material. When solid rock is encountered completely stop the water injection for a short period. The dry cuttings will pack into the collar cuttings in the fractured zone and form a temporary cement. This can hold long enough for the driller to insert a short length of casing after completing the borehole. Failure to provide proper borehole quality will substantially increase the potential of adverse blasting results inherent to any blasting operation. Four adverse blasting results and the drilling deviations associated with them are listed in table 18.24.

### Adverse Blasting Results Attributable to Poor Drilling

| Result | Deviation |
|--------|-----------|
| Misfires | Extreme subdrill |
| | Foreign substances |
| | Boreholes too shallow |
| Over pressure | Boreholes too shallow |
| | Insufficient burden |
| | Too close together |
| | Loaded too heavily |
| Flyrock | Boreholes too shallow |
| | Insufficient burden |
| | Overloaded |

*Table 18.24 — Adverse blasting results attributable to poor drilling.*

The recommendations in table 18.23 are offered to help the drilling crews meet the blasting crew's requirements, and minimize the potential of adverse blasting results that increase risk.

### Recommendations For The Driller To Meet Blast Design Requirements

| Recommendation |
|----------------|
| Separate the drilling by extension locations to previously drilled areas. |
| Start from the open face or the planned opening portion of the blast |
| Provide a borehole free path for survey vehicle. |
| Check accuracy of leveling indicate on a regular basis. |
| Keep survey crews access points clean of excess cuttings. |
| Adjust drill energy to match rock being drilled. |
| Flag drill boreholes in the face and crest areas area first. |
| Flag all boreholes upon completion. |
| Cover any boreholes subject to loose soil or broken rock. |
| Case any boreholes at risk of being flooded or backfilled with mud or cuttings. |
| Check off the boreholes on the pattern map as they are drilled |
| Consult with the blaster-in-charge to place any boreholes that cannot be drilled to design. |
| Check jacks for hydraulic leaks |

*Table 18.27 — Recommendations for the driller to meet blast design requirements.*

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## ADDITIONAL RESOURCES

Hustrulid, C., Scott, M. Peek, J. Zepernaki, J. 1991. Blastholes Deviations: Measurement, Mechanisms, and Impact on Drifting, International Society of Explosives Engineers (ISEE) Proceedings of the 17th Annual Conference on Explosives and Blasting Technique, February 3 – 7, Las Vegas, NV. ISEE. Cleveland, OH.

Forsyth, W., Kleine, T., Cameron, A., Iancetese Blaisthole Drilling. Golder Associates Ltd

Lyall Workman, Austin Powder

Von Oreun, H. 1997. Air Drilling Handbook, ABA publishing company. Pickering, OH.

Atlas Copco AB. 1978. Atlas Copco Manual, 3rd Edition, Atlas Copco AB. Stockholm, Sweden.

Baker Hughes. Mining Tools. 1999. Blenched Bit Handbook, Baker Hughes Mining Tools, Grand Prairie, TX.

Sandvik-tools, Robit files, Sandvik Rock Tools Bulletin HR-1754 ENG, Sandvik Tools.