nuclear density gauges are not accurate
nuclear density gauges are not accurate
(OP)
Not an argument but to see what others think. I dont think nuclear density gauges are accurate, any of them. I think they are no better than 85%. Prove me wrong, but after 26 years in the business i have no fate in the gauges.





RE: nuclear density gauges are not accurate
Always have at least one 'other' density measurement technique done per day of earthworks filling, and normally with clays this would be a core cutter every 5 to 10 NDG readings and run it on a rolling calibration. More difficult with granular soils ( I have very little faith in the SRD test), so would normally have a spec requirement for calibration boxes to be done on a regular basis. Where stabilised soils are being tested, this would probably be one per day. Then use the data from the calibration box as a rolling assessment. I also plot the results of the data on a rolling basis to see if there are any trends with the data which could indicate the gauge is giving erroneous readings.
One big point to note is that there are a LOT of older gauges out there at the moment due to the cost of de-commissioning them. From my own experience lots are still in circulation that should not be and a number have been 'given' away to other test houses who have less scruples. So when I start to doubt the readings one of the first questions is "how old is the gauge' and can I look at the 'daily checks'.
What you do get with a gauge is a lot of data in a short space of time. How much reliance you place on an individual reading will be a reflection of your own confidence in the kit, technician, material, and how critical the data is.
When I write specs, I always ask for additional in-situ assessments to be done at each and every NDG reading, which is normally either a set of hand vanes or mexe probes so I can make a judgement as to the value of the data. I would never rely on an uncorrected gauge in isolation (just writing a review on someone else's work where they have done this, and the numbers just don't stack up!!).
RE: nuclear density gauges are not accurate
If it has the magic number on it, then nothing can be wrong.
I get test reports back sometimes with the compaction at 110% Modified Proctor and other nonsense, but hey, it passed, so no worries.
RE: nuclear density gauges are not accurate
RE: nuclear density gauges are not accurate
RE: nuclear density gauges are not accurate
You have to calibrate the Nuke to the material for cross reference as both iandig and BigH noted. Back when Nuke gages were becoming popular, we calibrated them to sand cone density tests. Not a bad idea, but a direct volume comparison is even better (drive sleeve)...problem was that you can't use drive sleeve in all materials (material > 1/4"). Comparing several methods, you could get a good idea of the accuracy of the Nuke gage. Keep in mind that the gage calibration is to a standardized density block...not soil or asphalt. You still need to compare it to conventional methods of soil/asphalt density.
As for moisture, the gages have gotten better over the years. They used to be awful for moisture content. Almost completely unreliable in any material that had a reasonably high calcium content. Not great in most other materials either. We found that a Calcium Carbide moisture test was more accurate than the Nuke in most cases.
Almost any electronic or nondestructive test method must be calibrate to physical test measurements. At the least, just to give a bit of "peace of mind".
RE: nuclear density gauges are not accurate
brownbag-
I would agree with your statement...but also think "some" of the floating numbers begin in between sampling and the lab work done. I do however think the guage requires the contractor to provide good compactive effort at a reasonably close moisture content, when used correctly.
RE: nuclear density gauges are not accurate
RE: nuclear density gauges are not accurate
what other methods for comparing a nuke.
RE: nuclear density gauges are not accurate
I would encourage you to teach the other techs how to do the sand cone test. No, you won't use it very often in practice, but if they learn to run the test, they'll have a better understanding of soil density and compaction. One of the problems with electronic "black box" methods of testing is that the original basis of the testing is lost. That leads the technicians to believe anything the "black box" tells them. Understanding "why" is much more important than understand "how". Teach them "why".
RE: nuclear density gauges are not accurate
as far as teaching the crew, ex waffle house cooks just care about the hours, push the button, write it down, when quitting time.
This is not an argument just trying to pick everybody brain on proving nuke accurate.
RE: nuclear density gauges are not accurate
I would not want to use a testing laboratory that doesn't train its technicians.
RE: nuclear density gauges are not accurate
RE: nuclear density gauges are not accurate
RE: nuclear density gauges are not accurate
Many a time later on when a contractor would call and say he was being penalized unfairly by a competitor inspector using the nuke, it was pretty easy to show the nuke was wrong. That helped bring business our way, so maybe it was not so bad after all.
Anyhow, as for sand cone, tests have been done to check its accuracy and it has been found that how you leave the sides of the hole plays a big part in accuracy achieved. Slight disturbance makes big difference. Of course other things like sand being kept dry, etc. are important.
Then comes a soils professor out doing some, on the side, density testing work for a client. I happened to be on the job for the competitor, just watching. His sand jar was a canning jar. Cone diameter was maybe 4 inches. Can you imagine? Yet that was sold as one form of test gear. Makes one wonder if some of us use our heads out there. Maybe the profs should stick to the books.
No one so far has mentioned the size of the hole, cone size, etc. Maybe those having little faith in it use canning jars?
No one has mentioned rubber balloon either?
RE: nuclear density gauges are not accurate
Proofrolling is done to show areas of instability and should not be a final measure of density, unless of course you can correlate number of passes to achieved compaction.
I can show you equally many instances of proofrolling that show no instability but did not achieve compaction. One common example of such material is slightly silty, slightly clayey fine sand that is very hard on the dry side of optimum but often compaction is not achieved on the dry side. The result, when surface or groundwater moves in, is a significant loss of stability because of the lack of compaction.
You can achieve compaction without stability, but you cannot achieve true stability without compaction.
RE: nuclear density gauges are not accurate
RE: nuclear density gauges are not accurate
These methods do inlcude comparision between the guge and other in-situ methods, as well as the use of the calibration box.
I don't have a copy of this to hand, but will speak to a couple of labs I know and see if I can 'source' the exact methodologies for you if that will help. You do have to bear in mind that the way in which most ofthe BS's are written, it does assume you know what you are doing, following a recipe does not mean you are a good chef!
RE: nuclear density gauges are not accurate
RE: nuclear density gauges are not accurate
In your experience with a testing lab, you saw the variability of the quality of work of individuals. It's the same in most labs. The key is the quality of the review and unfortunately, some labs send out results with no review.
RE: nuclear density gauges are not accurate
We generally do not have the authority to reject test results.
My statement was pretty cynical and exaggerated, but there is some truth in it.
It is also frustrating that I do not get the test reports in many cases until two months later.
If there are a couple out of whack but the vast majority are OK, I usually don't say anything about it.
If there are more than a couple of tests that are out, I inform the owner, construction company, and other affected parties.
Most of the people that I inform simply don't care and don't seem to understand that 110% is not "better" than 100%.
RE: nuclear density gauges are not accurate
I bow to your greater experience, but as a contractor's engineer I am not sure that you "can achieve compaction without stability".
Generally when proof rolling I am looking for stability. Any significant movement and either recompaction is required or perhaps further excavation at formation level to take out any unsuitable material.
Many inspectors will look for any sign of slight instability and then choose that area for a density test (usually in my area sand cone or rubber balloon).
I would say that instabilty, in particular caused by the layer(s) below, will cause a compaction test to fail.
RE: nuclear density gauges are not accurate
Further, I've seen lots of clean fine sands achieve compaction while pumping....unstable, but compacted. A good example of why proofrolling is a good idea in many soils.
You are correct about underlying soil instability creating problems with upper compaction. The analogy I've used in testimony before is that you are trying to compact a layer of soil on top of a mattress...doesn't work. That's why we try to build successive layers for pavements from the bottom up, not the top down.
RE: nuclear density gauges are not accurate
How applicable are those proctor in the field anyway? In the real world, it is impossible to get the same soil throughout the site. So just trying to correlate the density in the field to a proctor in the lab (a statistical estimate) can pose a lot of errors- we would need several proctors, and will will also need special eyes and other skills to determine which proctor matches the soils in the field.
RE: nuclear density gauges are not accurate
That's called having properly trained technicians and geotechnical engineers willing to get their shoes dirty...both of which are becoming short in supply in many testing labs.
Yes, it's "possible" to get 110% compaction with current compaction methods, but not practicable. "In the real world" contractors are not likely to put enough effort into the compaction to achieve 110% compaction on a given soil, so it is much more likely that you are outside the range of the particular Proctor you are using.
The concept of moisture-density relationships is not a stochastic concept but a physical one. Statistical inference is not necessary for the technician to apply the laboratory result as a comparison for the field testing.
The concept is one of standardization of laboratory and field test methods so that a level of confidence can be achieved that the test procedure is validly repeatable by the laboratory (single operator precision and within-test variation are the statistical concepts that apply here). Those are company quality assurance issues, not field comparisons. So given that a laboratory knows how to run the tests properly, the chances of being outside the range of the Proctor are much greater than the chances of actually achieving 110% compaction.(Now that's a statistical concept)
So, bottom line, if I review field density test results and the technician shows 110% compaction, I start asking questions and requiring validation.
RE: nuclear density gauges are not accurate
Now, are the numbers right? Take the gauge to someone who will calibrate it on several materials, per the ASTM standard, at the interval indicated. Yes, look in the back, it is there.
Now, get a tech that understands the process, the gauge, the soils, and other test methods. Perform a moisture calibration for the material you are using. Apply the proper corrections. Have the tech observe the construction, as well as test it. Have the techs check results with a sand cone or drive cylinder (done correctly, same as for using the gauge)at a reasonable interval for the project to assess if the gauge is continuing to provide similar results.
You now have an accurate gauge and a smart tech (well, maybe smarter).
The next step is tougher. You now need to educate the engineers that oversee this type of work. Not just your company, but all of them. Get the guys that were too special to ever work a gauge, and the ones that never understood it when they did. Do not forget the ones that just have not worked with one hands on in the last 20 years, too. These are the people that make the gauge inaccurate or introduce poor logic into dealing with results.
I was at a seminar where a well respected solid waste engineer told everyone how much successive readings of the gauge could be. He showed 4 readings and explained how his grad student pressed the button, rotated the gauge 90 degrees, pressed the button . . . , until he had tested 360 degrees around the same hole in the ground (4 tests).
The problem is that this showed how the soil changed in a small area, as he had tested 4 different portions of soil, not the same soil. It did not show the fluctuations in readings that he had indicated. That was occurring as well, but that was should have been within the statistical limits identified be the manufacturer.
RE: nuclear density gauges are not accurate
The engineer with their PE called me about a site that I was reviewing (I work for the State of Indiana). They were being able to meet their density numbers with the nuclear test in backscatter mode, but not with the sand cone method. They called to see if they could disregard the sand cone test and just base everything on the nuclear test.
I found out that they were only using a vibratory roller on lifts of 2-3'. Thus they weren't getting sufficient compaction for the entire lift, but only at the surface. Since they were only using the nuclear test in back-scatter mode (i.e., they didn't have the probe penetrating the entire lift) they were only measuring the upper portion of the lift, that probably was compacted.
RE: nuclear density gauges are not accurate
RE: nuclear density gauges are not accurate
That is exactly the kind of thing I was griping about above.
It seems that a lot of testing agencies are only in business to produce paper reports to "pass" instead of actually providing a service to the project.
Ron's comment above about properly trained technicians and engineers willing to get dirty is spot on.
RE: nuclear density gauges are not accurate
By running a one point proctor (and there is a method for this that was written by the COE for use on thier projects) the technician could justify to me their reasons for chosing a particular laboratory curve. I never let the field one-point become the new maximum value without running another laboratory proctor.
As for ways to check the gauge in the field, I really belive that the sand cone, drive tube and balloon are your best choices. I saw your reservations about drive tubes abobve, but I always felt that in fine grained soils that the chances of human error in the drive tube test were less likely than those in the sand cone test.
RE: nuclear density gauges are not accurate
RE: nuclear density gauges are not accurate
However as someone stated earlier, I would say at least half the time we have compaction problems on the project it is because the proctor is wrong.
One of my big peeves is that proctors are in my opinion, overrated. It seems if we get to 95% the soil is suddenly fine as a bearing material. A fine to medium sand at 95% is good for whatever load we want to put on it, but a well blended gravel compacted to 94% is rejected! It would seem that we would be trying to coralte compaction to strength.
RE: nuclear density gauges are not accurate
In principal, I believe that there are 3 types of specification which can be used on a project:
1. Method (how the work is done)
2. End product (minimum degree of compaction, max air voids etc...)
3.End performance (how the material needs to act, how stiff etc...)
Now depending upon the level of risk associated with a project/material/construction etc... this should be reflected in which type of or combination of specification should be adopted, and in the case of this post, how it should be measured and what equipment is suitable to measure it.
If the possibility of a method of measurement being less accurate (nuclear gauge) which in turn may causing an unacceptable risk, change the method. The fact that for bulk earth filling, method placement is often acceptable (with a limited number of tests to check the method is working), this should be reflected in the manner in which the check tests are conducted, hence in this instance density checks using a gauge are appropriate. If however a catastrophic failure could occur then surely the specification should account for this and alternative tests completed to check not only the method, but also the end-product and the end-performance.
When I write specifications, I will use site trials to prove the proposed method can achieve the end-product and end-performance. The contractor then has a target to aim for (end-product) and checks are done to ensure the end-performance is met. The level of accuracy of the test equipment can then be 'built into' the type and frequency of tests, plus the acceptability limits. In my own experience, if you do enough tests you will get outliers, if there are no outliers, someone has 'normalised' the data. An engineer should be competent to be able to assess this along with the implications on the design (my view, bit of a soap-box moment sorry). At the moment I spend a lot of time reviewing other organisations earthworks specifications and identifying the conflicting requirements within them followed by many hours taking to the various parties trying to find out exactly what the material needs to achieve. 95% compaction does not mean a soil is strong enough, nor will not settle/heave. Less than 5% air voids does not mean a soil will have sufficient shear strength. Just because a plate load test does not settle more than 25mm under a load of 240kN/m² does not mean the soil has an allowable bearing pressure of 80 kN/m². You have to look at the whole picture.
In summary, in my opinion a gauge can be accurate enough (when calibrated and in conjunction with other tests) for many projects, but each case is different and each should be assessed on a risk basis.