Transcript Blast Design - Supplemental Teaching Resources

Designing Basic Blasting Shots
Part 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 cutoffs
• 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