Shot Design Example

©Dr. B. C. Paul 2000

Note – The example portrayed in these slides is an illustration prepared by the author using knowledge that the author believes is commonly held by those familiar with the

subject. The example includes graphics published with the program used for slide presentation as well as spreadsheet views from the Havablast Spreadsheet series

prepared by the author for use in the classes he instructs.

Example Problem

• A quarry is being developed for 5 million ton per year production in a hard limestone rock. The rock continues to a depth of 320 feet before encountering a fracture zone where rock quality is poor. The specific gravity of the rock is 2.7 and the p wave velocity is 16000 ft/sec. The primary crusher has a gap of 32 inches and the rock breakers on the grizzley can handle up to 10% oversize. The company currently has crawler mounted drills capable of drilling 2 to 4.5 inch holes with a 6% drill hole deviation. The company will also consider buying new drills capable of 3 to 7 inch holes with 3% deviation at a cost of $300,000 each or rotary drills for 6 to 9 inch holes with 2% deviation at a cost of $500,000. Hole conditions are dry but scaled distance is limited to 40 at a distance of 1250 feet. Local safety ordinance limits bench height to 80 feet maximum. You are required to select a common bench height and design the standard production shot pattern.

Picking Off Relevant Information

• What am I suppose to do?– Pick a single standard bench height– Design a single standard blast pattern

• What constraints do I have on that bench height?– Limited to 80 ft max– Drilling accuracy will influence my fragment size

limits and could change bench height• They slipped a pick a drill problem in here

More Picking Off Data

• More Bench Height Constraints– Hole cut-off could limit - they have provided me with

P-wave velocity data• I will still need the explosive velocity

– They slipped another one on me - pick the explosive

– Blast Vibration Limitation Could limit - they have provided me with limit data for maximum charge weight per delay

• Do you get the feeling that I’ve already picked my first problem target?

Why Did I Go for the Bench Height First?

• I felt like it - with many open ended engineering problems you go for the attack at your zone of greatest comfort with assumptions or available information

• I knew that bench height would influence my charge per delay and I couldn't do a pattern without that - also it controls my stiffness ratio which controls hole spacing so I couldn’t drill without that first

Start The Problem

• Review information outside of problem for assumptions that may help focus bench height decision– One constraint on bench height is that your

equipment must be able to dig to the top of the face or rubble pile.

– With typical quarry practice the face is collapsed by the shot so that it is not necessary for the loading equipment to have a digging height as great as the face height.

More Bench Height Data Review

• With respect to geology, one constraint is that the bench height should not mix high and low grade ore to where no selective mining is possible (unless a non-selective mass mining technique is intended). – In this case, this is a quarry and all the rock is

fairly homogeneous with respect to the key aggregate properties, thus there is no bench height constraint here.

More Thoughts on Bench Height

• A quarry may face layers of hard or soft rock or rock of high and low quality. It may be desirable to put certain rock layers at certain points in certain benches.

• In this case I will be looking for a number that gives a good fit of benches in the top 360 feet of rock without cutting into the fracture zone.

Bench Selection Background Data• I general, the fewer the benches the greater the

savings. Each level requires its own drilling patterns and primers, plus its own road access infrastructure.

• I desire to limit the number of benches by making benches as high as possible.

• Local ordinance limits me to 80 ft max. • I will try 80 foot benches. It meets a safety

constraint and gives me 4 decent benches while keeping me out of the fracture zone.

Things to Note

• Note that I made a decision on my first guess with only limited use of data provided by problem - most came from my own understanding

• I still need to check– Cut-off problems– Vibration Problems– Fragmentation Problems

• These require calculations that will use the supplied information

Getting Ready to Run Spreadsheet to Check Performance

• What hole size are you going to tell it?– I don’t have to - my spread sheet is so cool it checks a

wide range automatically

• Will you use inter-hole delay within a row?– Not if I can help it - it raises the powder factor and

makes the shot more complicated

• What explosive will you use - you have to say because you need an explosive velocity to check for cut-offs or weight of explosive in holes.

The Explosive Decision

• slower heaving explosives often work best on sedimentary rock

• since the holes are dry, I do not have to worry about water resistance

• ANFO is normally the most cost effective explosive.

• I choose ANFO for initial design attempts.– My spreadsheet has VOD data for ANFO

Essential Spreadsheet Inputs• My spreadsheet uses shaded areas to designate

cells for user input

• To get size restriction it need crusher gap and percent oversize (which we have)– if not an issue for you use a large number so the

problem will not be constrained

• To Check blast vibrations it needs scaled distance and distance to compliance point (which we have)– if not a issue to you - use a very large distance and

advise your attorney to prepare for a suite

Setting Spreadsheet Inputs

• Needs your bench height - (we just decided what we wanted to try)

• Needs rock properties– p wave velocity (Cat handbook has some typical ranges

if the problem hadn’t provided)– Rock Factor (Since we listened to Dr. Paul's wonderful

lectures we know what to try for a hard limestone– Rock Specific Gravity (if problem didn't say Cat

handbook would help)

More Spreadsheet Inputs

• Need to input Burden Adjustment Factor (which we know from the lectures)

• Need Explosive Parameters– Relative Weight Strength (100 for ANFO)

– Explosive Specific Gravity (0.85 for ANFO)

• Need Blast Design Features– Stem factor (0.7 if gravel 1.0 if cuttings)

– Inner Hole Delay (set 1 or 2) I’m not using

• Need Size Minimums if constraint on fines– I’ll just set so they won’t get in the way

Run The Spreadsheet

1 - Stiffness ratios greater than 7 give me heart burn on cut-offs - the hole needs to be 5.5 inches or more

2- The maximum column of explosive before cut-off is less thanbench for all holes below 8 inches

a- could double prime?3- Maximum hole size that meetsvibration constraint is 6.5 inches -this is not looking good.4- All the holes have too muchcoarse rock

So Why Does Life Suck?

• My Explosive Columns Kept Getting Cut-Off

• The Vibrator Police Thought my only decent shots shook things up too much

• I had too many big rocks

• Any Common Themes?

• Yes - My Explosive Columns Are to Big

Solving by Things You Can and Cannot Do

• The amount of time needed for cracks to propagate to the next row back is a function of the p-wave velocity in the rock (can’t be changed by the engineers) and the burden between holes (can be changed as a function of hole size). – Unfortunately the spreadsheet already considered

a range of hole sizes - no use beating a dead horse.

Reviewing Solutions

• We can try to solve the problem by making the explosive column detonate faster. This can be done by changing the velocity of the explosive which means changing the explosive

• - possibly to one that costs 3 or 4 times more than ANFO – - I don’t like it

More Solutions

• I could change the length of my explosive column

• I can change the length of the explosive column either by decking charges or reducing bench height.– Decking reduces the explosive in the holes and I

already have oversize problems– Decking is a lot harder to wire and I’m trying to

keep things simple

Its Choosing Time Boys and Girls

• Reduce Bench Height - it will limit charge size and length of explosive column

• Deck Your Charges (but please not with bows of Holly)

• Get a Faster Explosive - When you get them Faster it costs more money and you still have the vibrator police

• I Choose to Reduce My Bench Height (60 ft divides into 360 better anyway)

Running the Spreadsheet

1- Stiffness Ratios for 4.5 inches up are OK

2- Six inch holes or greater will notcut-off

3- 6 to 7 inch holes will meetvibration constraints

4- My best fragmentation was witha 5 inch hole. 6 to 7 inch holes arebetter than before but still a problem

Analyzing My Problems - Again

• The blasting fragmentation is not acceptable. • Checking the size distribution parameters, the

n values are all above 2 so the distribution is not unreasonably wide.

• The d50s are, however, in the 20 to 21 inch range which is pretty large. – D50s are a function of powder factor, drilling

accuracy, optimized spacing, and charged interval of hole.

The D50 Problem

• We are already using optimized spacing.

• Because we have avoided decking and are already using gravel to reduce stemmed interval we are already about a good as possible with charged interval. – (6 inch holes have the maximum charged

interval since stemming length is a function of burden which is a function of hole size).

More on the D50 Issue

• We are already using a new drill although I could reduce drill hole deviation by 33% by doubling the price of my new drill.

• I could also raise my powder factor.

• Many companies are very sensitive to capital costs so I choose to try to raise my powder factor to shift my d50 to a smaller size.

Manipulating Powder Factor• I need to increase my powder factor

• I can do this by lowering my burden (and thus my spacing) – I can adjust my burden with the burden adjustment

factor - the rock is harder relative to what I want.

• I could just lower my spacing. – If I lower my spacing while firing a row at a time I

may blow out the material between holes. My boss doesn’t like fly rock since I dropped one on his new Caddy.

Looking at Spacing Adjustments

• I could use inner hole delay– This allows me to reduce spacing without

bridging between holes

• It, however, turns out that I am already firing one hole at a time because of blast vibration constraints. Thus, I’m already using inner hole delay and I just need to set my spreadsheet to reflect this fact.

Running the Spreadsheet

1- The 6 to 7 inch interval that isok for cut-offs and blast vibrationsis still here

2- Most hole sizes including thefeasible range have acceptablefragmentation

Establish My Design• I choose 6 inch holes because they are a good size

for air pneumatic hammer types of drills and I get the finest and most uniform distribution. (I am paying a price in specific drilling and it is distinctly possible that others would make this choice differently).

• I will round my numbers for ease of field work. – Burden will be 12.5 ft. – Spacing will be 17.5 ft. – Hole depth will be 64 ft. – and stemming will be 9 ft. with gravel.

Select my Blasting Pattern• I choose a plow cut.

– The plow cut is straight forward and does not have confusing differences between as drilled and as fired burdens and spacing.

– This is a quarry and a longer muck pile will not be a problem for front end loaders. There will be less stacking of material into high hard to reach stacks.

– lesser confinement will make blast vibrations less which is important considering the proximity of many quarries to urban areas.

• My spacing to burden ratio of about 1.4 will not allow me to get the square pattern drill pattern advantage for which V cuts are noted.

Select the Primer Position

• I choose a bottom prime position. – It will help me pull the toe and avoid spreading

of rock that might occur with the toppling action of a top primed shot.

• If I had known of hard layers I might have positioned additional primers to get an overdrive effect on the ANFO

Design the Timing on the Shot• Timing on the shot must simultaneously satisfy the

following conditions.• - Inter Row Delay Times must be adequate for movement

of rock at the front of the shot and must provide for the desired shape and distribution of the muck pile.

• - Delay between holes in the same row must be appropriate for the rock being blasted.

• - Timing must be achievable with available equipment and initiation systems

• No more than the maximum allowed weight of powder must detonate in any 8 ms time interval.

Screening Applications of the Restrictions

• For 12.5 feet of burden and a normal muck pile distribution I want about 4 ms/ft.– 12.5 * 4 is about 50 ms.

• This time is readily achievable since caps commonly come with 25 and 50 ms spacings as can be seen from the attached table of Dupont millisecond delays.

Look at Inner Hole Delays

• For 17.5 feet of inner hole spacing I need to figure out my inner hole delay. For a hard limestone I want something around a 1.5 ms/ft delay– 17.5 * 1.5 = 26.25 ms

• This is very close to the 25 ms standard timing interval on blasting caps.

The Vibration Problem

• The problem with using 25 ms between holes and 50 ms between rows is that the 50 is an even multiple of 25 which will mean multiple charges on different rows will go off in sync with each other. This will violate the 8 ms between independent charges rule.

Possible Solutions

• To cope with this problem we will use an offset in the time by firing the rows 58 ms apart.– This can be achieved using a sequential

blasting machine with a standard interval setting of 33 ms. By changing one cap series number for 25 ms and using a 33 ms interval in electrical initiating impulses I can get 58 ms between rows.

Implementing Timing

• By making the shot 7 holes long I can use each of the first 8 cap series and cover timing on the first 2 rows of holes.

• As is typically done in a plow cut I will fire the holes at the end of each row last– This allows all the material in the row to move clear. This

will allow the end holes to fire into 2 free faces making the extra tearing action at the end of the shot more manageable.

• Within the row I will fire systematically from one side to the other.

Back Row Problems

• I can fire about 3 rows of holes on a plow cut without too much confinement in the back. If I fire 4 or more it is often desirable to either reduce burden to balance the more confined free face, or I can increase the timing between rows to allow more time for the rock to move clear.

Solutions for Back Row Timing

• I Choose to try to work with Timing rather than alter the drill pattern

• Casting tendencies start about 7 ms/ft and cut-off risk starts climbing around 6 ms/ft. I probably want something around this range. – 83 ms can be achieved with two 25 ms cap periods

and a 33 ms electric impulse interval. This will allow me about 4 rows deep for the shot, which is reasonable for a modest shot.

A Solution Drill Pattern