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Zero Air Voids Curve 14 lbs higher than Plotted Proctor Curve?

GE34 (Geotechnical) (OP)
26 Nov 08 12:23
I have a plotted Modified Proctor (ASTM D 1557-07) curve of fine sand (SP) with a plotted zero air voids curve that is 14 pounds above the peak and wet side of the Proctor curve.  The wet side of the curve is parallel to the plotted proctor curve and the curve does not cross to the right of the proctor curve.  But as  I stated the ZAV curve is much higher than the plotted proctor curve.  Has anyone seen such a case?  Is this normal with very poorly graded clean fine sands? Or does this invalidate the proctor curve?  I know there are possible variations in soils but 14 lb seems like a big gap.  However, the proctor results seem reasonable for this material.  The specific gravity test had been performed and is also reasonable.  Any thoughts on why the ZAV is not close to the curve?
civilperson (Structural)
26 Nov 08 16:08
Wrong specific gravity?
GE34 (Geotechnical) (OP)
26 Nov 08 16:57
Actually the Sp Gravity is 2.69 which is Reasonable.  I found the answer I was seeking on the Figure on page 115 of Holtz and Kovacs which corroborates a difference of 13 lb/ft3 b/t the ZAV curve and the peak proctor curve for SP sands.  So this is consistent with my plot.
woofar (Geotechnical)
27 Nov 08 18:18
The ZAV will always be above the proctor curve. How far above depends on the porosity of the material (which can be measured by water absorption). The reason is that the material in the proctor test has not achieved true ZAV, there are still some voids entrained in each particle of soil and aggregate. As a general rule you can use a 2% air voids line and it should come near enough to your proctor curve to be reliable for visual purposes. In a perfect world if you knew the exact figure for the entrained air voids, say 2.36% e.g., then your plotted voids line would go through each wet point of the proctor curve, however, it still wont be precise because the proctor curve is simply a line of best fit through four or more data points which each have their own vagaries of precision. What it comes down to is it doesn't matter where you plot your air voids line from 0%(well above proctor) to 5%+(well below proctor), the crucial point is that the wet side of the proctor curve should be close to parallel with the air voids line.
BigH (Geotechnical)
27 Nov 08 20:01
First off, ZAV means NO AIR - the material is all water and soil particles.  Full 100% saturation.  Any soil that is compacted will not be fully saturated - hence, as you are plotting dry unit weights, you must have air in many of the voids.  A material's plot will depend on the gradation of the material and on the material shape.  If uniform sand, you can only get so much "close packing" - remember your chemistry days.  Always plot the ZAV line for your lab compaction results - I've seen a very very reputable firm take a big legal hit because the field results plotted on the wrong side of the ZAV - hence the results were "suspect."
emmgjld (Geotechnical)
27 Nov 08 21:21
As an experiment, plot out two additional curves, at 86% & 96% Saturation for your Specific Gravity. The saturated 'down' portion of the proctor curve will usually plot within this 86% & 96% zone, my experience is usually below 94% saturation. I have found this helpful is validating some curves. It has also helped in some problems with the soil, such as high sulfate content, which can schew the moisture contents.
woofar (Geotechnical)
27 Nov 08 22:55
I hope this poat doesn't sound argumentative, it is not meant in that vain. The degree of saturation is a variable, (both from material to material and at different %compaction in the same material), and as such is not a reliable way of checking a proctor curve. To truly check the curve you need to do a Particle Density and Water Absorption test on each particle size present. You can then use these results, along with the Grading and Degree of Saturation to calculate the true line for that particular material. All the wet points should fall on this line.

That is a lot of work to do for very little gain. Using an assumed constant of 2% Air Voids is much simpler and an experienced operator can tell at a glance whether the proctor curve is right or wrong and which points should be ignored or re-done.
iandig (Civil/Environmental)
28 Nov 08 9:55
I am sure this has been covered above, but I would just like to add my own opinion to this. If you have a 'fine sand' this suggests that a significant part of the material under analysis will be single sized and be relatively free draining. You will only get dry densities close to the zero air void line if you can pack all the voids, however if as described you have a fine sand, there may be insufficient fines to achieve this. Plus, as the material is fairly free-draining the action of compaction can allow what fines there are to be transported by 'free' water away from the sample, further influencing the ability to 'fill the voids'.
From my own experience of fine sands [and silts] you generally get a flat compaction curve, air void contents on the wet side in the region of 5 to 10% [as opposed to less than 5% for a clay] and also it becomes very difficult to achieve many points beyond the optimum. Given that you describe the curve plotting parrallel to the air void line, I would take this as a good indication that you have completed the test correctly, however you do have to be careful you do not get a 'double' curve.
In theory if the particles were all the same size and spherical you would only get a mimimum air void content of 33%, no matter how much compaction you applied.
woofar (Geotechnical)
28 Nov 08 13:19
Hi iandig,

You are confusing the total voids ratio with the air voids ratio. Total voids includes the water, air voids does not. In your theoretical example the air void ratio would equal the absorption minus the degree of saturation divided by the mass multiplied by 100 which will come out as around 2% with most materials, including the theoretical spherical material you mentioned.

Your comment about flat curves with clean sands is extremely relevant. Clean sands should be tested with a Max/Min density index and not a proctor test, however, the cutoff line is not clearly defined. The usual cutoff line is 15% fines but if those fines are highly plastic then the percentage can be lowered. In the case of bentonite it is about 1%.
fattdad (Geotechnical)
29 Nov 08 11:26
Rather than look at the "height" of the zav above the max dry density, consider the position of the zav to the right of the max dry density/optimum moisture content.  You should be able to determine the percent saturation at the top of the curve and then determine whether this is abnormal or not.  You should be somewhere around 90 percent saturation.

If you are not familiar with this approach, let's say you have a maximum dry density of 110.0 at 12 percent.  Let's say the zav for 110 pcf is at 13.5 percent.  If you take 12 and divide it by 13.5 you'd then have 89 percent saturation for the sample to acheive maximum dry density.

Hope this helps.

f-d

¡papá gordo ain't no madre flaca!

woofar (Geotechnical)
29 Nov 08 17:50
Hi fattdad,

That calculation can't be done from the optimum, it is usually done from the wettest point but can be done on any point that is on the wet side of the curve.

At optimum there is still a very small amount of free air voids. It is only after you have passed optimum that the free air voids are totally expelled and the only voids left, are entrained air voids.
BigH (Geotechnical)
29 Nov 08 21:34
woofar - maybe I'm not seeing your point - but who cares about the entrained air (i.e., air voids within the grain particle) anyway? I suppose one must first concern themselves with the specific gravity that is used to determine the ZAV curve - does it, by definition include "water filled" voids (i.e., SSD) or "unfilled" air voids. Unless  you are chasing intellectia, I don't see it being of any relevance in normal practice.
fattdad (Geotechnical)
29 Nov 08 22:27
(Not agreeing with woofar.)

The ZAV by definition is the point of 100 percent saturation.  On any horizontal line from the ZAV (heading to the left) you are going from 100 percent saturation to a lesser value.  At zero moisture content, you are at zero saturation.  It's a liner relationship between moisture content and percent saturation.  That's a fact that has nothing to do with entrained air, or anything else. BigH knows where the body is burried:  You have to know the correct specific gravity.  If you do, then I stand by my original post.

f-d

¡papá gordo ain't no madre flaca!

woofar (Geotechnical)
30 Nov 08 5:51
Hi BigH,

The entrained air is very relevant, it is the reason why the ZAV line sits out to the right from the compaction curve or to be more precise the compaction curve sits to the left of the ZAV line. As you compact the soil on the dry side of the curve the free air is being expelled as a function of the moisture (under confined conditions the moisture is the strongest ingredient in the mix). As you get to the optimum you have expelled almost all of the free air and as you move on to the wet side of the curve the water has dispelled all of the free air and is now dispelling soil, that is what causes the drop in your compaction curve. The entrained air is still in the material and that is why the curve will always turn before it reaches the ZAV line. Zero air voids cannot be achieved with standard compaction the closest you will get is about 2% air voids. Modified compaction will force more moisture into the particles and as a result will compact to about 1.5% air voids.

As for relevance, this knowledge is necessary for anyone who needs to assess the validity of a compaction curve. Also for relevance this is the answer to the OP's question, which was basically, why is my zero air voids line higher than my proctor curve?

Regards
Michael
fattdad (Geotechnical)
30 Nov 08 11:25
Woofar,

We are describing two different things.  I am just making the point that you can determine the percent saturation from considering the positon of any point on the curve with respect to the ZAV.  You are describing how the curve cannot coincide with the ZAV owing to the presence of entrained air (and ultimatly the requirements for compaction via the Standard/Modified Proctor).  These two statements are not mutually exclusive.

f-d

¡papá gordo ain't no madre flaca!

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