Chapter Three - Blasthole Loading

Introduction

Blasthole loading involves placing all of the necessary ingredients into the blasthole, including the main explosive charge, deck charges, initiation systems, primers, and stemming. Blasthole loading techniques vary depending on borehole diameter, type of explosive, and size. In this discussion, boreholes have been arbitrarily classified as small diameter (less than 4 inches) and large diameter (greater than 4 inches). Small-diameter boreholes may be drilled at practically any inclination from vertically down to vertically up. Large-diameter blastholes are usually drilled vertically down, but in some cases are angled or horizontal.

Electrical Storms

Blastholes should never be loaded during the approach or progress of an electrical storm.

General descriptions of blasthole procedures are in the literature.

Checking the Blasthole Before Loading

Depth Verification

Before loading begins, the blastholes should be checked. Depending on the designed depth, either a weighted tape measure or a tamping pole should be used to check that the boreholes are at the proper depth.

• If a hole is deeper than planned: Drill cuttings or other stemming material should be used to bring the bottom of the hole up to the proper level. Loading an excessively deep blasthole is a waste of explosive and usually increases ground vibrations.

• If a hole is less than planned depth: Either clean out with the drill or compressed air, or redrill. Sometimes economics or equipment limitations may dictate that a shot be fired with a few short holes. The blasting foreman should make this decision.

Obstructed Boreholes

Occasionally a borehole may become obstructed. On a sunny day, a mirror may be used to check for obstructions. Obstructions in small holes may sometimes be dislodged with a tamping pole. In large, vertical holes, a heavy weight suspended on a rope and dropped repeatedly on the obstruction may clear a hole. Attempts to clear a hole that may have become obstructed after being partially loaded should be made with caution.

Redrilling

If it is necessary to redrill a hole adjacent to a blocked hole, the blocked hole should be filled with stemming. If this is not done, the new hole may shoot into the blocked hole and vent, causing excessive flyrock, airblast, and poor fragmentation.

A hole must not be redrilled where there is a danger of intersecting a loaded hole.

Checking Holes for Water

While checking the hole for proper depth, it is convenient to check for water in the borehole. With just a little experience, the blaster can closely estimate the level of water in a borehole by visually checking the tamping pole or weighted tape for wetness after the borehole depth check has been made. To get a more accurate check, the weighted end of the tape can be jiggled up and down at the water level. A splashing sound will indicate when the weight is at the water level.

Voids in the Blasthole

A blasthole may pass through or bottom into an opening. Where this opening is not unduly large, it may be filled with stemming material (figure 31). Where the opening is too large for this to be practical, the hole must either be left unloaded, redrilled in a nearby location, or plugged.

Plugging a Blasthole

A simple method for plugging a blasthole is as follows:

1. A stick is tied to the end of a rope, lowered into the void, and pulled back up so it lodges crosswise across the hole
2. The rope is staked securely at the borehole collar
3. Bulky materials such as empty powder bags or rags are then dropped down the hole
4. Dirt is then shovelled down the hole to form a solid bottom
5. After which explosive loading can proceed

Where voids are commonplace, you may want to develop a tailormade borehole plugging device.

Hotholes

In some districts hot holes may be encountered, although this is not very common. Hot holes may occur in anthracite mining or other areas of in situ seam fires. If there is reason to suspect a hot hole, the hole can be checked by suspending a thermometer in it for a few minutes.

Explosive materials should not be loaded into holes hotter than 150 degrees Fahrenheit.

The explosives manufacturer should be consulted when loading hotholes.

General Loading Procedures

Check Rise of Powder Column

Blastholes may be loaded with bulk or packaged products:

• Bulk products are either poured into the hole, augured, pumped, or blown through a loading hose
• Packaged products are either dropped into the hole, pushed in with a tamping pole or other loading device, or loaded through a pneumatic tube

It is a good idea to check the rise of the powder column frequently as loading progresses, using a tamping pole, weighted tape, or loading hose. This will give warning of a cavity or oversized hole that is causing a serious overcharge of explosive and will also ensure that sufficient room is left at the top for stemming.

When the powder column has reached the proper location, the primer is loaded into the borehole. It is important that the wires, tubes, or detonating cord leading from the primer are properly secured at the borehole collar in vertical or nearly vertical holes, using a rock or stake.

Charge Coupling

In almost all situations it is recommended that the explosive charge be totally coupled. Total coupling means that the charge completely fills the borehole diameter. Bulk loading of explosives ensures good coupling. When cartridge products are used, coupling is improved by slitting the cartridges and tamping them firmly into place.

There are three situations where cartridges or packages of explosives should not be tamped:

1. In controlled blasting, where string loads or even gaps between cartridges are used to reduce the charge load in the perimeter holes to prevent shattering
2. In water, where the package serves as protection for a non-water-resistant explosive product
3. A primed cartridge

Stemming

It is recommended that all blastholes be stemmed to improve the efficiency of the explosive and to reduce airblast and flyrock. As a rule of thumb, the length of stemming should be from 14 to 28 times the borehole diameter.

Sized crushed stone makes the most efficient stemming. However, for reasons of economy and convenience, drill cuttings are most commonly used.

Large rocks should never be used as stemming as they could become a dangerous source of flyrock and may also damage the wires, cord, or tubes of the initiation system.

Because it is inconvenient to stem horizontal holes, horizontal rounds are sometimes left unstemmed, although it is recommended that all blastholes be stemmed to improve blasting efficiency.

Care must be exercised in using detonating cord downlines in relatively small blastholes. See "Field Application" in the "Detonating Cord Initiation" section of Chapter 2 for recommended grain loads of detonating cord for various blasthole diameters.

Wet Boreholes

One solution to blasting in wet boreholes is to use a water-resistant explosive. However, economics often favour dewatering the blasthole and loading it with ANFO inside a protective plastic borehole liner. Although dewatering has been used mostly in large-diameter holes, it can be used in diameters below 4 inches.

Dewatering Procedure:

1. A pump is lowered to the bottom of the hole
2. When the water has been removed, the hole is lined with a plastic sleeve
3. A roll of hollow plastic tubing is brought to the collar of the hole
4. A rock is placed inside the end of the tubing and a knot is tied in the end of the tubing to hold the rock in place
5. The tubing is cut off at the collar allowing 4 to 6 feet of extra tubing for charge settlement
6. The ANFO and primer are loaded inside the tubing and the hole is stemmed

Where water is seeping into the borehole, it is important that the tubing and ANFO be loaded quickly to prevent the hole from refilling with water.

Small-Diameter Blastholes

Loading Small-Diameter Blastholes

When small-diameter blastholes are loaded, the primer cartridge is normally loaded at the bottom of the hole. This gives maximum confinement at the point of initiation and also guards against leaving undetonated explosive in the bottom of the borehole if it should become plugged during loading or cut off during the blasting process.

Some experts condone, or even recommend, a cushion stick or two, but the general recommendation is not to use a cushion stick.

To avoid having the detonator fall out of the primer cartridge, the cartridge should never be slit, rolled, or otherwise deformed. The primer cartridge should never be tamped.

Cartridged Products

Cartridged dynamites and slurries (water gels) are commonly used in small-diameter blastholes. These cartridges are usually slit, loaded by hand, and tamped to provide maximum coupling and loading density. One or two cartridges should be loaded after the primer before tamping begins. Tamping should be done firmly, but not excessively. Using the largest diameter cartridge compatible with the borehole diameter will increase coupling and loading density.

Pneumatic Systems for Water Gel Cartridges

Pneumatic systems for loading water gel cartridges are available. The cartridges are propelled through a loading hose at high velocity at a rate of up to one cartridge per second. The cartridges are automatically slit as they enter the blasthole and each cartridge splits upon impact.

Because of the high impact imparted to the cartridges, loading dynamites with this type of loading system is not permitted.

Pneumatic cartridge loaders are especially useful in loading holes that have been drilled upward.

Bulk Dry Blasting Agents

Bulk dry blasting agents, usually ANFO, may be loaded into small-diameter blastholes by pouring from a bag or by pneumatic loading through a loading hose. Poured charges in diameters less than 4 inches lose some efficiency because of ANFO's low density and its reduced detonation velocity at small diameters. As with all bulk loading, good coupling is achieved.

Caution should be exercised in using poured ANFO charges in diameters less than 2 inches, and water will compound the problem.

Pneumatic Loading of ANFO

Pneumatic loading of ANFO in small holes is recommended because of:

• Ease of handling
• Faster loading rates
• Improved performance of the ANFO caused by partial pulverizing of the prills, which gives a higher loading density and greater sensitivity

The two basic types of pneumatic loading systems are the pressure vessel and the ejector or venturi-type loader.

Pressure Vessel Type ANFO Loader

A pressure vessel type ANFO loader should have a pressure regulator so that the tank pressure does not exceed the manufacturer's recommendation, usually 30 pounds per square inch. This low-pressure type loader propels the prills into the borehole at a low velocity and high volume rate, loading the ANFO at a density slightly above its poured density with a minimum amount of prill breakage.

In a pressure vessel, the compartment containing the ANFO is under pressure during loading:

• Loading rates of over 100 pounds per minute can be achieved with some equipment
• Pressure vessels with ANFO capacities of 1,000 pounds are available
• Smaller and more portable pressure vessel loaders have loading rates of 15 to 50 pounds per minute and ANFO capacities of 75 to 200 pounds
• Pressure vessels larger than 1 cubic foot in volume should meet ASME construction specifications

Venturi-Type ANFO Loader

The ejector-type system uses the venturi principle to draw ANFO from the bottom of an open vessel and propel it at a high velocity but low volume rate into the borehole, pulverizing the prills and giving bulk loading densities near 1.00.

• Ejector systems operate from line pressures of 40 to 80 pounds per square inch
• Load at rates of 7 to 10 pounds per minute
• Combination loaders are available that force-feed a venturi from a pressurized pot
• This system gives the same high loading density and prill breakage as the straight venturi loader with an increase in loading rate

The detonation velocity of ANFO as a function of charge diameter for poured and pneumatically loaded charges is shown in Figure 32. The benefits of high-velocity pneumatic loading are significant at small borehole diameters.

Small Diameter Holes, Soft Formations, and High-Pressure Ejector Loader

A problem may arise where a high-pressure ejector loader is used to load ANFO in small holes in soft formations such as uranium ore. The pulverized prills may be dead pressed by the compression from adjacent charges fired on earlier delays. This can cause the ANFO not to fire.

Static Electricity and Pneumatic Loaders

Static electricity can be a hazard when loading ANFO pneumatically into small-diameter boreholes. Static electricity hazards can be reduced by:

• Using anti-static caps or nonelectrical initiators such as Hercudet, shock tube, or Detaline
• Using a semiconductive hose with a minimum resistance of 10,000 ohms total resistance, and a maximum total resistance of 2,000,000 ohms for the entire system
• Properly grounding the pneumatic loader

General Recommendations

• Homemade loading equipment should not be used
• All equipment should be operated at the proper pressure
• Gaps in the powder column can be avoided by keeping the hopper full and maintaining a constant standoff distance between the end of the loading hose and the column of ANFO
• Loading proficiency improves through operator experience

Pneumatic Loading and Wet Holes

The pneumatic loading tube is useful for blowing standing drill water from a horizontal borehole. However, if the borehole is "making water", external protection for the ANFO by means of a plastic sleeve is required.

Loading inside a plastic borehole sleeve is not recommended for underground work because of the static electricity hazard during loading and toxic fumes generated during blasting.

If plastic-sleeve protection with pneumatic loading in well-ventilated locations is required, a nonelectric detonating system should be used because the insulating effect of the sleeve is likely to cause a buildup of static electricity.

Bulk Slurries

Slurries may be bulk loaded into blasthole diameters as small as 2 inches. These products are frequently poured from bags, but occasionally bulk pumping units are used.

The sensitivity of slurries and hence the diameter at which they may be effectively used depends largely on their formulation. The use of bulk slurries in diameters below those intended for the product can result in substandard blasts or misfires. The manufacturer should be consulted when loading bulk slurries into small-diameter blastholes.

Large-Diameter Blastholes

Blasthole Diameters Larger Than 4 Inches

With few exceptions, economics and efficiency favour the use of bulk loading in blasthole diameters larger than 4 inches. The products are cheaper, loading is faster, and the well-coupled bulk charge gives better blasting efficiency. As described in the "Priming" section of Chapter 2, large-diameter blastholes may be top, center, or toe primed, or multiple primers may be used.

Packaged ANFO Products

Large-diameter dynamite cartridges are seldom used today except for occasional use as primers. ANFO and slurries are better economically in large-diameter blastholes.

When wet blastholes are encountered, the operator can use water-resistant polyburlap packages of partially pulverized, densified ANFO (figure 4 and 5). Densification is necessary so that the packages will sink in water.

ANFO packages should be carefully lowered into water-filled holes rather than dropped, because a broken bag will result in:

• Desensitized ANFO
• An interruption in the powder column
• Most likely, some unfired ANFO

A disadvantage of waterproof ANFO packages is that some borehole coupling is lost. Also the heat lost to the water will reduce the energy released. Where it is desired to use ANFO in wet boreholes, the option of borehole dewatering should be investigated.

Packaged Slurry Products

Slurries are available in polyethylene packaging in diameters up to 8 inches. Some of these products are semirigid and others are in dimensionless bags that will slump to fit the borehole diameter. With semirigid cartridges, the advantage of borehole coupling is lost.

Bulk Dry Blasting Agents

Bulk loading offers significant advantages over loading of packaged products in large-diameter blastholes, including:

• Cheaper products
• Faster loading
• Better use of the available space in the borehole

Bulk Loading Systems for ANFO

The bulk ANFO or prills are stored in overhead storage bins, from which they are loaded into the bulk trucks. The ANFO may be trucked to the blast site in premixed form or the oil may be metered into the prills as they are placed into the blasthole. Bulk loading systems for dry blasting agents (ANFO) may be auger or pneumatic type.

Auger Loading of ANFO

Auger loading gives the fastest loading rates. A side-boom auger is satisfactory for loading one row of holes at a time. Where it is desired to reach more holes from one setup, an overhead-boom auger with a 350 degree radius of swing can be used.

With this type of equipment, flexible tubing usually extends from the end of the auger boom to ground level. The amount of blasting agent delivered into the blasthole is sometimes indicated on a meter in the truck. In other situations a hopper with a given volume of capacity is hung at the end of the auger boom to measure the ANFO as it is loaded.

Pneumatic Loading of ANFO

Pneumatic loading is also used in large-diameter boreholes. Pneumatic units are especially useful in rough terrain, where a long loading hose is used to load numerous blastholes from a single setup.

Hand Pouring ANFO

Hand pouring ANFO from 25 kg bags is still practiced at operations where the expense of a bulk system cannot be justified. This, of course, gives the same complete coupling as bulk loading.

Bulk Slurries

Bulk slurry pumping is commonplace in large-diameter vertical-hole blasting. Some slurry trucks have pumping rates of 200 to 400 pounds per minute. A bulk slurry truck may bring a plant-mixed slurry to the borehole or it may carry separate ingredients for onsite mixing.

Onsite Slurry Mixing

Onsite slurry mixing is more complex than ANFO mixing and is usually done by a competent explosives distributor rather than the user. Plant mixing permits closer quality control in the blending of ingredients, whereas onsite mixing permits different energy densities to be loaded from hole to hole or in different locations within a single hole.

The slurry is pumped as a liquid and a cross-linking ingredient is added just as the slurry enters the loading hose. Cross-linking to a gelatinous consistency begins in the hose and is completed in the borehole. A meter on the bulk truck indicates the amount of slurry that has been loaded.

Hand Pouring of Slurry

Hand pouring of slurry from polyethylene packages is still practiced at operations where the volume of slurry used does not justify bulk loading. Loading the entire package gives complete borehole coupling.