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3. Rock Excavation Methods

Explosives

The type of explosive used in any project depends on the hardness and structural characteristics of the rock and the overall geometry of the cut (burden, depth, and width). There are numerous types of explosives, and for each type there are several different concentrations and mixtures. Properties to be considered when selecting an explosive include its sensitivity, density, strength, water resistance, fumes, price, and availability. Table 2 provides a general overview of explosives commonly used in the transportation industry.

Dynamite

Dynamite is the best known and most widely used explosive. It is classified according to its percentage by weight of nitroglycerin (percentages range from 15 to 60%). Strength does not increase linearly with proportion, however. For example, 60% dynamite is about 1.5 times stronger than 20% dynamite.

There are several variations in dynamite composition:

  • Straight dynamite consists of nitroglycerine, sodium nitrate, and a combustible absorbent (such as wood pulp) wrapped in strong paper to make a cylindrical cartridge.
  • Gelatin dynamite consists of a nitrocellulose-nitroglycerine gel. It is available in very high strengths (up to 90% nitroglycerin), making it useful for excavating extremely hard rock.
  • Ammonia dynamite has similar composition to straight dynamite, but a portion of the nitroglycerine content is replaced with ammonium nitrate to create more stable and less costly dynamite. It has a strength of approximately 85% of straight dynamite.
Ammonium Nitrate and Fuel Oil (ANFO)

This combination of a nitrogen fertilizer and fuel oil has largely replaced dynamite in medium and large borehole blasting. The explosiveness of ANFO greatly increases with the concentration of fuel oil (the maximum is 6%). Mixing can be done on-site or in the factory, although premixed compounds present concerns regarding handling and storage (premixed ANFO has been known to spontaneously combust when kept in storage for long periods of time). Straight ammonium nitrate can be shipped and stored the same way as any other blasting agent. ANFO does not combust well in water, but it can be sealed in bags to prevent water seepage.

Slurry (Water Gel)

Also known as a dense blasting agent (DBS), slurry is a mixture of a sensitizer, an oxidizer, water, and a thickener. The sensitizer can be any number of reducing chemicals, but is usually TNT (trinitrotoluene). The oxidizer is ammonium nitrate. The thickener is guar gum or starch. High-density slurry can remove a greater burden than ANFO, which allows for the use of smaller diameter boreholes (or wider borehole spacing) to obtain the same explosive power and fragmentation. However, the higher price of the slurry may offset the cost savings of drilling fewer holes. Slurries are reasonably insensitive, but temperature and density have large effects on this (i.e., slurries become less sensitive and less fluid as temperature decreases). Sensitivity can be increased by adding sensitizers to the composition. Slurries load about three times faster than conventional dynamite, making them more convenient and faster to use.

Table 2. Explosives commonly used in transportation projects.

Type Of Explosive Dynamite Ammonium Nitrate And Fuel Oil (ANFO) Slurry (Water Gel) Emulsion Explosives
Most Common Application(S) Most often used in smaller boreholes. Gelatin dynamites are useful for blasting extremely hard rock. Medium and large borehole blasting and cushion blasting. The most common general-purpose explosive in use today. Often used in place of dynamite because of safety and convenience. Has begun to replace dynamite, particularly in wet or submerged conditions.
Composition Straight dynamite contains nitroglycerine, sodium nitrate, and a combustible absorbent (e.g., wood pulp). Ammonia dynamites contain ammonium nitrate. Gelatin dynamites contain nitrocellulose to create the gelatinous consistency. Ammonium nitrate (a nitrogen fertilizer) mixed with up to 6% fuel oil. A sensitizer (typically TNT), an oxidizer (ammonium nitrate), water, and a thickener (such as guar gum or starch). An oxidizer solution (typically ammonium nitrate) and oil.
Strength Straight dynamite is the benchmark for explosive weight/strength comparisons. It is generally available in 15% to 60% concentrations of nitroglycerin (gelatin dynamite contains up to 90% nitroglycerin). Similar strength to straight dynamite. Stronger than ANFO, less strong than gelatin dynamite. Similar in strength to slurry explosives.
Impact Sensitivity Ammonia and gelatin dynamites are less volatile and sensitive to shock and friction than straight dynamite. Premixed compounds can be sensitive. If not pre-mixed, they are quite stable and insensitive. A lot less sensitive than dynamite but more sensitive than emulsion. More sensitive at higher temperatures. Emulsions are the least sensitive type of explosives. Cartridges are fairly resistant to rupturing during normal handling.
Water Resistance Straight dynamite has good water resistance. Gelatin dynamite is nearly waterproof. Ammonia dynamite has poor water resistance. Poor water resistance Good to excellent water resistance. Excellent water resistance.
Fumes Straight dynamite has some toxic fumes. Ammonia and gelatin dynamite fumes are less toxic. ANFO produces less toxic fumes than dynamite but more than slurry or emulsion explosives. Slurries and emulsions have a similar fume class. Slurries and emulsions have a similar fume class.
Price/ Availability Dynamite is easy to obtain and relatively inexpensive. The least expensive and most available explosive. Widely available. Less expensive than dynamite, more expensive than ANFO. Similar in both cost and availability to slurries.
Emulsion Explosives

Emulsions are a water-in-oil type of explosive consisting of microdroplets of super-saturated oxidizer solution within an oil matrix. The oxidizer is usually ammonium nitrate. Packaged in a thin, tough plastic film, emulsion cartridges have a good degree of rigidity and resistance to rupturing during normal handling but maintain the ability to rupture and spread when tamped.

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