Boulders can be broken up using either explosives or mechanical methods which can be implemented in many ways. Handling boulders hung up in crushers is inherently inefficient for the overall operation, and the techniques in handling them each have their own advantages and disadvantages.

Table 4 shows a comparison of explosive boulder breaking techniques. When these methods are used it is important to make certain that the crusher feed is empty of rock that could transfer shock energy from the explosive detonation and damage the crusher. The table summarizes economic evaluations of some blasting methods on the basis of drilling need, and type and amount of explosives needed to break a cubic yard of rock. For each method, either dynamite or binary (two component explosives) can be used. The methods will be explained in detail, but it must be emphasized that cost figures shown in Table 4 cannot adequately include the cost for any increased accident potential that could result.

This section covers techniques found in use in mines and quarries or likely to be in use. A later section describes new technology that are not presently well enough developed for use for boulder breaking.

Use of Explosives

Blockholing

Blockholing is the placing of an explosive into a shallow drill hole in a boulder. Tamping the explosive makes better use of the explosive’s energy, provided there is sufficient stemming material in the hole. Upon detonation, the explosive energy produces radial cracks about the drill-hole caused by shock waves. The radial cracks are then expanded by gases from the explosive’s chemical reaction. Increased efficiency over other methods can be achieved by the lesser amount of explosive needed to perform the work. The object is to use just enough explosive to crack the boulder with as little violence as possible.

The blockholing technique has many disadvantages. Drilling the required holes is time consuming and hazardous. The boulder, probably containing cracks from the primary blast, will be hazardous to drill because it could split without warning. Also, the drill hole can be easily overloaded adding to the danger. Even with care, damaging flyrock can be created, therefore, proper precautions must be taken as for any blasting operation.

Generally medium to low strength explosives such as 40% straight dynamite would be used for blockholing. Binary explosives for extra safety are also available in cartridge form but at a much higher cost. These explosives can be initiated with a number six blasting cap. Only a small amount of explosive is needed with this technique, and just enough explosive to cover the cap, if well confined. The objective is to dislodge the boulder, not to pulverize it.

Before any drilling is done, the boulder should be examined for stray powder and for the best hole location. A hole is then drilled to one-half the depth of the boulder or a little more. The explosive and cap are then placed at the bottom and the hole is stemmed to the top. The hole diameter should be large enough to allow ease of loading the explosive. An illustration of this method appears in Figure 10. The electric blasting cap should be placed into the explosive in the direction the shock energy is desired to travel. That is, for mudcapping, the leg wire end of the cap should be pointing up.

Mines visited that use blockholing first remove the boulder from the crusher and blast between shifts in order to reduce the danger of unauthorized personnel entering the area. The chief blaster is usually in charge of the operation. He judges where the hole should be placed, and loads the explosives. Electric blasting is practiced with 40% dynamite loaded in a 1¼ in. hole.

Snakeholing or Bombing

The method of snakeholing is not directly applicable to boulder blasting in crushers and grizzlies, but the method is analogous to placing a “bomb charge” under and tightly against a boulder. The main inefficiency would be in the lack of confinement for the explosive used in this manner because an access hole for placing the explosive is not used.

The “bombing” operation consists of wrapping explosive cartridges to a pole and placing it under the boulder. More explosives may be needed in that there is no confinement provided as in mudcapping. Explosive initiation can occur by any conventional means, though safety fuse is less desirable since the charge or boulder may shift after lighting and create a very dangerous situation.

Snakeholing may need to be used if access to the boulder from above is too dangerous or impossible. In this instance, safe access may be the major advantage to this system. The major disadvantages appear in the inefficient use of the explosive energy and in the greater possibility of damaging shock waves transmitted to the crusher. The air shock is

also very damaging to the surroundings. Also, an extra item is the pole usually needed for safely placing the explosive. Obtaining the pole creates a storage problem and extra downtime during the operation.

As with other methods, medium to low strength explosives would be used such as 40% strength dynamite. The system is very flexible and many explosive and initiating systems can be used as mentioned previously. For snakeholing, about 1 lb of explosive per cubic foot of rock opposite the charge is needed. When boulder blasting in crushers, less explosive should be used in that the boulder only needs to be cracked or jarred.

Materials needed to blast a boulder in this way are explosives, a wooden pole long enough for convenience of explosive placement, some material to tie the explosive to the pole, such as tape, rope or detonating cord, and an initiator. The explosive is fastened to the pole and the blasting cap inserted. Often the location of the explosive against the boulder is limited by safety of access or by space available. Leadwire can be stretched from the cap in the case of electric blasting, or a cap can be taped to the end of detonating cord used to fasten the explosives to the wooden pole.

Mudcapping

Mudcapping is a widely used, although not necessarily the safest, boulder blasting method. The method consists of placing an explosive on the boulder surface and then covering it with mud to provide greater confinement. The impulse action of the explosive causes breakage. The mudcap maintains the explosive detonation pressure which increases the impulse action and the explosive efficiency.

Advantages of mudcapping are evident with the reduction of downtime which is a beneficial economic factor. No drilling is necessary, but this is not offset economically due to the extra explosives needed. The elimination of drilling also provides greater safety by eliminating the driller’s exposure to falling or hazards of falling rock in the crusher hopper. Mudcapping has deleterious effects also. The method is noisy, creates flyrock and airblast, all of which are objectionable in a quarry, especially if located near a metropolitan area. Care must be taken to assure that any mud used does not contain stones or other potential projectiles. The mudcapping techniques are shown in Figure 11.

Many explosives can be used for mudcapping, but due to the inherent inefficiency of the system, a 40% straight gelatin gives a good balance between cost and high detonation pressure. Binary explosives are widely used for mudcapping for their added efficiency and safety. Explosive initiation can be accomplished in a number of ways using safety fuse, electric blasting caps, or detonating cord.

The amount of explosive needed greatly depends on the rock size. A range of 0.5 to 2 lb per cubic yard is a good estimate for use. Regulations in many states prevent initiating more than 10 lb at a time, but this amount should never be approached when blasting in the crusher. Multiple charges should not be necessary either.

In mudcapping the boulder should first be examined for a depression that can best accommodate the charge. To increase the density of the interface between the charge and the rock, the explosive can be removed from the wrapper if permitted by law, and molded in a conical pile on the rock surface. Mud could also be placed on the rock, and the explosive embedded. The cap is then embedded in the explosive which is then covered by the wrapper if the explosive is unwrapped. Ideally then, 12 inches of mud, clear of debris, is placed on the explosive so as to provide confinement.

A dolomite quarry was visited where mudcapping was practiced. The small double impact crusher in use there becomes plugged several times during the day, and Tovex 1¼ x 8 inch sticks are used to clear the rock. One half stick and an electric delay cap are utilized with one foot of mud. The crusher has a side access door which facilitates placement of explosives. The access door is wedged in place so there is little danger of it flying off during blasting. Although an experienced chief blaster is employed at the operation, the plant foreman performs the crusher blasting. Blast warnings were given by signs and verbal communication. Even though a 12 v pickup truck battery was used to detonate the electric cap (which is improper and illegal), no history of misfires existed. The mudcap method was used at this operation because the crusher’s limited access would not be conducive to other explosive methods or mechanical breaking methods.

A large crusher is planned for the future, and this is expected to reduce boulder blockage. The primary quarry blasts are well designed and utilize closely controlled drilling and loading procedures.

Blasting Mats

Blasting mats are devices for containing flyrock from a blast by catching the flyrock before it has a chance to escape. The mat is a tightly woven covering of heavy manila rope, wire rope, chain, used conveyor belt, or an old dump truck bed perforated with holes to vent the gas pressure produced by the blast.

The conveyor belt type blasting mat is inexpensive, and covers very well. It is made from three pieces of conveyor belt with the two outer pieces having a short piece of chain bolted to them. The center piece is layed out first with the two outer pieces hooked together and overlapping the first center piece. The two outer pieces are removed with a winch and a hook connected to the short chains.

In order to prevent the mat from getting blown into the air during use, one side should be anchored or fastened down. Then if the vent spaces are not large enough to relieve gas pressure, the mat can lift accordingly without being thrown from its location.