Maximum Dry Density 6

[Rookie2]

I am working on a residential subdivision in which the municipality requires 100% compaction of the aggregate base course under the curb and gutter. The contractor on this project constructed the curb and gutter about 8 months ago prior to having any compaction tests done. The municipality required him to tear up several sections of curb and gutter and perform the tests about 1 month ago. Depending on the methods used the geotechnical engineers gave us a result of between 88% and 96%. The contractor (and some of the geotechnical guys) are telling us that the base has lost moisture which has caused it to lose compaction. This brings me to my question. I don't understand how moisture content can affect 'DRY' density. Does this make sense? Also could freeze / thaw cycles affect the density? Is it common for a soil to lose compaction after sitting for 8 months? I would appreciate any input, We don't want the contractor to tear out the curb and gutter unless it is really necessary.

 

[cdh61]

Rookie2,

Best to have a look though ASTM D698. But overall, if the granular was compacted and the density was not impacted by seasonal changes then loss of moisture would not necessaryly decrease dry density value it can increase it. I am not sure what you mean by 88% to 96% depending on method used (D698 or D1557) is this what you mean? Freeze thaw can effect density values and will depend on many factors such as soil types, moisture contents.... This also applies to density values over time.

But the main question should be how are the curbs and gutters performing? Just because the specified density value may or may not have been met does not mean all has to be removed. Assessment of curbs and cutters should be completed and then based on the results a decision should be made to see if further action is required. Removal followed by replacement strictly on dry density values of underlain granular does not make sense if all is well with the curbs and cutters.

regards

 

[Rookie2]

Thanks for the response. Increasing the dry density makes more sense to me. Possibly because the soil would settle into the voids left by the loss of moisture? As for testing methods, I should have said testing sites and methods, they have tested with DHCP (I think this is correct) nuclear, and sand cone. Unfortunately the municipality will make the decision of whether it's torn out or not, we're just trying to determine if it is justified, and if there is an argument to allow it to remain. Also for my own benefit I'm just trying to understand how loss of moisture could cause a soil to decrease in dry density. Thanks Again

 

[BigH]

Welcome to the world of where everyone knows everthing about compaction. Unless the granular shrinks and swells with moisture (quite doubtful), then the dry density should be basically the same to the accuracy of such geotechnical measurements - none of which are actually accurate which is why you run correlations of nuke densities vs rubber balloon or sand cone.

There is little doubt that even 88% crushes stone under the kerb is not enough. Sure it should be okay. 'ell, here, they use 150mm blinding concrete under kerbs!! - ridiculous. Still, while I agree wholeheartedly with cdh61's comments on how is the kerb behaving, you have a contract that the contractor has stipulated when he signed the contract that he would achieve 100% MDD (modified or standard - you didn't say). As such, he must achieve it. If he doesn't, you shan't pay him. He must rectify the kerb "foundation" or he must offer a face-saving way out and offer a rebate in the unit rate and perhaps an extra year or so of defective work liability. This is more a contractual matter than technical - but, alas, that is what too many contracts are becoming these days. Easier to hide behind the words of the contract than practical technical aspects. All said and done, the contractor did agree to compact to 100%.

 

[cvg]

It seems that maybe the owner or inspector is a bit at fault here also. Where were they when the contractor was busy setting forms and pouring concrete? Why wasn't the compaction quality assurance test run? Around here, the contractors perform their quality control tests, but the owner has the final say "before" the concrete is poured.

If no tests were performed, I still don't see a reason to tear out and perform the test. It seems it would have been in everybodys best interest to wait and see what happens. If anything does, it likely would within the first year. And then the Contractors warranty bond would have been used to repair the work.

By the way, what is "150mm blinding concrete"? I haven't heard that term used here in the states.

 

[MPENN]

Hmm, "150mm blinding concrete". Would this be the approximation of 6 inches of concrete curb lock?

 

[BigH]

cvg - blinding concrete - same as "mud mat" in the States and Canada. It is an UK-use term (similar to kerb vs curb). Don't know what you mean by kerb-lock. Here, it is just a mud-mat of 150mm, then they slip-form the kerb with a kerbing machine on top - the kerb's base thickness is about 150 with the "kerb" rising 250 above. The blinding concrete (aka mud mat, aka pcc, aka lean concrete) is not 'part' of the kerb per se.

 

[Zambo]

also being from the UK I would call the 150mm "blinding concrete" or "lean concrete" this would typically have a compressive strength of around 10 to 15 N/mm2. So BigH I do not follow your concern as this is as high as a CSM road base and appears to me as a suitable base for a kerb.

If the road base under the kerb was layed separately from that under the road pavement with lighter compaction equipment then it is more than possible that the required compaction was not achieved.

Zambo

 

[dirtsqueezer]

Rookie2,

It sounds like from the unbelievably high city requirements and the sour mood of the engineers that your problem may go deeper than just the subgrade. A long time ago, in a galaxy far, far away, I have heard that contractors used to use nothing more than flowing water to compact granular sand. If you've ever walked barefoot on a beach, and felt how compacted the sand gets just from the waves, you can almost believe it. My point is, if your granular experienced any rain in the 8 months it sat, it's more likely that it's more compact, and not less, as BigH said. Not only that, but in my experience, it's rare to see any requirement whatsoever for curb/gutter compaction, and never on the order of 100% (90% is the highest I've seen). 100% is for landing 757s on; not SUVs.

My guess is that somewhere along the line some major earthwork problem came up for the city and the geotechs that lead to these impossibly high requirements and all this worry over a gutter. It's possible you have some swelling clays in your area, that would make any jurisdiction or firm jump out of it's skin, or there was some major oversight in quality control with a contractor that lead to the designers/owners going way over budget. If they've done all the earthwork as you say, DCP, nuclear, and even sand cones (!), it's possible there won't be much you can say. I think there was probably a problem before you ever got there, and you'll might just have to weather the storm. It's a heck of a lot of expense testing for a mere curb/gutter, and hopefully economics and common sense over time will prove the long-term winner.

There are two bright, shiny pennies for you! Best of luck.

 

[BigH]

Zambo - the blinding concrete is not a concern; I just think it is an overkill especially when the blinding concrete is sitting on WMM. (Actually, I'm not from the UK, but one of my favourite authors is Tomlinson and this is where I picked up the term of "not seeing".

p.s., How did you guys let the French off??? Two goals by Zidane in the injury period tacked on at the end and you lose????????????????????? Yikes . . . I watched it live - here at 0430am!

 

[Rookie2]

Thanks very much for taking the time to read and respond to my post. Dirtsqueezer is right, the 100% compaction is generally not required in this area but this particular city has one engineer, and his expertise lies in areas other than soils. I guees he would rather be on the conservative side. Thanks Again

 

[burao]

Hi,
If the contractor obtained the correct max dry density then any increase in moisture content would just affect only the optimum moisture content with a corresponding increase in the max dry density and overall it should not influence the 100 % proctor or whatever specification ( say 95 - 100 % ). The standards do not decrease with time.

Burao

 

[BigH]

Burao - If I read your post correctly: "... increase in moisture would just effect only the optimum moisture content" ... ? Don't think so. OMC is a "material" property. Standards don't change, but densities might (swell or shrink of materials - if clayey, say).

 

[burao]

Hi
Basically what I mean is if you look at the graph of dry density vs MC the peak gives us the OMC and the maximum dry density. If the peak of the curve can be increased with say modified proctor then the OMC will also change. So what I am saying is whether the soil natural MC is changed should not in any way modify the max dry density and the OMC. If the max dry density changes then the OMC will also change. The max dry density is influenced by the method of compaction. Rookie2 says the contractor (and some of the geotechnical guys) are telling them that the base has lost moisture which has caused it to lose compaction. I think that this is not valid. I hope I am clearer.

Burao

 

[BigH]

My friend - you are much clearer on the second post. You are correct in that if you change your "standard" of compaction level, the OMC does change since the compaction level changes. We must always be mindful that we are compacting to a standard. Sure, in the field, we might be using, say, a 50tonne roller and the "field MDD" is higher than even the modified MDD - but we are comparing to a model - namely the modified Proctor in my example. This is also why one can achieve 102% MDD standard Proctor - even though you are, say at 99% MDD modified Proctor.

 

[Ron]

100% compaction to Modified Proctor MDD is a bit high, but if to Standard Proctor, it is rather common. I know of municipalities that require 100 percent of the Modified Proctor MDD and it shouldn't be an issue. As BigH said...he bid it, make him do it.

For granular materials, the dry density does not change with changes in moisture content. That's why the standard is "dry density" not "wet density". The effect of the moisture content is negated. If shrink/swell potential exists in the material, then moist content change will cause a slight density change, but not much. If material has that much shrink/swell potential it shouldn't be used in that application.

You can have problems achieving 100 percent compaction in base materials if the subgrade is not compacted properly or doesn't have adequate stability. Sort of like compacting on top of a foam mattress.

Getting by the correlation issues BigH mentioned, the values given by the Geotech are reasonable and believable. The tests are reasonably accurate and repeatable if performed properly and the results are reliable. The compaction is likely low.

Compaction is done for reasons other that load capacity and stability. It is done to maintain material integrity and stability in the face of changing moisture levels and inundation. I would not specify less than 98 percent compaction in this case and would prefer 100 percent as well.

 

[DRC1]

First I disagree with Ron and Big H. Unless the contractor was also responsible for testing, which although is not unheard of, it is most often not the case, then the municipailty had a responsibility to have testing done while the material was being placed. At that point in time, the contractor could have easily corrected the problem by more compaction. All parties in a contract have to be mindful of their responsibilities. If there has been no failure of the curb line and the 96 % value is accepted as correct, The municipailty would be hard pressed to prove that even though the pavement did not meet the specs, it was not servicable. By forcing the contractor to remove the gutter, they may be setting themselves up for a claim based on the docterine of economic waste, which in the past have resulted in large settlements for contractors.
The fact that there is such a wide range in values for the same materials is disconcerning. The corralation should be better than that. I would not be happy at 88%, but might accept 96%. It is a big spread. As a first step, someone should determine the proper procter and in situ densities.
Finally it is really scary that your geotech guys are telling you that by loosing moisture the fill will lose compaction. You are correct that the compaction is based on dry density and the moisture changes do not effect compaction. The moisture content is important when placing a fill becuase at the optimum moisture content, the least compactive effort is required. For a well drained granular fill changes in the moisture content will not change the compaction. However, a compacted subase is only as good as the material it is founded on. It the base is poorly drained, subject to freeze thaw or expansion,does not have sufficent bearing, such as soft clays or peats, or is erroding into a drainage system, the previously well compacted subbase will lose compaction. This may be due to a construction or a design problem.
I STRONGLY suggest that before anyone does anything rash, the municipality or your firm hires an outside engineer to review the situation and determine :
1.) What is the actual compaction and the required compaction?
2.)Is the pavement as it exists serviceable?
3.)Is there a larger problem and if so what is the best solution?