Effects of Metal Shoring on Nuclear Gauge testing and Trench Offsets?

[redneckjep]

I am updating my company's training manual for compaction testing protocols using a portable nuclear gauge and hope to see if anyone could help me get clarification on using the Trench Offset function. Will a metal trench shield/trench box affect the gauge readings? If so, can it be corrected using a Trench Offset? Our gauges are the CPN MC 3 Elite. The manual did not provide any insight.

One of our engineers was trained with the idea that any metal or pipes within proximity to the gauge will have an impact on the tests. My understanding is that the gauge should only be recording radiation that is not absorbed by the soil between the probe and the detector (for density counts) and radiation that is reflected by the hydrogen atoms in the soil underneath the gauge (for moisture counts). I agree that metal pipes too close to the gauge would have an affect, but because the backscatter is so shallow, one would have to be way too close to another utility for it to matter. Let alone seeing an affect from anything that is above the gauge.

Please let me know if I am understanding the theory correctly.

 

[TigerGuy]

I've not touched a gauge in a few years. It seems that you cannot test within 10 feet of a vehicle, which I think the trench box would mimic to the gauge. I don't remember the reason for this, but I think it was the metals tend to reflect some of the radioactive waves and particles.

 

[redneckjep]

The gauge's manual only mentions to not perform standard counts on top of a truck tailgate, or other non-solid material. I don't know that is has much to do with the metal itself. In certain scenarios with minimal space to do my standard counts and my compaction testing, I have parked my work rig within 5 feet of the gauge as an extra layer of protection against vehicle traffic, and never seemed to have issues with the standard counts in those scenarios. That said, I could have been doing it wrong during those times, but got away with it.

 

[redneckjep]

I stand corrected on my reply. AGNGA recommends testing away from vehicles and large vertical objects. Their training videos do not mention shoring however. I may direct this question directly to the gauge manufacturer.