Are you a student? The maximum dry density is developed from a laboratory test - either the standard Proctor (or Light tamping for the Brits) or the modified Proctor (Heavy Tamping). Each soil and each gradation of a soil will have its own unique maximum dry density - although in reality - variations of gradations, etc. can give you the same result as well as different soils can have the same result (although the optimum moisture content may be different).
The big question is "To what level of relative compaction do I need to establish for my particular project and its needs?"
Here there will be major divergences between geotechnical engineers. I had a "huge" argument with one of my junior engineers one time when we were discussing the level of compaction for engineered road base course fill that was being placed to raise grade to support a footing. He wrote the draft report to say 100% standard Proctor for the fill compaction. I changed it to 97% modified Proctor - and then the "fight" began. He wanted the standard as, well, it is more standard. I wanted modified to impress the contractor that this was bloody important - I had seen too many contractors not take compaction seriously if standard Proctor values were used.
For landscaping fills - 90% relative compaction might be sufficient. For area fills that will be used for land developement, 95% relative compaction is typically specified. Road base subbase fills can range from 98 to 100% standard - or 97 to 100% modified depending on which "agency" and which "country" you are using. I find that hydro designers use lower relative compaction requirements for site roads than, say, a highway engineer would use. Earth dam engineers typically use 95% relative compaction - except for filters where they like to use a percent relative density. Also for larger sized aggregate - the Proctor tests are not really suitable due to oversize compared to the size of the laboratory molds. In such cases - for 100 to 500 mm sized stone, they might use a minimum unit weight determined from a site "ring" determination.
Years ago there was a paper written in Ground Engineering about the "95% Fixation" -
Don't forget, it you want some form of standard, compacting next to a retaining (basement or other) wall, that compacting can move that wall. Deciding what to to spec depends on the job details.
Thank you very much for your replies. Well, the thing is that design engineer specify that the specific gravity of the soil to be used for backfilling should be 1.6g/cm3. What I want to know is that how do they calculate to decide a soil of this much of specific gravity is needed.
No Okiryu, I don't have Smith and Gerber's full paper/research. Interesting that in 2013 they used the same phrase "95% Fixation" as in the Ground Engineering article . . .
Fill soils in many geotechnical applications must be defined by some material property. Often it's strength (i.e., friction angle), subgrade behavior (i.e., CBR) or compressibility (modulus). The field exploration program shows us the soils on the site and we get some idea what may be available for use in earthwork.
So, we know we want to use the on-site soils and we take bulk samples to the laboratory to evaluate the strength (or some other characteristic). How do we prepare the sample for testing? Well, we'd want to know the properties when compacted and we'd want to determine some specification level for how much compaction.
Proctor tests are done in the laboratory to evaluate the moisture-density relations. That means for some level of moisture content, under prescribed energy, you'll get some dry density for that soil. Under the most favorable moisture content you'll get some, "Maximum" density, which is directly related to the amount of, "Prescribed" energy (refer to ASTM D698 v, D1557. If I know the maximum dry density is 120 pcf (1.9 g/cc), I may want to fall off that in testing, 'cause it may not be likely that the contractor will achieve this level of compaction during production. So, I'll chose 95 percent compaction. I could chose 94 or 97, but I chose 95. I get the lab to prepare a bunch of samples at 95 percent compaction and they do testing. I get the strength at 95 percent and anything else I want. Now I do geotechnical engineering (slope stability, etc.) and specify the contractor use THOSE soils and place them at 95 percent compaction. Not just any compaction, but the compaction that we used in the lab - i.e., either Standard or Modified Proctor.
Next up, we can discuss compaction moisture content. There is an entire body of engineering that discusses whether geotechnical properties are influenced by the compaction moisture content in addition to the degree of compaction.
fattdad, yes, what you are explaining in the first portion of your post is what ideally we need to do. However, most of my projects do not have the budget and time to conduct tests in compacted soils. But of course, when there are chances to do this, we try to do this. Also, if you could expand on your second part of post, i.e. geotechnical properties affected by moisture content, it will be appreciated...Thanks !
Regarding first portion: Whether you have the budget or not, most correlations to compacted properties likely consider compaction to 95 percent standard Proctor. So, that alone may be a basis to require at least 95 percent compaction.
Regarding compaction moisture content: If you take two identical samples and compact them to the same dry density will they have the same geotechnical properties? Well, that answer depends on the compaction moisture content. If you take one sample and compact it on the dry side of optimum and the other sample and compact it on the wet side of optimum there is a chance they may show different strength, permeability, modulus, cohesion or other such property. All we know is the dry density is the same. No real reason to believe these other values will be the same!
f-d, I agree with you. For example, Japanese guidelines for CBR testing assume that the sample is compacted to 95% standard Proctor.
Also, I had the chance to do some testing in remolded soils and basically followed your suggestion (made a Proctor to check the compaction degree of the remolded samples). I remembered that I did this for some over consolidated fat clays and got higher modulus for the remolded samples than the undisturbed samples. I was expecting the opposite. We only did undrained tests. Understand that for compacted clays, drained strengths may be more important.
Sorry malan99, I know that the above may not be related to your question, but I think that the final application of your compacted soils is also good to know. For example, if you are using clays for embankments, besides the compaction degree, the drained shear strengths from remolded samples should be also obtained for stability analysis.
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