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Determine location of bottom of existing drilled pier foundation

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ajk1

Structural
Joined
Apr 22, 2011
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1,791
Location
CA
Does anyone know if there is a way to determine the founding depth (bottom) of an existing 24" diameter plain concrete pier that supports an existing reinforced concrete slab on grade that spans between the piers? We could drill down thru it I suppose, but that would be messy. The length of the pier is known to be between 4 and 8 feet. GPR will not determine the length and we are told that ultrasound and impact-echo are also not likely to work. We want to know the bottom elevation of the pier so that we do not undermine it when digging a 5 foot square hole, within about 2 feet of the pier face, to an underground drain line that requires repair of a sagging portion. The drain line is about 7 feet below the floor, and was originally installed about 50 years ago into a relatively narrow trench.
 
Why not just dig your hole slightly wider and locate the bottom of pier the old fashioned way. It's not like you won't be digging to the drain line anyway.
 
To Jayrod: that may be what we end up doing, but the problem is that if the caisson is found to be up at an elevation that it will be undermined when we dig on down to the pipe, we will face a problem of what to do without incurring delays, extras, etc. If there is a way to predetermine the elevation of the bottom nondestructively, that would be preferable in my opinion.

To Ingenuity: ok, I will check that out. Thanks.
 
Straight shaft.
 
What kind of soils? My farm boy brain is saying drive a length of rebar at an angle from various distances until you stop hitting the pier
 
To Jayrod - my city brain was starting to tell me the same thing! It is a sandy-silt till with gravel, about 12 blows per foot until it suddenly changes to a very dense slightly cohesive silty-sand with some gravel sizes at 60 blows per foot. I believe the caisson is most probably founded at an elevation such that we will not undermine it, but I just wanted to be sure. Thanks for your help. Much appreciated.
 
When pounding the rod at an angle, I'd measure that angle, mark off the rod lengths and calculate about where the resistance should change when you hit it. That way any other hard resistances can be dismissed. Move areounda bit from one test probe to another so that possible disturbances don't goof you up.
 
There are established test methods available for this situation where are small diameter hole is drilled down adjacent to the pile (at a distance of about 500mm) and a probe lowered. It works better if the pile is reinforced but there are methods for unreinforced piles. Sonic integrity testing may also work.
 
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