Maximum Dry Density
Maximum Dry Density
(OP)
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.





RE: Maximum Dry Density
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
RE: Maximum Dry Density
RE: Maximum Dry Density
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%.
RE: Maximum Dry Density
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.
RE: Maximum Dry Density
RE: Maximum Dry Density
RE: Maximum Dry Density
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
RE: Maximum Dry Density
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.
RE: Maximum Dry Density
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!
RE: Maximum Dry Density
RE: Maximum Dry Density
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
RE: Maximum Dry Density
RE: Maximum Dry Density
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
RE: Maximum Dry Density
RE: Maximum Dry Density
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.
RE: Maximum Dry Density
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?