Why 95% Compaction 11

[1967pradyot]

Hi

Why for any compaction of soil, it is stated that 95% of Modified Proctor or Standard Proctor dry density is to be obtained ?
If we specify 85% or 90% and do obtain the required strength, whether it is OK? I want to know what exactly the justification of specifying that specfic ( generally 95%)degree of compaction.

 

[solutioninc]

Focht3 has raised some valid points. When Proctor came up with a compaction control test way back when, the equipment available for field compaction were relatively light and the Standard test as it was known later (as opposed to modifies) was adequate for the intended purpose, namely compaction control in the field. With the advent of heavier machinery, the percent compaction in the field often exceeded 100 and that led to the development of another compaction test with compactive effort 4.5 times larger than the original one. As of today I am not aware of a percent compaction exceeding 100 if modified proctor is used as the "compaction control" test. I wrote down all these just to highlight that the percent compaction is not the be all and end all of an engineer's responsibility while constructing an engineered fill. I just pointed out two engineering properties in my earlier post which the engineer aims for. A percnt compaction cannot ensure a fill with the required properties simply because the compaction control tests give different maximum densities depending on the compactive effort. So use of a percent compaction will result in different densities (and therefore different engineering properties) depending on the compaction control test used. For achieving different engineering properties including those mentioned in my earlier posting and that of Focht3, both the molding moisture content and the resulting density are the important factors. Also, as mentioned earlier, the standard proctor and the modified proctor lie on either side of the compactive effort of currently available field compactors. Therefore it is left to the engineer to choose a field compaction control test to achieve his/her objective. I conclude by restating that one has to do quality assurance tests on samples from the engineered fill to verify that the design requirements are met. Thease laboratory tests along with the percent compaction data available at a much larger frquency are the ONLY assurance to the engineer of a uniform fill with the required properties.

 

[BigH]

Maybe what we should be focusing on is not the MDD but the desired void ratio at compaction! See the article 95% Fixation that I've offered to forward.

 

[drfefe]

Hi
Why 95% of compaction of the Standard Proctor like reference for the compaction of grounds were chosen?. In the time that engineer R.R. Proctor invented the logical and practices form to make a compaction, the equipment used in the field was light and was Difficult obtain always the 100% of compaction of Proctor Standard, Besides all the tests did not mach a uniform result. Then the percentage of 95% liked a practical minimum good percentage accompanied with humidity near the optimal of the proctor, At the same time involved an uniformed compaction. In World War II, started making heavy equipment, and was creating the necessity to establish the test modified Proctor with greater energy of compaction, and continued being conserved the criterion of the percentage of 95%.

Excuse me for my bad English

FEFE

 

[geodan]

BigH Good lord if you wish to monitor void ratio during fill placement (most moisture-density gauges) then you must come up with AASHTO method of determining the required void ratio for each individual soil type... ahhh!
incidently in sw washington, for sandy silt/silty sand: 100% of the standard proctor is equal to 95% of the modified as rough numbers. After many *very* tests conducted on local ML/SM (flood deposit material) we spec 95% of the modified as the contractor usually has no problem achieving +100% of the standard and as somebody said... "stronger is better"

>> silt/sand in our area (avg of many tests) undisturbed native (1 to 4 feet)= dd 80 to 90, e = 0.9 to 1.2 standard proctor mdd = 95 to 105 17-20mc
what contractor usually achieves in the field at 14-16mc after approximately 10 passes? = 110 to 115, e = .45 to .6


What is the main reason I dont like 95% of standard? Even in areas where compacted fill is visibly transfering shear stress (wave motion under loaded dump truck) generally 24 to 28% mc, the dd is still 90-95pcf at e = 0.7-0.9 resulting in +95% and adequate compaction according to standard. Although walking on the ground can indicate "pumping"

Just a bit of my experience with 95%.

 

[BigH]

Geodan - I didn't say to monitor void ratio during fill placement but that in choosing the proctor value to which to construct; the void ratio, many times is the desired product of what we want to achieve - it might be reasonable to base your criteria on the void ratio desired. Send me your e-mail and I'll forward the article to you.

 

[cvg]

Big H

You are correct, when it comes to a flood control dam, levee or other types of embankments that are designed to retain water, maximumizing the in-place density and minimizing voids is a primary concern. Strength is also important to resist deformation caused by earthquakes, but factors of safety are generally large for slope stability.

 

[geodan]

Yes I agree Big H, I was just being a bit facetious. As a company policy we have required our techs to record the void ratio (when using the nuclear gauge for fill QA/QC testing) for each test taken as it helps me to review their work.

However the void ratio varys greatly with the material. Most crushed aggregate is placed at e = .1 to .2, whereas the maximum for various poorly graded sand can range from e = .7 to 1.0

 

[g7mann]

g7mann:

This is a most interesting discussion. My understanding is that the Modified Proctor was developed as a result of an increase in airplane size, weight and undercarriage assembly. Larger loadings imposed more severe stresses of the fills underlying airstrip pavements and this led to increased damage to those pavements. The oribinal intent was, i believe, to generarte a "denser" and firmer compacted fill subgrade, and to take advantage of the increasing compactive effort generated by improved compaction equipment.

So much for the origin! It should also be clearly understood that fill source materials, either imported or in-situ, typically vary in composition, sometimes considerably. One, or even two or three, Modified Proctor samples will often not generate enough data to "accurately" test all of the placed and compacted fill. Thus, the "requirement" of 95% compaction helps "force" the earthwork contractor to expend reasonable effort to achieve a well compacted, competent and [hopefully] unyilding fill mass. If the variation in material composition tends to prevent the achievement of this degree of compaction it is always possible to "back off" the 95% requirement providing the contractor is truly exercising his best effort to achieve this goal. Thus, the 95% requirement is a means of generating a competent fill mass and, given the origin of the test requirement, often a more than competent fill for lightly loaded development, such a residential of light retail. Practically speaking, the requirement to achieve 95% compaction is a clean and reasonable means of holding an earthwork contractors' feet to the fire.

The "requirement" for this specification appears to me, based onm my more than 35 years experience, results from public agency pressure. Many, if not most, public agencies make this requirement in an effort to cover their liability [real or percieved] and do not understand what this means in time, effort, cost,or even result. They are tied to having paper test results just so thay can point at an engineer or technician and say "they said it was ok." Although a generalization, many public agency employees do not understand the reasonaing behind fill compaction, or wahy variations in the results [degree of compaction achieved] means. As a result the achievement of 95% of Modified Proctor compaction is ultimately often a paperwork exercise.

Also, and this is a pet peeve of mine, virtually none of the technicians I have been involved with have ever plotted the laboratory proctor test curves as "family" on a single sheet. Typically, the technician is provided with an optimum moisture content and a maximum dry density and he [or she] simply inputs this to the nuclear density gauge and goes merrily on his [or her] way. This does not work. I typically insit on haveing a family of curves plotted and then have the technician plot ever field density test on this family of curves. This will immediately show which of the test curves the tested soil meets [or does not] and will provide a more accurate indicator of fill densification. Many times I have been required to explain to a contractor why the field tests indicate a "failing" result [<95%] but the compactor is bouncing on the fill surface and is shaking the adjacent houses. It is generally because the technician only has the numbers for a material, and that the material being tested in the field is not the material tested in the laboratory. Well, that's off my chest for now.

Lastly, the achievement of any degree of compaction is determined by the soil composition [particularly the percentage of silt and clay fines], the materials' moisture content, the amount of compactive effort being imposed on the fill mass, the thickness of the layer being compacted, and the suitability of the compaction equipment. Ultimately, if all of the above is taken into account, regardless of the specified degree of compaction, the end product should be a firm, competetn and generally unyielding fill mass.

 

[Focht3]

[blue]g7mann[/blue]:

Welcome to [green]Eng-Tips[/green]!

I agree with you - but the technician's ignorance on this point isn't all bad. I customarily input the results into a spreadsheet to calculate the apparent degree of saturation as well as allow me to plot the results. "Outliers" have a habit of occurring when techs "pencil whip" results to satisfy the contractor. (They're not smart enough to adjust both the water content and density; they usually only change the reported dry density.) Sometimes it's a crooked tech, sometimes it's a sense that the test result is wrong. But it doesn't matter why; it's a clear warning that a particular technician - or job site - bears closer scrutiny.

Keep posting - your experience is a real asset. Again, Welcome!



Please see FAQ731-376 for great suggestions on how to make the best use of Eng-Tips Fora.

 

[BigH]

Seldom, too, does anyone ever plot the ZAV on laboratory compaction curves. This can get you trouble if your field results are "on the wrong side." Happened to a very very reputable firm.

 

[Focht3]

Yup. Yet it's so easy with modern geotechnical software...



Please see FAQ731-376 for great suggestions on how to make the best use of Eng-Tips Fora. See faq158-922 for recommendations regarding the question, "How Do You Evaluate Fill Settlement Beneath Structures?"

 

[dirtsqueezer]

baobab,

Just picking up some of the table-scraps here. I've run both density specifications for soils using the modified proctor, and found some differences that I think affected our work.

On an FHWA project for a summer I performed the mod AASHTO where I had previously only used ASTM D1557 in the company lab. The method uses the same weight, drop, and basic moisture distribution (though different soil types for different weights, actually), the major difference being that the AASHTO method re-uses material from previous moisture points for subsequent moisture points. The ASTM method specifically prohibits this. I happened to be working near Mt. St. Helens at the time, and tested a good amount of native fill that consisted lightweight pumice. You can imagine how ground up that component was throughout the course of testing. I actually ran a few ASTM D1557s for comparison (because there was no one else there to tell me not to , and consistently got up to 2-3% lower numbers using only pristine samples with the porous rock intact. I'm not sure how the pumice would have reacted in actual lifts in the field, but it seemed to me at the very least debatable.

Another difference was the grain size division between the 12" and 18" drop methods, though I don't remember thinking results would have been affected. Also, I think there was a minor difference in the accounting of oversize material. One method of the AASHTO I believe specifies complete removal of oversize w/o adding it back in. I have both methods, and would have to go back and be sure, but I could get more specific in the wording, if you're interested.

Also just interesting to note with the pumice fill, because of the extreme low specific gravity- actually less than 1 (the rock floated), an oversized rock correction would have actually dropped the proctor value! That is, again, assuming again the rock wasn't ground up during compaction. We didn't happen to have that much larger pumice in our soil, but it was fun to consider.

 

[Focht3]

An excellent point! It's important to remember, too, that all of our basic tests (consolidation, unconfined compression, direct shear, pressuremeter, DMT, CPT, etc.) assume that the soil grains are incompressible - which is debatable (or outright wrong) when dealing with oolitic sands and some volcanic materials.



Please see FAQ731-376 for great suggestions on how to make the best use of Eng-Tips Fora. See faq158-922 for recommendations regarding the question, "How Do You Evaluate Fill Settlement Beneath Structures?"