Chapter Four - Blast Design

Introduction to Blast Design

Blast Design Is Not a Precise Science

Blast design is not a precise science. Because of the widely varying nature of rock, geologic structure, and explosives, it is impossible to set down a series of equations which will enable the blaster to design the ideal blast without some field testing. Tradeoffs must frequently be made in designing the best blast for a given situation. This chapter will describe the fundamental concepts that are useful as a first approximation for blast design and also in trouble-shooting the cause of a bad blast. Field testing is necessary to refine the individual blast dimensions.

Major Principles of Blast Design

Throughout the blast design process, two overriding principles must be kept in mind:

1. Explosives function best when there is a free face approximately parallel to the explosive column at the time of detonation
2. There must be adequate space into which the broken rock can move and expand

Excessive confinement of explosives is the leading cause of poor blasting results such as backbreak, ground vibrations, airblast, unbroken toe, flyrock, and poor fragmentation.

Properties and Geology of the Rock Mass

The character of the rock mass is a critical variable affecting the design and results of a blast. The nature of rock is very qualitative and cannot be quantified numerically. Rock character often varies greatly from one part of a mine to another or from one end of a construction job to another. Decisions on explosive selection, blast design, and delay pattern must take into account firsthand knowledge of the rock mass. For this reason, the onsite blaster usually has a significant advantage over an outside consultant in designing a blast.

Keys to Characterizing the Rock Mass

The keys to characterizing the rock mass are a good geologist and a good driller. The geologist concentrates on obtaining data from the rock surface. Jointing is probably the most significant geologic feature of the rock. The geologist should document the direction, severity, and spacing between the joint sets. In most sedimentary rocks there are at least three joint sets, one dominant and two less severe. The strike and dip of bedding planes are also documented by the geologist. The presence of major zones of weakness such as faults, open beds, solution cavities, or zones of incompetent rock or unconsolidated material are also determined. Samples of freshly broken rock can be used to determine the hardness and density of the rock.

Drilling Information

An observant driller can be of great help in assessing rock variations that are not apparent from the surface:

• Slow penetration and excessive drill noise and vibration indicate a hard rock that will be difficult to break
• Fast penetration and a quiet drill indicate a softer, more easily broken zone of rock
• Total lack of resistance to penetration, accompanied by a lack of cuttings or return water or air, means that the drill has hit a void zone
• Lack of cuttings or return water may also indicate the presence of an open bedding plane or other crack

A detailed drill log indicating the depth at which these various conditions exist can be very helpful to the person designing the blast. The driller should also document changes in colour or nature of the drill cuttings, which will tell the blaster the location of various beds in the formation.

Rock Density

Some amount of displacement is required to prepare a muckpile for efficient excavation. The density of rock is a major factor in determining how much explosive is needed to displace a given volume of rock (powder factor). The burden-to-charge diameter ratio varies with rock density, causing the change in powder factor.

Rock Hardness

The hardness or brittleness of rock can have a strong effect on blasting results:

• Soft rock is much more "forgiving" than hard rock. If soft rock is slightly underblasted, you will probably still be able to dig it. If soft rock is slightly overblasted, excessive violence will not usually occur.
• Hard rock requires closer control and tighter tolerances. Slight underblasting often results in a tight muckpile that is difficult to dig. Overblasting is likely to cause excessive flyrock and airblast.

Voids and Incompetent Zones

Unforeseen voids and zones of weakness such as solution cavities, underground workings, mud seams, and faults are serious problems in blasting. Explosive energy always seeks the path of least resistance. Where the rock burden is composed of alternate zones of hard material and incompetent material or voids, the explosive energy will be vented through the incompetent zones, resulting in poor fragmentation. Depending on the orientation of the zones of weakness with respect to free faces, excessive violence in the form of airblast and flyrock may occur.

A particular problem occurs when the blasthole intersects a void. The void will be loaded with a heavy concentration of explosive, resulting in excessive airblast and flyrock.

Improving Fragmentation in Voids and Zones of Weakness

If these voids and zones of weakness can be identified, steps can be taken during borehole loading to improve fragmentation and avoid violence:

1. Use a good drill log to document the depths of voids and incompetent zones
2. Have the geologist plot the trends of mud seams and faults
3. Load inert stemming material, rather than explosives, through weak zones
4. Fill voids with stemming (where practical)
5. Block the hole just above the void before continuing the explosive column (where voids are too large)

Rise of the Powder Column

Where the condition of the borehole is in doubt, the rise of the powder column should be checked frequently as loading proceeds:

• If the column fails to rise as expected, there is probably a void - load a deck of inert stemming material before continuing
• If the column rises more rapidly than expected, frequent checking will ensure adequate space for stemming

Alternate Zones of Competent and Incompetent Rock

Alternate zones of competent and incompetent rock usually result in unacceptable blocky fragmentation. A higher powder factor will seldom correct this problem; it will merely cause the blocks to be displaced farther. Usually the best way to alleviate this situation is to use smaller blastholes with smaller blast pattern dimensions to get better powder distribution. The explosive charges should be concentrated in the competent rock, with the incompetent zones being stemmed through wherever possible.

Jointing Effects

Jointing can have a pronounced effect on both fragmentation and the stability of the perimeter of the excavation:

• Close jointing usually results in good fragmentation
• Widely spaced jointing, especially where pronounced, often results in a very blocky muckpile because the joint planes tend to isolate large blocks in place

Where the fragmentation is unacceptable, the best solution is to use smaller blast pattern dimensions. This extra drilling and blasting expense will be more than justified by the savings in loading, hauling, and crushing costs and the savings in secondary blasting.

Perimeter Holes

Where possible, the perimeter holes of a blast should be aligned with the principal joint sets. This will tend to produce a more stable excavation, whereas rows of holes perpendicular to a primary joint set will tend to produce a more ragged, unstable perimeter. Jointing will often determine how the corners at the back of the blast will break out.

Bedding Effects

Bedding can also have an effect on both the fragmentation and the stability of the excavation perimeter:

• Open bedding planes or beds of weak material should be treated as zones of weakness
• Stemming, rather than explosives, should be loaded into the borehole at the location of these zones
• In a bed of hard material, it is often beneficial to load an explosive of higher density than used in the remainder of the borehole
• To break an isolated bed of hard material near the collar, a deck charge is recommended
• Satellite holes (short holes, usually smaller in diameter) can be used to help break a hard zone in the upper part of the burden

A pronounced bedding plane is frequently a convenient location for the floor of the bench. It not only gives a smoother floor but also may reduce subdrilling requirements.

Dipping Beds

Dipping beds frequently cause stability problems and difficulty in breaking the toe of the burden:

• Beds dipping into the excavation wall enhance slope stability but create toe problems as the toe material tends to break out along the bedding planes
• Beds dipping outward from the wall form slip planes that increase the likelihood of slope deterioration and blasthole cutoffs caused by differential bed movement

Beds dipping outward from the final slope should be avoided wherever possible.

Surface Blasting

Blasthole Size Selection

The size of blasthole is the first consideration of any blast design. The blasthole diameter, along with the type of explosive being used and the type of rock being blasted, will determine the burden. All other blast dimensions are a function of the burden.

Large Diameter Blastholes

Practical blasthole diameters for surface mining range from 2 to 17 inches. As a general rule, large blasthole diameters yield low drilling and blasting costs because:

• Large holes are cheaper to drill per unit volume
• Less sensitive, cheaper blasting agents can be used in larger diameters

However, larger diameter blastholes also result in:

• Large burdens, spacings, and collar distances
• Coarser fragmentation

Comparison: Large vs. Small Blastholes

Four-Hole Pattern (20-Inch Diameter Holes)

• Relatively low drilling and blasting costs
• Broken material will be blocky and nonuniform in size
• Higher loading, hauling, and crushing costs
• More secondary breakage required
• Possible insufficient breakage at the toe

400-Hole Pattern (2-inch Diameter Holes)

• High drilling and blasting costs
• Finer, more uniform fragmentation
• Lower loading, hauling, and crushing costs
• Minimized secondary blasting and toe problems

Geologic Structure

Geologic structure is a major factor in determining blasthole diameter. Planes of weakness such as joints and beds, or zones of soft, incompetent rock tend to isolate large blocks of rock in the burden. The larger the blast pattern, the more likely these blocks are to be thrown unbroken into the muckpile.

Collar Distance and Environmental Problems

As more blasting operations are carried out near populated areas, environmental problems such as airblast and flyrock often occur because of insufficient collar distance above the explosive charge. As the blasthole diameter increases, the collar distance required to prevent violence increases.

Controlled Ground Vibrations

Ground vibrations are controlled by reducing the weight of explosive fired per delay interval. This is more easily done with small blastholes than with larger blastholes. In many situations where an operator uses large-diameter blastholes near populated areas, several delayed decks must be used within each hole to control vibrations.

Ideal Conditions for Large Blastholes

An operation with the following characteristics is ideal for large holes with large blast patterns:

• Large volume of material to be moved
• Large loading, hauling, and crushing equipment
• No requirement for fine uniform fragmentation
• An easily broken toe
• Few ground vibration or airblast problems
• Relatively homogeneous, easily fragmented rock without excessive, widely spaced planes of weakness or voids

Drill Patterns

There are three commonly used drill patterns:

1. Square - Equal burdens and spacings
2. Rectangular - Holes of each row lined up directly behind holes in the preceding row
3. Staggered - Holes in each row positioned in the middle of the spacings of the preceding row

The staggered drilling pattern is used for row-on-row firing. The square or rectangular drilling patterns are used for firing V-cut or echelon rounds.

Burden

The burden is defined as the distance from the blasthole to the nearest free face at the instant of detonation. In multiple row blasts, the burden for a blasthole is not necessarily measured in the direction of the original free face. One must take into account the free faces developed by blastholes fired on lower delay periods.

Calculating Burden

It is very important that the proper burden be calculated, taking into account:

• Blasthole diameter
• Relative density of the rock and the explosive
• Length of the blasthole (to some degree)

Consequences of Improper Burden:

• Insufficient burden: Excessive airblast and flyrock
• Too large a burden: Inadequate fragmentation, toe problems, and excessive ground vibrations

Charge Diameter and Burden

The burden dimension is a function of the charge diameter:

The burden-to-charge diameter ratio is seldom less than 20 or more than 40, even in extreme cases.

Approximate B/D Ratios for Bench Blasting

ANFO (density ~0.85 g/cm³):

Slurry/Dynamite (density ~1.2 g/cm³):

Burden Flexing and Rock Fragmentation

High speed photographs of blasts have shown that flexing of the burden plays an important role in rock fragmentation. A relatively long, slender burden flexes and breaks more easily than a short, stiffer burden.

Subdrilling

Subdrilling is the depth to which a blasthole extends below grade level to obtain a floor that is relatively free of humps and unbroken toe rock. Since the confining pressure is greatest at the bottom of the burden, where the rock tends to "lock" into the formation below the grade line, a greater concentration of powder is needed at that location.

Subdrilling Guidelines

• Subdrilling of 20 to 30 percent of the burden should be adequate for all but the most difficult conditions
• Subdrill should not exceed 50 percent of the burden even under the most difficult conditions
• If the toe cannot be pulled with a subdrill-to-burden ratio of 0.5, the fault probably lies in too large a burden

Subdrilling and Priming

Priming the explosive column at the toe level gives maximum confinement and normally gives the best breakage. Other factors being equal, toe priming usually requires less subdrilling than collar priming.

Excessive Subdrilling

Too much subdrilling is a waste of drilling and blasting expense and may also:

• Cause excessive ground vibrations owing to high confinement at the bottom
• Cause undue fracturing in the upper portion of the bench below in multiple bench operations
• Create difficulties in collaring holes in the lower bench

Insufficient subdrilling will cause high bottom, resulting in increased expensive secondary blasting.

Collar Distance

Collar distance is the distance from the top of the explosive charge to the collar of the blasthole. This zone is usually filled with an inert material called stemming to give confinement to the explosive gases and reduce airblast.

Effects of Collar Distance

• Too small: Excessive violence (airblast and flyrock), possible backbreak
• Too large: Boulders in the upper part of the bench

Collar Distance Recommendations

• Field experience shows that a collar distance equal to 100 percent of the burden is a good first approximation (except where collar priming is used)
• Collar priming normally causes more violence than center or toe priming, requiring longer collar distance
• Collar distances greater than the burden are seldom necessary

Where adequate fragmentation in the collar zone cannot be attained while still controlling airblast and flyrock, deck charges or satellite holes may be required.

Deck Charges

A deck charge is an explosives charge near the top of the blasthole, separated from the main charge by inert stemming. If boulders are being created in the collar zone but the operator fears less stemming would cause violence:

1. Reduce the main charge slightly
2. Add a deck charge
3. Fire the deck charge on the same delay as the main charge or one delay later

Care must be exercised not to place the deck charge too close to the collar of the blasthole, or excessive flyrock may result.

Public Relations and Collar Distance

From the standpoint of public relations, collar distance is a very important blast design variable. One violent blast can permanently alienate neighbours. In a delicate situation, it may be best to start with a collar distance equal to the burden and gradually reduce this if conditions permit.

Spacing

Spacing is defined as the distance between adjacent blastholes, measured perpendicular to the burden.

Effects of Improper Spacing

• Too close: Crushing and cratering between holes, boulders in the burden, toe problems
• Too wide: Inadequate fracturing between holes, humps on the face, toe problems between holes

Spacing Guidelines

Firing holes in a row on the same delay period:

• A spacing equal to twice the burden will usually pull the round satisfactorily

Using millisecond delays between holes in a row:

• Better fragmentation and reduced ground vibrations
• Spacing-to-burden ratio must be reduced to 1.2-1.8 (1.5 is a good first approximation)
• Large-diameter blastholes require lower spacing-to-burden ratios (1.2-1.5)
• Small-diameter blastholes can use higher ratios (1.5-1.8)

Except when using controlled blasting techniques, the spacing should never be less than the burden.

Hole Depth-To-Burden Ratio

As a rule of thumb for bench blasting, the hole depth-to-burden ratio should be between 1.5 and 4.0.

Ratios Less Than 1.5

• Cause excessive airblast and flyrock
• Give coarse, uneven fragmentation due to short, thick burden shape
• If required, place the primer at the toe for maximum confinement
• Consider increasing bench height or using a smaller drill

Ratios Greater Than 4.0

• Increased error in hole location at toe level
• Poorly controlled blast
• Extremely long, slender holes may even intersect
• Greater potential for cutoffs in the explosive column

Bench Height Considerations

High benches with short burdens create hazards such as:

• Small drill having to put in front row holes near the edge of a high ledge
• Small shovel having to dig at the toe of a precariously high face

Lower benches give more efficient blasting results, lower drilling costs, fewer chances for cutoffs, and are safer from an equipment operation standpoint.

Millisecond Delays

Millisecond delays are used between charges in a blast round for three reasons:

1. To ensure that a proper free face is developed
2. To enhance fragmentation between adjacent holes
3. To reduce the ground vibrations created by the blast

Optimum Delay Intervals for Bench Blasting

Based on Andrews (duPont) field investigations:

1. Delay time between holes in a row: 1-5 milliseconds per foot of burden

   • Less than 1 ms/ft causes premature shearing, resulting in coarse fragmentation
   • 3 ms/ft gives good results in many kinds of rock
   • Excessive delay prevents additional fracturing between holes

2. Delay time between rows: 2-3 times the delay time between holes in a row

   • Sufficient delay needed for burden from previously fired holes to move forward
   • If too short, movement in back rows will be upward rather than outward

3. For airblast control: At least 2 milliseconds per foot of spacing between holes in a row

4. For ground vibration control: Most regulatory authorities consider charges separated by 9 milliseconds or more to be separate events

In-Hole Delays

When using surface delay systems (detonating cord connectors, sequential timing machines), chances for cutoffs increase. To solve this:

• Use in-hole delays in addition to surface delays
• Example: 100 millisecond delay in each hole with surface connectors
• Avoid delays shorter than 75-100 milliseconds with sequential timers

Delayed Decks

It is sometimes necessary, when firing large blastholes in populated areas, to use two or more delayed decks within a blasthole to reduce ground vibrations. These blast rounds can become quite complex and should be designed under the guidance of a competent person.

Delay Accuracy

All currently used delay detonators employ pyrotechnic delay elements. Although reasonably accurate, overlaps have been known to occur. Therefore, when it is essential that one charge fires before an adjacent charge (such as in a tight corner), skip a delay period.

Powder Factor

Powder factor is a necessary calculation for cost accounting purposes.

Expression Methods

• Construction/stripping: Pounds of explosive per cubic yard of material broken
• Mining valuable materials: Pounds of explosive per ton of rock, or tons of rock per pound of explosive

Typical Powder Factors for Surface Blasting

Powder Factor Formula

P.F. = L × (0.3405d) × (D²) / [(B × S × H) / 27]

Where:

• P.F. = powder factor (lb/yd³)
• L = length of explosive charge (ft)
• d = density of explosive (g/cm³)
• D = charge diameter (in)
• B = burden dimension (ft)
• S = spacing dimension (ft)
• H = bench height (ft)

Factors Contributing to Lower Powder Factor

• Higher energy explosives (containing aluminum)
• Soft, light rock
• Large blastholes (larger proportion of stemming)
• Rock with numerous, closely spaced geologic flaws
• More free faces (corner cut < box cut < sinking cut)

Secondary Blasting

Some primary blasts, no matter how well designed, will leave boulders that are too large to be handled efficiently by loading equipment or that could cause plug-ups in crushers or preparation plants.

Four Techniques for Secondary Fragmentation

1. Drop Ball Method

   • Heavy ball dropped repeatedly from a crane
   • Relatively inefficient; may take considerable time for large or tough rock
   • Adequate where boulder production is not excessive

2. Wedging Device

   • Hole drilled into boulder; wedging device inserted to split it
   • Slow method; satisfactory for limited secondary fragmentation
   • Advantage: No flyrock associated with explosive techniques

3. Mudcap/Plaster/Adobe Charge

   • Loose explosive packed into crack or depression, covered with damp earth
   • Inefficient due to lack of confinement; requires large amounts of explosive
   • Results: Considerable noise, flyrock, often inadequately broken boulder
   • Hazardous: Primed charge prone to accidental initiation by impact
   • Use only where drilling a hole is impractical

4. Small Boreholes (Recommended)

   • Most efficient method using 1-3 inch boreholes
   • Hole drilled 2/3 to 3/4 through the rock
   • Less explosive required (approximately 1/4 lb/yd³)
   • For larger boulders, drill several holes to distribute charge
   • All secondary blastholes should be stemmed

Safety Precautions for Secondary Blasts

• Secondary blasts are usually more violent than primary blasts
• Flyrock is often more severe and more difficult to predict
• Require at least as much care in guarding as primary blasts
• Secondary blasting is truly an art, with experience being key to success

Controlled Blasting Techniques

The term controlled blasting describes several techniques for improving the competence of the rock at the perimeter of an excavation. The purpose is to reduce perimeter cracking and increase the stability of the opening.

Line Drilling

Line drilling involves drilling a row of closely spaced holes along the final excavation line. The line-drilled holes are not loaded with explosive.

Specifications:

Characteristics:

• Provides a plane of weakness for the final row of blastholes to break to
• Reflects a portion of the blast's stress wave
• Maximum practical depth: ~30 feet (limited by drilling accuracy)
• Best results in homogeneous rock with little jointing or bedding

Limitations:

• Use limited to jobs where even a light load of explosives would cause unacceptable damage
• Results are unpredictable
• Cost of drilling is high
• Results heavily dependent on drilling accuracy

Presplitting

Presplitting (preshearing) is similar to line drilling except that holes are drilled farther apart and a very light load of explosive is used. Presplit holes are fired before the main blast.

Specifications:

Recommendations:

• Completely stem around and between cartridges in the borehole
• Desirable to fire all presplit holes on the same delay period
• Maximum depth: ~50 feet (limited by drilling accuracy)
• Deviation greater than 6 inches from desired plane gives inferior results
• Avoid presplitting far ahead of production blast

Smooth Blasting

Smooth blasting (contour/perimeter/sculpture blasting) is the most widely used method for controlling overbreak in underground openings. Similar to presplitting, but perimeter holes are fired after the main blast.

Specifications:

Characteristics:

• Involves string loading slender cartridges
• Perimeter holes fired one delay later than last hole in main round
• Burden on perimeter holes: ~1.5 times the spacing
• Reduces overbreak and need for ground support

Cushion Blasting

Cushion blasting is surface blasting's equivalent to smooth blasting. Holes are fired after the main excavation is removed.

Specifications:

Advantages:

• Larger holes allow larger spacings, reducing drilling costs
• Better results in unconsolidated formations than presplitting
• Larger holes permit better alignment at depth
• Permits depths up to 90 feet

Ditch Blasting

Preparation

Blasting can excavate ditches quickly and economically in most types of soil. Advantages over mechanical excavation:

• Speed and simplicity
• Lower costs (no heavy machinery required)
• Eliminates large spoil banks
• Allows ditching in ground too wet or soft for machines
• Permits variable ditch dimensions

Most effective in:

• Moist or wet loam
• Shallow muskeg
• Clay that is not too sticky
• Ground with a hard layer just below proposed ditch bottom

Not effective in:

• Loose, dry sand or gravel
• Dry hard-packed earth
• Fluid muck or silt

Recommendations

• Clear the ditch line beforehand to prevent ejected material from being blocked by overhanging branches
• Fire test shots of at least 8 meters to determine ground reaction
• Do not blast too early in spring (deep frost inhibits depth and clearing action)
• Start blasting at lowest elevation and work uphill when draining flooded areas
• Limit section length to about 100 meters
• All personnel should be more than 100 meters from any blast

Methods of Ditching

Single Line Method

• Requires least time and labour
• Holes uniformly spaced with charges at proper elevation
• Primer cartridge always loaded first

Specifications:

Cross Section Method

• Variation of single line method for wide but shallow ditches
• Cross rows placed at every other hole at right angles to centre-line

Post Hole Method

• Used for ditches deeper than 1.5-1.8 meters
• Single line method with heavier charges concentrated at bottom of large diameter hole
• About 0.6 kg explosive per cubic metre of material
• Charge concentrated at depth 2/3 that of required ditch

Relief Method

• Used where heavy sod or root mat exists
• Relief ditches blasted first along parallel lines
• Main centre-line charges blasted after relief ditches

Pond Blasting

Explosives can be used effectively to blast ponds for water storage or wildlife habitat improvement. Techniques are similar to ditch blasting.

Limitations:

• Maximum width: 9-12 metres
• May be exceeded under favourable conditions with strong cross wind
• Under-loading on a windless day causes blasted material to drop back

Methods:

• Shallow ponds: Variations of cross section method
• Deeper ponds: Post hole method

Safety:

• All personnel should be located upwind and at least 300 metres from the blast site

Stump Blasting

Preparation

Most effective in firm, moist soil; less effective in dry, sandy or frozen soil.

Root Types:

• Lateral rooted (roots flat and close to surface): Cedar, fir, hemlock, maple, spruce, white pine
• Taprooted (main root grows straight down): Hickory, red pine, Norway pine
• Both types: Chestnut, elm, gum, white oak

Estimating the Charge

Under average conditions: 1/2 to 2/3 cartridges (32mm × 200mm) per 2.5cm stump diameter (measured 30cm above ground)

Adjustments:

• Rotted stumps: Lighter charges
• Green stumps: Heavier charges (up to twice as large)
• Dry, sandy or loose soil: Increase charges by 10-15%

Locating the Charge

• Lateral rooted stumps: Single charge in soil below and just past centre of stump
• Taprooted stumps: Charge in or directly against main taproot (may need charges on both sides)
• Multiple charges: Required for semi-taprooted or large lateral rooted stumps; must be detonated simultaneously

Loading Procedure

1. Check clearance and depth with loading stick
2. Push primer cartridge gently to bottom (DO NOT TAMP)
3. Load balance of charge one cartridge at a time, tamping each lightly
4. Stem hole right to top

Boulder Blasting

Three methods of boulder blasting:

Blockhole Method

Most expensive (requires drilling) but most efficient.

• Holes drilled slightly more than 1/2 way through boulder
• 1/4 cartridge for boulders up to 1 meter thick
• At least 1 cartridge in 45cm deep hole for boulders up to 1.5 meters thick
• Stem all holes with damp clay or sand

Mudcap Method

• Charge placed against surface of boulder and covered with mud or clay
• More explosive required but no drilling
• Most effective on flat surface or slight depression
• Lay cartridges side-by-side for maximum rock contact

Snakehole Method

• Used for partially or completely buried boulders
• Explosive consumption about half that of mudcapping
• Hole required under and against lower side of boulder
• Primer cartridge loaded first; balance compacted firmly against rock surface

Ice Blasting

Emergency Surface Method

• Place large charges of dynamite on ice
• Mudcap with wet-packed snow
• Fire all charges simultaneously for best results
• Typical charges: 4.5-9 kg depending on ice thickness
• Ice jams 9-12 meters thick may need 454-682 kg

Under-Ice Method (More Efficient)

• About 8 times more efficient than surface blasting
• Less flying ice
• Charge about 2 feet below bottom surface of ice
• Holes can be closely spaced (~1.6m) for propagation through water

Making Ice Channels

• Charges placed 1-2 feet under ice surface
• Rows about 1 meter apart
• Charges of about 2.5 kg each
• Produces channel about 1 meter wide in ice up to 6 feet thick

Safety Precautions

• Flying ice fragments are extremely dangerous missiles
• Allow 30-60 minutes for cracks to form after blasting
• Don't rush back onto ice after blasting
• Wear woollen gloves (wet wool sticks well to ice for pulling yourself out)
• Carry loading pole as lifesaver if you break through

Blasting Old Foundations

Old walls, foundations, and machinery mounts of brick or concrete can be removed economically by explosives provided the masonry slab is a minimum of 12 inches thick.

Brick Walls

• Drill row of holes near bottom, 3/4 of the way through
• Spacing: 3-4 feet between holes
• Loading: 1/2 to 1 cartridge depending on conditions
• Use loading ratio of 0.25 lb/yd³ as guide

Concrete

• Same procedure as brick but requires more explosive
• Close spacings must be employed
• Reinforced concrete: More difficult, more flying missiles
• May need to cut reinforcing rods with torch

Foundations Inside Buildings

• Blast only one hole at a time
• Use small deck-loaded charges
• For thick foundations: Remove by benching in 1-1.2 meter lifts
• Use more holes on close spacings (0.8-1 meter)
• Cover with blasting mats
• Open all doors and windows to minimize air blast damage

Demolition of Bridges/Piers

• Most economically removed by blasting in isolated locations
• May need two blasts: dry portion first, underwater second

Loading Ratios

• Dry work: 0.23-0.34 kg per cubic meter
• Flyrock hazard: Reduce to 0.11 kg or lower
• Underwater: 0.5-0.6 kg per cubic meter

Recommendations

• Small-diameter holes on close spacing give best results
• Hold charges well down in boreholes (5-6 feet below waterline minimum)
• Detonating cord initiation generally preferred for non-delay firing
• Use water gels underwater to reduce propagation potential
• Adequate matting mandatory for congested areas

Trenching

For vertical wall trenches (water/sewer lines), standard ditch blasting methods are not applicable.

Primary Requirements

1. Minimize overbreak
2. Minimize noise
3. Minimize vibration
4. Minimize flying material

Modern Trenching Practices

1. Track drills for 2.5-3.5 inch diameter holes
2. Small number of holes per blast
3. Minimum explosive per hole (including decking through mud seams)
4. Delay electric blasting caps
5. Water gel explosives (avoid propagation)
6. Blasting mats

Typical Pattern

• Trench 1.6 meters wide
• Holes drilled 1-1.8 meters on zipper pattern
• Holes about 1 foot from desired boundary lines
• Subdrilling: 1-2 feet

Explosive Loadings

• Easy-shooting rock with open face: 0.5 kg/m³
• Hard rock with no free face: 2 kg/m³

Well Shooting

Occasionally used to fracture and loosen producing formations to increase flow. Work should be carried out by experienced persons.

Complications

• Uncertainty of results
• Possible property damage from ejected material
• Structural damage from shock
• Well may be damaged or ruined from caving or casing collapse

Requirements

• High-strength explosive with high water resistance
• Electric blasting cap initiation typically used
• Oil and gas well shooting requires special equipment and high degree of skill