nuclear density gauges are not accurate 3

[bb29510]

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.

 

[jgailla]

Ron,
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%.

 

[Zambo]

Ron,

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.

 

[Ron]

Zambo..when I'm referring to stability here, I'm talking about a quantifiable stability value such as a CBR or some other discrete measure of stability. For example, to stabilize a subgrade material for pavements, you might want a CBR value of 25 or 30. Depending on the material, you can achieve compaction, but not necessarily achieve the required stability. Stability is generally a material+compaction issue, so compacting the material ad infinitum might give you a specified compaction, but still not meet the requirement of stability for the purpose.

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.

 

[Riggly]

jgailla - It is possible to get compaction exceeding 110% of the standard or modified proctor. Remember the standard or modified proctor is NOT the maximum density, but the maximum for the compaction effort and moisture conditions. As with the accuracy of the nuclear guage, I don't believe it is, but I believe it is a convenient and quick tool to give an indication of degree of compaction. As mentioned in other post, I also use visual observation to see whether the foundation is stabilizing during compaction. But there is also human errors in using the equipment, and in some soil conditions it is just difficult to get a clean flat surface for the guage to rest on.
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.

 

[Ron]

will also need special eyes and other skills to determine which proctor matches the soils in the field.

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.

 

[TDAA]

Read your manual. That will give you statistics on how accurate your gauge is reading.

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.

 

[zelgar]

I think some engineers need to make certain they know the limitations of the equipment they'll using.

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.

 

[Ron]

Having a P.E. license doesn't prevent you being an idiot sometimes!

 

[jgailla]

zelgar,
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.

 

[ncarolinageo]

I know I am late to the party here - but I dodn't see where anyone mentioned the use of a one-point proctor to aid the technician in his selection of the most appropriate laboratory proctor. I agree that it does require the techcnician to take more equipment in the field, and there is always room for human error, but in my 10 years as a manager of field technicians (all of whom eventually used a nuke gauge) I found this to be one of the most useful tools in teaching them about density testing.

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.

 

[Ron]

NCGeo..the one-point proctor is certainly a valid method of checking the field material against the lab material. My biggest disagreement with the one point proctor, if run in the field, is the proper determination of the moisture content. Further, I would want to check the actual technician's capability to replicate, with a manual proctor hammer, the automatic proctor hammer results most likely used in the lab.

 

[DRC1]

I used to ron troxlers many years ago. If you followed all the protocall, I feel they were in general pretty accurate. They were not appropriate in certian soils, Not the best tool in narrow damp trenches and were subject to odd readings ocasionally. Prior to use they needed to be calibrated to the standard block. If it was out of tolerance, what do you think your readings will be? Also how you prepared the bed for the gauge was important. It was also important to remember excatly what information you get from the gauge. It does not measure the dry density of the soil. It measures the average total density between the base of the gauge and the tip of the probe. It measures the water content in the upper 2 inches of soil, weighted toward the surface. It then computes the dry density. Thusif the top were somewhat wet, as from spraying an ajoining area, the top 2 inches may be wetter than the next four. This will significantly lower the dry density. I liked them, had confidence in them, and when they give odd answers am always suspicious of technique.
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.

 

[iandig]

Just to go back to the original post, which was basically Nuclear density gauges are not accurate. Well, from reading through all the posts (many times, including my own comments), I was struck by one fundamental comment which was missing, what level of accuracy (degree of certainty) is appropriate. The answer will be different for different projects and uses, which in turn should be reflected by the person/organisation writing the specification.
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.