Expected 'flatness' tolerance for blasting bedrock level pad 1

[atrizzy]

What is a reasonable expected tolerance to expect an earthworks contractor to be able to meet if blasting a flat pad out of exposed bedrock?

Intuitively, i would think that you might expect typical variations of +/- 500mm or so, but I'm trying to justify this somehow for estimating purposes (to calculate how much fill we'd need to import to raise the blasted surface to a given elevation).

Any thoughts are appreciated, and thanks ahead of time.

 

[miningman]

Absolutely futile question unless you have knowledge of the local geology, diameter of blasthole , blast geometry etc etc

 

[IRstuff]

Seems to me that it's highly dependent on how much money you're spending. I would take a wild guess that the cheapest scope would result in 1-2 meters of height change

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[oldestguy]

Questions like this don't deserve an answer. Start with all the details that you know of and then maybe some help will come. Start with continent, then country, climate, etc.

 

[atrizzy]

OK, fair enough. How silly of me.

First, I should rephrase that I'm looking for a reasonable tolerance to instruct a contractor to achieve. I therefore don't have any information on blast geometry, etc. I'm aware that you could achieve better tolerance with more effort, etc. I'm looking to split the difference between blasting and infill costs.

Secondly, the location is in the Canadian arctic. The rock typically consists of banded iron formations hosted in greywacke and mudstone. RMR89 around 65. UCS around 120 MPa. The first meter or so is heavily freeze-thaw weathered, but we expect to go approximately 3m deep to competent rock.

Thanks.

 

[cvg]

suggest you find ways to remove the rock without blasting. chemical splitting might be easier to control. overbreak, overrun, blasting plan, monitoring, safety and everything else make blasting an expensive and high risk option

 

[oldestguy]

Nice going finally. Under the circumstances as to where it is and how large-an area (house size? Athletic field size? city block size?) if you don't get an experienced engineer here with knowledge, I'd go local and ask the largest contractor there what to expect. Off hand without distinct beds and their orientation I'd only use the word of someone that does that work in that rock.

 

[miningman]

I wouldnt entertain anything other other than explosives myself........ BIF and permafrost make for tough conditions. What is the approx volume of rock?? This will determine the optimum size of the drill rig , as well as likely hole diameter. Is this out on the tundra or within city limits...... blast mats necessary or not?? Can you accomodate minor hi spots....this would have a big impact on blast geometery

 

[racookpe1978]

What are you going to use the blasted area for?

The "too deep" blasted holes will be there: Will they fill with water, snow, or mosquito ponds and trash and debris if not flattened later and filled in? If so, does it matter? How much will it cost to fill and flatten them if they are exposed?

Will the "pad" be filled with concrete for a road, bridge abutment, or parking lot or building foundation? How much "extra" will each cubic yard/meter of concrete cost as-poured and finished? If the "blast holes" are rough, how will that affect your sludge and muck removal costs? Can you afford to fill blasted excess holes with rebar and MORE cleanup to get a working surface?

In the Hoover Dam bypass tunnels, the Six Companies were paid only toi blast out their 10,000 feet of tunnels holes and concrete for a 56 foot diameter circle, with 3 feet of tunnel wall. ANYTHING extra past that 3 foot wall was extra expense underground to fill back with concrete and extra time that was not paid for by the contract.

Also, what is the "cost" of not blasting/chipping/grinding enough rock out the first time? If six spots are too high, how much will it cost you to re-survey the blasted area, mark the 6 spots (or 8 or 12 or 16 or 600 spots), individually go back and take out those 6, 8, 16, or 600 spots, then re-clean the much, re-survey the re-blasated areas, and fix what is still not right?

 

[capliard]

We've had an occurence in the Abitibi where a pad was blasted for a large mill and a big garage. The average achieved grade was about -1.2m with 4'' blastholes. The job was done buy mining D&B contractors without specifications from the construction group. As mining guys, we wanted to make sure we had no mounds above grade, for those would have required reblasts and it is hard to remove those mounds.

The construction guys expected a perfect grade. They weren't happy. They had to pour light concrete to achive a workable surface grade where the building footing was to be poured. Inside the footing, gravel was placed before pouring the slab, except where the Huge SAG grinding mill was to be place where its foundation went directly to bedrock.

If I to lead a group that had to do such a job. I'd join the D&B with the concrete pour job. Then your contractor has to work extra if not carefull. As Racookpe in his Damn Dam job.

Big Diameter -> cheap -> big subrill -> Uneven surface.
Small Diameter -> More Costly -> Small subdrill -> More evens surface.



Ingenieur Minier. QuTbec, Canada.

 

[PEinc]

Just had a pump station project with a 92' diameter, 42' deep, braced SSP cofferdam that needed about 15' to 20' of very hard, granitic gneiss rock to be blasted. The site got over-blasted by up to about 8' which had to be removed and filled in with lean concrete.

http://www.PeirceEngineering.com

 

[GSI2722]

For payment use a paylimit of 1 foot below subgrade. The blaster will space blastholes based on the depth below subgrade anticipating that the surface will be irregular due to the conical depressions that will result from fragmentation. Fill any cavities with a lean concrete mudmat to subgrade.