Nuclear Densometer Problems (organics 1

[MDHEngSol]

I am wondering if organics will throw off the Moisture content readings? I am getting at least 100% compaction with a MC of 34%. My proctor results show that the Optimum MC is 28%. Thanks For your time.
LBI

 

[rconner]

It may be advisable to ask the manufacturer of the equipment this question. However, while not claiming to be an expert in this field I believe I remember way back from my school days that the Proctor test involves multiple tests at different moisture contents, but with only a specific, well-defined compactive effort. I'm not sure it means that if the contractor were to apply e.g. a much higher relative compactive effort/energy, he could not achieve an effective degree of soil particle compaction at even a little higher than "optimum" (per Proctor curve) moisture content.

 

[eric1037]

I am familiar with the Troxler guage. I think that the moisture content is determined by counting the hydrogen atoms. Since organic soils contain hydrocarbons, they will throw off the moisture readings. Since the dry density is calculated in the machine by using the moisture content, the percent compaction is thrown off too.

See this document for more info on the Troxler moisture corrections:

http://www.troxlerlabs.com/pdf files/MoistCorrectAppBrief.pdf

Are you using the Standard or Modified Proctor number? It is possible to have numbers in the 99 to 100 percent range if you are using the Standard Proctor. I become suspect if I see numbers of 99% or more with the Modified.

 

[GeoPaveTraffic]

Orgaincs may cause the gauge to read a higher than actual moisture content, they will not change the wet density as measured by the gauge.

Based on your OMC (i.e. 28%) I assume you are using a standard Proctor curve. In that case it is possible that you could get high degrees of compaction while being up to 6% over OMC. However, I would check to make sure that the soil that the Proctor was ran on is the same one you are seeing in the field.

 

[TDAA]

What speed / depth are you running the test at? These could both be introducing additional error in the test. Do the moisture correction as described above as well. You can have many things influence the moisture content portion of the test. Allways burn one to be sure it is correct.

 

[looksatstars]

you can get 100% compaction with +6% over optimium. With todays compaction equipment, you can get alot higher than 100%.
Field moisture values will be higher as well than field values because the soil in the field can drain more readily.

BUT I think the main part of your problem (more than the moisture content) is that you are seeing organics in the compacted fill. Organics over time will rot and disinigrate leaving voids in the fill which will settle.

 

[GeoPaveTraffic]

looksatstars,

I have to disagree with the part of your comment about field moisture values being higher (or for that matter different) than laboratory values "because the soil in the field can drain more readily". Drainage has nothing to do with the compaction relationship shown on a Proctor curve.

 

[looksatstars]

Geopavetraffic:
Your entitled to your opinion, but with my field and lab observations with proctors and densities, it has been my experience that there is a difference. With a moisture content of 28%, he is looking at a clayey soil, and that soil in-situ has moisture content from groundwater or surface water. The soil that was tested in the lab was dried and then water mixed into the soil. The water penetration would not be the same. The moisture content in the lab should only be used as a guideline out in the field as it is only a small representable sample. It just seems to me that LBI is thinking that there is something wrong because he got 100% compaction even though his MC was +6%. Even if the soil has 14% more moisture, he can still get the 100% compaction but will need probably 120% compactive effort comparing to the standard proctor compactive effort (which is not that hard with todays equipment)
On a side note,with graded gravel samples, I have found the proctor values to be very close to the field values but definately not in clays.

I appreciate your opinions on this forum as you do have alot of experience, but i feel there is no right or wrong answer in most soils problems and this forum can opens eyes to other people's experiences that they have found in their work experience. But nothing should be taken as being 100% right because I have not seen the soil or the conditions.

 

[eric1037]

It is very difficult to get sufficient compaction if you are over 5% over optimum. In addition, your moistures seem really high, even for clays. I would run another proctor and get some real in-situ moisture contents. If things are out of whack, do the moisture correction on your nuke as recommended by the manufacturer.

If there are only small amount of organics (like occasional small root fibers below the stripped topsoil), then it's unlikely to throw off your moisture counts. However, if you have high amounts of organic, you may have other problems. Generally, we recommend not using soil that has more than 4% organics for engineered fill purpose.

 

[TDAA]

"Even if the soil has 14% more moisture, he can still get the 100% compaction"

Looksatstars: Do you draw the zero air voids curve when you do Proctors? The cure represents a maximum weight for various amounts of water and soil, based on the specific gravity of the soil particles (and will vary slightly based on the specific gravity). If you have used the correct SG for the material, then it is highly unlikely that there is sufficient allowance for an additional 14% moisture, while maintaining the max dry density. It has been a while since I have looked at a Proctor curve, but as I recall, you might get more in the range of 5% over, and as eric1037 points out, this get hard to achieve. The curve slopes down with increased moisture, not up! I really don’t care how good the equipment is now, I have yet to see someone compact mud to a high density.

MDH: How does the soil feel? Does it feel “wet”, or does it feel “dry”? Do you also have mica in the soil? If the soil is actually on the dry side, you could be getting higher compaction, while something such as mica could be increasing the “moisture” measured by the gauge. The net result could be 100% compaction at a high moisture content, but it would not be right. The point is, there are a lot of variables that must be looked at to see if the results are correct. As stated before, the use of an oven, microwave, or field stove will help determine the accuracy of the gauge. That is the first step.

 

[looksatstars]

TDAA you are correct that 14% above optimium would probably be mud. It was a bad and wrong percentage to use.
I have only used the zero air voids to show that the proctor curve was in the allowable limits.
As more water is introduced into the sample, the water starts to take up more room and then more of the density which will infact lower the overall max. density. Its the basic principle of the Proctor.
The field compaction with a vibrating sheepsfoot is alot different than a standard proctor hammer.
I have experienced alot of people from the office that have 100% trust in the densometer which I don't believe in. Its a great tool and highly accurate but there are certain aspect of the densometer that you should be aware of.
It is known that on older densometers they only read the top 2" of the soil being tested even though the probe goes into the soil 10". If there is a time delay between placement and testing the top layer can dry out giving a false reading.
Also if the soil is wet with a dense layer underneath the water can migrate to the top of the soil, also giving an error in the reading.
I typically dig up the area i tested if I get a weird reading and usually find some other material mixed in.

I have also found that if there are organics in the soil then the density would go down as well.

 

[TDAA]

Looksatstars: It is good to hear that you actually poke around the soil to determine if it is “relatively uniform”. I too have seen people ignore what they see and go based on the gauge. That is what I was trying to portray to MDH.

As far as the depth of moisture measurement (at least for Troxler), the radiation source is contained within the gauge, not on the probe, so the depth of the probe has nothing to do with moisture. The manual for the 3440 gauges has a formula for approximately how deep the gauge will read for moisture. At about 25% moisture, the depth is around 6 inches. With higher moisture, it will not go as deep, and with less, it will go deeper.

 

[boffintech]

looksatstars wrote, "It is known that on older densometers they only read the top 2" of the soil being tested even though the probe goes into the soil 10"."

I'm going to sugar coat this so as to say this as gently as I know how: Wrong, completely wrong. Sorry. There was never any nuke gauge that only tested the top 2" even though the probe goes 10" into the soil.

Back to the question at hand: MDHEngSol asked if organics will throw off the Moisture content readings? He wrote, "I am getting at least 100% compaction with a MC of 34%."

The answer is, of course, yes. Organics do throw off nuke gauge MC readings because, as eric1037 pointed out, organics contain hydrocarbons which are counted in the MC reading.

Something else is wrong if he is getting 100% compaction with 34% MC. Probably a wet layer over a dense drier layer.

Use probe rod. Use available equipment to proof roll area. Dig test pit. Perform correlation MC tests using an oven. Investigate.

 

[looksatstars]

boffintech-you say my advice is wrong but give nothing to back it up?
coincidentally, I found this old (2005) eng-tip forum topic on moisture content on nuclear densometers and the way I read it is that people that use the densometer in the field say it is only accurate to ~2" but people in the office state the owner's manual and say it useful to the entire depth. Ironically, you were part of that post.

http://www.eng-tips.com/viewthread.cfm?qid=117761&page=1

As i stated before my views are just opinions, it would be nice if you can investigate but some times you just don't have the time or the option to run lab tests for a high MC so you have to use your field experience to make a call.

Also, MDHENGsol, were you getting consistent MC reading with the 100% compaction? Organics are usually not consistent throughout backfilled soil, but if it is subgrade there could be a layer of organics just below the surface.

 

[zelgar]

As with any tool, a nuclear densometer can be used incorrectly. I've seen people try to use values for density with a nuclear densometer in backscatter mode when the sand cone method gave vastly different results.

I've also seen people try to state that a nuclear densometer in backscatter mode could be used to verify that a crack did not penetrate a concrete slab.

Nondestructive tools can be helpful, if they are used in the correct manner and FIRST verified using a destructive method. If the destructive method indicates values that are greatly different than the non-destructive ones, you need to evaluate what is different. Did taking the sample for destructive method change the sample or is the testing apparatus different than the on-site conditions. Also, for the non-destructive method, read the manual & call the manufacturer if you have any doubts about the results.

 

[boffintech]

looksatstars I never said your advice was wrong. I said your assertion that there was in existence at any point in time a nuke gauge that “only read the top 2" of the soil being tested even though the probe goes into the soil 10” was wrong. There never was any such gauge.

As eric1037 alluded, this must not be structural fill if it has a lot of organics in it.