The International Information Center for Geotechnical Engineers

Summary of Surface Blasting and Damages with Analysis of Two Mitigation Techniques – Presplit and Smooth Blasting

4.0 - Typical Components and Terms of Blasting

Approximately eight parameters can be used to describe most blasts, despite the location or use of the blast.  These are listed in Table 4.1.  For the scope of this paper, each is only delved into enough as to give the reader basic understanding, and therefore, some aspects of blasting are left out altogether. Most of the parameters are interdependent on one another and changing one will affect the magnitude of the others.

Table 4.1: Parameters of Blasting

1.       Explosives

2.       Blast Hole Diameter and Drilling

3.       Burden

4.       Bench Height

5.       Spacing

6.       Detonation

7.       Stemming

8.       Properties of the Rock Mass

4.1. Explosives

The term explosive refers to a “chemical compound or mixture of compounds that undergoes a very rapid decomposition when initiated by energy in the form of heat, impact, friction, or shock.” (Dick 1982).  A typical law of nature is that unstable substances want to become more stable, and as such, the explosives will become so by converting into a very hot gas and releasing large amounts of energy, which causes the immense pressures and the associated shockwaves responsible for the failure mechanisms of rock. 

A list of different explosives and some of their properties and ingredient compositions can be found in Figure 4.1 (USACE 1972).  Two types of explosives can be used, low explosives and high explosives, which are defined by the velocity in which the chemical reaction takes place.  For a low explosive, the reaction moves below the speed of sound, termed deflagration, while a high explosive’s reaction occurs faster than the speed of sound, called detonation (Dick 1982).  An example of a low explosive is black powder and is the main low explosive used commercially although not very effective at fragmenting rock (Dick 1982); all other explosives used in commercial blasting are normally high explosives.  As such, detonation velocity is one of the most important characteristics in classifying an explosive.

Figure 4.1

Figure 4.1: Explosive Ingredients and Properties. USACE (1972)

Detonation velocity is a function of the density of the explosive, as shown in Figure 4.1, the less dense an explosive, the lower the detonation velocity.  Density serves another purpose; which is to determine the amount of charge placed in a hole.  Typically, the amount is given in weight per length, and this varies according to the density of each explosive and how that explosive is packed in the hole.  Density and water resistance are important, too, when a high water table is expected or blasting underwater is needed.  If the explosive is less dense than water it will not sink into the blast hole, and an explosive that is not resistant to water cannot be used at all.

Another classification of explosives used is strength, and this is typically measured based on weight or volume.  However, it does not give a good measurement of the ability of the charge to do work and can be misleading (USACE 1972).  Another important property that should be considered when blasting in enclosed spaces, like a tunnel, is fume class, which classifies explosives based on if poisonous gases are released.  Explosives classifying as one that gives off poisonous gases should not be used in tunnels but typically pose little concern to open-surface blasting.

A general formula provided by Langefors (1978) equates the quantity of charge needed to break the burden as a function of geometry, explosive, and the rock parameters.  Because these parameters are interdependent, this equation can be rearranged and some variables held as constant to define other parameters.  This is important to remember for each individual term following and in a design.

Q = f(V, K, E, h, d, s, p, u, ci)

Where: V= burden, K= bench height, E=spacing between blast holes, h= height of the charge, d= hole diameter, s= weight strength, p= density of explosive, u=detonation velocity, ci= rock characterization

4.2. Blast Hole Diameter and Drilling

The blast hole diameter is the leading factor in determining other parameters, mostly because burden is a direct function of the diameter, and then other parameters are determined from the burden (Dick 1982).  With that said, one should note that if two moderately differing diameters are loaded with the same quantity of explosive in the hole’s bottom or with an equal, uniform charge per length of the hole, then little variation is noted in breaking force between them (Langefors 1978).  While this can seem counterintuitive to the previous statement, the change in diameter size is still relevant.

What a change in diameter allows for is a greater concentration by weight of charge at the bottom of the hole or per length, therefore increasing the size of the burden that can be blasted.  Thus, as diameter increases, burden increases and so does the maximum height of the bench.  Inversely, as the diameter increases, the cost decreases to a level because it is cheaper per unit volume to drill and cheaper, bulk explosives can be used (Dick 1982).  This is limited, though, because large diameter holes will have fragmented rock that is more expensive to remove, and a compromise must be reached optimizing the benefits of large and small diameters. In addition, the size of the hole drilled is dependent upon factors such as site conditions, drill rigs and labor available, costs, and rock type (USACE 1972).  These limitations are what keep contractors from simply drilling the larger diameter holes and blasting the larger amounts of rock, which is typical of mining because they have the ability to drill large diameters and remove the coarser fragments.

Drilling can be accomplished by percussive drills, rotary drills, and various other procedures which in turn can be performed by varying drill bits and drill rigs, each with their own advantages and disadvantages.  Regardless of type and technique, the drilling accuracy is what distinguishes theory from actuality because these deviations can result in variations of burden as seen in Figure 4.2 (Langefors 1978).  Depending on the deviation, the burden can become too thin or too thick at the toe, creating the unintended new free face represented by the dotted line in Figure 4.2.  This issue compounds with multiple rows, as the effective burden of the next row increases or decreases.  Section 4.3 describes the issues related to overly thick or too thin burden lengths.  

The angle at which a hole is drilled will also affect burden dimensions, and drilling parallel to a sloped free face is often done to reduce unevenness of burden at the top and toe of the free face experienced when the drilling is vertical.  Additionally, the borehole is drilled below the intended final floor elevation.  This is termed the subdrill depth and helps to pull all the rock from the floor.  The subdrill depth ranges from from about 0.2 to 0.3 times the burden, seen as J in Figure 4.3.  This depth is reduced if angled drilling is used as a result of the burden at the toe being more easily removed (USACE 1972).

Figure 4.2

Figure 4.2: Result of Deviated Drill holes. Langefors (1978)

4.3. Burden

The burden can be defined as the distance between the drill hole and the free face.  In a typical bench blast, Figure 4.3, and a more complicated blast design, Figure 4.4, the burden is labeled as B (Dick 1982).  Burden also applies to the distance between two rows of blast holes if multiple rows are detonated in sequence.  The burden can be related for bench blasting, in general, directly to the diameter of the blast hole.  A very easy rule of thumb, accounting for some drill error, is simply for every inch of diameter, you have 1-meter of burden (Langefors 1978).  Thus, if you have a 1-inch diameter hole, your burden is 1-meter; a 2-inch diameter hole gives you 2-meters and so on.  While this mixes English units with metric units, it is easily remembered when doing initial calculations for a rough estimate. 

If the height of the bench falls below 1.8 * B, then the total burden length allowed becomes a function of diameter and height of the bench, instead of just diameter (Langefors 1978). Decreasing the height while keeping the diameter constant will decrease the burden width. Therefore, it is beneficial to have a bench height above 1.8 * B because more rock can be blasted for each detonation. Other considerations that affect burden length are multiple rows and direction of the free face.

Underestimating or overestimating the burden poses different issues, respectively. Underestimating the burden will create airblast and fly rock. Overestimating the burden, on the other hand, results in blocky fragmentation, issues at the toe, and unnecessary ground vibrations (Dick 1982). As such, a semi Factor of Safety is built into calculations of the burden to reduce the maximum value to an effective value based on typical inaccuracies of drilling (Langefors 1978). No corrections are made for underestimating, but when airblast and fly rock are detrimental, like urban environments, other precautions can be taken.

Figure 4.3 Burden spacing subdrill etc

Figure 4.3: Isometric View of Typical Bench Blasting Diagram. Dick (1982)

4.4. Bench Height

As already mentioned, bench height can have significant consequences on the burden thickness. It is measured, simply, as the depth of the drilled hole, identified as H in Figure 4.3. Typical height to burden ratios are between 1.5 to 4.0 (Dick 1982). While this is below the 1.8 recommended by Langefors, a ratio of 1.5 is more easily remembered and variability is expected. Going below a ratio of 1.5 will have the effect of producing excessive airblast and fly rock as well as uneven or poor fragmentation of the burden (Dick 1982). A ratio greater than 4.0 leads to a greater chance of the drill hole at the bottom of the bench height deviating from its intended position, or the explosive charge being cutoff along the length, resulting in unexploded charges (Dick 1982). In situations where location and geometry necessitate an undesirable height, solutions are available to correct for the above disadvantages.

4.5. Spacing

In Figure 4.3, the spacing is seen as S between the blast holes for a single row. However, as seen in Figure 4.4, the blast patterns are not always so simple, and for those cases spacing is perpendicular to the burden. Like the previous parameters, typical values are given as a function of other parameters, in this case a ratio of spacing to burden, where a good first estimate is 1.5 (Dick 1982). Spacing is also affected by the timing of detonation, and a single blast can have delays between multiple rows or delays among individual blast holes in a single row. If no delay is used, the simultaneous detonation of a single row allows for a spacing to burden ratio of about 2.0 (Dick 1982). When blasting for a final face, spacing is important because too great a spacing will create a wave pattern along the face due to incomplete fracturing while too small of spacing creates its own issues by cratering and crushing rock between the drill holes (Dick 1982).

 Figure 4.4. Staggered spacing

Figure 4.4: Plan View of Sapcing and Burden for Different Free Face Orientations. Dick (1982)

4.6. Detonation

The process of initiating the main charges begins with an electrical surge sent into a blasting cap. The blasting cap then ignites a detonating cord or the main charge itself. A primer, or catalyst, is another explosive set off by the blasting cap or detonating cord that quickens the reaction and increases the efficiency of the main blast (USACE 1972). Detonation of blasts is measured on the same order of magnitude as the chemical reaction and mechanical fracturing of the rock, typically in milliseconds.

With such a minute time-scale, the sequence in which the charges are detonated affects the overall blast. As was mentioned in spacing, detonation of all charges in a single row at once allows for greater spacing, due to the energy being released simultaneously. However, the use of delays from row to row and between individual blast holes is beneficial because it creates free surfaces for the next charge to take advantage of, will optimize fragmentation, and reduces the ground vibrations. Reduction of ground vibrations is proportional to reduction of damage to the rock mass, as well as reducing damages to surrounding structures.

4.7. Stemming

A blast hole is loaded intentionally with space between the top of the charge and the opening of the hole. The opening at the top is called the collar, T in Figure 4.3. The space is then packed with an inert substance to confine the charge below, known as stemming (Dick 1982). The additional confinement focuses the blast energy into the rock mass and in so doing, reduces airblast which, in regards to the surrounding neighbors' perceptions of blasting, is vital because the violent noise from airblast can cause an influx of complaints (Dick 1982). A median must be reached between too little a stemming distance and too great, similar to the other parameters. The result of a stemming too small creates flyrock and airblast, part of the issue with short bench heights, and too large a stemming distance will result in boulders at the top of the burden as well as overconfinement. A rough estimate provided by Dick gives a good initial estimate for stemming distance equal to 0.7 times the burden (Dick 1982).

4.8. Properies of the Rock Mass

Properties of the rock mass play a vital role in determining all of the previous parameters. Looking at it from the perspective of the mechanics of a blast, strength and degree of jointing and fracturing affect the way it fails. As discussed earlier, the compressive strength is much greater than the tensile strength, and the ratio of the two is a called the blasting coefficient. Values of the blasting coefficient can be 9 to 15 for quartzite, between 10 and 17 for most igneous rocks like granite and basalt, and range from 17 to 23 for sedimentary rocks such as limestone (USACE 1972). The blasting coefficient is a good indicator of the degree of spalling, with a higher number indicating greater spalling in the rock.

Another parameter linking the properties of the rock mass to blasting is the powder factor, or the weight of explosive in pounds required to blast a cubic yard (USACE 1972). A dense rock requires more explosive to displace the rock, and vice versa. In addition, the hardness of a rock mass is important in determining charge. Placing too great a charge in a very hard rock can result in excessive flyrock and airblast while an over-charged blast hole in soft rock is less likely to have this happen. On the other hand, if a hard rock is under-blasted then removing the resulting material is difficult relative to a soft rock, where removal is likely to still be possible (Dick1982).

Fracturing and jointing of a rock, like other aspects of rock engineering, create planes of weakness for failure to occur. This can be beneficial to a blast if the rock mass is heavily fractured with close-spaced jointing because the development of new fractures is not needed, and a smaller amount of explosive can be used to heave the rock. However, highly jointed rocks can pose an issue for presplitting because the gases will infiltrate the joints and loosen them (USACE 1972). For more widely spaced joints, the fragmentation can become uneven and result in large blocks remaining. Adjusting the spacing is a useful way to limit this. Smaller spacing will increase the chance that individual blocks of rock will be penetrated, and in so doing, form new fractures within the block. Compared to larger spacing, the blocks formed may remain largely intact from no direct contact with the explosives and favoring of the blast energy to escape along the existing joints.

Figure 4.5 from Dick gives a visual representation of how to prevent the explosive force from penetrating the existing weaknesses, resulting in more favorable fragmentation.  This is accomplished by a careful borehole log and then stemming where the joints are thought to be (Dick 1982).  Again, these joints can be crucial in pre-splitting to prevent an unstable final face.  Alternating layers of strong and weak material behave similar to jointing because the energy will want to escape through where it is easiest, leaving the stronger material intact. 

Figure 4.5 jointing

Figure 4.5: Stemming of Planes of Weakness. Dick (1982)

Add comment

NOTE: The symbol < is not allowed in comments. If you use it, the comment will not be published correctly.

Security code
Refresh
*Please insert the above-shown characters in the field below.

The Geoengineer.org Corporate Sponsors: