interpretation of nuclear densometer test

[optimumP]

We want to compact sand to at least 95 % proctor. We have done laboratory tests on the material and have obtain the optimum density and an optimum water content of 11 %.

We are now doing nuclear density tests on the sand and the reading indicated a compaction of 98 to 100 % but for water content of 4 %.

My question is this: when reading the nucleodensometer what value should you be paying attention to, is it the compaction value or the water content ?

 

[BigH]

I presume when you say you have 98 to 100% compaction, you mean relative to the optimum dry density at whichever one you are using (i.e., Standard Proctor (ASTM D698) (or British "Light" compaction) or Modified Proctor (ASTM D1556 (or British "Heavy" compaction)). Make sure you are consistent.
If you are reaching 98 to 100% and the specification says that 95% is required, then you have met the requirement - unless there is also (as sometimes is) a requirement of the compaction being carried out at optimum +/- 2% (or other numbers). Keep in mind, though, that with sand there will be a dry out period so that the moisture content you are determining by the "machine" (or subsequently in the lab) may not be indicative of the moisture content at the time of compaction (time in delay of test to placment and compaction). Also, I found that you really should calibrate the nuclear density moisture content with the laboratory one. Back in the late 70s when our machine read water in lbs/ft3, I found that the machine was reading 30% too high - in other words, the lab moisture was 5% and the machine was reading 7%. You can see that by the machine reading "high", the density determined would be low by 1 to 2% depending on the material.
Another thing to keep in mind, some specifications also have a requirement that the average of the compaction readings must be greater than the specified value by some statistical coefficient. For 1 failure in 20, you would add (stdev * 1.65 + [ 1.65 / (sqrt[N]) ] where N is the number of compaction tests you took for the layer and stdev is the standard deviation of the "set" of compaction tests for a particular layer. If N < 10, there are methods to determine the standard deviation based on the range of values - if you think you have to go that way.

 

[Rjeffery]

Do a sand cone test to determine if the gage is giving you the proper data. This will confirm the M% and density...

The best way to test something is to squeeze it, slowly, until it breaks!

 

[oldestguy]

Isn't there an ASTM "Requirement" that you calibrate the nuke for the site soils you are testing? that's where the sand col]ne comes in, as indicated above.

This means calibrating as BigH indicates, not taking some "standard" reading in the lab on a concrete block or a paraffin block.

This form of testing is prone to all sorts of errors and they generally are on the low side, causing more grief on the jobs than necessary.

Being faster and easier does not relate to being better.

 

[iandig]

Ignoring the use of the gauge to determine the density, I take it from your opening comment that the concern relates to being able to achieve 98% compaction but at a low moisture content. Have you looked at the grading of the material, the shape of the compaction curve and the air void contents?.
To better explain this, there are a number of characterisitcs which may provide a high % compaction over a wide range of moisture contents, this is often the case with single sized materials such as sands. In affect, all the grains of sand are touching, which means that you get the same density over a wide range of moistures, however, with increasing moisture the voids fill with water and you reduce the air void content. This would be exhibited by a flat curve from the compaction. As a check to see if this is happening, you can also schedule compaction tests using the 2.5kg and 4.5kg rammer [different names in different countries] you may well find both curves overlie each other. The grading of the material can also be used to aid the assessment, try also calculating the uniformity coefficient for the sand.

 

[BigH]

iandig makes a point but we need to know how long after compaction the moisture was taken. In hot climates, if he waited 1 day to take the tests, it is likely the moisture is far less than that at the time of compaction.

 

[genomty]

In my opinion there are basiclly two or three reasons because of the results you are having can occur and I would check them in order to find the real reason to explain what is happening.

1.-Material being compacted in field is different to that tested to determine the Maximum dry density in lab, the material in field could contain oversize particles that not were included within the lab test for instance, proctor maximum density and moisture content should be corrected by oversize fraction according to ASTM D-4718 in order that readings provided by the nuke are right, or lab material could contain a higher fraction passinf No. 200 sieve.

2.-Compaction equipment used in field compaction is heavier than requiered, heavier equipments will provide higher compaction energies, it means that effective efort used is grater than requiered, consequently, for the sime soil as compaction energy increases you need less moisture to reach certain density, this not mean that nuke or lab test are wrong, simply means that you have to improve field quality control program, If material is the same tested in lab, mositure content in field should be within an adequate range, lets say as BIG H plus or minus 2 for sandy soils, in order that your soil is being compacted as requiered, remember that as far as you appart your moisture content from theorical saturation curves as grater is the mechanical behavior drop in case of a suddenly moisture increase, that's the main reason to established and control a moisture content range

3.- Nuclear gauges has two radiological sources, the one located in the tip of the rod is used to measure density by the radiological attenuation principle and the mositure is measured by thermalization of hydrogen electrones, is like you bomb the H2O molecules using electrons the impáct termalized the electrons of hydrogen and produce an energiy emmision taht is measured by the gauge sensor, nevertheless this bombing not reach China, it is limited and the moisture content reading given by the nuke just reach a couple of inches as much as three, if you ar in a hot wheather location and the loss of moiture due to wind and solar radiation is high this can be affecting the reading, this thermalization also has a problem wich id that some kind of materials may produce false readings in moisture content and consequently dry density, for instance soils trated with hydrated lime (see the excess of hydrogen) will induce an error in the test, nevertheless there are correction that can be done to make the reading reliable, moisture, density, trench etc.. and usually correlate the nuke to sand cone and mositure content detrmined oven or direct heat dried samples are quite recommended

using the nuke requieres certain expertise degree by technician, because most of them use the value provided by lab and the reading provided by the machine and forgot the basics

 

[DirtDelight]

How much oversize is in sand ????

 

[fattdad]

(What everybody else said.)

When in doubt however, take a bulk sample at a nuke gauge location and pound a one point (dry side of optimum) and see if the one-point for the soil at the density test location falls on the original (i.e., reference Proctor) curve. If it does, then you are good. If it doesn't then you are referencing the wrong Proctor.

I also echo the comment that if you are testing much after the fill placement, the soil may have dried below the compaction moisture content. Dry density remains unchanged, but the measured moisture content is no reflection on the placement moisture content.

Fraught with problems, these compaction test projects.

f-d

¡papá gordo ain’t no madre flaca!

 

[erich]

The other thing to realize is that most nukes test moisture on;y for the top 2" or so of material but the density is measured over thewhatever the depth of embendment is due to the placement of the sources. When working with "live" sands the top inch or so may dry and fluff very quickly, but over the thicknes of the lift minumum densities are maintained.