Designing Basic Blasting ShotsPart II

©Dr. B. C. Paul 2000

Note – The topics covered in these slides represent the author’s summary of information familiar to those well studied in the field. Ideas expressed in these slides

draw heavily on the book Surface Blast Design by Calvin Conya

Picking Stemming Length

• Stemming is the inert material places at the top of hole to constrain fly rock and noise

• T = 0.7 to 1 * B– where T is stemming in feet– use 0.7 for river gravel (digs into side of

hole and holds better)– use 1 for drill cuttings

Problems with River Gravel

• Gravel must be size to tightly fill the hole and not get hung-up (leaving void space where inert fill was required)

• Hole Diameter should be 20 times hole for good fill and no hang-ups– Sz = 0.05 * De

• Where Sz is the size in inches for gravel• If lot of gravel is finer it is probably drill cuttings and

won’t dig into side of hole

Get Subgrade

• Subgrade is length of drilling below level of next bench needed to pull the toe and keep a level bench surface

• J = 0.3 * B– where J is subgrade in ft

• Example– 0.3 * 6.5 = 2 ft of subgrade

Hole Spacing

• Depends on Stiffness Ratio and the Delay Timing on the Shot

• If SR < 4 and entire row is on single delay (no delay between holes)– S = ( L + 2*B) /3

• where S is the spacing

– Example - 15 foot bench with 6.5 ft burden• (15 + 13) /3 = 9 ft 4 inches

• Spacing to Burden Ratio (9.333/ 6.5 ) = 1.44

More Hole Spacing

• If SR < 4 but interhole delay is practiced

• S = ( L + 7*B) / 8

• Example– ( 15 + 7 * 6.5 )/8 = 7.562 ft

• Checking Spacing to Burden Ratio– 7.562 / 6.5 = 1.163

Additional Hole Spacings

• If SR > 4 and row is instantly fired– S = 2 * B

• If SR > 4 and row uses interhole delay– S = 1.4 * B

Observations about Hole Spacing

• Note that instantaneous delays require greater hole spacing– problem is bridging between holes - bridge

crater and loose forward throw– inner hole delay reduces problem– also gives us a clue on how to pre-split

• Lower SR allows holes closer– Lower SR comes from larger holes with less

powder and less tendency to bridge

Choosing a Primer Position

• Bottom Prime is Standard– Primer overdrives explosive and bottom of hole

and better kicks out toe– Kicks out bottom and collapses top for better

collected muckpile near face

• Top Prime– May overdrive explosive below stemming zone

and improve caprock break-up– Topples rock away from face to spread out

muck pile

Other Primer Placement Strategies

• Place Extra Boosters just below hard rock intervals– Overdrives the explosive for breaking up

rock

• Double Prime hole if powder column is too long or one zone produces cut-offs

• May have multiple explosive decks with inner deck stemming in single hole

The Cut-Off Risk Problem

• Cut-off involves a break in the detonation continuity of a powder column– Effects fragmentation performance– Can be a hazard to later mining operations

• Control is based on crack propagation from one hole to the next or to the face– Assume that cracks propagate at 20% of the

velocity of the p wave through rock

Cut-Off Formula

• Pcmax = 5 * B * Ve/ Vp + J

– Ve = velocity of detonation of explosive

– Vp = p wave velocity in rock

– Pcmax = longest powder column that can reliably detonate without risk of cut-offs

• Note that this will tend to form an upper limit on bench height for a given hole size - while cratering will form a lower limit

Review that 3 inch hole on a 40 ft bench where SR was > 6

• 5 * 6.5 * 11,000/ 12,000 + 2 = 31.79 ft

• But the powder column on a 40 foot bench is 37.5 feet!

• This bench is prone to cut-offs

• If can’t shorten bench or increase holes size then may want to consider double priming

Calculate Delay Timing

• Delays between rows in blast– Chosen based on avoiding backbreak,

vibration and cut-offs and creating the desired muck-pile shape

• Must be > 2 ms/ ft of burden– will backbreak from inadequate time for rock

movement if less

• 3 ms/ ft will cause pile high and close to the face

Timing Between Rows

• 4 ms/ft gives an average muck pile distribution and is usually safe from cut-offs

• 6 ms/ft gives a spread out muck pile with some cut-off risk– material starts out fast and slows - later rows

pile up into material blast front and are held back

– long delay times get front out of way of later material

Long Timing Between Rows

• 7 to 14 ms/ft is the range used for cast blasting– warning above 8 ms/ft the cut-off risk rises

rapidly

• 10 to 20 ms/ft is used on deep back rows to allow material in front to move and avoid backbreak without alteration of the drilling pattern