# The Caisson Dilemma



## mstovall (Jan 6, 2007)

Hi,
I am a GC in Middle TN. 

I have a project overlooking the lake coming up and I am currently working out the budget. The house is going to be on 8x8 posts (44 of them) , with CB88's on 16" diameter sonotubes with the caissons (same dia.) extending 2' into solid rock with a 3' deep probe hole to insure no voids. The site has between 2' and 4' of Colluvium over weathered limestone. Geologic survey for the area shows the weathered limestone may only be 1 foot thick with a colluvium layer before hitting solid rock (probably the reason that a $2 million house close by has moved 1').

I am having a tough time finding anyone local who has a rig that can or wants to operate on the slope (20%). There are few here that have a track drill and those that I've talked to really don't want to do it (the site is 6 miles back on a fairly narrow ridge road). 

Okay...now the question (bet you wondered if I'd ever get to it). Has anyone here ever used rock drills and jack hammers to drill caisson holes on tough access sites? I can get a backhoe down the slope without too much trouble, so I can dig a series of trenches to expose the rock. I do know that there will be the danger of busting off the "edge" of your bore which may then necessitate epoxying dowels.

Any Ideas (I'm trying to keep the budget somewhat reasonable).

Thanks
Michael


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## cexcavation (Apr 17, 2008)

Liability seems to be the issue as I read this. Even if I knew what you were talking about in detail, I wouldn't touch it with a 10ft pole let alone 44 of them If you can get a backhoe down there, then you would have zero problems getting a track drill down. Again, the trick is finding the guy who wants to have his name attached to a project that could settle 1' like the $2million dollar one next door:blink:.........unless an engineer stamps it and takes all liability in writing, a guy would be crazy to take that on in my opinion. Houses on stilts scare me, so I am very biased:thumbsup:


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## genecarp (Mar 16, 2008)

sounds like a challenging project, i always liked those. go for it. move cautiously. post photos. good luck. there are some good site contractors on this site. i am thinking of the spyder.


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## john1066 (Dec 26, 2007)

When i was at Con Expo there was a machine there for drilling caisons called the baby drill. it was the size of a mini excavator and made for working inside buildings i think with that and maybe the spyder you could do this job. and as far as going the 2 feet into the rock i would go a little more as an insurance policy for y self especially if it is easy to drill into. other than that the only other thing to do is drill and blast but that could cause problems. No matter what the cost of the caisons is going to be high ot begin with never mind the rest of the work.


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## ch0mpie (Nov 30, 2005)

Sounds steep for a rig, but if you can get a backhoe in there, what about and excavator with a hoe ram and a rock drill? Strip back all the soil, lay out the footers, drill your probe holes, then excavate the rock with the hoe ram. Talk to your engineer about spread footings, it sounds crazy to mobe a drill rig for such small caissons.


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## Nac (Apr 16, 2006)

Why not talk to the eng. and see if you can use mini piles (Titan anchors) can be installed with a small air track or a mini x with a rock drill. If i read rite you need 16" holes in soild rock? so you need a 16" rock socket. That will take a fairly big rig with a large DTH hammer and a huge compressor. What kind of load are looking for per pier? or you couls install 6" mini piles with a DTH setup with a cassing and grout the casing. If you were not on rock you could use helicals but that wont work.


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