How to determine pier depths?

[ckissick]

I need to find out how deep some existing drilled piers are. The house addition was built without a permit, and there are no records related to the drilled piers. They are 18-inch diameter concrete piers with rebar, located along an exterior wall of the house.

I could use a back hoe or hand-digging, but the piers should be over 10 feet deep. Has anyone here successfully used some kind of remote sensing technique?

I thought maybe attaching a geophone to the top of the pier and whacking the pier and measuring the return signal. But a local geophysics company has never attempted this. And you would need to know the seismic velocity of the concrete.

A utility location company thought they might be able to put an electronic signal on the rebar and then sense the depth of the rebar at depth. But they have never tried this.

So, has anybody had success with this, without having to dig?

 

[bridgebuster]

Maybe this will work a friend of mine used impact-echo testing to determine the stem thickness of a retaining wall.

 

[aeoliantexan]

Here is a low-technology, low cost approach: try to track down the general contractor, pier drilling contractor, or geotechnical engineer and ask them. If you are flying blind, call all the pier drilling firms in the area.

 

[MiketheEngineer]

Hire a mole

 

[FixedEarth]

Primary consolidation takes about 5 years, so if the house was completed less than 5 years, you could consider supplemental piers with corbel placed near the exterior footings. These new piers can be spaced based on your new investigation test results, but probably not more than 12 ft on center. However, if the house was completed more than 5 years ago and there are no visible distress, you most likely are in the clear. Good luck.

http://www.FoundEng.com

 

[TXStructural]

Olson Engineering in Colorado Springs (

http://olsonengineering.com)

worked on testing related to my masters project. The methods (impact echo, as your describe, along with other methods) accurately determined drilled shaft depth up to a length-to-diameter ratio limit somewhere around 40:1. They sell (and may rent) the instrumentation required for this testing, IIRC. Rudimentary impact echo testing consists of placing an accelerometer on the end of the concrete, hitting the end of the rebar (or concrete) and looking at the oscilloscope for "harmonics" which indicate things like rebar ribs or the end of the concrete. Not really easy to interpret unless you know what to look for.

http://www.olsoninstruments.com/sonic-echo-impulse-response-ndt.php

Alternatively, you can drill a small diameter soil boring parallel to the pier with very little effect on the existing capacity, or drill down the center of the concrete (yes, we have done this to great depths.)