GPR as inspection tool

[smwpe]

Has anyone out there had experience in using GPR in building inspection? Do you know a contractor in the Florida West Coast area who does this work? Any cost data you have would be appreciated.

A client has requested testing of his project now under construction for subgrade compaction and presence of organic material, and placement of reinforcing steel.

We have never used GPR as an inspection tool; we do have a copy of the Construction Specifications Institute article "Infrared Imaging and Ground Penetrating Radar as QA/QC Procedures".

 

[geodan]

smwpe

I don't understand the need for GPR. You have been retained for QA/QC construction inspection/testing of;

-subgrade compaction
-percent organics
-placement of reinforcing steel

These inspections/tests can be done using standard methods. Where do you need to use GPR?

 

[smwpe]

Geodan:

Thank you for the reply. Most site work is completed, the walls are up and roof framing is in progress. The client has been dealing directly with the builder, relying on the local building department inspections for QA/QC.

The client now feels he needs third party assurance that the work has been done properly,and he is willing to pay for this assurance.

We're doing our due diligence in preparing a proposal for him. We can of course go the destructive testing route. However, the CSI article I referenced above indicates GPR might work, but at a large cost.

Thanks again for any advice.

 

[BurntOnDirt]

As long as the required testing has been performed properly, and the geotech. recommendations have been followed. Why is there a concern? Building is past any problem being solved now. How much fill was placed to require GPR?

 

[geodan]

smwpe

now I understand. However, what if your "findings" indicate bad soil, and overall negative results? Is he willing or ready to accept these results? Or is he only willing to pay if you give him "assurance"? Prehaps he is just looking for cheap insurance.

If that is resolved, I would recommend 4 or 6" borings. Relatively non-destructive and you will be able to get a real first hand look and underlying soil conditions. Also you will be able to run actual laboratorty tests and back up your findings with hard raw test results. Just keep in mind and mention in your report that it only pertains to the EXACT borings or locations you tested! who knows what could be below the ground elsewhere.

 

[SirAl]

I have seen GPR applied quite effectively in mapping the thicknesses of pavement structure (ACP & GBC) beneath a large ACP paved parking/access area. In this case, some premature failures prompted suspicion by the general contractor that deficiencies in thickness were present. The data was presented in a contour format showing first thickness of the ACP then, on a separate plot, thickness of the GBC. The results were impressively accurate (as verified by cores). The findings were not disputed by the paving subcontractor and the deficiencies were rectified with an overlay.

More relevant to your question, compaction is generally a comparison of dry density (field vs. lab standard). As such, moisture content would have to be established. In addition, I doubt that GPR can provide the density precision necessary for verification of compaction density.

 

[yogibear1]

I am in west central Florida (Hillsborough County) and have used a GPR firm; I think they were from Pinellas County with a name like Subsurface Detection or Investigations, Inc. We used them before we placed a large diameter prestressed concrete water storage tank. The GPR was useful to show the differing ground layers. One thing we looked was sinkholes, and they are good for that. I don't recall how far the GPR can sidescan - which it sounds like something you now need.

 

[BurntOnDirt]

Is there a geotechnical report for the site? Borings are the only thing I know of that might hold up in a court. I take it this is a private bldg.where the owner played construction manager?... If you do use GPR, back it up with borings around the bldg footprint and inside if possible.I'm guessing you might have done some test pits at certain locations or compaction tests? If there were no inspections on the subgrade prior to fill(if any)by an independant lab/inspection agency,then the owner has learned a lesson in "covering ones butt"."an ounce of prevention is worth a pound of cure"

 

[smwpe]

Thanks to everyone who took the time to respond. All of your comments are very helpful and I will put a copy in the project file for future reference. Yogibear1 you were absolutely right. We located Michael Garman at his firm in Tampa, Subsurface Evaluations, Inc. (813-353-9093).

As to what the client will do if we find significant problems, we will have to give that some thought. A lot of thought.

Thank you again.

 

[Focht3]

[blue]smwpe[/blue]:

I don't envy your position. Good luck, and remember that a key decision was the owner's - s/he chose to forego verification testing during construction. A very unwise choice. The presence of a technician and nuclear density gage really keeps a lot of contractors on the "straight and narrow." It could have prevented the present circumstance by forcing the contractor to do the job in accordance with the specs -

Of course, the contractor is clearly responsible for the quality of their work. If it isn't in accordance with the specs, it has to be remedied. At the contractor's expense -



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