Chapter 33: Surface Blasting

Surface blasting is done for either (1) mining or (2) construction excavations. The goal of surface mining is to recover useful minerals from the earth in an economical, safe and environmentally sound manner. Surface mining consists of removing the overburden or waste material from the top of the relatively shallow ore deposits to allow the excavation and removal of the ore itself. This is accomplished by mechanical means (bulldozers, 1998; Society of Mining Engineers (SME) Mining Engineering Handbook, 1992). Surface construction blasting is discussed later in this chapter.

The selection of a particular surface mining method depends on the shape, size, spatial position of mineral deposits, spatial distribution of mineral values, the proportion of ore and "country" rock, the topography, economic factors, safety, government regulations, and environmental concerns. Surface mining methods are as varied as the product that is mined. Regardless of the mining method the process will involve the extraction of ore, waste materials and/or overburden.

In almost all cases, mining will involve blasting. Several mining issues such as fragmentation, muckpile characteristics and overbreak are dependent on blasting outcomes (Scott et al. 1996, chapter 7 and chapter 32).

FRAGMENTATION

The primary goal of blasting is to fracture the in-place rock into fragments that can be excavated by a given piece of equipment and handled efficiently for processing into a final product (See chapter 7). The rock mass properties and structure will play a major role in controlling the size and shape of the fragments (See chapter 8). This muckpile resulting from a blast is made up of both blocks that are simply separated from the original rock and by fragments of newly fractured rock.

The desired size distribution of fragments is dependent on the purpose of blasting and the end use of the product being mined. For example, blasting fragmentation for construction aggregate is not a big concern as long as the material is loosened and fragmented to match the loading capacity and ability of the excavator. On the other hand, blasting in mining for metallic ores may also have strict downstream processing requirements that mandate a certain maximum feed size to crushing equipment or a maximum fragment size for autogenous grinding.

The capacity and performance of the comminution process will be influenced by the run-of-mine fragment size distribution. Crushers require a certain range of feed size to operate efficiently.

Oversize, fines and excavation efficiency are outcomes of blasting fragmentation that can influence the economics of a mining operation.

Oversize

Blasted rock fragments that cannot be handled by the excavator or passed through the crusher are considered oversize. Oversize will require to be rehandled for secondary breakage or for clearing the loading area (See figure 33.1). Rehandling of the oversize is time consuming and expensive and will often require stoppage of the loading and crushing operations. Oversize should be monitored as a measure of blast performance that can be improved upon (See chapter 7 and chapter 32).

Fines

Fines may be defined as fragments that are too small to be used as a final product and present downstream difficulties in handling and processing. Fines may be the result of excessive blasting fragmentation or simply due to the nature of the formation blasted. Although fines do not present problems for the excavation process or overburden removal, they can present, depending on the material, problems in transportation and processing. For example, in the quarry industry small product size that has no market value is considered fines and will have to be disposed of at additional cost. Fines with high moisture content will also tend to cake conveyor systems. Attempts to minimize the generation of excessive fines should be made where fines present a major economic impact on the processing efficiency and the value of the products.

Excavation

Fragment size distribution should allow for the efficient excavation of the blasted material by the excavator being used. Fragments should be able to fit in the excavator bucket without causing increase in loading cycle time or causing a reduction in bucket fill factor. Bucket fill factor and bucket fill time are indicators of blast performance.

MUCKPILE

Muckpile geometries are as varied as the surface mining methods and the type of excavator that are being used. Muckpile characteristics will depend on the original bench size and geometry, available space, muckpile movement constraints, desired swell distribution and excavator characteristics.

Shape and Size

The available physical space will dictate the size and direction of the blast and will consequently also control the size and shape of the resulting muckpile. The blasted rock will move, self-organize and rest at an angle of repose generally of 35°. For ore grade control, pile movement may be restricted by blasting against an unexcavated muckpile. Explosive energy, burden, bench height and timing affect burden velocity, consequently they must be carefully controlled for muckpile profile modifications (Chiappetta, 1991).

Swell

The in-place rock mass once blasted will fragment, move and come to rest with an increase in bulk volume. The increase in volume or swell of the muckpile is not evenly distributed throughout the blast. The swell will tend to be the highest toward the front of the blast. The swell is an indication of how loose the muckpile is. Swell, together with fragmentation, will dictate the ease by which a given excavator can dig the muck pile efficiently.

Excavation

The excavation requirements for the muck pile shape, size and swell will depend on the type of excavator that is being used. A front end loader is a very mobile excavator that for efficiency requires a large working area where the muckpile is shallow, well fragmented, loose, and spread out on the floor (See figure 33.2).

Figure 33.2 – Typical muck pile profile for front end loader operations (Blast Dynamics, 1992).

Power shovels or rope shovels require fixed operations with limited mobility, and they are most efficient when their movements are minimized. Muckpiles should be steep and as high as shovels' maximum digging height to facilitate bucket fill (See figure 33.3). Power shovels can dig through tight muckpiles, but oversize should be minimized since oversize will require stoppage of the loading operation for clean up. Muckpiles in front of the power shovel should be large enough to minimize the frequency of shovel relocations.

Figure 33.3 – Typical muck pile profile for power shovel operations (Blast Dynamics, 1992).

Hydraulic excavators are more mobile than the power shovels and can efficiently excavate less steep and shallower muck piles. With a backhoe attachment they can work from the top of a spread out muck pile with lengths no more than the lengths of the shovel stick (See figure 33.4). Hydraulic excavators are more flexible in terms of bench geometry, loading configurations and can be used when required bench heights are less than the maximum digging height for the power shovel.

Draglines are designed to dig down while sitting on top of a bench. Blasting is only done to loosen the bench material for dragline excavation. Unless the operations uses cast blasting (See chapter 34) to move some of the spoil into the spoil out area, the overburden is blasted in place and rock movement is kept at a minimum. The blasted bench should be loosened and stable enough for the dragline to operate safely and efficiently.

Figure 33.4 Hydraulic backhoe working on muck pile. (Courtesy: M. Katsabanis)

OVERBREAK

The quantity of explosive and the rate of energy release for a given blast geometry controls the extent of fragmentation. Overly confined blasts due to poor blast geometry or sequencing (See chapter 14) can result in a transfer of energy to the surrounding rock and cause excessive damage to the final walls. Excessive damage or overbreak can jeopardize the integrity of the bench. Tension cracks on the bench surface and loose rock fragments on the bench face create safety hazards. Overbreak at the bench crest results in an uneven burden for the next round. In multiple bench operations, crest damage may also result from excessive subdilling into the bench below and/or from using a heavy toe charge (See figure 33.5).

Figure 33.5 – Possible damage due to excessive subdrilling (Scott et al. 1996).

Overbreak may be exacerbated when blasting a jointed rock mass (See chapters 8 and 14). Joints dipping toward the face at 25° to 65° can cause sliding along the joints and when dipping away of the face at 85° to 110° can lead to block toppling (See figure 33.6).

Figure 33.6 – Effects of jointing on bench overbreak (Hagan, 1977).

Overall pit slope stability can also be adversely affected by the low frequency, low amplitude surface waves produced by a blast. The factor of safety of the slope against failure must be assessed taking into consideration the dynamic loading due to blasting (Hook and Bray, 1981). When and if pit slope and bench stability becomes a problem controlled blasting techniques must be used (See chapter 36) to improve stability.

Figure 33.7 – Features of a successful shored operation blast (Hook and Bray, 1981).

In surface mining the blasting engineer is faced with conflicting requirements of providing large quantities of well-fragmented muckpile that can be efficiently excavated and loaded, and of minimizing damage to the host environment. This must be done without sacrificing the economics of the blast (See chapter 33). Critical evaluation of fragmentation and muckpile characteristics should be performed. For example figure 33.7 depicts some of the desired features of a successful production blast for a shovel operation.

The front row should have moved out evenly, but not excessively, to leave a high muckpile and minimum swell at the production floor. The main charge should have moved the muckpile evenly with no centering. The back of the blast should have dropped indicating a good forward movement of the free face. Tension cracks should be visible in front of the final dig-line. There should not be excessive cracking behind the final dig-line and there should be good fragmentation of the toe. All these features will significantly contribute to an efficient operation (Hook and Bray, 1981).

TYPES OF BLASTS

In this chapter, a cut is defined to be the depth to which material is to be excavated to bring the surface to a predetermined grade (American Geological Institute, 1997).

In surface mining, blasting cuts can be categorized as either box-cuts or corner-cuts. Box-cuts are blasts with only one free surface. Box-cuts are very confined where there is no choice in the direction of rock mass movement. Sinking-cut blasts (See figure 33.8) and bench slot-cut blasts (See figure 33.9) are two examples of box-cuts.

Figure 33.8 – Sinking-cut blast (Bucyrus-Erie, 1979).

Figure 33.9 – Bench slot-cut blast (Bucyrus-Erie, 1979).

Corner-cuts are blasts with two free faces. Corner-cuts provide more flexibility in the direction and movement of the blast. Figures 33.10 and 33.11 are illustrations of corner-cuts.

Figure 33.10 – Bench blast (Bucyrus-Erie, 1979).

Figure 33.11 – Corner blast (Bucyrus-Erie, 1979).

Sinking-Cut

Sinking-cuts are used when opening a new surface mine or deepening an existing mine (See figure 33.8). A sinking blast for starting a new mine varies from most blasts since there is no open face for relief, and the direction of blasted rock movement is vertical (See figure 33.12). The entire blast will be very confined consequently high levels of vibrations and flyrock can to be expected. In most formations it will be necessary to decrease the burden and spacing for the initial boreholes in the delay pattern in order to open an area of relief to which the remaining holes may fire (See figure 33.13).

Figure 33.13 – Sinking-cut blast with center axis relief delay pattern. (Adapted from ISEE Blasters' Handbook™, 17th Ed. figure 23.18)

To deepen a surface mine, new bench levels must be established from a sinking ramp (See figure 33.8). This type of blast requires special attention since the depth of the boreholes will increase as the pattern progresses down the ramp when the ramp is intended to be a permanent ramp. If the ramp is intended to be a temporary ramp, the ramp can be blasted to full bench depth and dug to grade. In the first the remainder can be excavated. Sinking cut ramp design, to establish new bench levels, has been proposed by Chiung (1982) and expanded by Hustralid (1999). Figure 33.14 depicts the initiation sequence for a sinking-cut to blast a ramp. The initiation is started at the deep end of the cut or by instantaneously firing two corners holes to provide initial relief. Control blasting can be used on the sidewall of the cut to minimize overbreak of the side will be part of the final pit slope. (See chapter 36 for a discussion of final wall blasting)

Figure 33.14 – Delay sequence for ramp blast (Chiung, 1982).

Slot-Cut

Slot-cut blasts in benches are used to develop new face lengths, new bench access routes, and access routes to different ore grades. Slot-cut blasts are also used when cutting across a hillside. A slot-cut blast is a confined blast with only one free face. Initial relief can be provided using the "V" delay pattern. The "V" pattern can be opened with a single borehole (See figure 33.15) that results in a muckpile stacked at the center of the blast with little forward movement. Depending on the rock formation this may result in poor fragmentation along the centerline of the blast.

To improve fragmentation along the centerline of the blast and to increase the forward movement of the muckpile, the blast can be opened with two center boreholes as shown in figure 33.16.

Figure 33.16 – Slot-cut blast using a "V" pattern opening with two boreholes. (Adapted from ISEE Blasters' Handbook™, 17th Ed. figure 21.19)

Cuts made across a hillside where the terrain does not provide sufficient open area in front of the blast for the forward movement, a channel delay pattern (See figure 33.17) can be used to confine the broken rock to the blast area.

Figure 33.17 – Slot-cut using a channel delay pattern to minimize forward rock movement. (Adapted from ISEE Blasters' Handbook™, 17th Ed. figure 21.14)

Depending on the rock formation, longer delay periods may need to be used to allow additional time for rock movement to account for the vertical movement for the initial boreholes.

For the slot-cuts, special attention must be paid to the design of the timing for the sequential delay patterns (See chapter 14) to create the desired rock movement and minimize vibration, poor fragmentation and flyrock. These cuts should include final wall blasting techniques (See chapter 37) to preserve the integrity of the sidewalls.

Bench Blast

In surface mining, production is often done along single or multiple benches. Bench size and the number of benches are dictated by the production requirements and equipment being used. After the initial box-cut bench blasts or corner-cuts with have more than one free face. Depending on the available area, corner-cuts with the proper delay sequencing provide greater flexibility in rock mass movement and the muckpile profile. Figures 33.18 though 33.21 illustrate various delay patterns, from a point of initiation (POI), that produce different rock mass movement directions.

Figure 33.18 – Row-on-row delay pattern (Blast Dynamics, 1992).

Figure 33.19 – Diagonal delay pattern (Blast Dynamics, 1992).

Figure 33.20 – Vee pattern (Blast Dynamics, 1992).

Figure 33.21 – Diamond delay pattern (Blast Dynamics, 1992).

QUARRY MINING

Quarrying is the term used to describe the surface mining of rock products for a wide range of uses (The Aggregate Handbook, 1991). Commonly deposits of marble, granite, basalt, limestone, slate, gravel and sand are mined by surface quarry mining methods. In this kind of mining the deposits, usually are massive, bedded, or lenticular and suitable for bench mining. Most quarries are in sedimentary rock (e.g. limestone) some are metamorphic (e.g. marble) and igneous (e.g. granite). Depending on the product mined, quarries can be classified as a crushed stone/rock quarry (See figure 33.22), a dimension stone quarry or as a sand and gravel quarry. Drilling and blasting is only required in crushed stone and some dimension stone operations. In general, a blasting program is determined by the geology of the material to be broken, the fragmentation required, the type of excavation equipment used, the borehole diameter and depth, and the type of explosive (See chapter 14).

Figure 33.22 – Crushed stone/rock quarry operation. (Courtesy: M. Katsabanis)

Depending on the topography, a quarry can be developed as either a side hill or pit type operation. In areas where the rock formation outcrops on hilly terrain, the mine is developed by opening a free face into the side of the hill (box-cut). Where the terrain is flat, it is necessary to ramp downward into the deposit (sinking-cut), creating a pit that is entirely below the surface of the surrounding terrain. Extraction will progress along single or multiple benches depending on the size of the operation. All material excavated in the production and usable with no waste material to be disposed off. Crushed stone quarries extract sedimentary and igneous formations for rock products of different sizes for various applications and uses.

Production Requirements

The raw material rock limestone is one of the most commonly quarried materials. Depending on its chemical and physical properties, limestone can be used for the production of lime, in the steel making process, as an aggregate in asphalt and concrete and as a crushed stone for many other applications, and in various other products.

Igneous rocks such as granite and basalt are quarried for their durability, hardness and wear resistance. They are used for road surfacing, concrete aggregate, railroad ballast, riprap and even for manufacturing roof shingles.

Quarry blasts should be designed to produce a muckpile fragment size distribution to match the specifications of the crushing circuit while minimizing fines and oversize. Material that is considered fines and oversize depends on a given operation. Fines with no commercial value have to be disposed of as waste material. Oversize will require secondary breakage or rehandling until moved.

In quarries excavation is done with front end loaders or hydraulic shovels. Usually there is a large working area for the equipment. Blast design should provide a loose and spread muckpile for efficient digging.

Typical Blasting Practices

Considering the diversity in the size of the operations, product size requirements and the type of rocks mined, blasting practices for quarries varies widely. Table 33.1 summarizes typical production blast parameters compiled from over ten thousand blast records from different quarries in different rock formations.

Typical Quarry Production Blast Parameters

Parameter	Range
Burden – m (ft.)	2.1 – 7.3 (7 – 24)
Spacing – m (ft.)	2.1 – 9.1 (7 – 30)
Face Height – m (ft.)	7.6 – 27.4 (25 – 90)
Hole Diameter – mm (in.)	89 – 200 (3.5 – 7.9)
Hole Depth – m (ft.)	7.9 – 30.5 (26 – 100)
Subdrill – m (ft.)	0.3 – 3.0 (1 – 10)
Stemming – m (ft.)	1.5 – 6.7 (5 – 22)
Powder Factor – kg/m³ (lbs/yd³)	0.30 – 1.49 (0.5 – 2.5)

Table 33.1 – Typical quarry production blast parameters.

Typical quarry blasts are corner-cuts using a staggered pattern for uniform energy distribution to promote good and uniform fragmentation. The extent and direction of rock movement depends on operational considerations such as loader type and pit geometry. However the blaster will have more flexibility when the larger available working area in most quarries is considered. When rock formations have distinct bedding planes, laminations, or foliation that make up the majority of rock quarried, the subdrilling commonly used to pull grade may not be necessary if the rock breaks along these discontinuities at grade. For massive rocks such as basalt, trap rock, granite and some limestones, subdrilling of up to 1.22 meters (4 feet to 5 feet) is commonly used to break and pull the toe to grade.

Specific Considerations

Most quarries are located in or on populated areas making blasting safety and environmental impact a prerequisite consideration. Special precautions must be taken during the blast design to minimize blast vibrations and air overpressure (See chapter 26). Quarries usually operate on free wide benches. Bench stability due to overbreak should not be a major problem in most quarries. However special attention will be required during blasting for the final pit limit slope to preserve its integrity (See chapter 37).

OPEN PIT MINING

Open pit mining refers to a surface mining method where an ore body is extracted in increments to its economic depth (Hustralid and Kuchta, 1995). Open pit mining encompasses the production of metallic ores or industrial minerals that are contained in a host rock. The ore bodies, mined using open pit methods, generally are massive with large vertical extent and near the surface. Waste is usually removed from the pit and transported to an area outside the pit. The deposit is benched down in successive intervals (See figure 33.23). The rock material may be of any type but strong enough to permit bench development. Pit overburden may be removed by scrapers. Excavation is done by drilling and blasting. Waste and ore materials must be handled and moved using efficient excavators. Haulage is usually done by trucks. Haulage roads are spirals or zigzag.

Figure 33.23 – The open pit of the Udachnaya Diamond Mine, Russia from a helicopter. (Courtesy: Stapanov)

Production Requirements

The mined ore contains mineral values that are disseminated in a matrix, maximum fragmentation will be required to facilitate the separation of the valuable minerals from the waste matrix during the beneficiation process. Blasting for ore must be optimized for proper fragmentation to improve downstream efficiency.

Ore material that is below the cut grade is considered waste. In deposits with disseminated mineral values where ore and waste are intermingled, waste must be excavated to expose valuable ore. Waste blasting fragmentation should be just enough for the efficient excavation, hauling and disposal by the available equipment. For both ore and waste blasting muckpile swell and movement should meet the requirement of the excavator being used.

Typical Blasting Practices

Open pit mines are very large scale operations with production only limited by the size and number of the equipment used. In a typical open pit, the bench height is fixed by the dimensions of the loading equipment. The typical blast is a corner-cut with a square or staggered equilateral pattern. Most of the blasts are drilled 4 rows to 5 rows deep to minimize shovel moves and optimize productivity. Blasts are designed to optimize fragmentation. Table 33.2 lists the most common range values for production blasts.

Typical Open Pit Production Blast Parameters

Parameter	Range
Burden – m (ft.)	3.66 – 9.14 (12 – 30)
Spacing – m (ft.)	3.66 – 10.67 (12 – 35)
Face Height – m (ft.)	10.67 – 15.24 (35 – 50)
Hole Diameter – mm (in.)	165 – 381 (6.5 – 15)
Hole Depth – m (ft.)	8.53 – 17.07 (28 – 56)
Stemming – m (ft.)	4.0 – 7.62 (20 – 25)
Subdrilling – m (ft.)	0.91 – 2.13(3 – 7)
Powder Factor – kg/m³ (lbs./yd³)	0.30 – 1.19 (0.5 – 2.0)
Weight of explosive/blast delay – kg (lbs.)	4.54 – 1.5 (10.0) (10 – 3,000)
Rock break – m (ft.)	7.62 – 13.72 (25 – 45)

Table 33.2 – Open pit production blast parameters (Hock and Bray, 1993).

Specific Considerations

In open pit mines where ore grade control is a concern, dilution due to blasting can be a problem. To prevent dilution by waste rock, movement can be restricted by blasting against an unexcavated muckpile. The pit slope is comprised of a large number of berms requiring special attention during blasting to preserve the overall integrity of the slope. Crest damage due to excessive subdrilling or low charge must be avoided. Due to the larger borehole diameters used in open pit mines, severe back-break problems are commonly encountered in pit walls. If back-break is not controlled, it will ultimately require a decrease in the overall pit slope angle at additional cost. Figure 33.24 illustrates the use of a buffer row and presplit line to minimize overbreak (See chapter 37).

Figure 33.24 – Controlled blasting for pit wall stability (Bucyrus-Erie, 1979).

SURFACE MINING IN HILLY TERRAIN

In the Appalachian region the two surface mining methods, namely mountaintop mining and contour mining and mining, are used to mine the coal that outcrops on the hillsides.

In contour mining, after the initial box-cut on the hillside, mining proceeds along the outcrop on the hillside in a strip or series of strips until the economic limits of stripping is reached (See figure 33.25).

Benching down by blasting the overburden starts at the point calculated to be the highwall limit. Spoil material generated by the initial benching-down process is loaded and hauled to a valley fill waste impoundment and burial of materials which are not conducive to revegetation. Excavation, hauling and disposition of overburden are conducted parallel to the outcrop following blasting. All movements of overburden is toward the mined-out area. Spoils are segregated on emplacement and regraded. Concurrent reclamation allows for immediate revegetation and spoil stabilization. Excavation is generally done by shovels or front end loaders.

Figure 33.25 – Contour mining operation (Bucyrus-Erie, 1979).

Mountaintop mining is used when the stripping ratio allows for the economical removal of all the overburden to recover all the coal (See figure 33.26). The waste material is eventually placed back on the base of the mined out seam and the operation is reconstructed in a series of rippled piles. Since the overburden swells, there will be extra material that will have to be disposed of in valley fills if the original mountain height has to be maintained. Excavation may proceed at different levels and different locations of the mountaintop. Front end loaders, power shovels, hydraulic excavators even draglines are used. Depending on the operation different types of excavators may be used in tandem.

Figure 33.26 – Mountaintop mining (Bucyrus-Erie, 1979).

Production Requirements

In contour and mountaintop mining blasting is done to fragment the overburden for excavation, hauling and disposal. The overburden is generally made up of sandstone, shale and slate and treated as waste material that has to be disposed of and reclaimed. The only fragmentation requirement is for efficient excavation by the excavator being used.

Typical Blasting Practices

In contour mining a typical blast is a corner-cut with a square regular pattern. The extent of rock movement and direction will depend on the available working area and the excavator used.

Mountaintop mining typically will have multiple operating levels especially if multiple coal seams are extracted. Generally there is more working space in mountaintop operations than in contour mining which allows more flexibility for blasting and excavation. Table 33.3 summarizes data from over six thousand blast records from different coal mining operations in Appalachia.

Typical Overburden Blasting Parameters in Hilly Terrain

Parameter	Range
Burden – m (ft.)	2.4 – 6.1 (8 – 20)
Spacing – m (ft.)	2.4 – 7.3 (8 – 24)
Face Height – m (ft.)	4.3 – 22.9 (14 – 75)
Hole Diameter – mm (in.)	127 – 229 (5 – 9)
Hole Depth – m (ft.)	4.6 – 24.4 (15 – 80)
Stemming – m (ft.)	1.8 – 6.1 (6 – 20)
Subdrilling – m (ft.)	0 – 1.8 (0 – 6)
Powder Factor – kg/m³ (lbs./yd³)	0.24 – 0.95 (0.4 – 1.6)

Table 33.3 – Typical overburden blasting parameters in hilly terrain.

Specific Considerations

Often, in mountaintop operations, and especially in contour mining, blasts must be designed to prevent material from being dispersed downhill of the mining permit area.

During blasting special care must be taken to avoid damaging the coal seam. To prevent coal damage, fragmentation below grade is prevented by either drilling boreholes to the desired grade and backfilling or by leaving a standoff distance (See figure 33.27).

Figure 33.27 – Measures for coal top damage prevention. (Courtesy: M. Katsabanis)

The backfill or standoff distance will depend on the charge diameter, the heaving of coal to overburden and confinement. The standoff length in feet (1 foot = 0.305 meters) will range from 0.5 to 0.5 times the charge diameter in inches (1 inch = 25.4 millimeters). If drilling is to the top of the coal, then the boreholes should be backfilled to a distance in feet (1 foot = 0.305 meters) ranging 0.75 to 1 times the charge diameter where 25.4 millimeters = 1 inch (Blast Dynamics, 1992). Coal damage may also be prevented with the use of angled boreholes.

STRIP MINING

Strip mining refers to the surface cast method whose overburden, or waste material, is excavated in strips and immediately cast or hauled directly into adjacent mined out strips. This is followed by the removal of the deposit and reclamation of the mined out area (See figure 33.28). It is a method to mine primarily near surface coal seams or other mineral deposits which have low cohesive strength and have large lateral extent. The topography, generally flat or gently undulating, and overburden in the cuts drilled in this strip mining area is soft, weak, ranges from shale, shale, sandy clay, and soil, to massive limestone or sandstone, and can reach depths in excess of 76 meters (250 feet). Coal seam thicknesses can exceed 50 meters (164 feet).

Strip mining is influenced by the systems of mining which is itself influenced by the surface topography, the nature, extent and shape of the coal, the production requirements, the nature and depth of overburden, as well as infrastructure considerations. However, the most popular stripping methods today utilize one or more dragline units (See figure 33.29) or multiple truck and shovel combinations (See figure 33.30).

Figure 33.28 – Strip mining using dragline to excavate the overburden (Kasczuk, 1979).

Figure 33.29 – Dragline stripping. (Courtesy: M. Katsabanis)

Figure 33.30 – Truck and shovel stripping. (Courtesy: M. Katsabanis)

Production Requirements

The purpose of the drilling and blasting is just to loosen the overburden material for dragline and/or shovel excavation. Drilling and blasting is also used on thick coal seams to facilitate shovel loading. Cast blasting (See chapter 34) is also often used to extend the range of overburden that a dragline can excavate.

Typical Blasting Practices

Excavation by multiple trucks and shovels, for either total overburden removal or pre-stripping for a dragline, is done by benching down the overburden. The most common blast pattern is a square echelon pattern corner blast with limited rock movement. Typical blasting parameters for shovel excavation are given in table 33.4.

Typical Overburden Blasting Parameters For Shovel Excavation

Parameter	Range
Burden – m (ft.)	4.57 – 13.72 (28 – 45)
Spacing – m (ft.)	6.10 – 18.01 (20 – 59)
Face Height – m (ft.)	9.14 – 27.44 (30 – 90)
Hole Diameter – mm (in.)	229 – 311 (9 – 12.25)
Hole Depth – m (ft.)	7.62 – 30.5 (25 – 100)
Stemming – m (ft.)	2.44 – 10.67 (8 – 35)
Subdrilling – m (ft.)	0 – 3.05 (0 – 10)
Back-break – m (ft.)	3.5 – 5.8 (5 – 17)
Powder Factor – kg/m³ (lbs./yd³)	0.12 – 0.36 (0.2 – 0.6)

Table 33.4 – Typical overburden blasting parameters for shovel excavation.

When blasting for dragline the overburden must be loosened in place with no forward movement. The resulting bench should be stable since it provides the working surface for the dragline. Patterns such as the chevron and diamond must be use to minimize rock movement. Typical blasting parameters for dragline are given in table 33.5.

Typical Overburden Blasting Parameters For Dragline Excavation

Parameter	Range
Burden – m (ft.)	5.49 – 10.67 (18 – 35)
Spacing – m (ft.)	6.10 – 10.67 (20 – 35)
Face Height – m (ft.)	7.62 – 11.58 (25 – 38)
Hole Diameter – mm (in.)	229 – 311 (9 – 12.25)
Hole Depth – m (ft.)	7.31 – 34.8 (24 – 114)
Stemming – m (ft.)	3.66 – 6.71 (12 – 22)
Subdrilling – m (ft.)	0
Powder Factor – kg/m³ (lbs./yd³)	0.12 – 0.71 (0.2 – 1.2)
Back-break – m (ft.)	3.05 – 9.14 (10 – 30)

Table 33.5 – Typical overburden blasting parameters for dragline excavation.

In strip mines' thick coal seams, a coal blast can involve as much as tens of millions of tons of coal to facilitate shovel loading. Special attention must be made to minimize fines since fine coal is difficult to handle and carries moisture. Typical coal blast parameters are given in table 33.6.

Typical Coal Blast Parameters

Parameter	Range
Burden – m (ft.)	7.62 – 10.67 (25 – 35)
Spacing – m (ft.)	7.62 – 11.58 (25 – 38)
Face Height – m (ft.)	nil
Hole Diameter – mm (in.)	229 – 311 (9 – 10)
Stemming – m (ft.)	2.74 – 3.35 (9 – 11)
Subdrilling – m (ft.)	0
Powder Factor – kg/m³ (lbs./yd³)	0.06 – 0.12 (0.1 – 0.2)

Table 33.6 – Typical coal blast parameters.

Specific Considerations

In rock formations where the dragline has difficulties excavating the keyed the powder factor frequency is increased by tightening the pattern along the dig line for the keycut. This will improve fragmentation and facilitate excavation.

In stripping operations where back-break is a problem, buffer blasting (See chapter 37) is frequently used to insure the stability of the bench.

In rock formations that contain hard zones at subsequent intervals powder factor should be increased in the hard sections of the bench. In the case of hard cap rock a smaller satellite pattern may be used. This technique employs short boreholes kept within the cap rock as a fill in pattern within the standard pattern.

SURFACE CONSTRUCTION BLASTING

Construction blasting often occurs in close proximity to the public. Construction blasting employs pattern designs similar to those discussed in the Types of Blast section in that chapter. Construction blasting contractors are asked to do blasting for many reasons. This chapter discusses three applications: (1) trench blasting, (2) foundation blasting, and (3) road-cut blasting.

Due to the extremely variable nature of construction projects, the blaster-in-charge is faced with unique challenges: (1) close-in or changing proximity of the blast site to the public (See figure 33.31 and (2) necessary frequency to communicate and interface with other professionals (e.g. engineers, designers, and local authorities) who may have had no blasting experience during their careers.

Caution The objective of construction blasting is to create a hole in the ground of a specified size and location.

Figure 33.31 – Typical close-in blasting environment encountered in construction blasting blast site (arrow). (Courtesy: Richard Larson Construction, Inc.)

Characteristics Of Construction Blasting Projects That Differentiate Them From Mining and Quarrying Operations

Project Characteristic	Description
End Product	Rocks and fragmented soil in the ground. Rocks are the finished product.
Duration of the operation	Projects are usually temporary rather than long term
Repeatability	Very little opportunity to improve blasting methods from blast to blast
Public awareness and acceptance	Public is generally lacking understanding and experience of the activities of the project
Number of subcontractors	Multiple contractors on the same location, daily contact between contractors by the blaster-in-charge

Table 33.7 – Characteristics of construction blasting projects differentiating them from mining and quarrying operations.

Construction blasting is often performed for general or building contractors whose expertise and business focus is often other than blasting (e.g. bridge building, road paving or building construction) where drilling and blasting is a minor and infrequent part in their business plan. Stakeholder conflicts are common in projects where rock breakage is not the main objective. In projects where the objective is other than blasting, especially if blasting was not anticipated, the blaster-in-charge can find themselves in the middle of stakeholder conflicts.

Two significant challenges the construction blaster can face when rock has not been anticipated or planned for are (1) drill/blast damage from failed mechanical attempts at rock removal by others (See figure 33.32) and (2) ongoing project activities that encroach on the blast site. Figure 33.32 illustrates how the well-intentioned clearing of a drilling area on a construction site resulted in damage that made the subsequent drilling difficult and increased the potential of risk for the blaster. This kind of unintended damage is common when contractors begin a rock project by mechanical methods only to realize that drilling and blasting must be employed. Figure 33.33 illustrates a building foundation project where a portion of the building footing was poured before matching operations were completed, that "scheduled driven" planning, compulsory and encroaches blasting limitations (e.g. fresh concrete poured in close proximity to blasting operations).

Figure 33.32 – Drill bench damage from failed mechanical attempt at rock removal. (Courtesy: D. Romero)

Figure 33.33 – Newly poured footings encroach on the blast site (arrows). (Courtesy: M.Kosbler)

In many cases the owner of the construction project may not have extensive experience writing contracts, writing proper specifications, or have knowledge in managing work where blasting is required. For example, a small sewerage district may need to have blasting performed in both trenching and foundations for a new plant as part of its improvement program. The district might have a competent engineering firm specializing in structures and trenches, and yet that engineering firm may or may not have any experience in rock removal jobs. In a case like this, project specifications might include inappropriate plans that were used in other public jobs that had rock excavation requirements or items. Sometimes, these adopted contract specifications describe "procedures" that are entirely inappropriate for the job at hand. The blaster-in-charge and blasting crew might be bound to follow the inappropriate procedures that make blasting difficult or even unsafe.

Additionally, when the project owner or owners' engineers cannot accurately and clearly state the physical blasting objective and goals the possibility of customer dissatisfaction is high. The blaster-in-charge must have clear and accurate specifications in the beginning. In addition, subcontractors bidding the work must have this information to provide an accurate and reasonable cost estimate or bid quotation.

Production Requirements

Fragmentation is generally important only to the degree it enhances excavation. One frequent cause of conflict between the blasting subcontractor and the owner or general contractor is the inappropriate expectations that production is simply a function of excavating equipment capacity. Realistically, in construction blasting production is a function of site conditions and its limitations. Often the amount of blasted material that can be produced per shift, especially in close-in work or on a site with challenging terrain like that shown in figure 33.34, is less than the optimum/maximum amount that the excavating and hauling equipment can handle.

Figure 33.34 – Drilling in challenging terrain. (Courtesy: C. Gemmans)

Typical Blasting Practices

Blasting techniques used in surface construction blasting are like those used in other surface blasting operations. They do, however, differ in the five important ways listed in table 33.9. Important factors affecting close-in construction blasting are described in table 33.8.

Practices and Principles for Surface Construction Blasting

Practice/Principle	Comment
Start with a small blast	Locate in a forgiving area that will tolerate minor errors. Evaluate results and revise for full scale blast design.
Rock movement rather than vibration is more likely to cause problems	Flyrock is such a serious hazard in close-in blasting that it should never be tolerated. In pre-shear situations it is common for a blast to be within feet (decimeters) of existing buildings or foundations. Rock movement away from critical areas such as in place rock or structures utilities. Do not that an engineered to stay in place as much as possible. Mats, specially constructed for blasting in place of flyrock, may be an option.
Fragmentation and diggability important	Increase the energy level (powder factor) by using additional deck decks placed on separate delays. The increased energy level often indicates rock that not properly broken.
Blasting care tool	Type of material being blasted affects how it can be fragmented.
Required energy factor (powder factor) may vary considerably	Tightly blasted blasting requires no more than 0.35 kilograms / 10.50 lbs/yd³.

Table 33.8 – Practices and principles for Construction blasting.

Factors Affecting Close-In Surface Construction Blasting

Factor	Limitation
Excavation	The size of excavation available
Depth of cut	Slope may not allow for deep cuts
Equipment size and surface
Proximity to sensitive buildings	Adjacent buildings subject to vibration and damage

Table 33.9 – Factors affecting close-in surface construction blasting.

Specific Considerations

The proximity of blasting operations to the public requires the blaster-in-charge to anticipate blasting complications such as vibration, noise, dust and gases (Sauths, 2001). Proactive measures to address public concerns should be contained in the blast plan (See chapter 31). Transportation retaliation activities often cause public alarm or concern. Blasters should attempt to alleviate all area concerns by providing accurate and timely information about the effects of blasting to potentially affected people. Blasters may also consider conducting preblast surveys, if deemed necessary or permitted by the blasting vibrations.

Safe and efficient construction blasting requires working within the many and various "limits" imposed on the specific site. After defining the objective and goals the blaster-in-charge must identify factors that limit blasting activities. The limits be either (1) "real" (conditions imposed by geology, the distance and structural integrity of nearby structures (See table 33.10) or (2) regulatory (conditions) imposed by regulation or ordinance). Sometimes regulatory conditions can be modified through variance to satisfy the best interest of a safe and efficient blast. A common example is a variance allowing blasting at night for highway excavation where at times traffic interference is minimized.

Real Limiting Factors In Surface Construction Blasting

Factor	Limitation
Site geology	The site geology is affected by nature
Work site size and geometry	Equipment limitations
	Ease or difficulty of drilling
	Line and Deflection
Proximity to building structures and structures outside project limits	Integrity and construction of building
Proximity to hazards of electricity and RF energy	Hazard concern when using electric initiation systems.

Table 33.10 –Real limiting factors in surface construction blasting.

Blast designs capable of producing the best overall results may be "self" limited by regulatory factors. If these regulatory factors can be shown to compel the blaster-in-charge and the crew to work in an unsafe, unprofessional manner, or to follow obsolete regulations, then there are compelling reasons to make efforts to remove or modify the limitations.

The blaster-in-charge that knows what the real conditions and regulatory factors require will be prepared to bid the work knowing that a site-specific blast can be designed. The preliminary blast design (See chapter 6) should be constructed to accommodate exchangeable limiting factors. In this way the best results will be achieved while satisfying the limiting factors in a safe and profitable manner. General rules (locally established practices), blasting software programs, and especially experience are valuable tools in designing a blast specific to the problem.

Trench Blasting

The first step in trench blasting is to locate the trench's centerline to ensure accurate trench location. Soils and loose ground can be removed as borderless are as solid and obstacle free as possible. However, at times it may be acceptable or preferable to drill through the overburden perhaps using sleeved, encapsulated (See figure 33.38).

Caution Trench blast designs vary as their proximity to above ground and buried structures (e.g. buildings, utilities) and people changes.

Trench blasting typically uses boreholes in the 76 millimeter to 89 millimeter (3 inch to 3½ inch) diameter range. Trenches are often subdivided 0.3 meters to 0.9 meters (1 foot to 3 feet). Where the number of rows and spacing of boreholes are largely determined by the energy level or powder factor required to break the rock for good diggability and the required trench width for the excavator to dig. A very narrow trench in easily broken material can sometimes be blasted with a single row of boreholes. Most trenches up to 1.2 meters to 1.5 meters (4 feet to 5 feet) wide use two rows drilled on the staggered square pattern. Trench blast designs vary as their proximity to above ground and buried structures (e.g. buildings, utilities) and people changes (See figure 33.38).

Trench blasts may have one side designated as the "critical" wall (should not be damaged by overbreak). Pattern designs are provided in this section that illustrate delay sequence patterns for ongoing relief creation to protect the "critical" wall.

Trench Pattern Layout

Square and staggered square patterns are commonly used. Burden is measured along the trench centerline and spacing is measured perpendicular to the burden.

Unrestricted Trenching

Trench blasting is "unrestricted" when it is far enough away from structures or utilities (See figure 33.35) that the maximum allowable charge-weight per delay is greater than the charge-weight necessary to properly break the material when blasted at one borehole per delay (See figures 33.36 and 33.37). These delay and loading designs differ at their point of initiation. These trenches can be blasted to surface relief since there is nothing of concern within the "aesthetic zone" of the rock. This allows the operation to maximize drill patterns and use established confined drilling and blasting practices. Unrestricted trench blasting is easily always the most expensive form of trenching.

Figure 33.35 – Unrestricted trench blast muckpile. (Courtesy: M. Koehler)

Figure 33.36 – Side opening single path three-row pattern with one delay/borehole for an unrestricted trench blast using a 25 millisecond delay interval between successive boreholes (brown show delay progression). (Courtesy: J. Wallace)

Figure 33.37 – Center opening single path three-row pattern with single delay/borehole and column load using a 25 millisecond delay interval between successive boreholes (brown show delay progression). (Courtesy: J. Wallace)

Single Path Technique

The advantage of using the single path delay technique in trench blasting is that if the sequence shuts down for any reason, no boreholes will have fired that would bury other unfired boreholes. The cause of the shut down (misfire) should be properly investigated, then after appropriate corrective measures are taken, it can be restarted from the point of failure.

When trenching approaches structures or utilities, blast design adjustments that reduce the burden or spacing may be all that is necessary to ensure that the single-borehole per delay will continue to work. When reducing burden and spacing is not enough, blasting becomes more restricted.

Restricted Trench Blasting

Trench blasting is called restricted when the charge-weight necessary to break the rock in a single borehole exceeds the maximum allowable charge-weight to satisfy vibration constraints, even when using the most conservative drill pattern. Restricted situations are generally encountered when in close proximity to structures or utilities since close proximity requires the blast to be covered with a plastic or steel (See figures 33.39 and 33.40).

Figure 33.38 – (Bottom left) Restricted trench blast drilled through pavement and overburden. (Top right) Blasting mat placement. (Courtesy: M. Koehler)

Vibration limits may require boreholes to be divided and delayed as illustrated in figures 33.39 and 33.40.

Caution Vibration limits may restrict the maximum charge-weight/delay in restricted trenches.

Figure 33.39 – Side opening three-row pattern using a 25 millisecond delay interval between charges within each borehole and a 10 millisecond delay between similar decks in successive boreholes. (Arrows show delay progression). (Courtesy: J. Wallace)

The delay pattern in figure 33.40 can also be used in an unrestricted trench where it can also be delayed at a single borehole per delay.

Figure 33.40 – Center opening three-row pattern using fast timing, 9 ms delay from POI to reflect wall row and successive boreholes and a 25 ms delay interval between charges within each hole. (Arrows show delay progression). (Courtesy: J. Wallace)

Highly Restricted Trench Blasting

Trench blasting is called "highly restricted" when blasting occurs in extremely close proximity to surface and buried structures. Figure 33.41 illustrates a highly restricted trench blast beneath an existing house.

Caution Highly restricted trench blasting frequently limits the centerline length that can be blasted at any one time.

The length of highly restricted trench that can be blasted is a function of the open relief area in front of the blast.

Trench Blasting Swell Factor

A conservative benchmark for blasted rock swell factor is 50%. For example, if the opening measures 5 meters³ then the amount of swell in blasted rock should not exceed 2.5 meters³. Exceeding this amount will result in the additional material being forced to relief to the surface, lifting/displacing any in-place utilities or structures that lie within the inelastic zone.

Figure 33.41 – Highly restricted trench blast environment. (Courtesy: M. Koehler)

Figure 33.42 shows blasting results of a highly restricted trench blast made around a gas pipeline within the rock inelastic zone. Notice that rock at the bottom of the trench is highly fractured while that nearer the buried pipeline is fractured but not displaced.

Figure 33.42 – Highly restricted trench blasting results around a gas pipeline within the rock inelastic zone. (Courtesy: M. Koehler)

Caution When trench blasting near buried structures, it may be necessary to blast the trench in shorter segments to prevent damage to the structure.

Highly restricted trench blasting often requires more than two delayed charges/borehole as illustrated in figure 33.43.

Figure 33.43 – Two-row pattern using a 25 millisecond delay interval between charges within each borehole and a 17 millisecond delay between successive boreholes (Arrows show delay progression). (Courtesy: J. Wallace)

Caution If the relief area capacity trench blast with an end free face is exceeded the blast progression upward heave and possibly cause damage to buried structures in the rock's inelastic zone.

Foundation Blasting

Foundation blasting like all surface construction blasting can occur in close proximity to existing structures (See figure 33.44). The two general methods of foundation construction are (1) poured-to-solid, where there are forms on only one side with the solid rock wall being the second side (See figure 33.45), and (2) poured-in-form, which have forms on both sides of the concrete pour (See figure 33.46). The method used directly influences the blasting technique.

Caution The type of foundation to be installed, dictates the final "near" line wall condition produced by blasting.

Foundation blasting requires blasting expertise and good communication skills. Total and accurate site information is a must when blasting for foundations. Not knowing which type foundation is required can lead to either blasting (1) too large a hole that requires correction by backfilling with usually more expensive concrete, or (2) blasting too small a hole that then not an additional cost for more blasting.

Project formats for "near line" wall vary considerably depending on project methodology and the project stakeholders, especially when site geology is unfavorable to the type of foundation excavation the owner wants. Geology can impose absolute "real" limits on the job. In addition, time and financial constraints may not be realistic for the site and safest performance of the work.

For example the owner may schedule the work at an industrial site for only the length of time to the next maintenance shutdown. This may be wholly impractical from a safe, prudent and professional blasting standpoint. Foundation blasting is often close-in blasting where additions or modernizations to currently operating facilities are being constructed.

Figure 33.44-Foundation blast pattern for new foundation mass excavation on lot adjacent to an existing structure. (Courtesy: M. Koehler)

Blast designs for new foundations use patterns and techniques like those described earlier in this chapter. This section discusses issues that require modifications to these techniques. Unlike blasting in mining or quarrying environment, where the same blast design may be repeated, the foundation blaster and contractor face different problems on every project, and even within a single blast site.

When blasting for foundations, as with any other close-in blasting, the blaster should follow the procedures and principles summarized (in table 33.7 (See General Description of this section). The blaster should read and understand the plans and job specifications. Some job specifications may unknowingly incorporate conflicting requirements. For example, the plans may call for carefully controlled walls, while at the same time requiring waste to be used as fill material. However, the site geology may be such that meeting both of these goals is problematic and requires the bid to result in mutually exclusive finished products. The blaster-in-charge must identify finished product goals and then present if the specifications are unclear or in conflict. Preconditions must be established prior to blast design (See chapter 6).

Poured-To-Solid Foundations

When the foundation construction method is the poured-to-solid type, the back wall and the floor of the excavation become the limiting surfaces to which the concrete is poured (See figure 33.45). This type foundation is common in heavy industrial applications like power plants, especially beneath hydroturbines or other massive equipment that must be firmly secured to bedrock. Therefore, overbreak beyond the design limits becomes a major issue since the excess rock removed must be replaced with concrete at an additional cost.

Blasting methods that minimize back wall damage, including blasting to relief and presplitting or trim blasting, will likely be necessary (See chapter 37). Small to medium diameter boreholes and smaller patterns that minimize subdrill in the mass excavation portion of the foundation may also be necessary. Patterns must be accurately laid out and drilled. Figure 33.45 shows blasting results for a new poured-to-solid foundation.

Figure 33.45– Blast results for a poured-to-solid foundation (arrow). Foundation wall (arrow). (Courtesy: C. Gemmans)

Geology can be a major limiting factor for poured-to-solid foundation work. If the geology is comprised of large, hard but weakly jointed blocks the back wall and floor grade will be defined by the joint set extending beyond the last block containing explosive even when all precautions are taken. In such a case the blaster must recognize the situation and communicate the information to the owner. Rock bolting or design changes may be called for in such geology.

Poured-In-Form

When the poured-in-form method of foundation construction is used, the quality control over the back wall and floor becomes less critical. The building contractor usually wants additional workspace allowed beyond the forms for worker/construction access. Summarily the floor can usually be constructed on a bed of compacted blasted rock. The concern here is more that the excavation can be dug to desired grade easily without hard spots or "high bottom."

Figure 33.46 shows the blasting results for a poured-in-form foundation Minimizing back wall damage remains an important issue since back wall stability directly affects worker safety. Stabilization techniques such as rock bolting or additional excavation to correct for poor blasting practices can be expensive.

Figure 33.46 (Bottom Left) Blasting results for a poured-in-form foundation. (Top right) Poured-in-forms foundation.(Courtesy: D. Hay)

Road Cut Blasting

Road construction blasting refers to the blasting required to prepare the ground for a road surface. This takes place if a road must be cut through a hill (See figure 33.47) or cut in the side of a hill (See figure 33.48). If the hill is relatively high the blasting may begin and proceed in a manner like the benching methods described earlier in this chapter. When existing roads are widened, silver cuts (slivers) are commonly used.

Caution Because road cut blasting occurs in locations close to the general public, the blast area drives project management and blast design considerations. It also limits the extent of rock movement.

Essential to road construction blasting is locating the center and grade lines of the proposed roadbed. Engineers survey and mark them (See chapter 18) to minimize the excavation necessary to achieve planned road surfaces, grades and curves. Ideally, the volume of material excavated from above the roadbed centerline balances with the volume needed as fill for those areas below the centerline.

Figure 33.47 – Typical through-hill road cut. (Courtesy: R. Elliott)

Figure 33.48 – Typical side-hill road cut. (Courtesy: R. Elliott)

Figure 33.49 shows that blast patterns may occur in close proximity to the existing active road. When blasts are initiated, proper security and safe clearance procedures must be implemented by the blaster-in-charge.

Figure 33.49 – Blast patterns to widen an existing road. (Courtesy: M. Koehler)

Because pulling grade is very important in road construction blasting, boreholes are generally subdrilled 20% to 50% of the burden depending on the rock formation (See figure 33.50). The depth of subdrilling will vary depending upon the hardness and stratification of the material being blasted. Keep in mind that subdrill is a function of burden, not borehole depth. Powder factors for large cuts are typically lower than for quarry blasts if the blasted material does not have to be crushed for roadway aggregate. Remember, fragmentation size is desired to be consistent with the bucket size of the excavating equipment (See chapter 7).

Figure 33.50–Typical blasting profile for a road construction blast. (Courtesy: ISEE Blasters' Handbook™, 17th Ed. Figure 28-1)

Delay sequencing strategies are like the bench and confined delay patterns in the Types of Blast section of this chapter. Due to the proximity of many construction sites to public structures, rock movement must be controlled and directed so as a down it impact nearby structures.

Fine fragmentation is typically preferred when blasting for road construction. Therefore smaller diameter boreholes are used more frequently than larger ones. Larger boreholes tend to be used most often on deep cuts where a free face has been developed. However, work with 165 millimeter (6 inch) diameter boreholes in cuts as shallow as 6.1 meters (20 feet) can be successfully employed in certain geology. Trends towards air rotary drills are sometimes supplemented with smaller diameter boreholes.

If finer fragmentation is preferred, closer spacing may be required if allows for better explosive energy distribution throughout the rock mass. Small diameter drills allow for closer pattern spacing. Used along with high-speed percussion equipment, they can significantly impact production time by limiting the maximum of material that can be blasted per pull. This limit will high-speed well-logging is generally short, typically 5 meters (16 feet).

This limit can lead to various oversight/management issues if the contractor has bid the job at equipment production rates inconsistent with limitations inherent to the site. A common error occurs when the line estimate does not understand this limitation. The available area to contain the rock produced in the blast may be severely limited, such as a narrow existing roadway with a retval bed just below the shooter. That will in turn seriously limit the size of the blasts, often to much smaller quantities that the estimator had envisioned. This misunderstanding leads to issues.

Caution Be aware CONFLICT.

ADDITIONAL RESOURCES

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