# Blasting Swellage



## mudpad (Dec 26, 2008)

cexcavation said:


> I don't blast for a living so I personally have no clue, but to think a guy is hosing you by intentionally overblasting means you should have gotten multiple bids if you weren't comfortable with this guy. If you did get multiple bids and they were all in the ballpark, then it is what it is.


I don't think anybody is intentionally trying to hose me, but I expect this sub to absorb some of the risk. This is a hard bid government job, so of course we got multiple bids, and actually chose not to take the low guy, or even the 2nd low guy, because we wanted a competent excavator knowing the complexity of the job. The blaster is a subcontractor for the excavator, I.E. a sub of a sub, which is never a good situation IMHO.




cexcavation said:


> P.S. How are you determining where the fracturing truly ends and who is making that determination. I have dug in rock similar to your pics and it was completely undisturbed prior. You'll hit nodular sections that are impossible to dig through and then go back to crumbly material. What is to say that the blasting had anything to do with. Furthermore, what is going on these footings??? If it s a simple house, I would think hiring a geo-tech engineer could result in less over-ex.


This is definitely fractured from blasting, as determined by the geo tech of record (and myself, this ain't my first rodeo.) The geo tech has the final say, but his (and my) definition of solid rock is rock that can't be removed with a backhoe. 

This is a music rehearsal hall for a college campus. Concrete frame split level building excavated into hillside. 1st floor is open to roof structure above at the front, the back half has a mechanical mezzanine at +14.00 and a second floor level at +28.50. The cuts to subgrade are from 28' at the back to 4' at the front. Bearing capacity required by structural engineer is 10 kips


----------



## mudpad (Dec 26, 2008)

PipeGuy said:


> So....given there's no observed evidence of over-drilling and no reported incidence of excessive fly-rock or heave (as might be expected from over application of explosives) then if shooting to 6" below a relatively shallow subgrade (cut 6.5'?) resulted in 5+ feet of over-excavation (cut 12'?) to what depth should drilling have been limited in order to facilitate only the desired amount of excavation (cut 6'?)? Something just doesn't add up.


No evidence of over drilling, yet- but I do suspect that is what has happened.


----------



## mudpad (Dec 26, 2008)

cexcavation said:


> Thats kind of what I was thinking.........you'd have to really hit the hole ridiculously hard in order to send a blast an extra 6' down when you only have 6.5' of cover. The neighbors should have had rocks dropping through their roof if that was the case!!!:clap:
> 
> Maybe the guy should have laid some dynamite on the ground with a 6.5' deep pile of sand on top of it in order to nail the cut depth..............inquiring minds want to know more.


The only neighbors are other buildings on the same campus, on each side within 75'. Specs for blasting PPV were half what we would usually have 1" per second instead of the usual 2 that most specs (and the state) allow. Monitors on each side of site, pre blast and post blast survey of all buldings within 300 feet (total of seven multi story buildings) all verified seismic was kept well below acceptable limits.


----------



## mudpad (Dec 26, 2008)

ConstSvcs said:


> Good point by someone...........what was the initial starting depth of the overall cut to the desired bottom?


The cut was basically a wedge, varies from 28' to 4'. The problem seems to be worse in the shallow cuts. The deep cuts are pretty much right on (1' deep to 6" high)

Here is a picture from a week ago before we started digging the footings in question.


----------



## rino1494 (Jan 31, 2006)

mudpad said:


> The problem seems to be worse in the shallow cuts. The deep cuts are pretty much right on (1' deep to 6" high)


Sounds like the blasting contractor used too much ammonium nitrate. Did he use blasting mats ?


----------

