Chapter 36: Final Wall Blasting

Final wall blasting techniques are controlled blasting techniques used to minimize overbreak (or the fracturing of rock) beyond the designed boundary of main blasting or excavation areas. The success of these techniques, which are used in both underground and surface operations, depends primarily on the geology of the rock formation being blasted. In hard, massive rock modified production blasting techniques can produce satisfactory results. In less competent rock, Excessive application of final wall techniques may not be needed. In fractured, jointed, or incompetent formations that will not support themselves, consistently good results may not be possible. The five methods of final wall blasting discussed in this chapter are listed in table 36.1.

Final Wall Blasting Techniques

Table 36.1 – Five final wall blasting techniques.

These techniques are not a cure-all. Slopes should still be designed to maximize the rock's inherent geological strength. When using final control wall blasting methods, it is recommended that conservative trials be devised to determine whether the method can be successfully applied and, if so, to establish the optimum borehole and drill hole spacings. Several controlled blasting techniques are used to reduce overbreak. They have the common objective to minimize fracturing and loosening of the rock beyond the "neat" excavation line.

From the first use of explosives in the mining and construction industries, attempts were made to develop methods to control overbreak. In recent years, the approaches have become more sophisticated with formulas now available to assist in the initial design of a project. However, as far as practical field application is concerned, there is still a large element of trial and error involved. This is not surprising, considering the geologic variables involved in blasting. For example, it is unrealistic to believe that the same blasting technique would be equally successful in massive granite formation and in highly jointed sedimentary formations.

Final wall blasting applications are required on construction sites, quarries, and surface mines of all types. Underground wall control is critical for roof and pillar stability. In fact, final wall applications may be required for all three in some operations in order to maximize backbreak and maintain stable pillar walls. Please keep in mind that the success of final wall applications may be limited by a number of factors. Poor geology, ground water, and inappropriate drilling equipment are a few of the potential difficulties a blaster-in-charge may encounter.

SAFETY FOR FINAL WALL BLASTING

Final walls may stand for years to come and must be shot with controlled methods to minimize backbreak and ensure competent walls. Public roads, haul roads, and working operations may be located at the top or along the crest of these walls exposing equipment and/or personnel to potential rock falls or wall collapse.

Final Wall Control Factors

Efficient wall control blast design can be defined as achieving a safe and stable slope for the lowest possible cost. From a production standpoint the goal of final wall control blasting is to make the transition from a well fragmented rock mass to an undamaged slope in as short a distance as possible. This can be quite challenging due to the many factors that influence wall damage. To develop efficient designs, one must have a basic understanding of wall failure mechanisms, as well as the limitations of the various wall control procedures. In addition, it is imperative that the design be precisely implemented, evaluated, and refined on a continuous basis.

The time and effort spent in developing and implementing efficient designs is insurance against future wall failures. The four (4) site factors that control stability are given in table 36.2.

Site Factors Of Final Wall Control Blasting

Table 36.2 – Site factors of final wall control blasting.

Obviously, the geology of the site will influence both the slope and blast designs. It is important that close attention be paid to the geologic conditions of the wall at the blast site to develop blasts that will limit damage. The key geologic factors are the rock mass structure and strength. When evaluating the structural integrity of the rock mass the type, size, persistence, orientation (strike and dip), in-fill and fracture frequency of the defects must be taken into account.

The strength of the rock mass under shear, tensile, and compressional loading will also dictate the overall stability of the slope. In most cases, the final slope design is modified during the excavation process, as the site conditions become more understood. In addition, major plan changes can alter the slope design. The four (4) key slope design parameters are (1) overall slope, (2) bench height, (3) batter angle, (including the berm width that defines the overall slope), and (4) top loading or surcharge (the soil or other material lying on the top of the rock layers). These parameters are illustrated in figure 36.2. Several blast design factors influence the stability of the wall and are listed in table 36.3.

Blast Design Factors That Influence Wall Stability

Table 36.3 – Blast design factors that influence wall stability.

The last major factor that controls wall stability is the implementation of the mine plan. Even well-conceived damage control programs will not perform properly if there is no commitment to quality. Quality, in this case, refers to proper blast design, accurate drilling, and precise charging of the boreholes.

Blast Induced Damage Mechanisms That Affect Final Wall Blasting

Stress failure occurs when the stress intensity is greater than the strength of the rock. Three general types of blast induced breakage mechanisms that affect final wall blasting are (1) rock stress (compression, tensile, and/or shear) failure, (2) crack extension, and (3) cratering.

Detonation of a column load adjacent to the slope boundary places the final wall under compression. When this load is released, the rock mass bounces back; this can cause separations along geologic discontinuity and structure planes. This type of damage has been observed in excess of 30 meters (100 feet) beyond the boundary of the blast. Over confinement of the explosives adjacent to the slope accelerates this load damage.

Final Wall Failure Due To Existing Stresses

There are three important criteria to consider when using final wall blasting techniques: (1) drilling accuracy, (2) type of explosive used, and (3) the fact that loading techniques differ from short to long boreholes. The techniques for non-vertical wall and any deviation from planned alignment can require changes to cut-over for adjacent boreholes. Drillers should be trained to recognize angle indicators and take extra care during both setup and production. Blasters-in-charge should follow applicable regulatory standards and should not allow drilling to occur in close proximity to the column of exposed detonating fuse or downline in any of the perimeter boreholes. This may require special procedures to cover exposed detonating cord for noise control, as well as covering or matting a blast within close proximity to protect nearby structures or equipment.

Final Wall Failure Due To Crack Extension

The second major cause of wall damage is detonation crack extension. As the explosives adjacent to the slope detonate, high-pressure gases expand into rock cracks and structural defects in the rock and cause them to expand, as if a wedge is driven into them. The damage done by this wedging action is determined by the strength of the rock and the duration of the pressure pulse. Wedge failure can occur in materials from the sandiest to the hardest of rock.

Final Wall Failure Due To Cratering

The third type of rock wall failure mechanism is cratering. Cratering is a fragmentation mechanism that can cause wall damage. Basically, cratering occurs when the explosive pressure generated to the slope is not adequately relieved away from the wall. This results in cratering beyond the desired pit limit. Block heaving is also a major factor in damage along weak loads (swell) formations.

FINAL WALL BLAST DESIGN

It is typically not practical or cost-effective to use one type of wall control procedure for every area of the excavation. Instead, the sensitivity and importance of each blast site should be evaluated to assist with the development of the most appropriate design. Before using a specific type of final wall blasting technique, the factors in table 36.4 should be evaluated.

Final Wall Blast Evaluation Factors

a. Other features like mud seams, discontinuities, and voids sometimes have lateral dimensions like the natural structures and can be mapped.

Table 36.4 – Final wall blast evaluation factors.

Once rock quality zones have been determined, specific plans can be developed to address the conditions of each blast site. The designs should be continuously refined based on the analysis of each blast's performance. Ideally, the initial blasts should be evaluated in noncritical areas so the design can be refined to match the existing rock conditions, prior to blasting in critical areas. Correctly applying the three (3) blast design principles (See chapter 14) in table 36.5 provides for effective and efficient wall control blast performance. In sensitive zones, each of these factors must be in balance with the others to efficiently protect the wall.

Blast Design Principles For Effective Final Wall Blasting

Table 36.5 – Blast design principles for effective final wall blasting.

The energy distribution is determined by the charge diameter and blast pattern used. Excessive charge diameters can increase slope damage by creating a nonuniform energy distribution. In many cases, it is necessary to air deck such holes to improve the distribution of energy and reduce damage.

MODIFIED PRODUCTION BLASTING

Modified production blasts using a reduced charge and subdrill in the last row is most successful in competent rock masses or on slopes designed with a high factor of safety. The primary disadvantage of modified production blasting is that the wall is not protected from crack dilation, gas penetration and block heaving. In modified production blasting, the energy level is decreased adjacent to the wall to reduce overbreak. This is often achieved by simply reducing the charge weight by 30% to 60% in the row nearest the slope as shown in figures 36.4 and 36.5.

Modified Production Blast Design Limitations

Most rock types require additional design modifications to minimize damage. These modifications can include switching, reducing the burden and spacing dimensions, minimizing subdrill length, and increasing the delay interval between the last two rows of boreholes. These potential design changes are shown in figure 36.5. When modified production designs are used, the excavator digs from the free face and works toward the wall. This practice will limit the damage that results from heavy back pasting to a standoff of the last row of loaders. The Standoff is defined as the distance from the last row of laders to the final slope. This offset controls both the wall stability and ease of excavation of the toe.

The excavated standoff distance depends on the strength and structure of the rock mass and should be determined by carefully analyzing blast performance. The following guidelines are for initial production design modification guidelines.

Initial Production Blast Design Modification Guidelines

Table 36.6 – Initial production blast design modification guidelines.

The guidelines in table 36.6 were developed for a wide range of rock types and geological structural considerations.

The performance of this initial modified production design should be evaluated in terms of overbreak, diggability, and cost. In some cases, it may not be necessary to apply all of the recommended design modifications to achieve good results. While modified production blasting generally provides the best excavator productivity of all of the wall control methods, it is unlikely that it will be suitable for all of the rock types and structures within the excavation. In most modified production blasts, the explosive energy in the last row of boreholes is over confined and will damage weak or sensitive walls.

PRESPLITTING

Presplitting involves a single row of holes drilled along the "neat" excavation line. In construction projects, the boreholes are usually the same diameter 51 millimeters to 101 millimeters (2 inches to 4 inches), all loaded in most cases with 22 millimeter to 25 millimeter (7/8 inch to 1 inch) or larger diameter explosives, and initiated before any of the adjoining main blast is initiated.

The empty annulus around the small-diameter explosive cushions the explosive shock wave, thus reducing the crushing and radial cracking of the rock around the borehole. The theory of presplitting is that some of the radial cracks from a lightly shot borehole either join an adjacent borehole or other radial crack from an adjacent hole to form a plane of broken rock between the boreholes.

Caution If boreholes are overloaded, the shear zone will extend to and beyond the tension zone.

In presplitting, two charges are initiated simultaneously in adjacent boreholes the collision of shock waves between the boreholes places the web in tension and causes cracking that produces a sheared zone between the boreholes as illustrated in figure 36.7.

This cracking between boreholes, initially produced by the shock wave of the explosive, is subsequently extended and widened by the presplitting gases based on these three factors: (1) properties and conditions of the rock, (2) spacing between boreholes, and (3) amount and type of explosives in the holes. The fractured zone between the boreholes will be easily split to give a shear zone of fractured rock. This split or crack in the rock forms a discontinuous zone that both (1) minimizes or eliminates overbreak from the subsequent primary blast and (2) produces a smooth finished rock wall.

Presplitting Limitations

When presplitting, it is difficult to determine results until excavation of the primary blast is complete to the finished wall. Since presplitting is done before primary blasts are made, it is not possible to take advantage of the knowledge of local rock conditions gained in the primary blasts.

An alternative to the presplitting technique widely used (particularly in large borehole operations) is airdocking. This method uses a base charge in the borehole and a plug at the top of the borehole. The air gap remaining in the borehole provides for low impact to the final wall.

Presplitting Explosives Products

Presplit boreholes may be loaded with a variety of explosive products. These are water gel and emulsion products made in a variety of small diameters and produced in coils for easy use in cast applications in the borehole. Some products have a built-in detonating cord downline as shown in figure 36.11. Cartridged dynamites and emulsions in rigid tube shells with built-in connecting sleeves are also available, in diameters ranging from 19 millimeters to 32 millimeters (3/4 inches to 1¼ inches). Detonating cord may also be used as a primary load and comes in heavy core loads if desired. Smaller products are used when the airdocking technique is used.

Field experience has shown that using the continuous small cartridge products in deep boreholes provides excellent presplitting results while permitting increased loading rates and reduced labor costs. This unique product loads much easier than cartridged explosives as single or decked boreholes because of the improved control it offers in handling. Occasionally, in large diameter holes vacuum cartridge sizes are taped to detonating cord downlines at predetermined spaces. On occasions, low density ANFO loads have been tried in presplit holes, but results are seldom satisfactory because of crushing around the borehole circumference.

These charges are simultaneously initiated with detonating cord and trunklines (See figure 36.10) or instantaneous detonators.

Loading is generally carried out to a shear factor ranging from 0.2 kilogram/meter to 1.24 kilograms/meter (0.04 pounds/foot² to 0.25 pounds/foot²) of final wall area.

If excessively long presplit lines are shot, or the maximum pounds per delay must be reduced for blast vibration control, sections should be delayed with MS delays or MS connectors. Where conditions preclude firing the presplit holes in advance of drilling the primary holes, presplitting can be accomplished by delaying the presplit holes to allow the presplit holes to fire first.

Slope Relief

Slope relievers are boreholes drilled on the slope in a depth just above the presplit boreholes for the purpose of this discussion.

If the configuration of the blast area is adequate, the best results can usually be obtained by blasting the presplit holes before the primary holes are drilled. Most presplit holes on highway construction are drilled on the slope line. On a 1 to 1 (a slope, the bottom of a 12.2 meter (40 feet) long presplit borehole would be 3 meters (10 feet) horizontally displaced from the top of the borehole. Both the first production borehole(s) in the primary blast and the slope reliever can be disturbed by the presplit boreholes. Rock shelf could result in undamaged explosives being flung in the muckpile. A minimum burden to bench height ratio of 1:1 is generally required. For areas with weak rock, inconsistent discontinuities, or heavy water conditions, the ratio should also be increased to 1:1. In some cases, the explosives charge in the slope relievers and the first row of primary boreholes can be propagated by the presplit borehole resulting an excessive backbreak and flyrock.

The optimum spacing for presplit boreholes can only be determined after a trial blast. However, in a relatively competent formation a spacing of 0.8 meters (30 inches) for a 76 millimeter (3 inch) diameter borehole loaded at a loading density of 0.37 kilogram/meter (0.25 pounds/foot) will produce acceptable results for the trial blast. In most formations the desired results can be obtained by adjusting the borehole spacing and still retaining a column loading density of 0.37 kilogram/meter (0.25 pounds/foot). In a very soft, weathered rock formation it is occasionally necessary to reduce both the borehole spacing and the column loading density. In very extreme conditions the spacing may be as close as 30 centimeter (12 inches) and loading 85 grams/meter using (400 grain/foot) detonating cord as the entire explosive charge.

The amount of stemming above the explosive column will vary depending on the rock formation and the borehole depth. In most cases, the stemming will be 1.2 meters to 1.8 meters (4 feet to 6 feet) in length. The borehole should be blocked above the explosive column to prevent the stemming from falling in the annulus around the explosive column. The charge in the bottom of the borehole is sometimes increased to assure that the toe (the bottom of the borehole) is pulled. The required amount of bottom load varies with the formation and the depth of the hole. A borehole 4.6 meters (50 feet) or deeper about 0.3 meters to 0.9 meters (1 foot to 2-4 feet) of the same charge used in the bottom of the primary is sufficient.

The depth that can be presplit at one time depends upon the driller's ability to maintain accurate borehole alignment. A small error from the proper drill angle causes deviation in the borehole alignment. This deviation becomes more pronounced as the borehole depth increases. Approximately 15.2 meters (50 feet) is generally the maximum depth for 61 millimeters to 89 millimeters (2½ to 3½ inch) diameter boreholes without significant change in results due to borehole deviation. Theoretically, the length of a presplit shot is unlimited. In practice, however, shooting too far ahead of the primary excavation can cause trouble if the rock characteristics change. The presplit line may not be established if the holes are too far apart. On the other hand, if the holes are too close together, overbreak will occur. By carrying the presplit only one shot in advance of the primary blasting (See figure 36.12), blasters can gain knowledge from the primary blasts regarding the rock and apply it to subsequent presplit blasts.

Caution Deviation greater than 15 centimeters (6 inches) from the desired plane of shear may give inferior results.

In weak and soft rock formations, results may be improved by using guide or relief holes. Guide holes are unloaded holes located between loaded holes for the purpose of promoting fracturing along the desired plane and in the desired direction like a perforation. For example, guide holes are recommended to direct cracking in corner shots or reduce backbreak, but they do not normally permit increased spacings between loaded holes (See figure 36.13). In fact, they probably decrease this distance because the unloaded holes tend to terminate the crack.

Line-drilled holes as guide holes can be located between the normally spaced holes to help control backbreak. These line holes may vary in depth from the top few feet to the full depth of the presplit boreholes. Backbreak is more likely at the top of a bench or lift. Consequently, line drilling between presplit holes for the top few feet reduces the chance of overbreak in all types of formations.

Large borehole presplitting may be done by utilizing existing drilling equipment available at a mine. The large holes can be loaded with large-diameter presplit explosives, by taping packaged material on detonating cord or lattice (lath), or possibly by air decking the borehole.

Underground Applications

Presplitting has some applications in underground headings and stopes to control overbreak, to improve back and rib stability, and to reduce concrete requirements.

If the perimeter boreholes of a heading are drilled on the presplit principle, loaded lightly, and initiated simultaneously ahead of the main round, overbreak can be minimized. In horizontal boreholes it is usually necessary that some form of stemming be used at the collar of the borehole to prevent excessive rifling or blowing out of the explosive force. Although theoretically sound, presplitting techniques are not often employed in underground headings because of possible cutoff problems with the close spacing and burden that are required in the primary blast. Good results can be obtained using presplitting techniques in underground headings. However, time must be taken to determine optimum explosive loads to prevent cutoffs.

Also, the presplit explosive is not contribute any significant energy to break and move the rock between the perimeter boreholes and the nearest production boreholes. Consequently, the strain charge must be loaded very close to the presplit line to ensure proper breakage to it. These lines in heading rounds in many cases must be within 2 to 4 borehole diameters from the presplit line. To overcome this, most operators use smooth wall techniques and initiate these perimeter holes after the main charge has moved the bulk of the rock and created a free face.

One application of presplitting in underground work that is proving satisfactory is for cave control in block-caving operations. By presplitting the ore body limits, ore dilution in the caving process is minimized.

Presplitting the slope limits also promotes initial caving of the ore. Boreholes as large as 127 millimeters to 254 millimeters (5 inches to 10 inches) in diameter have been used successfully. These large boreholes can be used because shatter around presplit boreholes and does not, on the finished presplit line is not as significant. The main objective is to create a good parting between the ore and the rock. Long hole and down-the-hole drills have made this technique more feasible.

Specialty Presplit Applications

The presplit technique is used as the primary blasting technique for many dimensional stone quarries and for specialty projects such as Mr. Rushmore and Crazy Horse monuments in the United States (See figure 36.15). Loads may include standard presplit explosives or be reduced to detonating cord only. These applications may involve splitting the rock on all sides utilizing horizontal boreholes on the bottom of the shot.

Figure 36.16 depicts presplit on a concrete lock and dam project to allow-new panels to be attached for future use.

SMOOTH BLASTING

Smooth blasting (sometimes referred to as post splitting, contour blasting, perimeter blasting, or sculpture blasting) was introduced in Sweden. It is the most widely accepted method for controlling overbreak in underground headings and stopes. Smooth blasting techniques have application in both underground and open surface work. In smooth blasting the boreholes are drilled along the excavation limits, lightly loaded with well-distributed charges, and initiated after the main excavation is removed. By initiating instantaneously or with minimum delay between the holes, a shearing action is obtained which gives smooth walls with minimum overbreak. The borehole spacing in smooth blasting can be greater than in presplitting. This reduces drilling and explosives costs.

Smooth Blasting Limitations

The limitations in smooth blasting are that it (1) usually involves more perimeter holes than conventional methods and (2) does not work in all formations (if the ground is too weak to support itself, smooth blasting will not completely eliminate the need for back supports or bolting).

Underground Applications

In underground headings where the rock and ribs slough and cave because of unconsolidated material, overbreak is common because of the shattering action from blasting. By employing smooth blasting techniques with light, well-distributed explosive loads in the perimeter holes, fewer supports are required and less overbreak occurs. Even in the harder and more heterogeneous formations, smooth blasting provides smoother and firmer backs and ribs.

Many operations smooth blast the back of their heading rounds to reduce falling rock hazards. This is especially true in main haulage drifts, in areas where two passes will be taken, in places where miners must reenter and work under the roof at a later date, and in poorly consolidated ground.

Smooth blasting in heading rounds is done by drilling perimeter holes on a burden-to-spacing ratio of approximately "1:1.5 to one"; loading with light well-distributed charges; and firing with, or after, the last delay period in the round (See figure 36.18). These holes are fired after the lifter holes (bottom holes in a heading round) to ensure that the broken rock is displaced sufficiently and to offer a free face and maximum relief for the smooth blast holes. This relief and internal free face permits unrestricted movement and good fragmentation of the final rock broken and results in less shatter beyond the excavation limit. To ensure minimum relief, a pilot heading is sometimes used. After the pilot heading has been completely excavated, the final cross section is drilled and shot. In this case depths greater than the length of a single round can be smooth blasted. The pilot heading method allows the use of smooth blasting around a greater portion of the periphery of a heading. When shooting smooth blast holes in a round, the confinement relief is limited to the arch and partially down the rib because of muck pile-up. Therefore, good smooth blasting results generally are not obtained lower in the ribs.

Although the 1.5:1 burden-spacing ratio is recommended as a starting point, the formation being blasted may warrant modifications. Also, firing the smooth blast holes with minimum delay between holes is not always necessary. The well-distributed light loads in the perimeter holes with conventional patterns and delays will often produce satisfactory results.

Since it is not convenient or practical to attach charges to detonating cord lines in horizontal holes, smooth blasting is usually done by utilizing explosives especially designed for presplitting. These are usually tube shells of dynamite or emulsion explosives with attached connectors to ensure a continuous column of explosives. It is usually necessary to stem these holes with tamping plugs or use an anchor on the product to secure the explosive column in the borehole (See figure 36.17). If the smooth boreholes are not stemmed or anchored, the string-loaded charges can be cutioned out as the previously delayed boreholes detonate. Stemming can prevent excessive rifling and permits the use of lighter charges. Initial trials with small-diameter, string-loaded charges are often unsuccessful because of loading too close to the collar. By so doing, overbreak from previously delayed holes pulls the explosive charges out into the muck before they have time to detonate.

Smooth-wall blasting techniques also produce good results in shaft and winze rounds. In this work, the holes may be shot with each round or after the rounds shooting to a pilot opening or to a larger, bored opening. Continuous water gel explosives or cartridged products explosives may be used with detonating cord lines to accomplish long-hole, smooth-wall work of this type.

Smooth blasting underground is accomplished by drilling perimeter holes on a burden-to-spacing ratio of approximately "1.5:1" loading with light, well-distributed charges and firing with, or after, the last delay period in the round.

Large underground hard rock operations can utilize the post splitting technique to assist in roof and/or rib control. The heights in these headings are generally 6.1 meters to 6.1 meters (15 feet to 20 feet) and may reach depths of 30 meters (100 feet) if multiple lifts are taken out. Through geologic analysis and shot results, it can be determined if the roof or the ribs should be shot last. This technique can substantially minimize scaling time and provide safe working conditions.

The principal advantages of smooth blasting are listed in table 36.7.

Advantages Of Smooth Wall Blasting

Table 36.7 – Advantages of smooth wall blasting. (Courtesy: M. Koehler)

CUSHION BLASTING

Cushion blasting (sometimes referred to as trimming, slabbing, or slashing) was introduced in Canada. Like smooth wall blasting, a single row of holes is drilled along the "neat" excavation line, loaded with light, well-distributed charges, and initiated after the main excavation is removed. Unlike smooth wall blasting, the annular space in the boreholes is filled with crusite since the entire column length. This "cushions" the shock from the finished wall as the holes are detonated and minimizes the stresses and fractures in the finished wall.

This technique is seldom used today because the air annulus around the small-diameter charges generally produces equal results and reduces the loading time.

The only application for cushion blasting today is in the situation where large-diameter cartridges are taped on detonating cord downlines at planned intervals. The cushion holes are fired with minimum delay between holes. This shares the rock web between holes and yields a smooth wall with maximum overbreak. Detonating cord trunk lines or instantaneous detonators are used to initiate the detonating cord downlines if noise is a problem.

A combination of techniques usually provides better results.

The fact that the main cut area is removed in cushion blasting leaves a minimum buffer in front of the next excavation line. The cushion holes can be drilled prior to or after any primary blasting. The burden and spacing will vary with the borehole diameter being used. The burden-to-spacing relationship will vary with different formations, but the spacing should always be less than the width of the burden being removed to obtain maximum shearing between holes. When a full-length, detonating cord downline is present/utilized by taping the charges on it, the stemming is added after placement of the entire charge. In this case sand, crushed stone, or gravel can serve as stemming provided that it is sufficiently free flowing. Raising and lowering the detonator slightly in the stemming as the loading process proceeds will help compact and position the stemming. The "lock" of very fine stemming against the side of the borehole serves to support and cushion the decked explosives in any angled hole. The length of stemming and the amount of top stemming will vary with the formation that is being shot. Close borehole spacing is required when cushion blasting around curved areas or corner than when blasting a straight wall, so 90° corners, a combination of cushion blasting techniques usually produces better results than reliance on a single technique. Small-diameter, unloaded guide holes placed between loaded holes also can be used to advantage when cushion blasting around curved areas and corners where it is difficult to hold a consistent wall, unloaded guide holes between cushion holes are recommended. Generally, small-diameter guide holes are employed to reduce drilling costs.

Where only the top of the formation is weathered, the guide holes need to be drilled only to that depth and not to the full depth of the cushion holes. This procedure is common on the first lift or bench since back-break is more probable there than on lower benches.

Figure 36.23 shows results of a combination of cushion blasting and guide holes where the latter were drilled to full depth.

Cushion blasting in open work has application to inclined and vertical boreholes. In both cases good borehole alignment is essential.

LINE DRILLING

In line drilling, a single row of closely spaced, unloaded, small diameter holes is drilled along the next excavation line. This provides a plane of weakness to which the primary blast can break and to some extent reflects the shock waves created by the blast, reducing the shattering and stressing in the finished wall.

For many years, line drilling was the only technique used for overbreak control. Line drilling may be defined as a "single row of unloaded, closely spaced holes drilled along the next excavation line to provide a plane of weakness toward which the blast can break." Over the years, modifications in line drilling have resulted in related techniques, which are referred to by many names as listed in table 36.1. These techniques differ chiefly from the original line drilling principle in that some or all of the drill holes are loaded with relatively light, well-distributed charges of explosives. Due to the fact that initiation of these charges tends to crack or split the rock between the boreholes, wider borehole spacings than those of line drilling are permitted. Consequently, drilling costs are reduced and in many cases better control of overbreak is obtained.

Line drilling is best suited to homogeneous formations where bedding planes, joints, and seams are at a minimum. Natural planes of weakness tend to permeate break through the line-drilled holes into the finished wall. Therefore, thin-bedded, sedimentary, foliated metamorphic formations are not well suited to line drilling for overbreak control unless drilling can be done perpendicular to the strike of the formation. This, however, is not practical in most excavation work. Figure 36.24 shows a typical pattern for line drilling. Best results are obtained when the primary excavation is removed to within one to three rows of holes of the next excavation line. The last row (or rows) of boreholes in them slabbed away from the line drilled holes using delay detonators or MS connectors. This procedure gives maximum relief at front of the finished wall, allows the rock to move forward, and creates less backpressure, which could cause overbreak beyond the line drilling.

Line Drilling Strategies

Line drill holes are generally 38 millimeters to 76 millimeters (1.5 inches to 3 inches) in diameter and are spaced from 2 to 4 times the borehole diameter apart along the excavation line. Boreholes larger than 76 millimeters (3 inches) in diameter are seldom used in line drilling since the higher drilling costs cannot be offset by the increased spacings.

The boreholes directly adjacent to the line drill holes are generally loaded lighter and are more closely spaced than the other holes in the primary pattern. The distance between the line drilled holes and the directly adjacent boreholes is usually 50% to 75% of the normal burden. A common practice is to reduce the spacings of the adjacent boreholes the same amount with a 50% reduction in the explosives load. The explosives should be well distributed in the borehole using decks and detonating cord downlines.

Line Drilling Limitations

Line drilling is very limited in application. The only place where it is applicable is in areas where even the light explosive loads associated with other controlled blasting techniques may cause damage beyond the excavation limit, or where line drilling is used between loaded holes to promote shearing and guide the presplit line. Some of the disadvantages are (1) unpredictability except in very homogeneous formations; and (2) high drilling costs because of the close spacings required.

SUMMARY

In final wall blasting the degree of confinement of the explosive energy adjacent to the slope plays a major role in the amount of wall damage produced. The blast designer should always provide the explosive energy with a path of least resistance (relief) directed away from the wall. The goal of final wall control blasting is to make the transition from a well-fragmented rock mass to an undamaged slope in as short a distance as possible. This can be a difficult process and the blast designer must avoid the false notion that to limit blast damage the explosive energy must always be minimized. The challenge is to apply the explosive energy in ways that limit damage with little or no adverse impact on fragmentation and productivity.

REFERENCES

International Society of Explosives Engineers (ISEE). 1998. ISEE Blasters Handbook™, 17th edition. ISEE, Cleveland, OH.