Sensitivity of Compaction 15

[merryfull]

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 +/- %)

 

[Focht3]

"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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[DRC1]

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.

 

[digger242j]

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...

 

[Focht3]

[blue]digger242j[/blue]:

First, welcome to [green]Eng-Tips[/green]!

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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[cvg]

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.

 

[PBroad]

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.

 

[dirtsqueezer]

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...

 

[DRC1]

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?

 

[Focht3]

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:[ol][li]There are (usually) two kinds of settlement to evaluate: total and differential.[/li][li]Total settlement doesn't typically pose a problem unless the magnitude exceeds 4 inches (100 mm.)[/li][li]Differential settlement is usually the concern - this causes the structural distress (cracks, sticking doors, etc.)[/li][li]The thicker the fill body, the more important the quality of the fill placement becomes.[/li][li]Fill bodies with varying thickness almost certainly will settle unevenly.[/li][li]Consistent fill density should reduce the differential settlement for fills of uniform thickness.[/li][li]Increasing the percent fill compaction will reduce - but not eliminate - the magnitude of total fill settlement.[/li][li]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.[/li][/ol]Some of the "rules of thumb" that I use for fill shrinkage of a 'well placed' fill body are as follows:[ul][li]Crushed rock ~ [½]%[/li][li]Sands and gravels ~ [½]-1%[/li][li]Sandy and silty clays (CL soils) ~ 1-2%[/li][li]Fat clays (CH soils) ~ 2-4%[/li][/ul]And, of course, judgment is a big part of every fill body evaluation...



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[digger242j]

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.

 

[BigH]

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.

 

[KRSServices]

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?



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[cphi]

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.

 

[Laser28]

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.

 

[BigRedGeo]

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.

 

[cphi]

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.

 

[BigRedGeo]

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.