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Expert: - 10/1/2008


Question

trenching with boulder
Good day Mnr. Cummings

the Boulder Buster works wonders with loose boulders,but i need to blast a 1.5 meter trench out of a solid rock for water pipes.the granite rock is 4 meters long and 2 meters high with a slope to ground level!what is the best drilling patterns and were do i start to blast to get the best results
hope you can help me with this matter!

Answer
When you say 1.5 meter trench I assume you are talking about the depth, not the width because you are only laying pipe.  That impacts the width of the drilling pattern.  In most cases you would need a trench width of at least 1 ft in order to access the bottom for cleaning and to properly bed even small pipes.  I will assume yours will need to be 1-1.5 ft wide.

You did not include the trench profile in your sketch, but I think we can safely assume the rock extends below grade at either end so whatever depth the trench has to be at each end would be added to the height of the rock above grade.  For example, if the pipes are to be 0.6 m (2 ft) below grade at the ends then your trench depth in the middle of the rock would have to be as great as 2.6 m.  That's a lot for a Boulder Buster.  In this case it might be worth it to hire a blaster, consider renting a hydraulic splitter, or a hoe-ram.  You can use the Boulder Buster but you will need to maintain relief for the rock to break to, and you may need to do this in two lifts.  For the moment I will assume that the granite is generally fresh and relatively sparsely fractured to unfractured, and that you have a Boulder Buster and want to use it.

The following opinion is offered as a starting point; you will doubtless need to adapt the approach as you progress.

I think you can probably do this if you maintain a trench width of 0.5 m.  Start from the low side and work across the rock towards the rock's high point, and adapt your approach as you go.  Use a staggered pattern with the drill holes aligned along the trench sides.  If you are going to use the Boulder Buster in such a case I think you might be able to get away with about a 1.5 ft spacing (measured along each side -- remember to stagger) where the height is less than 1 m or so, but you may have to reduce this to 1 ft to maintain a clean break.  You will almost certainly have to reduce the spacing to about 1 ft where the height is between 1 m and say 2 m.  You may not find the Boulder Buster effective where the height is more than 2 m, even if you use the secondary cartridges.  If this is the case, you will need divide the cut into two lifts.  

You will greatly improve effectiveness if you fill the holes with water.  You will find that if you do not keep the burden low (in other words, narrow the hole spacing so each shot breaks cleanly to the bottom with no back break) you will not be able to keep water in the next holes because of the backbreak, and you will end up using silica gel or sleeving the holes with plastic or PVC to keep water in them.  If you shoot dry holes with the Boulder Buster in hard rock you are pretty much wasting your time and energy.  If it were me, I would leave some of the loose rock in place against the faces each time, to encourage the breakage in shear along the sides.

You should anticipate needing 0.5 ft or more of subdrill.  You may be tempted to cut back on your drilling to save effort but don't cheat on the subdrill -- it is a real hassle getting high spots out of the bottom of a narrow trench and you will quickly wish you had drilled deep enough in the first place.  I would suggest drilling over-depth at first and then adjusting; much better than finding out after the fact that you can't get the pipe in because you left the trench bottom too high.  

If your sides keep coming out ragged, you can pre-drill them on 6-in. centers and fill them with water, and shoot every other one.  These "guide holes" as they are termed in blasting will help direct energy, but are really only effective if they are filled with water to help the energy propagate along the sides.

I would be curious to know how this works out.

Robert Cummings, P.E.

Expertise

I can answer questions related to rock blasting, rock and soil excavation (such as tunnels and highway cuts), stability of such excavations, and foundations in rock and soil. I can also answer questions related to geology and mining.

Experience

30+ years as a geotechnical engineer and minerals engineer. Active consulting practice in rock blasting, geotechnical engineering, and rock mechanics for mining and heavy construction.

Organizations
Society of Mining Engineers, Deep Foundations Institute, Association of Engineering Geologists, and International Society of Explosives Engineers.

Publications
Mining Engineering, AEG Bulletin.

Education/Credentials
BS and MS Geological Engineering

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