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
RE: Sensitivity of Compaction
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
RE: Sensitivity of Compaction
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
RE: Sensitivity of Compaction
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
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
RE: Sensitivity of Compaction
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
RE: Sensitivity of Compaction
If you don't have access advise and I can scan and forward copy.
Best Regards.
RE: Sensitivity of Compaction
KRS Services
www.krs-services.com
RE: Sensitivity of Compaction
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
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
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
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
Sounds like a very useful article. Would you please forward to dave_adkins@msn.com?
Thanks mucho,
DPA
RE: Sensitivity of Compaction
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
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
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.
Please see FAQ731-376 for great suggestions on how to make the best use of Eng-Tips Fora.
RE: Sensitivity of Compaction
RE: Sensitivity of Compaction
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
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...
Please see FAQ731-376 for great suggestions on how to make the best use of Eng-Tips Fora.
RE: Sensitivity of Compaction
RE: Sensitivity of Compaction
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
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
For roadways this is probably okay, but I don't understand it for building pads and foundations.
Any comments?
RE: Sensitivity of Compaction
To set the tone for this discussion, let's establish a few items that all should be able to agree with:
- There are (usually) two kinds of settlement to evaluate: total and differential.
- Total settlement doesn't typically pose a problem unless the magnitude exceeds 4 inches (100 mm.)
- Differential settlement is usually the concern - this causes the structural distress (cracks, sticking doors, etc.)
- The thicker the fill body, the more important the quality of the fill placement becomes.
- Fill bodies with varying thickness almost certainly will settle unevenly.
- Consistent fill density should reduce the differential settlement for fills of uniform thickness.
- Increasing the percent fill compaction will reduce - but not eliminate - the magnitude of total fill settlement.
- 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...Please see FAQ731-376 for great suggestions on how to make the best use of Eng-Tips Fora.
RE: Sensitivity of Compaction
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 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.
RE: Sensitivity of Compaction
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
www.krs-services.com
RE: Sensitivity of Compaction
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
RE: Sensitivity of Compaction
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
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
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.