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Old 06-09-2009, 10:36 AM   #1
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How to cut a hole in a concrete floor slab

I have a septic tank that I'll need to have cleaned sometime in the next 6-12 months as a prerequisite to selling our house.

Before we bought the house, the tank was under the dirt floor of the outdoor carport next to the original house. The "genius" who owned it remodeled the house several years before we bought it, and poured a concrete slab completely over the entire dirt floor of the carport and turned it into part of a new interior addition to the house. So now the septic tank opening is somewhere beneath the carpet in our home office! Not to mention somewhere beneath the concrete floor beneath the carpet!

I knew this when I bought the house. When I pull back the carpet, I can see where the "genius" either jackhammered or sledge hammered a hole to get at the tank opening, and then repoured concrete to fill the hole when he was done.

I want to do it in a better more repeatable fashion. I want to cut a fairly neat hole in the slab, and before I fill it, I want to have a local welder fashion some sort of metal frame I can insert and mount inside my opening in the slab, and then lay a metal plate (I'm thinking plain steel about 1/4" thick will be fine) inside the frame, so that the next guy doesn't have to do what I'll have to do this time!

So here's my question: what sort of tools or techniques are there for cutting this kind of hole? The part of the slab where I need to do this is in a small alcove in the office, not in the middle of the room. So I don't have room, I think, for those massive diamond bladed concrete saws you drive by on the endless summer concrete roadwork repair jobs. And I can't imagine a huge air compressor-powered jackhammer tearing up my house! What else is there, other than hiring some hulk to just bash the crap out of it with a sledge hammer?


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Old 06-09-2009, 10:42 AM   #2
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It may take a while, and creates tons of dust, but you can buy a masonary blade for a circular saw.


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Old 06-09-2009, 11:01 AM   #3
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I never knew that! thx
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Old 06-09-2009, 11:07 AM   #4
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A diamond blade in a circle saw will probably only cut 2" deep, probably not near enough. I would spend that money (for 7" blade) on renting a electric concrete saw (14") & diamond blade. It's far faster than a circle saw also. You can safely run them with water on the blade to eliminate almost all dust. It may require a third person, though, wet-vaccing the sludge up behind so it doesn't flow too far away & wreck the surroun ding area.
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Old 06-09-2009, 11:19 AM   #5
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Good advice, jomama. Sounds like it's getting close to being out of my range, though. Time to search the yellow pages for an experienced guy...

Can you give me an approx. idea of the max dimensions of the space that the concrete saw you're thinking of would take? I'm dealing with the need to cut the hole in a fairly small corner of my room, no more than 47" wide by 19" deep. I don't know at the moment how big the opening I need would be, or where in that 47 x 19 rectangular footprint it'd be, so I don't know how close any of the sides of the opening will be to the sides of the 47 x 19 opening I have to put it in. I'm worried that I wouldn't be able to get the saw close enough to the back or sides of the 47 x 19 opening for the hole I cut to wind up right over the opening to the septic tank below the slab!
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Old 06-09-2009, 12:23 PM   #6
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See response in other thread about a cap.
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Old 06-10-2009, 01:21 PM   #7
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Google 'concrete chainsaw'... seems that would be your best bet to hire someone with one of those suckers for tight areas.

...if that doesnt do what you need I dont know what would

Last edited by creamaster; 06-10-2009 at 05:35 PM.
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Old 06-10-2009, 02:32 PM   #8
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I believe your best bet is to hire a drill/bore company, I have used one a few times and they do great work, and a lot quicker than you could do it. Also just a thought on the steel lid, fumes and rust will be a problem, so some sort of gasket and stainless steel plate and ancor bolts will be needed. good luck with your project.


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