PRIMER & BOOSTER

 Figure 1: Primer and Booster in Borehole

PRIMER

Selection Criteria for Primer

The two most critical criteria in primer selection are primer composition and primer size. The primer composition determines the detonation pressure which is directly responsible for the initiation of the main charge. Research conducted by Norm Junk at the Atlas Powder Company had demonstrated that primer composition significantly affected the performance of ANFO charges. Figure 3 is a graph illustrating the effect of detonation pressure for a 3-inch diameter ANFO charge and response of the ANFO at various distance from the primer. It will be noted that thermal primers of low detonation pressure actually caused a burning reaction to start rather than a detonation.

Primer size is also important to obtain a proper reaction. Very small diameter primers are not as effective as large diameter units. Figure 4 demonstrates the effect of primer diameter on ANFO response in 3-inch diameter charges at various distances from the primers. This research conducted by Norm Junk from Atlas Powder Company, indicated that small diameter primers become ineffective regardless of the composition of the material used.

 Figure 2: Effect of Primer on Velocity of Detonation (VOD) - Olofsson

Figure 3: Explosive Composition and Primer Performance (after Junk)

Figure 4: Primer Diameter and Primer Performance (after Junk)
 

Note:

The steady state velocity depends on the density, the confinement and particle size of ANFO as well as the blasthole diameter. The VOD increases as the blasthole diameter increases and reaches its highest value at a blasthole diameter of 300 mm as shown in table below.

Table 1: The Steady State Velocity of ANFO for Different Blasthole Diameters

Primer Selection Guideline

The following are some general guidelines for priming:

1. The detonation pressure of a primer must be above the level necessary to cause the main charge to detonate at or above its normal velocity. The specific gravity and confined detonation velocity can be used as indicators of detonation pressure if detonation pressure values are not available. A primer that has a specific gravity of approximately 1.2 with a confined detonation velocity greater than 15,000 ft/s would normally be adequate when priming non-cap sensitive explosives, materials such as ANFOs, blasting agents and most water gels. This combination of density and velocity produces a detonation pressure of about 60 kilobars. For explosives such as emulsions, which would detonate at higher velocites, more energetic primers would produce better results. A specif gravity of primer of 1.3 with a confined detonation velocity greater than 17,000 ft/s would be adequate to more quickly achieve the explosive's normal velocity. This combination of density and velocity produces a detonation pressure of about 80 kilobars.
2. The diameter of the primer should be larger than the critical diameter of the explosive used for the main column charge.
3. The primer must be sensitive to the initiator. A wide variety of the products are used as primers. These primers have different sensitivities. Some may be initiated by low energy detonating cord, while others may be insensitive to these initiators. It is important that the operator understand the sensitivity of the primer to ensure that detonation in the main column charge will properly occur.
4. The explosive in the primer must reach its rated velocity of detonation within the length of the cartridge. If this is achieved, then additional cartridges of primer explosive serve no useful purpose.
5. For most blasting applications, no more than two primers per blasthole are needed. The second primer, although technically not needed, is commonly used as a backup system should the first primer fail or fail to shoot the entire charge.

BOOSTER

Booster are used to intensify the explosive reaction at a particular location within the explosive column. Booster are sometimes used between each cartridge of detonating explosive to ensure a detonation transfer across the ties of the cartridge. This is normally a poor excuse for the use of boosters, since booster cost can be considerable. The selection of an explosive in a cartridge which would not require a booster between each cartridge may be a more economical solution.

In general, boosters are used to put more energy into a hard layer within the rock column. They are sometimes also used to intensify the reaction around the primer which will put more energy at the primer location. This is commonly used when primers are near the bottom of the hole, since the bottom of the hole is the hardest place to break. Using a booster at hole bottom normally allows the increase in the burden dimension and better breakage at the toe of the shot. Boosters can be made of similar explosive materials as primer. Their sole function is to place more energy at point locations within the explosive column.

 Figure 5: Primers/Boosters

SLURRY EXPLOSIVES

A slurry explosive is a mixture of ammonium nitrate or other nitrates and fuel sensitizers which can either be a hydrocarbon or hydrocarbons and aluminum. In some cases explosive sensitizers, such as TNT or nitrocellulose, along with varying amounts of water are used (Figure 1). An emulsion is somewhat different from water gel or slurry in characteristics, but the composition contains similar ingredients and functions similarly in the blasthole (Figure 2). In general, emulsions have a somewhat higher detonation velocity and in some cases, may tend to be wet or adhere to be blasthole causing difficulties in bulk loading. For discussion purpose, emulsions and water gels will be treated under the generic family of slurries.

Figure 1: Slurry Formations

Figure 2: Emulsion Explosives

Slurries, in general, contain larger amounts of ammonium nitrate and are made water-resistant through the use of gum, waxes, cross linking agents or emulsifiers. A number of varieties of slurries exist, and it must be remembered that different slurries will exhibit different characteristics in the field. Some slurries may be classified as high explosives while others are classified as blasting agents since they are not sensitive to a number 8 blasting cap. Slurries functioned as blasting agents can be delivered as separate ingredients for on-site mixing. The separate ingredients brought to the job site in large tank trucks is non-explosive until mixed at the blasthole. The bulk loading of slurries can greatly reduce the time and cost of loading large quantities of explosives (Figure 3). However, slurries can be broken down into two general classifications, cartridge and bulk.

Figure 3: Slurry Bulk Loading Truck

Cartridged Slurries

Cartridge slurries come in both large and small diameter cartridges. In general, cartridges less than two inches in diameter are normally made cap-sensitive so that they can be substituted for dynamite. The temperature sensitivity of slurries and their lower sensitivity can cause problems when substituted for some dynamite applications. The blaster must be aware of some of the limitations before he tries a one-for-one substitution. The larger diameter cartridge slurries may not be cap-sensitive and must be primed with cap-sensitive explosives. In general, large diameter slurries are the least sensitive. Cartridge slurries are normally sensitized with monometholamine nitrate or aluminum, and air sensitized in the case of emulsions. Air sensitizing is accomplished by the addition of microspheres or entrapping air during the mixing process itself.

Figure 4: Slurry Explosive Manufactured by Gulf Oil Corporatin Limited

Bulk Slurries

Bulk slurries are sensitized by one of three methods. Air sensitizing can be accomplished by the addition of gassing agents which after being pumped into the blasthole produce small gas bubbles throughout the mixture. The addition of powdered or scrap grade aluminum to the mixture also increases sensitivity. The addition of nitrocellulose or TNT to the mixture will sensitize it to initiation. Slurries containing neither aluminum nor explosive sensitizers are the cheapest. They are often the least dense and the least powerful. In wet conditions where dewatering is not practiced, low cost slurries offer competition to ANFO. It should be pointed out that these low cost slurries have less energy than ANFO. Aluminized slurries and those containing significant amounts of other high explosive sensitizers produce significantly more energy and are used for blasting harder dense rock. The alternative to using high energy slurries is pumping blasthole, where possible, with submersible blasthole pumps (Figure 5 & Figure 6) and using polyethylene blasthole liners within the hole with ammonium nitrate as the explosive (Figure 6).  In most applications, the use of pumping with polyethylene blasthole liners and ammonium nitrate will produce blasting cost which are significantly less than would result from using higher priced slurries.

Figure 5: Pumping Blastholes

Figure 6: Legra Blast Hole Dewatering Pumps

Figure 7: Polyethylene Blasthole Liner (or Sleeve) with ANFO

Figure 8: Polyethylene Blasthole Liners

BRIEF INTRODUCTION OF HEAVY ANFO

Heavy ANFO or ammonium nitrate blends are mixtures of ammonium nitrate prills, fuel oil, and slurries. The ratio of the amount of slurry mixed with the ANFO can be changed to offer either a higher energy load or a load which is water resistant. The cost of heavy ANFO rises with increasing amounts of slurry. A disadvantage using the blends is that since the explosive occupies the entire volume of the blasthole any water in the hole is forced upward. This means that one may have to use the blend in the entire hole.

Some operators try to use heavy ANFO in wet holes, however, they don't use mixtures which contain sufficient slurry. To provide the necessary water resistance, it is recommended that at least 50% slurry be used in heavy ANFO which is to be used under wet boreholes conditions.

Figure 1: Heavy ANFO Bulk Loading Truck

Note:
Sensitivity/density of poured heavy ANFO in the blasthole can vary with the addition of microballoons or chemical gassification. In order to fulfill required density, some supplier give recommendation of lag time before stemming.

Figure 2: Example Product of Heavy ANFO

Figure 3: Another Sample Product of Heavy ANFO

DRY BLASTING AGENTS

Introduction of Ammonium Nitrate

Before learning what dry blasting agents is, we have to know what ammonium nitrate (AN) is. Ammonium nitrate production is started from reaction of anhydrous ammonia gas and concentrated nitric acid, which about 83% concentration is then produced in the AN solution. Ather that, excess water is evaporated off to leave an AN concentration of 95 to 99.9%.

A small percentage of water is left in the solution when the prills are formed. When the prills are dried, the water is removed and it leaves voids in the prills. This is where the diesel fuel goes when added to the prills.

Dry Blasting Agent or ANFO

Dry blasting agents are the most common of all explosives used today. Approximately 80% of the explosives used in this country are dry blasting agents. The term dry blasting agent describes any material in which no water is used in this formulation. Early dry blasting agents employed fuels of solid carbon or coal combined with ammonium nitrate in various forms. Through experimentation, it was found that solid fuels tend to segregate in transportation and provide less than optimum blasting results. It was found that diesel oil mixed with porous ammonium nitrate prills gave the best overall blasting results. The term ANFO (ammonium nitrate and fuel oil) has become synonymous with dry blasting agents. An oxygen balanced mixture of ANFO is the cheapest source of explosive energy available today (Figure 1). Adding finely divided aluminum to dry blasting agents increases the energy output ans also increases cost. Dry blasting agents can be broken down into two categories, cartridged and bulk.

Figure 1: Blasting Agent Formulations

Cartridged  Blasting Agents

For wet hole use, where blasthole are not pumped, an aluminized or densified ANFO cartridge can be used (Figure 2). Densified ANFO is made by either crushing approximately 20% of the prills and adding them into the normal prill mixture or by adding iron compounds to increase the density of the cartridge. In both cases, the object is to produce an explosive with a density greater than one so that it will sink in water. Another type of ANFO cartridge is made from the normal bulk ANFO with a density of 0.8. This cartridge will not sink in water, however, it is advantageous to use this type of cartridged ANFO when placing them in wet holes that were recently pumped and contain only small amounts of water.

Cartridge loading of explosives is more tedious and require more personnel since the cartridges have to be physically taken into the blast site and stacked by each hole. The cartridges are then dropped into the borehole during the loading process.

 Figure 2: Cartridged ANFO

Bulk ANFO

Bulk ANFo is prilled ammonium nitrate and fuel oil. It is often either blown or augered into the blasthole from a bulk truck. The mixed ANFO can be placed in some trucks which the dry ammonium nitrate (AN) and diesel oil (FO) can be mixed in the field as the material is being placed in the borehole. The blasting industry has a great dependence on dry blasting agents because of the large volume used. Dry blasting agent will not function properly if placed in wet holes for extended periods of time. For this reason, the blaster should know the limitation of his product.

Water Resistance of Ammonium Nitrate

Ammonium nitrate, which is bulk loaded into a blasthole, has no water resistance. If the product is placed in water and shot within a very short period of time, detonation can occur with the production of rust colored fumes of nitrous oxide. The energy produced from such detonation is significantly less than the product would be capable of producing under normal conditions. For this reason, blasthole geyser, flyrock is thrown, and other problems arise from using ammonium nitrate fuel oil mixtures in wet blastholes. If ammonium nitrate is placed in wet blastholes, it will absorb water. When the water content reaches approximately 9%, it is questionable whether the ammonium nitrate will detonate regardless of the size primer used. Figure 3 indicated the effect of water content on the performance of ammonium nitrate. It indicates that as water content increases, minimum booster values also increases and detonation velocity decreases significantly.

 Figure 3: Effects of Water in ANFO

Energy Output of ANFO

When ammonium nitrate fuel oil mixtures are made in the field, variations in oil content can easily occur. Bagged mixtures received from some distributors have similar problems. The amount of fuel oil placed on the ammonium nitrate is extremely critical from the standpoint of efficient detonation (Figure 4). To get optimum energy release, one would want about a 94.5% ammonium nitrate with a 5.5% diesel oil mixture (or waste oil filtered).

 Figure 4: Effects of Fuel Oil Content on ANFO

 Figure 5: VOD vs. Borehole Diameter Charged by ANFO

The critical diameter of poured ANFO is about 2.0 - 2.5 inches. Under this it will not detonate. If it is loaded pneumatically, the critical diameter can be increased to 1.0 inch.

THE CYCLING OF AMMONIUM NITRATE

For its weight,it supplies more gas volume upon detonation than any other explosive. In pure form, ammonium nitrate (AN) is almost inert and is composed of 60% oxygen by weight, 33% nitrogen, and 7% hydrogen. With the addition of fuel oil, the ideal oxygen balanced reaction is:

Two characteristic make this compound both unpredictable and dangerous. Ammonium nitrate is water soluble and if uncoated can attract water from the atmosphere and slowly dissolve itself. For this reason, the spherical particles, prills, have a protective coating of silica flour, which offers some amount of water resistance. The second and most important characteristic is a phenomena called cycling. Cycling is the ability of a material to change its crystal form with temperature.

The two temperatures at which cycling will occur under normal conditions are 0 F and 90 F. This is to say that products which are stored over the winter or for a period of time during the summer most likely will undergo some amount of cycling. During the summer in a poorly ventilated powder magazine or storage bin located in the sun, the cycling temperature may be reached daily. The effect of cycling on AN when isolated from the humidity in the air is that the prills break down into finer and finer particles.

The prills are made up of pseudotetragonal crystals. When the temperature exceeds 90 F, each crystal breaks into smaller crystals of orthorhombic structure. When the temperature again falls below 90 F, the small crystals break into even finer crystals of the pseudotetragonal form. This process can continue until the density is no longer near 0.8 g/cm3, but can reach a density near 1.2 g/cm3. The density increase can make the product more sensitive and contain more energy per unit volume.

After the AN has undergone cycling, the water-resistant coating is broken and the water vapor in the air condenses the particles. As cycling continues water collects on the particles and the mass starts to dissolve, recrystalizing into large crystals can occur with a reduction of temperature.

Therefore, it is evident that a volume of AN after cycling may have very dense areas and areas of large crystals. This performance of this product may range from that of a very powerful explosive to one that deflagrates or one that will not shoot at all.

Figure 1: Cycled Prills

MECHANICS OF ROCK BREAKAGE

Confined Charges in Boreholes

Three basic mechanisms contribute to rock breakage with charges confined in boreholes. The first and least significant mechanism of breakage is caused by the shock wave. At most, the shock wave causes microfractures to form on the borehole walls and initiates microfractures at discontinuities in the burden. This transient pressure pulse quickly diminishes with distance from the borehole and since the propagation velocity of the pulse is approximately 2.5 to 5 times the maximum crack propagation velocity, the pulse quickly outruns the fracture propagation.

After that, when the solid explosive is transformed into a gas during the detonation process, the borehole acts similar to a cylindrical pressure vessel. Failures in pressure vessels, such as water pipes or hydraulic lines, offer an analogy to this mechanism of rock breakage. When the pressure vessel is over-pressurized, the pressure exerted perpendicular to the confining vessel's walls will cause a fracture to occur at the weakest point in the pressure vessel. In the case of frozen water pipes, a longitudinal split occurs parallel to the axis of the pipe (Figure 1).

Figure 1: Fracture of Frozen Water Pipe

The same phenomenon occurs in other cylindrical pressure vessels due to the generation of hoop stresses. If a borehole is considered a pressure vessel, one would expect fractures to orient themselves parallel to the axis of the borehole. The major difference between pressurizing a borehole and pressurizing a water pipe is rate of loading. A borehole is over-pressurized almost instantaneously and therefore does not fail in many locations. Each resulting fracture will be oriented parallel to the axis of the borehole. Failure by this mechanism has been recognized for many years and is commonly called radial cracking (Figure 2).

 Figure 2: Radial Cracking in Plexiglass

Direction and extent of the radial crack system can be controlled by the selection of the proper distance from the borehole to the face (burden) (Figure 3).

Figure 3: Influence of Distance to Face on Radial Crack System

The second major breakage mechanism occurs after the radial cracking has been completed. There is a time lag before the second breakage mechanism goes into play. The second mechanism influences the breakage perpendicular to the axis of the charge.

Before the second breakage mechanism is discussed, form a mental picture of what has happened during the radial cracking process. Stress wave energy (shock) has caused minor cracking or microfracturing on the borehole walls and at discontinuities throughout the burden. The sustained gas pressure, which follows the shock pressure, puts the borehole walls into tension due to hoop stresses generated and causes the existing microfractures to grow. The high pressure gases extend fractures throughout the burden. The burden in massive rock is transformed from a solid rock mass into one that is broken by the radial cracks in many wedge-shaped or pie-shaped pieces. These wedges function as columns, supporting the burden weight. Columns become weaker if their length to diameter ratio or slenderness ratio increases. Therefore, once the massive burden is transformed into pie-shaped pieces with fixed bench height, it has been severely weakened due to the fact that its slenderness ratio has increased.

The work process has not yet been completed since the expanding borehole still contains very high pressure gases. These gases subject the wedges to forces acting perpendicular to the axis of the hole. One can say they are pushing towards relief or towards the line of least resistance. This concept of relief perpendicular to the axis of the hole has been known for well over a hundred years. Relief must be available perpendicular to the axis of the hole for borehole charges to function properly.

Bench Stiffness

In most blasting operations, the first visible movement occurs when the face bows outward near the center. In other words, the center portion of the face is moving faster than the top or bottom of the burden (Figure 4).

Figure 4: Axisymmetric Bending Diagram

 Figure 5: Cantilever Bending Diagram

This type of bowing or bending action does not always occur. One can find cases where instead of the center bowing outward, the top or bottom portion of the burden is cantilevering outward (Figure 5).

In either of these cases, the differential movement causes the burden to break in the third dimension. This breakage mechanism has been called flexural rupture or flexural failure. To properly discuss flexural failure, one must realize that these individual pie-shaped columns of rock caused by the radial cracking will also be influenced by a force perpendicular to the length of the column. This would be similar to beam loading conditions. When one discusses beam loading, the stiffness ratio is significant. The stiffness ration relates the thickness of the beam to its length. The effect of the stiffness can be explained by using, as an example, a full-length pencil. It is quite easy to break a pencil with the force exerted with one's fingers. However, if the same force is exerted on a two-inch long pencil, it becomes more difficult to break. The pencil's diameter has not changed, the only thing that has changed is its length. A similar stiffness phenomenon also occurs in blasting. The burden rock is more difficult to break by flexural failure when bench heights approach the burden dimension in length. When bench heights are many times the burden in length, the burden rock is more easily broken.

Two general modes of flexural failure of the burden exist. In one case, the burden bends outward or bulges in the center more quickly than it does on the top or bottom. In the second case, the top or the bottom of the burden moves at a higher rate than the center. When the burden rock bulges at its center, tensile stresses result at the face and compression results near the charge. Under this type of bending condition, the rock will break from the face back toward the hole (Figure 4). This mode of failure generally leads to desirable breakage.

In the second case, the rock is cantilevered outward (Figure 5) and the face is put into compression and the borehole walls are in tension.

This second case is undesirable. This mechanism occurs when cracks between blastholes link before the burden is broken and is normally caused by insufficient blasthole spacings. When the cracks between holes reach the surface, gases can be prematurely vented before they have accomplished all potential work. Air blast and flyrock can result along with potential bottom problems.

The bending mechanism or flexural failure is controlled by selecting the proper blasthole spacing and initiation time of adjacent holes. When blasthole timing results in charges being delayed from one another along a row of holes, the spacing must be less than that required if all the holes in a row were fired simultaneously. The selection of the proper spacing is further complicated by the stiffness ratio. As bench heights are reduced compared to the burden, one must also reduce the spacing between holes to overcome the problems of stiffness. 

Effect of Blasthole Length

The rock breakage process occurs in four distinctive steps. As the explosives detonates, a stress wave moves through the rock uniformly in all directions around the charge. Radial cracks then propagate predominantly towards the free face. After the radial cracking process is finished, high pressure gases penetrate into the cracks approximately 2/3 of the distance from the hole to the face throughout the radial crack system. Only after the gas has time to penetrate into the crack system are the stresses on the face of sufficient magnitude, to cause the face to move outward. Before the face begins to move and bend outward, fractures are created in the third dimension as a result of the flexural failure or bending.

In order to better understand the rock breakage process, a finite element model was created to closely resemble the radial crack network before burden movement occurs. The model was unique in that it allowed the study of redial cracks which were partially pressurized. An interim technique was used to recalculate and update the borehole pressures as borehole volume increased. The model was designed to study the important aspects of bench blasting. 

>>>Blasting Parameters

In order to compare the model's behavior with that of actual field results, parameters were chosen so that actual burden movement could be predicted. The model consisted of a single hole, four inches in diameter.The burden was fixed at ten feet, stemming and subdrilling were eight feet and four feet respectively and the bench height was varied from twelve to one hundred feet. Therefore, the stiffness ration (bench height divided by burden) changed from 1.2 to 10. The explosive parameters used in the model were those of ammonium nitrate and fuel oil. The borehole was initially pressurized with explosive gases to 425,000 psi.

>>>Stiffness Analysis

The burden was held constant at 10 feet throughout the analysis, therefore, changes in the stiffness (L/B) ratio resulted from varying the bench height. As the L/B ratio increased from 1.2 to 10, the bench height increased from 12 to 100 feet. Fourteen different models were used for the displacement analysis. Displacements and outer fiber stresses were calculated for nodes, which were located on the face, in the middle of the bench and in the direction of the burden face.

For discussion purposes, four different L/B ratios, 1.2, 2.4, 3.6, and 4.0 will be considered. With an L/B ratio of 1.2, there was no displacement on the face of the shot, instead local crushing occurred around the hole. As the bench height increased and the L/B ration became 2.4, less local crushing occurred and the model indicated a maximum displacement on the face of the 43 inches. For an L/B ratio of 3.6, the model indicated a displacement of 186 inches and at L/B ratio of 4.0 and the displacement was 279 inches or 23.2 feet. Figure 6 shows the deformed geometry configuration of this model and Figure 7 shows scaled displacements.

Figure 6: XZ-View of The Deformed Geometry Configuration as L/B Ratio Changes from 1.2 to 4.0

Figure 7: Actual Displacements for XZ-View as L/B Changes from 1.2 to 4.0

Further analysis was conducted applying beam bending theory in an effort to quantitatively explain the behavior of burden rock subjected to explosive gas pressure loads. A rectangular cross section was selected having a depth of 120 inches and a width of 1 inch. The beam length would be equivalent to the bench height. The deflection at the middle of the beams were calculated and compered to the results of the finite element model. A close correlation between the finite element model and the rectangular cross section was observed as shown in Figure 8.

 Figure 8: Comparison Between Calculated Finite Element Displacements and Rectangular Cross Section Deflection

The graph of L/B ratio vs. displacement for both the finite element model and the rectangular cross sectional model produce a better understanding of why this ratio is so important in bench blasting. The graph shows that the displacement increased at a slow rate when L/B ratio varied between 1 and 3.5. At 3.5, there was a distinct change in slope of the curve, which indicated that for the same gas pressure there was additional displacement. When the L/B ratio was greater than 6, there were significant increases in displacement with small changes in the L/B ratio.

Research conducted by Konya in 1967 and empirical data from the field indicate that a practical upper limit for L/B ratio is approximately 4. Above 4, the effects of stiffness are minimal and are no longer considered in field work. In this finite element analysis, the curve made two slope changes, one at approximately 3.5 and the other at 6. In both of these transitional zones, there was a significant increase in displacement for small changes in the L/B ratio.

The results of these analysis further confirm the hypothesis that rock breaks as a result of the gas pressure moving into the radial fractures and causing a burden displacement, and breakage as a result of flexural failure. 

Source: 
1) Rock Blasting and Overbreak Control (U.S Department of Transportation)
2) Book 2 
3) Book 3 
 
 

DESIGN OF SHOTCRETE SUPPORT IN UNDERGROUND MINING

There are many examples (very few of which are documented) where shotcrete has been used as a last act of desperation in an effort to stabilize the failing rock around a tunnel and, to most people's surprise, it has worked. The complex interaction between the failing rock mass around an underground opening, and a layer of shotcrete of varying thickness with properties which change as it hardens, defies most attempts at theoretical analysis. It is only in recent years, with the development of powerful numerical tools, that it has been possible to contemplate realistic analyses, which will explore the possible support-interaction behaviour of shotcrete. A clear understanding of shotcrete behaviour will require many more years of experience in the use of and in the interpretation of the results obtained from these programs. It is also important to recognize that shotcrete is very seldom alone and its use in combination with rockbolts, cablebolts, lattice girders, or steel sets further complicates the problem of analyzing its contribution to support.

 Figure 1: Rockbolt

Figure 2: Cablebolt

Figure 3: Lattice Girders

Current shotcrete 'design' methodology relies very heavily upon rules of thumbs and precedent experience. Wickham et al (1972) related the thickness of a shotcrete tunnel lining to their Rock Structure Rating (RSR). Bieniawski (1989) gave recommendations on shotcrete thickness (in conjunction with rockbolts or steel sets) for different Rock Mass Ratings (RMR) for a 10 m span opening. Grimstad and Barton (1993) have published an updated relating different support systems, including shotcrete and fibre reinforced shotcrete, to the Tunneling Quality Index Q. Vandewalle (1993) collected various rules of thumb from a variety of sources and included them in his monograph.

Table 1, Table 2, Table 3 and Table 4 are a compilation of current shotcrete practice, combining all of empirical rules. The reader is warned that this table can only be used as an approximate guide when deciding upon the type and thickness of shotcrete to be applied in a specific application. Modifications will almost certainly be required to deal with local variations in rock conditions and shotcrete quality.

Table 1: Summary of Recommended Shotcrete Application in Underground Mining

Table 2: Summary of Recommended Shotcrete Application in Underground Mining

(continuing Table 1)

Table 3: Summary of Recommended Shotcrete Application in Underground Mining
(continuing Table 1 and Table 2)

Table 4: Summary of Recommended Shotcrete Application in Underground Mining
(continuing Table 1,Table 2 and Table 3)