Blast- Pattern Parameters

Burden

The BURDEN is the distance from a single row to the face of the excavation, or between rows in the usual case where rows are fired in sequence.

The proper burden dimension to use in any given individual blast can be calculated by taking into account hole diameter, relative rock density, and the explosive that will be used in the blast. Too small a burden can result in excessive airblast and flyrock; on the other hand, too large a burden can result in improper fragmentation, toe problems, and excessive ground vibrations. The burden, in turn, is the basis for calculating spacing, stemming, and subdriling.

Spacing

Spacing is the distance between adjacent blastholes in a row, measured perpendicular to the burden. In row-to-row shooting, spacing is measured between holes in a row; when the shot progresses at an angle to the free face, the spacing is measured at that angle.

Spacing may be somewhat dependent on the timing, but is most often a function of the burden. Close spacing cause crushing and cratering between holes, boulders, and toe problems. Holes spaced too far apart will result in inadequate fragmentation.

Stemming

Stemming contains explosive energy within a blasthole, so that it will break and move the rock without generating flyrock. Sized crushed stone or drill cuttings should be used as stemming.

Subdrilling

“Subdrilling” is the distance drilled below the floor level (or actual required blast depth), in order to ensure that the full face of the rock is capable of being removed to the desired excavation limit.

Subdrilling may be required to achieve a smooth pit floor. The subdrill portion of a borehole is generally backfilled with drill cuttings or other stemming material. Explosive is not loaded in subdrill. Excessive confinement will lead to high peak particle velocity ground vibrations.

Decking

A process called “decking” is often used to reduce either the charge load per hole, the amount of explosives detonated per delay, or both. Decking is also used to get explosives into harder rock zones or to keep them out of weak zones such as mud seams.

Using the decking process, the top deck in a blasthole is normally shot one delay period after the bottom deck in the hole. The idea is to keep the explosives in the bottom deck from propagating through and detonating the top deck.

To achieve this, an air deck or deck of inert stemming is inserted between the top and bottom decks. The length of the inert deck will vary depending upon borehole conditions. Increase the deck length in wet holes to reduce the chances of propagation between decks.

Blast Patterns

A square blast pattern has drilled spacings that are equal to drilled burdens.

A rectangular blast pattern has drilled spacings that are larger than drilled burdens.

In a staggered blast pattern, the drilled spacings of each row are offset such that the holes in one row are positioned in the middle of the spacings of the holes in the preceding row. In addition, the drilled spacings are larger than the drilled burdens.  A staggered blast pattern is used for row firing, where the holes in one row are fired before the holes in the row immediately behind them.

DELAY PATTERNS

Delay patterns, and varying the hole array to fit natural excavation topography, allow for more efficient use of the explosive energy in the blast. Benches may be designed and carried forth with more than one face so that simple blasting patterns can be used to remove the rock.

Typical bench cut with two free faces and fired with one delay per row

Direction of throw of blasted rock

BLAST DESIGN

RULES OF THUMB FOR BLAST DESIGN

RULE 1: The detonation velocity of the explosive should match, as closely as possible, the sonic velocity of the rock to be blasted.

RULE 2: Generally select the most dense explosive possible, consistent with water, loading conditions, and desired results.

RULE 3: Select explosives according to the characteristics of the rock formation to be blasted

RULE 4 – When using slurry or water gel explosives, always determine the critical temperature below which the explosive will fail to reliably detonate.

Guidelines FOR BLASTING GEOMETRY

RULE 1: The distance between holes (spacing) should not be greater than one-half the depth of the borehole

RULE 2: In any blast where there is hole-for-hole delay, the spacing to burden relationship should be seven to five.

RULE 3: Stemming should be equal to the burden.

RULE 4: Subdrill (if necessary) should be between .3 and .5 of spacing.

Keywords- Burden, Spacing, Stemming, Subdrilling, Decking

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