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What is the difference between 95% and 98% Proctor Compaction?
6

What is the difference between 95% and 98% Proctor Compaction?

What is the difference between 95% and 98% Proctor Compaction?

(OP)
What is the difference, as far as load bearing capacity between a specified 95% and 98% max proctor? Is there anything to worry with a 95%?

RE: What is the difference between 95% and 98% Proctor Compaction?

4
3% - Just had to say that

RE: What is the difference between 95% and 98% Proctor Compaction?

main differences would be in potential for settlement and reduction in permeability

RE: What is the difference between 95% and 98% Proctor Compaction?

(OP)
Is 95% compaction acceptable as foundation subgrade?

RE: What is the difference between 95% and 98% Proctor Compaction?

usually is - but ask your geotech

RE: What is the difference between 95% and 98% Proctor Compaction?

Specification of percent compaction is typically done to relate to laboratory behavior.  If an engineer has a set of data (i.e., strength, CBR, permeability, etc.), that is based on 95 percent compaction and this is not acheived in the field, there would be no correlation between what is anticipated and what is provided.  Now I'm not sure why somebody would specify 98 percent compaction, but fundamentally if the civil/structural designer specified this and the contractor accepted the responsbility to build what was specified, some additional measure of compaction may be needed.

f-d

¡papá gordo ain't no madre flaca!

RE: What is the difference between 95% and 98% Proctor Compaction?

(OP)
Isn't 98% standard proctor compaction a standard for fill where foundations will be set?  Do you know a good reference that discuss this subject?

RE: What is the difference between 95% and 98% Proctor Compaction?

I practiced for a long time in an area where silty lean clay was prevalent, and the usual specification for building pads was minimum 95% Standard at water contents of optimum + or - 3%.  Typical allowable bearing pressure for footings was 2500 - 3000 psf. I think this practice is pretty widespread.

I learned that paving subgrades tended to rut under truck traffic during paving at optimum +3%, but not at optimum +2%. With the local material, 98% compaction could be acheived at optimum +3, but not at optimum +2.  I began to specify 98% at optimum -3 to +2% for paving subgrades. I was really after the lower water content, but the higher compaction requirement helped assure that the water content would be lower.

Also, after digging up fills that had become saturated after compaction at 95%, I found that they were just stiff, pretty marginal for a bearing pressure of 3000 psf.  Compaction to 98% leaves less void space for water absorption, so the saturated strength will be higher.  The higher compaction requirement will help assure good foundation performance, especially if loads are heavy and bearing pressures of 4000 psf or so are desirable.

These principles don't necessarily apply if the fill is highly plastic and potentially expansive.

RE: What is the difference between 95% and 98% Proctor Compaction?

As others have said it depends entirely on the soil type. Some soils will give you hardly any bearing capacity even at 98% compaction and others will be rock hard. So what you need to know is what bearing capacity are you getting at 95% and 98% compaction. Of course it also depends how much of the stuff you have under the foundation and the shape of the applied load or foundation.

Carl Bauer
www.bauerconsultbotswana.com

RE: What is the difference between 95% and 98% Proctor Compaction?

One thing to keep in mind, too, is that actual compaction to a spec if rather more than the minimum required.  Say the spec is for 95%MDD Std Proctor.  Likely, the actual compaction will be more than 95% - say 96 to 97% on average; and, particularly for cohesionless soils, compaction of overlying layers will increase the compaction even more.  So, in many ways - this it is a misnomer.  The engineer, say, for bearing on an engineered fill specifies the minimum he would be 'happy' with for the bearing pressures designed (and it is another question as to how he really came up with the percent MDD needed for the proposed bearing pressures).  In actual fact, you will likely get better behaviour because of the higher levels of compaction as achieved.  As for 95% MDD or 98% MDD - likely a matter of the engineer's experience and what he or his company has always specified. I'd be more interested in knowing the behaviour under 97% MDD vs 98% MDD - and whether it is really an issue.

RE: What is the difference between 95% and 98% Proctor Compaction?

(OP)
BigH you have hit exactly my question. What is the behavior difference between 95% or 98% compaction? Is it really an issue?

RE: What is the difference between 95% and 98% Proctor Compaction?

you are only testing at a few spots and compaction will vary.  the standard deviation with 95% requirement might only give you 90% in some areas which may not be acceptable.  With 98% requirement, the standard deviation is reduced and your minimum values of compaction may come closer to the the 95% that you are looking for...

RE: What is the difference between 95% and 98% Proctor Compaction?

we typically test for 95% std and recommend 98% std at top to help with exposure to weather, disturbance due to construction activity etc.

98% is better than 95%...benefit may be negligible or may be just enough depending on the scenario. ask the geotech.

RE: What is the difference between 95% and 98% Proctor Compaction?

While you are on this subject folks, do some testing of density of natural ground at the cut location.  I REFER TO CLAY SOILS. If you are then building on that natural CLAY ground at that density at say 3,000 psf, then why do you spec a higher density for the fill?

I'll bet you won't find the percent compaction (if you will)of that natural CLAY ground anywhere near 95, but more like 85.

Makes one wonder why such high requirements.

Also, making the contractor beat the stuff into those densities, adds to the job cost. Why do it?

Something to think about.

RE: What is the difference between 95% and 98% Proctor Compaction?

we try to avoid testing residual soils (we have mostly silts here with limited amounts of clays) since the in place density will likely fall low. sometimes, building officials require density tests because they don't fully understand what they're asking for so we then must test it (but we tell the contractor to recompact the top prior to testing. the compaction recommendation is always included because more often than not, the subgrade will become disturbed anyway either during grading or foundation excavation. for pavements, i will usually tell the contractor to recompact everything anyway since pavements are much more susceptable to problems to the surficial conditions. if it's a cut in residual soils and the subgrade proofrolls okay, then i will quite often not bother with making the contractor recompact for density testing. if the geology or planned construction type is cause for concern, then recompact it, test it and gone on.

and to go back to your question about why spec compaction when residual soils have an okay bearing with lower in place densities (and this assumes that "natural" is referring to residual--so this discussion might be different in areas up in northern u.s. where glacial till occurred but since i'm not extremely familiar with that scenario, i'll stick with the geology i know): you must keep in mind that residual soils have residual stresses built up over millions of years. once we take a CAT D8 through there cut it out and push it around, all those residual stresses have been broken so then you must put a CAT 815 on it to compact the lift to beat the voids out of it. the fill gets its strength from rearrangement of particle packed together. residual soil gets it strength from the inherent properties of the soils which built up over a very long time and quite often under immense pressure/temperature (but then weathered out to soil)--the last statement is not always the case depending on the geology, location, etc.

there's also the concept that if you require 95%, you know that at least some spots will not be to this compaction...say 90% for some locations that happened to not show instability or went at the exact location of the test. if the spec happened to be 90%, then maybe some locations would get down closer to 80%. for some projects, i've seen the compaction spec'd to 98% simply because they wanted to minimize those rogue 90% densities and make sure most everything was at least 95%. this was not my job (i'm not even sure it was my company) since i would simply increase the testing frequency. however, i think the spec was put out there because they were fairly confident they would not be performing the construction testing...and it was a "cheap" exploration on top of that.

and lets be "real" here, how much could it possibly add to the cost of a project to compact one layer of soil (even for something huge like a 250,000 sf building)? essentially zero compared to the overall cost of the project. but how much will it cost to remediate a bad subgrade after the fact for the same sized building?

and one last comment in my coffee driven post here: there's some residual soils that i specifically recommend the contractor NOT compact and tell them to keep all construction off of simply because it would go from stable before compaction to a gooey mess after compaction (which then might require undercutting and replacement). and all these points are from my location and geology...others likely have other "things" that are more common in their areas.

RE: What is the difference between 95% and 98% Proctor Compaction?

One other dimension to this topic:  While the geotechnical engineer may "recommend" one measure of compaction in the geotechnical engieering study, typically, it's the civil engineer that produces the specification.  Whether the geotechnical engineer feels that lesser compaction is o.k. or not, during construction the contractor is bound to perform to the requirements of the specifications.  If the geotechnical engineer doing the compaction testing were to "approve" lesser compaction, there would be a trasfer of liability, which may or may not be a good thing (likely not).

Just a nervious Nelly sometimes that it. . . .

f-d

¡papá gordo ain't no madre flaca!

RE: What is the difference between 95% and 98% Proctor Compaction?

very good point...probably just as much or more important than my commentary above. your point is sort of the heart of what we (professionals) must be careful of...taking the liability of someone else's work. 95% std Proctor compaction is not always appropriate for every single situation but more often than not, it's okay and has enough safety factor that even when the contractor sneaks in something "marginal" occassionally without our knowing it, things are usually still okay.

i still remember my structures professor (speaking with a heavy lebanese accent) talking about how engineers absolutely must be chickens...chicken factor he called it--referring to safety factor (both applied and implied). we must be chickens of all the things we know have some variability and can apply an appropriate safety factor to calculations. we must also be chickens of things we should know might try to sneak one over on us (referring to contractors) for which we cannot apply a direct safety factor. he said, we must be the professional chicken...if you're not scared as an engineer, you have absolutely no business in this field of work. -and having been a contractor in a past life seeing what goes on and now as engineer constantly dueling with contractors, i absolutely agree with his comments.

RE: What is the difference between 95% and 98% Proctor Compaction?

Two more points on 95 vs 98:

If you require 98, moisture control will be more critical, since it's harder to get 98 if it's more than two points wet of optimum - too close to the zero air voids curve.

It may require more fooling around with rollers, lift thickness, and moisture control - not a big deal if the project is a million CY of dam embankment, but could be a disproportionate headache on a smaller project if the 98 percent isn't really needed for compressibility or whatever.

 

RE: What is the difference between 95% and 98% Proctor Compaction?

I agree with msucog; there are many soils (alluvium, loess and sometimes till as well as residual soils,) that have more strength and stiffness in their natural condition than when recompacted.  Once we excavate or disturb these soils, they must be compacted to a higher density than their natural density, often at a reduced moisture content, to develop similar strength and modulus.  Simply restoring the natural density is  insufficient.

RE: What is the difference between 95% and 98% Proctor Compaction?

Guys:

regarding to original question, it depends on the consideration taken by the geotechnical engineer, or the objective to achive. I have seen that certain engineers specified for instace, fill material compacted to a minimum of 95 % MDD Proctor Mod. should provide at least 112 pcf, It was given by an engineer from other state where soils has a volcanic origin and produce low densities when tested in accordance to Proctor STD (90 to 96 pcf), the problem in the project was related to expasive clays and the purpose was to provide a minimal surcharge on soils, it was explained in the soils report, in this case the engineer was not familiar with the local materials that produce MDD Proctor std around 131 pcf (95% is about 125 pcf), he decided not to change change his specification, but there was anthother detail, he also specified mod. proctor for preparing subgrade and it has to be compacted to a 95 percent, so imagine the mess when moisture increase occurs in expansive clays.  

I'm agree with dgillete, and perhaps we are missing a point, $$$, a 3 percent increment in compaction from 95 to 98 may sound like a minimal increment, if we take this to lab scale and you are reproducing a sample, let's say for CBR, remmember Proctor STD 3 layers 56 blows per layer to achieve 100 %, obviously assuming an excelent moisture control (we are in lab scale), wel in order to reach about 90 % MDD Proctor you need around 15 blows per layer, now to increase from 90 to around 95 % you have to applied something like 26 to 28 blow per layer, now to reach 98 percent, well I've never tried this compaction target but perhaps you need something about 48 to 50 blows per layer. Lest's now supouse that your 100% MDD CBR is 10, 90% MDD CBR is 3, 95% MDD is 6 and 98% CBR is 8, is this incretment really sustantial to increase spec requierement, what happend if instead increase compaction we add lime or cement or lime-fly-ash and compact this mix to a 90 percent?

So, if we bring this to real scale, means that you will need almost twice of energy to reach 98 percent insted 95 %, this decision requires not only a better moisture control, perhaps you have to reduce layer thickess or bring to site a larger compaction equipment. I'd say that cost for increasing that 3 percent should have a very good back-up in order to demonstrate that the cost for increasing compaction is justified by the property improvement range reached.

Sometime I heard about a firm that use to specified 95 % MDD Proctor Modified, apparently it was like a cook recipe, well, someday,I can remmember what exactly happend but i believe that some contratctor that tried to get revange convinced to someone who had expended a lot of money when built a project, remmeber court ask for another professional opinions, as i said, I do not remmember all details however i can remmember that project owner won in court, since was demonstrated that 95 % MDD Proctor Std was enough for the building requierements and purposes.

RE: What is the difference between 95% and 98% Proctor Compaction?

The differentiation has not been made by the originator of the thread - are we talking about standard or modified proctor?  A 95% modified proctor may be more difficult to get than a 98% standard proctor, depending on the soils.  Looking at QLA, a 98% is certainly going to have *much* less qualitative failure than a 95%.

It depends on the penalty for failure.  What are you supporting?

RE: What is the difference between 95% and 98% Proctor Compaction?

LCruiser has a point in that the "type" of standard (modified (heavy) or standard (light)) was not stated.  Rather than how the specified value was chosen - i.e., 95% Mod MDD or 98% Mod MDD, I think the question is what is the real behaviour difference between a material compacted at either of the two efforts. And as important what kind of material with which we are dealing.  The differences may be more remarkable in clayey soils where the effects of porewater pressures during compaction can have an effect than in gravelly sands or well graded crushed stone.  In the latter, the material hardness also plays a part in that higher compactive effort will break down the aggregate - or knock off the sharp angled edges thereby reducing the interlock.  

I have not seen anything definitive to say that at 98% MDD for a given material (say a well graded gravelly sand (SW)) will settle only 11 mm under a given footing load whereby if the compaction was to 95%, it would settle 14, 15 or 16 mm.  

Sadly (or happily depending on the point of view) chosen levels of compaction by different engineers is based on experience, judgment, what the company has always done successfully and the like.  There is scant few real case histories.  As I indicated earlier, you cannot, say, take the behaviour at a specified level of 95% and believe that it will reflect the actual behaviour - insofar as one layer might have been 96%, a second layer 95%, the third 98%, the fourth 94% (and does it really mean anything so long as the entire fill layer is 95% or more??).  These are the philosophical questions, in my mind.

RE: What is the difference between 95% and 98% Proctor Compaction?

Yes, and they only represent a percentage of a specified density standard.  For instance, 95% modified can be 105% standard.

RE: What is the difference between 95% and 98% Proctor Compaction?

One more aspect to consider:  Just what is the accuracy of a field density test?  My opinion is that specifying 98 percent comaction probably is what it takes to get the outliers to fall above 95 percent compaction.  I'm fully confident that the precision of any field density test is not to the single unit.

On the matter of Modified v. Standard - I've always taken it as a 5 percent difference.  95 modified is about 100 standard.  Actually the modified proctor test was developed to keep folks from worrying when they calculated over 100 percent compaction (can't of course have over 100 percent, eh?)

f-d

¡papá gordo ain't no madre flaca!

RE: What is the difference between 95% and 98% Proctor Compaction?

Modified is also to help people get the right moisture content when they try to get high compaction.  (It's that zero-air-voids thingy.)

RE: What is the difference between 95% and 98% Proctor Compaction?

fattdad hit my point a bit better than I originally did.  I believe the reason for specifying higher percentage compaction is often (and primarily) to counter what is considered to be poor quality control or even the inability to check compaction on every square foot of the project.  If you are only checking compaction every 1,000 cubic yards, you have a lot of fill that is not being checked.  Some of those areas will not meet your specified densities.  The material properties may not match the samples tested in the lab either.  By specifiying that additional 3% density as an acceptance criteria, the engineer has greater assurance that the entire fill has been placed to a "minimum" of 95%.  Basically, you are going to pay the contractor to beat on it a bit more so you can rest assured that the entire mass is closer to your desired criteria.   

RE: What is the difference between 95% and 98% Proctor Compaction?

I am cognizant of the means of specifying, why it is done (rightly and wrongly), the variations, etc.  but, if I have the same material at 95% MDD (Std or Modified) and compare it to 98% MDD (corresponding Std or Modified) - what is the difference in phi?  in deformation modulus?  That's the original question I believe.  The relationship of Std vs Modified (light vs heavy for the Brits) varies according to the material.  I have had it in the order of 3% - 100% std MDD = 97% mod MDD for an Ontario crushed stone subbase course - but can be more so up to 5% doesn't seem unreasonable.

RE: What is the difference between 95% and 98% Proctor Compaction?

I think what you are really thinking of is How to correlate 95%/98% proctor compaction to N value, or results of plate load test or CPT. It would answer your question about how little more 98% can carry than 95% and how much can 95% carry.

There may well be such correlations somewhere which I will be very interested to know too!

RE: What is the difference between 95% and 98% Proctor Compaction?

I practice in the Piedmont province where a large portion of the near-surface soils are fine-grained and contain a good portion of clay and silt (CL, MH and ML).  As part of an internal experiment and since we were somewhat slow in our work load, I used a pressuremeter to test controlled fill sites to determine the soil bearing capacity and settlement potential.  The fill sites were graded under our observation to a minimum 95% Standard Protor MDD at +/- 3% of optimum moisture content.  We would create a pilot hole with a specialized shelby tube and run the pressuremeter test within the newly created pilot hole.  The shelby tube would then be transported back to the lab to determine % compaction.  We tested over 20 sites.  With the exception of a few soft layers (no grading contractor is perfect), the pressuremeter results indicated the presence of soils capable of supporting 3,000 psf.  The soils had compactions ranging between 95 to 100% Standard Proctor MDD.  What this means is that, in my opinion and for the soils in my geographic area, I do not see the need to spec higher than 95% Standard Proctor WITH MOISTURE CONTROL LIMITS for conventional foundation support for single to low-story construction.



 

RE: What is the difference between 95% and 98% Proctor Compaction?

We have typically seen geotechs recommend 92 to 95 % when compacting fill for footings to bear on up to 6 or 8 feet in depth. For large amounts of fill say 20 feet or so, we have seen geotechs calling for 98%. I assume that was spec'd to control the total amount of settlement.  

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