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Sensitivity of Compaction
15

Sensitivity of Compaction

Sensitivity of Compaction

(OP)
When a specification calls for a percent compaction eg 95% what would be the effect if you only achieved 90%. In other words how critical is the the asked for figure and what is considered an acceptable variation from that figure (in terms of +/- %)

RE: Sensitivity of Compaction

It depends on the reason specified for compaction.  Most fill soils are compacted beneath slabs and building foundations because compaction increases soil shear strength and reduces its compressibility, and is usually fairly easily achieved in sandy soils.  95% compaction is a typical standard for soil compaction.  Failure to achieve this compaction means that the soil in question would theoretically have a higher degree of compressibility and lower shear strength than a properly compacted soil; however, the difference may be insignificant if the undercompacted zone is fairly thin. If, on the other hand, the poorly-compacted zone is 10 feet thick, this could result in a problem. This is best evaluated by the geotechnical engineer of record. Other soils are compacted to reduce their permeability.  In this case, failure to achieve compaction may result in more severe problems (such as in landfill liner situations)due to increased permeability. Failure to achieve uniform compaction is usually an indicator that the contractor's methods are not acceptable, or that the soil is not at the right moisture content.

RE: Sensitivity of Compaction

Just some additional information for Merryfull - when I saw your posting, I felt inclined to contribute a little something... as a geotechnical engineer, there is a reason why given compactions are specified.  Typically, 100% is specified for fill beneath industrial and institutional buildings, 98% is specified for fill beneath commercial and residential buildings, 95% is specified for fill beneath pavements and pipe bedding.  The intention is to specify a compaction that will lead to minimal settlement in the fill material.

Typically, if the specified compaction is not met, it is because of excess moisture in the soil.  Drying or blending the soils often solves this problem, although it is not fool-proof.  If the specified compaction still can't be met - the safest plan is to bring imported (granular-if possible) material.  

As for an acceptable variation on the specified compaction - that depends entirely on the engineer who has to sign off on it.  They may be more flexible on road fill, knowing that it will be dug out for servicing, than they would for fill material beneath a new school.

RE: Sensitivity of Compaction

Compacting soil on the dry side is another and perhaps more common reason for not achieving compaction.  Contractors sometimes don't want to rent water trucks.  Most specifications require that soil be compacted in a range of -2 to +4 points of optimum moisture as determined by a standard or modified Proctor.  My experience is that under slabs and paved areas, the standard is 95-98%.  I've seen 100% used a few times, but it is hard to achieve and expensive.

RE: Sensitivity of Compaction

How critical compaction is depends on a number of items:
1.  what is going on it
2.  what material are you compacting
3.  what is the thickness of the poorly compacted layer
4.  what is the thickness of the overall fill
5.  what is the depth to bedrock

Where I work we deal almost exclusively with clays.  Compaction and moisture control is very critical with clay because of it's tendency for differencial settlement.  Also good compaction will help with shrink/swell problems.  For the most part (unless bedrock is extreemly shallow, or the fill is less than a foot thick) the numbers for under the building pad are absoulute.  We will make a contractor rework a pad until it passes.  We are more giving in pavment and landscape areas, even to the point of allowing some bad fill material (such as topsoil) to be wasted in the fill in thin (less than 2 inch) layers.  

Usually when compaction cannot be achieved and the engineer is willing to compromise, the site is proofrolled with a fully loaded dual or triaxal dump truck.  At ths point we watch for "pumping and rutting" which is esentially soil movement indicating soft spots which must be undercut and repaired

RE: Sensitivity of Compaction

I work for a grading contractor.  It is not uncommon to see compaction go as high as 110 % .  Just because a certain standard is set in a lab, does not mean that different outcomes are possible.  At the same time, there are many instances where we can not reach 70 %.  From my experience, when the specs say 95%, there is still a lot of variance in what you actually get.

RE: Sensitivity of Compaction

If you consistently see 110% there is something wrong with your proctor.  Especially if you are using a modified proctor.  Still although it is not uncommon to go above 100% of a standard proctor, equipment is better than when that test was designed,  doing so consistently is cause for concern.

If you cannot get compaction, yet moisture is where it should be and the site is proofrolling well then again there is likely something wrong with the proctor.

Often though the cause for too high compaction or too low is the moisture being out of spec.

RE: Sensitivity of Compaction

6
If I were a contractor, and a compaction result of 70% was reported, I'd start looking for a new geotech, unless, of course it was April 1st.  But seriously.....
The Proctor test is a physical test, brought to the world by, you guessed it, Mr. Proctor (circa 1933).  The test proved useful in stabilizing soils for military applications, especially impromptu aircraft landing strips.  Proctor (the man) believed that density increased as water, acting as a lubricant, reduced the friction between soil grains, allowing for a more efficient particle arrangement via void filling by the smaller grain sized particles.  Additional work by Hogentogler (1937), and Lambe (1958), and the man considered by many to be the master of soil mechanics, Terzaghi, increased the understanding of soil behavior to the point that, under carefully controlled conditions, soil and soil-aggregate composites can be relied on to behave properly under a very wide range of conditions.  Over the years, the basic laboratory Proctor test has not changed much. What has changed are the challenges that geotechnical engineers face when evaluating a site.  The combination of a good geotechnical engineer and a good structural engineer, working in concert, allow construction to take place at locations that previously were considered 'unbuildable',  It would take a detailed understanding to ever allow a recommended compaction requirement to be waived, lowered, or increased.  The best course of action is to determine why a spefied relative density is not occuring.  Any deviations from the specified relative densities should be brought to the attention of BOTH the geotechnical engineer and the structural engineer for analysis.  It may or may not be acceptable.  I'll leave you with some examples.

Highly compacted, unyielding soil may be required beneath one type of pipe to avoid shear failure of the pipe, but cause it in another type.
Highly compacted expansive soils, especially when compacted on the dry side of optimum, may heave and cause significant damage when lightly loaded and subjected to moisture infiltration.
Highly compacted fine grained, silty soils, may  consolidate when placed and compacted at moisture contents below optimum,  This is especially problematic in trenches.
Wide variations in density, even though minimum requirements are met, may lead to a non-uniform soil support system for pavements.
Other, equally important considerations have already been mentioned in this forum.  I hope you can see that the question you posed is not nearly as easy to answer as it may have seemed.  Whenever I hear it said that the "engineer overdesigned" a project, I always reply that the engineer knew instinctively that someone would try to under build it.

RE: Sensitivity of Compaction

If it was a paved road, and there was frost penetration, the results would be disasterous.  The road would not settle uniformly, and numerous ruts and deformations would occur.  Besides a costly maintenance headache, if there was a warranty period in the contract, the contractor and/or the testing firm (if recommending approval) could be held liable for the repair costs.

RE: Sensitivity of Compaction

2
Suggest all look, too, at thread 274-5909 on the geotechnical engineering and other related topics forum.  We have been having a go at compaction there, too.

One item begging a question in the original query was picked up on by others -  what is the underlying theme to the question?  For kind of fill and what use of the fill are we discussing?  A second item that none of the replies queried was whether the % compaction was modified (heavy) or standard (light).  This should have been part of the original query's background too.  

There is a good article I saw in Ground Engineering a number of years ago about the "95% Fixation".  When I go to the office tomorrow, I will find it and reference it in another reply.

I do want to emphasize a point that I made in the abovementioned thread.  For embankment construction, one should, unless the fills are very high, take into consideration too the nature of the foundation soil in design and specifying a level of compaction to achieve.  If you have 5 to 10m of soft to firm normally consolidated clay, settlements of the foundation will be much more critical than any self settlement of the compacted embankment fill (say several hundred millimetres or more vs 20mm, perhaps).  Further, if properly compacted in a professional manner even to less than 95% standard compaction, most fills <5m or more high wouldn't have any self shear distress either.  The foundation in such cases is critical, not the fill.

I do warn the young lions getting their first tastes of the field that regardless of engineering practicality and judgment (i.e., whether or not it REALLY makes a difference), always be up and front about any testing results with respect to the project specfications.  Some poor junior field engineer in Ontario was really hauled up on the stick recently for "fudging" a test result of the top soil layer under an approach slab - the test was lower than specified and he willfully made it "good" as the slab poor was already underway.  We could all argue whether this really made any difference to the performance of the fill/approach slab and that it was bad judgment to start the pour before really knowing the testing result, but any willful misreprentation of such results . . . NO.

Best regards to all.

RE: Sensitivity of Compaction

Good question, as stated by others it is not easy to answer.  Keep in mind that if you are using a nuclear densometer (Troxler) that it is a machine, certainly not as smart and/or instinctive than you or other engineers.  I have been on jobs where the material seemed bullet proof but failed to reach 95% and down the road using different material (different Proctor) the material passed but still seemed soft and rutted easily.  The numbers are for the most part a guide which when combined with experience will allow the engineer to make the right decision(s).  Good Luck.

RE: Sensitivity of Compaction

As indicated, you might all wish to get a copy of paper by Charles, J.A., H.D. Skinner and K.S. Watts "The specification of fills to support buildings on shallow foundaions: the "95% fixation"."  Ground Engineering, January 1998.

If you don't have access advise and I can scan and forward copy.

Best Regards.

RE: Sensitivity of Compaction

Geomaritimer hit it on the head.  Many municpalities and contract documents specify 95%, 97%, 98% SPD or modified due to the fact, and in my experience, that a specified density is the only measurable method to evidence re-compaction of disturbed material, embankment fill, and trench fill.  AS everyone has sort of pointed out, once native soils are disturbed, various soils become mixed (any organics and unsuitables removed) such as clays, sand and maybe silt lens.  Unless a proctor is taken for each excavation bucket, some other method has to be employed.  A combination of expertise and sound judgement is used to enable the contractor to achieve the required densities.  Many of my contracts allow for remove of unsuitables and the importing of borrow material.  Moisture content is also important as well as a good working knowledge of the material.  I always specify test strips be placed prior to placement of base course on a road project, and encourage test areas in trenching to ensure the protor being used is actually representative of the material.  I have seen variations in gravel stockpiles.  In those cases, I have erred on the side of caution and obtained a new proctor.  However that being stated, in most cases, the geotech has always been very knowledgeable and readily able to recommend the proper course of action.  Again, nothing substitutes for experience, therefore if in doubt or uncertain, ask someone.  I hope this helps.   

KRS Services
www.krs-services.com

RE: Sensitivity of Compaction

Hi
For project in Geotechnical lab i need ASAP nice photo with standard compaction apparatus.
Specialy for Proctor modified compaction test as per ASTM standard
Thank you in advance

RE: Sensitivity of Compaction

Hi
For project in Geotechnical lab i need ASAP nice photo with standard compaction apparatus.
Specialy for Proctor modified compaction test as per ASTM standard
Thank you in advance
my email : dgojgic@yahoo.com

RE: Sensitivity of Compaction

Personally I have never understood the procter test. Someone has finally hit on one of my pet peeves and waken the sleeping beast.
 First off, The proctor test, if done by an experienced technician is fairly reproducable, but not perfectly. I would guess it is reproducible around 3%.
It is not a perfect test. However, based on the number of completely screwed up procters I have seen, good technicians are apparently in inceasingly short supply.
 But my biggest gripe is that the proctor does not give us a number to which to design. If I were testing native material for suitability to support load by testing weather the insitu dry density excceded 95% of the modified procter you would call me a nut (or worse). Yet if I pick up a pile of soil and move it 50 feet across the site, this becomes the only criteria for acceptance. We do not know the bearing capacity friction angle, cohesion or any other properties. We do not konw what the capacity of the soil is verses the load we apply to it, yet if we compact it to 95% of an test standard that we do not have any hope of correlating to our field compaction equipment, we trust everything will be fine, and if it is only at 94% our work will collapse. I guess my big question is now that we have the fill at 95% of modified proctor, how do we know from a design standpoint that  the fill has sufficent strength to support the load?

RE: Sensitivity of Compaction

3
That's a complicated subject; have you read all the other threads on this site?  

It appears to me that much of what you complain about comes from a basic misunderstanding of soils in general.  You can't specify them the way you can steel or concrete - they're variable and quite non-linear.  You may not be able to separate different soils solely on the basis of color or texture.  And they don't come with a yield stress stamped on them...

It's unrealistic to expect a simpler specification than the common relative compaction standard - in the design and specification process, you have to look for a range of compactive effort that provides suitable performance.  Your statement that ... my biggest gripe is that the proctor does not give us a number to which to design suggests that your expectations may not be realistic.

You can correlate field compaction with the lab test, but it only works for a given borrow source, site, compaction equipment and compaction procedures.  Change one or more of these and the correlation breaks down.

Only a fool - with no understanding of the test and its' history - would arbitrarily say that a project would collapse at 94% relative compaction, yet is acceptable at 95%.  I don't know who told you that, or why, but I suggest that you go back to that person and ask him/her to explain the logic of that statement.

Keep in mind that we have two issues to deal with on most jobs: design/performance needs and contractual obligations.  It's terribly important not to confuse the two -



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

RE: Sensitivity of Compaction

Big H,

  Sounds like a very useful article.  Would you please forward to dave_adkins@msn.com?

Thanks mucho,
DPA

RE: Sensitivity of Compaction

DRC1:

You make some interesting comments.  The Proctor test is valuable for geotechnical engineers in conveying the idea that a particular soil that will be placed as fill needs to be compacted to a state where it will have the appropriate density, strength and stiffness to meet the expectations of the design engineer.  It is not a perfect method (as with most of what is done for geotechnical engineering) but it does help the field people to focus on the issues that are most critical to achieving reasonable performance.  The soils need to be at a moisture content that would lend itself to an appropriate degree of compaction.  The soils need to be placed in lifts of proper thickness so that the compactive effort is adequate and the right equipment needs to be selected to accomplish the compaction.

In various parts of the world with various soils, local geotechnical practice is developed over time with anecdotal performance information that generally achieves the objectives.  What we all need to look out for is a geotechnical engineer going into a new area with new soils that specifies compaction criteria without an understanding of what is achievable.

Field compaction control is really somewhat of an art where we have to keep in mind our objectives as others in the thread have pointed out.

I hope this helps.

Glen

RE: Sensitivity of Compaction

I would like to say in my own defense that I do have an understanding of soils and soil mechanics, and I feel Focht3 and ganderson make good points.My point is which gets back to the orginal question of the thread - How do you know, without additional testing, that a particular percentage of a procter value will give you the performance you want, especially when the percentage is specified prior to the selection of the material?
Focht3 also raises an interesting point -

"Keep in mind that we have two issues to deal with on most jobs: design/performance needs and contractual obligations.  It's terribly important not to confuse the two - "

This is true of construction but should it be so? If the owner is paying for work that is specified, but does not advance the design/perfomance needs of the project, what benifit does the owner recieve?

An interesting digression.

RE: Sensitivity of Compaction

"Keep in mind that we have two issues to deal with on most jobs: design/performance needs and contractual obligations.  It's terribly important not to confuse the two - "

I'm glad you picked up on that!

The contractor is typically required to meet or exceed some percentage of a compaction standard; say, 95% of the optimum dry unit weight as determined by ASTM D1557 ("modified Proctor".)  Some contracts (er, owners, construction managers and/or architects) do not allow for any "low" tests - the geotechnical engineer of record has no discretion in accepting or rejecting any part of the fill body.  However, most owners recognize the geotechnical engineer may accept a few "bad" tests using his/her engineering judgment as well as knowledge of the site and project.  This practice varies by nation, region, local practice, owner - and engineering firm.  Project type, too -

There are many projects for which a fill body compacted to only 90 percent of the modified Proctor optimum dry density will serve just as well as 95+ percent.  The 95+ percent requirement may have been included due to local practices or the owner's expectations, and not because the higher compactive effort was needed.

Stupid practice, you say?  Yup.

All too common, you say?  Yup.



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RE: Sensitivity of Compaction

Thanks Focht3. I feel beter now. Field comapction always makes me nuts, and I needed to do a litle venting. Always look forward to your coments.

RE: Sensitivity of Compaction

I stumbled across this site only a few days ago and I've found quite a lot of interesting topics being discussed. I must admit that this one is the one that most caught my attention. I must also admit that I'm not what most of you would call an "Engineer". "Operating Engineer", perhaps. I've spent the last 25 years in the trenches, so to speak, so I do have a clue (although how much of one is open to conjecture.) I've found that hanging around people who are smarter, and/or better educated than yourself is always beneficial. I hope none of you mind if, while I'm hanging around absorbing the wisdom I put in my 2 cents worth every now and then.

The way I read Merryfull's initial post, between the lines was a plea for a reason to become "emotionally invested", if you will, in getting that last 5% of compaction. I've felt the same need myself. There's always a strong desire to do the job right, but it's balanced by how badly we've got to beat our brains out in the process. The discussion that's ensued hasn't given me much that emotional investment. Moreso, it seems that what's at play here is more a mindset of, "This is a science. We've got to specify something exacting, and we've got to have a method of measuring whether that specification has been reached, and this is what we use."

Please don't take that as a criticism. On an intuitive level, I realize that there's art, and there's science. To approve results that are outside of the numbers specified is to get artistic with the work. Art is subjective, and science is objective, and it's the nature of the engineer's business, and the nature of the engineer himself, to attack these things objectively.

Like DRC1 said though, it can make you nuts sometimes...

RE: Sensitivity of Compaction

digger242j:

First, welcome to Eng-Tips!

Second, please re-read my post yesterday (March 10, 2004.)  I tried to address the issue of "the last 5 percent"; perhaps I wasn't clear in my description of "how things really are."  The topic of compactive effort is still more art than science, and not every engineer is good 'at it.'  Judgment is the real key - something that is more than a set of written instructions or specifications.  What still bothers you?

Before you answer, why not start a new thread?  Be sure and give it a "proper" title.  This thread's getting a bit too long (some of that length is definitely my fault!)  You will get responses...



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RE: Sensitivity of Compaction

What bothers me is that often times judgement is not allowed at all - generally by public works inspectors - who aren't geotechnical engineers.  They are covering their hind ends by pointing to the standard spec and saying 95% is required.  It takes an act of congress to allow anything less than the minimum specified.  And most geotechs just don't want to fight that battle.

RE: Sensitivity of Compaction

CVG you're right on.  But if we allow uninformed inspectors to merely act "by the book" there is no flexibility when new materials require differant standards.  If our only job is to keep the engine spinning at 95% critical then we have only ourselves to blame if we find ourselves locked in the boilerroom.

It seems that the respect the world once had for engineers began to decline about the time state regulatory bodies were first founded.  If so it would seem that Regulation is doing more damage to our profession than benefit (and damage to the public we "protect").  Being self-regulating as Canadian Provinces are still does not eliminate the code-quoting inspector who thinks that certification equates to perfection rather than papering over design fundemental flaws.  If the system is antiquated it can be expected to have fatigue cracks and these will not cease to exist simply because we choose to ignore them.

If we accept "Enron-itise" as being the North-American way we condone poor management and become part of the problem.  Are we the same people who during our salad days in University claimed that "Engineers rule the world"?     

The art portion of engineering as important as the scientific and we should speakout when others claim we are merely "protecting our turf" if we exclude those who do not understand.

RE: Sensitivity of Compaction

Ah, the unbelievers...
I've been testing compaction for almost 4 years using nuke gaues, sand cones, proctor testing apparati, ect.  In my estimation, using a consistant fill, with the judgement of a competent testing lab, tests produced by any of these methods are reproducable, reliable, and therefore can be used economically.  If you are getting varied results, perhaps the talent of your testers lies in his redirection of the cause, and not his use of the equipment.  

I see the thread of this discussion as which soil types are stable?  Will 95% compaction on a silty SP w/ gravel hold the same load as 95% of an ML?  You're basically getting into whether to use import or are the native soils good enough.  It's a good question!  Barring the obvious, like wood debris or organics, where do you draw the line?  Don't we have soils engineers that are supposed to tell us these things?  I thought that the uniformity coefficient and all the other soils models were designed to tell us information in place of field data like the shear modulus.  

I think Karl Terzaghi must be rolling over in his grave...

RE: Sensitivity of Compaction

AHH - we are slowly getting to my point. Compaction testing only establishes that a certian relative density of a material has been reached, It does not tell us if this material at this density is capable of carying that load. Yet many inspectors and engineers are instant that compacion reach a particular value which in  general is not arrived at by any rational anaysis.
For roadways this is probably okay, but I don't understand it for building pads and foundations.
Any comments?

RE: Sensitivity of Compaction

That is an interesting topic!

To set the tone for this discussion, let's establish a few items that all should be able to agree with:
  1. There are (usually) two kinds of settlement to evaluate: total and differential.
  2. Total settlement doesn't typically pose a problem unless the magnitude exceeds 4 inches (100 mm.)
  3. Differential settlement is usually the concern - this causes the structural distress (cracks, sticking doors, etc.)
  4. The thicker the fill body, the more important the quality of the fill placement becomes.
  5. Fill bodies with varying thickness almost certainly will settle unevenly.
  6. Consistent fill density should reduce the differential settlement for fills of uniform thickness.
  7. Increasing the percent fill compaction will reduce - but not eliminate - the magnitude of total fill settlement.
  8. As the particle size of the soil comprising the fill decreases, the likely "fill shrinkage" (post-compaction settlement of the fill body under self weight, usually when it is wetted) increases.
Some of the "rules of thumb" that I use for fill shrinkage of a 'well placed' fill body are as follows:
  • Crushed rock ~ ½%
  • Sands and gravels ~ ½-1%
  • Sandy and silty clays (CL soils) ~ 1-2%
  • Fat clays (CH soils) ~ 2-4%
And, of course, judgment is a big part of every fill body evaluation...



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RE: Sensitivity of Compaction

Focht3 asked me, "What still bothers you?"

DRC's reply above pretty well sums it up: "Yet many inspectors and engineers are instant that compacion reach a particular value which in  general is not arrived at by any rational anaysis...For roadways this is probably okay,..."

I've been fortunate that in my career most of my work has been in residential, or light commercial construction, and in that portion of it, my responsibility has been directly to the builders I've been working for. Apparently it's been *thier* judgement that in the majority of cases no outside engineering expertise has been required to evaluate the soil's ability to bear the loads that have been imposed. Where conditions have obviously been unsuitable from the outset, and engineering studies have been required, they've resorted to caissons and grade beams to bear the weight of the buildings. In only one case has there been any testing of compaction (in a driveway fill), and that was at the request, and expense of the owner. In all other cases it's been "seat of the pants" judgement that's been relied on.

Actually, more often than not it's a case of asking me, "Do you think we're on undisturbed dirt there? Do you think it looks ok?" Either the soil, or the seat of my pants, or some combination of the two has been very good to me, because I'm not aware of anything that's fallen down yet. (And believe me, it's *not* always a situation I'm comfortable with, and I've expressed on more than one occasion that I'm *not* qualified to be making those judgment calls.)

The particular case I had in mind when I commented that the process can drive you nuts sometimes, had to do with a very heated exchange between my then employer, the soils engineer, the architect, the general contractor, the school district's "clerk of the works", and anyone else who might've been within earshot. In question were those last few percentage points of compaction, under a proposed tennis court. Of course, the people who controlled the checking account won that battle, but I'm not convinced that what they were asking for was necessary. *That's* where I have trouble becoming "emotionally invested" in what the specs sometimes require.

As an aside, I've given a little more thought to my comment about art vs. science as it applies to engineering. Though nobody seemed to take great offense at it, I'll make an apology anyway. There's a great deal of engineering that's at least artful, if not downright elegant. What I was trying to express is that the engineer is still tied down to the numbers that the laws of physics dictate to him, while the artist is free to indulge in fantasy.




RE: Sensitivity of Compaction

I find the choosing of the level of compaction to be an interesting topic.  Most use % values from previous experience without even trying to see whether these are "required" or not.  Sometimes, chosen values represent the "degree" of difficulty that you want to impart to a contractor - he may be more careful if you say 95% MDD of modified rather than 100% MDD standard.  There really isn't much difference in, say 97% and 96% for a given soil.  I, too, often find that, say, in deep fills, you get 4 layers of 98% compaction (say 95% minimum) but you get 1 layer of 93% - and you reject it.  The chosen values at times are made to minimize the self-settlement under succeeding fills.  Does the one layer at 93% defeat the miminimizing if you have 80% of the layers well over the 95% value??  Something to ponder.
   I enjoyed the point about the flexibility.  The member who indicated that the geotechs - or other experienced staff - cannot use their judgments because of someone applying the spec to the letter.  This is always a sore point - and I see it more often because the job I'm on has all sorts of auditors (non-engineers for the most point) who don't know the difference in a liquid limit reported as 56.4% rather than 56% or that even if 70% is the maximum LL, a soil having a value of 71% hasn't "fallen off the table."
  Good thread - some great points.   to Focht3 again on many great insights that we all need to be reminded of from time to time.

RE: Sensitivity of Compaction

One final point that I may add, is that I use the compaction requirement not so much from a structural perspective, as evidenced by the previous posts, but more so from a contract administration viewpoint.  Other than the standard compaction test, as accepted by the nuke measurements, there really is not any other way to verify the QA/QC of the contractor's efforts, which of course relate directly to the measurement and payment clauses and enforceability of the warranty period.  

These values provide a quantification and measurable guage as to the effort required by the contractor.  In any of my contracts, these numbers have been reviewed by competant geotechnical engineers and are now common to the point of general acceptance.  If a geotechnical engineer is willing to seal a document stating that for example, a given road subgrade is acceptable at 93% versus the required 100%, I'll accept that.  If it fails, he accepts all liability for repairs.  Either way, I'm not out any additional costs.

That example is silly I realize, because I have never found any competent geotechnical engineer ever willing to take on this liability, and often, on very expensive cross sections, their recommendations tend to exceed the "norm".

Finally, if any of you, as owners are willing to accept less, it's your call and your cost.  If any of you engineers (representing the owner or contractor) are willing to put your "beaver" on the line to recommend less than the spec'd values, that is your decision.  Be advised though that your insurance company may walk if you are held to be negligent, because in a nasty court battle involving a failure, one of the tests that will be used by the court will be the reference to the "venerably tried and true" density test we are referring to.  I don't think it is worth it, do you?

KRS Services
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RE: Sensitivity of Compaction

Third year anniversary of this thread!

A clayey soil or a  silty soil compacted to a density of 95% of the Standard (or Modified ) Proctor density  at the optimum moisture content will perform differently than the same soil compacted to a density of 95% of the Standard (or Modified) density at a different moisture content.

RE: Sensitivity of Compaction

As most of my experience is in roads, I have to say that the use of in-situ testing is a great tool for compliance testing, but a poor tool for performance testing.  I always try to get my clients to insist on proofrolling roads and parking lots, and after each layer if possible.  I tend to roll the in-situ soil with an empty tandem truck, the granular layers with a loaded truck, and look for the soft spots and excessive rutting.  It amounts to almost 100% inspection, but it is visible proof of having obtained adequate and uniform compaction.  I think uniformity is very important, and cannot be proven with spot testing.

RE: Sensitivity of Compaction

I'd say proofrolling is in-situ testing, albeit a subjective method.

Sometimes moisture/density is very important, and sometimes it is less important.  A fat clay compacted at a low water content will not likely fail a proofroll.  But I wouldn't want to be the owner of that floor slab come the next rainy season!  

Performance tests are good, but they do not typically consider long-term effects.  Knowing the soil type and its behavior, density and moisture testing can give an indication of how the soil will perform under different conditions.  

RE: Sensitivity of Compaction

BigRedGeo,
Your comments should not be limited to only fat clay (CH). In my experience, I have not worked with soils (fine grained or coarse grained) where the moisture content at the time of compaction is not important, with the possible exception of open graded crushed aggregate such as #57 stone.  

RE: Sensitivity of Compaction

cphi,

I agree with you to an extent, as I said

"Sometimes moisture/density is very important, and sometimes it is less important"   I didn't say not important.  

Moisture content in clean granular soils is usually not an issue to obtain compaction unless the moisture is near the bulking moisture content.  

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