Specification of percent compaction is typically done to relate to laboratory behavior. If an engineer has a set of data (i.e., strength, CBR, permeability, etc.), that is based on 95 percent compaction and this is not acheived in the field, there would be no correlation between what is anticipated and what is provided. Now I'm not sure why somebody would specify 98 percent compaction, but fundamentally if the civil/structural designer specified this and the contractor accepted the responsbility to build what was specified, some additional measure of compaction may be needed.
I practiced for a long time in an area where silty lean clay was prevalent, and the usual specification for building pads was minimum 95% Standard at water contents of optimum + or - 3%. Typical allowable bearing pressure for footings was 2500 - 3000 psf. I think this practice is pretty widespread.
I learned that paving subgrades tended to rut under truck traffic during paving at optimum +3%, but not at optimum +2%. With the local material, 98% compaction could be acheived at optimum +3, but not at optimum +2. I began to specify 98% at optimum -3 to +2% for paving subgrades. I was really after the lower water content, but the higher compaction requirement helped assure that the water content would be lower.
Also, after digging up fills that had become saturated after compaction at 95%, I found that they were just stiff, pretty marginal for a bearing pressure of 3000 psf. Compaction to 98% leaves less void space for water absorption, so the saturated strength will be higher. The higher compaction requirement will help assure good foundation performance, especially if loads are heavy and bearing pressures of 4000 psf or so are desirable.
These principles don't necessarily apply if the fill is highly plastic and potentially expansive.
As others have said it depends entirely on the soil type. Some soils will give you hardly any bearing capacity even at 98% compaction and others will be rock hard. So what you need to know is what bearing capacity are you getting at 95% and 98% compaction. Of course it also depends how much of the stuff you have under the foundation and the shape of the applied load or foundation.
One thing to keep in mind, too, is that actual compaction to a spec if rather more than the minimum required. Say the spec is for 95%MDD Std Proctor. Likely, the actual compaction will be more than 95% - say 96 to 97% on average; and, particularly for cohesionless soils, compaction of overlying layers will increase the compaction even more. So, in many ways - this it is a misnomer. The engineer, say, for bearing on an engineered fill specifies the minimum he would be 'happy' with for the bearing pressures designed (and it is another question as to how he really came up with the percent MDD needed for the proposed bearing pressures). In actual fact, you will likely get better behaviour because of the higher levels of compaction as achieved. As for 95% MDD or 98% MDD - likely a matter of the engineer's experience and what he or his company has always specified. I'd be more interested in knowing the behaviour under 97% MDD vs 98% MDD - and whether it is really an issue.
you are only testing at a few spots and compaction will vary. the standard deviation with 95% requirement might only give you 90% in some areas which may not be acceptable. With 98% requirement, the standard deviation is reduced and your minimum values of compaction may come closer to the the 95% that you are looking for...
While you are on this subject folks, do some testing of density of natural ground at the cut location. I REFER TO CLAY SOILS. If you are then building on that natural CLAY ground at that density at say 3,000 psf, then why do you spec a higher density for the fill?
I'll bet you won't find the percent compaction (if you will)of that natural CLAY ground anywhere near 95, but more like 85.
Makes one wonder why such high requirements.
Also, making the contractor beat the stuff into those densities, adds to the job cost. Why do it?
we try to avoid testing residual soils (we have mostly silts here with limited amounts of clays) since the in place density will likely fall low. sometimes, building officials require density tests because they don't fully understand what they're asking for so we then must test it (but we tell the contractor to recompact the top prior to testing. the compaction recommendation is always included because more often than not, the subgrade will become disturbed anyway either during grading or foundation excavation. for pavements, i will usually tell the contractor to recompact everything anyway since pavements are much more susceptable to problems to the surficial conditions. if it's a cut in residual soils and the subgrade proofrolls okay, then i will quite often not bother with making the contractor recompact for density testing. if the geology or planned construction type is cause for concern, then recompact it, test it and gone on.
and to go back to your question about why spec compaction when residual soils have an okay bearing with lower in place densities (and this assumes that "natural" is referring to residual--so this discussion might be different in areas up in northern u.s. where glacial till occurred but since i'm not extremely familiar with that scenario, i'll stick with the geology i know): you must keep in mind that residual soils have residual stresses built up over millions of years. once we take a CAT D8 through there cut it out and push it around, all those residual stresses have been broken so then you must put a CAT 815 on it to compact the lift to beat the voids out of it. the fill gets its strength from rearrangement of particle packed together. residual soil gets it strength from the inherent properties of the soils which built up over a very long time and quite often under immense pressure/temperature (but then weathered out to soil)--the last statement is not always the case depending on the geology, location, etc.
there's also the concept that if you require 95%, you know that at least some spots will not be to this compaction...say 90% for some locations that happened to not show instability or went at the exact location of the test. if the spec happened to be 90%, then maybe some locations would get down closer to 80%. for some projects, i've seen the compaction spec'd to 98% simply because they wanted to minimize those rogue 90% densities and make sure most everything was at least 95%. this was not my job (i'm not even sure it was my company) since i would simply increase the testing frequency. however, i think the spec was put out there because they were fairly confident they would not be performing the construction testing...and it was a "cheap" exploration on top of that.
and lets be "real" here, how much could it possibly add to the cost of a project to compact one layer of soil (even for something huge like a 250,000 sf building)? essentially zero compared to the overall cost of the project. but how much will it cost to remediate a bad subgrade after the fact for the same sized building?
and one last comment in my coffee driven post here: there's some residual soils that i specifically recommend the contractor NOT compact and tell them to keep all construction off of simply because it would go from stable before compaction to a gooey mess after compaction (which then might require undercutting and replacement). and all these points are from my location and geology...others likely have other "things" that are more common in their areas.
One other dimension to this topic: While the geotechnical engineer may "recommend" one measure of compaction in the geotechnical engieering study, typically, it's the civil engineer that produces the specification. Whether the geotechnical engineer feels that lesser compaction is o.k. or not, during construction the contractor is bound to perform to the requirements of the specifications. If the geotechnical engineer doing the compaction testing were to "approve" lesser compaction, there would be a trasfer of liability, which may or may not be a good thing (likely not).
very good point...probably just as much or more important than my commentary above. your point is sort of the heart of what we (professionals) must be careful of...taking the liability of someone else's work. 95% std Proctor compaction is not always appropriate for every single situation but more often than not, it's okay and has enough safety factor that even when the contractor sneaks in something "marginal" occassionally without our knowing it, things are usually still okay.
i still remember my structures professor (speaking with a heavy lebanese accent) talking about how engineers absolutely must be chickens...chicken factor he called it--referring to safety factor (both applied and implied). we must be chickens of all the things we know have some variability and can apply an appropriate safety factor to calculations. we must also be chickens of things we should know might try to sneak one over on us (referring to contractors) for which we cannot apply a direct safety factor. he said, we must be the professional chicken...if you're not scared as an engineer, you have absolutely no business in this field of work. -and having been a contractor in a past life seeing what goes on and now as engineer constantly dueling with contractors, i absolutely agree with his comments.
If you require 98, moisture control will be more critical, since it's harder to get 98 if it's more than two points wet of optimum - too close to the zero air voids curve.
It may require more fooling around with rollers, lift thickness, and moisture control - not a big deal if the project is a million CY of dam embankment, but could be a disproportionate headache on a smaller project if the 98 percent isn't really needed for compressibility or whatever.
I agree with msucog; there are many soils (alluvium, loess and sometimes till as well as residual soils,) that have more strength and stiffness in their natural condition than when recompacted. Once we excavate or disturb these soils, they must be compacted to a higher density than their natural density, often at a reduced moisture content, to develop similar strength and modulus. Simply restoring the natural density is insufficient.
regarding to original question, it depends on the consideration taken by the geotechnical engineer, or the objective to achive. I have seen that certain engineers specified for instace, fill material compacted to a minimum of 95 % MDD Proctor Mod. should provide at least 112 pcf, It was given by an engineer from other state where soils has a volcanic origin and produce low densities when tested in accordance to Proctor STD (90 to 96 pcf), the problem in the project was related to expasive clays and the purpose was to provide a minimal surcharge on soils, it was explained in the soils report, in this case the engineer was not familiar with the local materials that produce MDD Proctor std around 131 pcf (95% is about 125 pcf), he decided not to change change his specification, but there was anthother detail, he also specified mod. proctor for preparing subgrade and it has to be compacted to a 95 percent, so imagine the mess when moisture increase occurs in expansive clays.
I'm agree with dgillete, and perhaps we are missing a point, $$$, a 3 percent increment in compaction from 95 to 98 may sound like a minimal increment, if we take this to lab scale and you are reproducing a sample, let's say for CBR, remmember Proctor STD 3 layers 56 blows per layer to achieve 100 %, obviously assuming an excelent moisture control (we are in lab scale), wel in order to reach about 90 % MDD Proctor you need around 15 blows per layer, now to increase from 90 to around 95 % you have to applied something like 26 to 28 blow per layer, now to reach 98 percent, well I've never tried this compaction target but perhaps you need something about 48 to 50 blows per layer. Lest's now supouse that your 100% MDD CBR is 10, 90% MDD CBR is 3, 95% MDD is 6 and 98% CBR is 8, is this incretment really sustantial to increase spec requierement, what happend if instead increase compaction we add lime or cement or lime-fly-ash and compact this mix to a 90 percent?
So, if we bring this to real scale, means that you will need almost twice of energy to reach 98 percent insted 95 %, this decision requires not only a better moisture control, perhaps you have to reduce layer thickess or bring to site a larger compaction equipment. I'd say that cost for increasing that 3 percent should have a very good back-up in order to demonstrate that the cost for increasing compaction is justified by the property improvement range reached.
Sometime I heard about a firm that use to specified 95 % MDD Proctor Modified, apparently it was like a cook recipe, well, someday,I can remmember what exactly happend but i believe that some contratctor that tried to get revange convinced to someone who had expended a lot of money when built a project, remmeber court ask for another professional opinions, as i said, I do not remmember all details however i can remmember that project owner won in court, since was demonstrated that 95 % MDD Proctor Std was enough for the building requierements and purposes.
