More Compaction (and Settlement) questions 6

[ben1212]

First of all, thanks to shin25, hokie66, and msucog for your responses to my last compaction questions.

I have several questions this time regarding compaction and settlement.

1) If structural fill (most of the time for me it's some sort of crushed limestone, like a road base material) is compacted to the Standard Proctor within 95% of the dry density what does that number mean in terms of future, long-term settlement of the fill? Would that imply that there is a potential to settle the remaining 5%? And that is in terms of density, how do you equate to depth or inches?

2) What if it is compacted to 100%?

3) Can it be compacted greater than 100% of the dry density?

4) We design a lot of residential projects with slab-on-grade type foundations on hillsides where the downhill foundation wall or grade beam is sometimes in excess of 10 to 15 feet. We do not rely on the compaction of the backfill but instead support the interior grade beams on drilled piers bearing on limestone. Thus, when the structural fill, which was placed in an uncontrolled manner, settles in the long term the drilled piers and perimeter grade beams support the loads. I typically set the "cut-off" depth at about 4 feet of fill. After the fill is deep than that the drilled piers are required. The problem is that this takes a lot of work and coordination with the contractor to ensure that the piers are put in at the right depth. I have had contractors test the compaction of the fill that was placed in an uncontrolled manner and it tests between 95% and 98% with the Standard Proctor, I guess due to the fact of how it was placed. Dump trucks drive over a portion of it while dumping it and then a Bobcat pushes it around. So the fill is actually placed in lifts. Although it can differ from site to site. This is all done within the formwork of the perimeter beams which is usually bags of fill stacked to form the inside face of the grade beams. So I can't imagine that the compaction next to these bags being very good. Anyway, this is not so much a question as an invitation for discussion.

Thanks everyone, I haven't been on Eng-tips in a while but have enjoyed the past few days.

 

[msucog]

1. no. the 95% doesn't mean 5% settlement. typically, i see well compacted fill settlement very little (and it usually happened during/immediately after placement). the percent compaction refers to compaction relative to a test method. this was determined from many many field tests and lab tests. for the most part 90% typically doesn't see much settlement. the 95% is used for many reasons but more or less--safety factor. proctors of the same material can vary by a few pounds or percent moisture. essentially, there is no difference between 90 and 95 percent under ideal conditions (which never happen). also, you can use modified Proctor as a comparison. 90% modified is roughly 95% standard due to the higher compaction effort and total energy applied.
2. to get 100% std Proctor compaction, moisture here in ga must be within 1% and is extremely difficult to get due to the silty and micaceous soils. 100% modified is reserved for quarry material since it can be controlled.
3. some soils can. silty/micaceous soils slip and slide on top so 100% is nearly impossible unless you remove the upper few inches and then it's still tough.
4. if you get 95-98% with a bobcat, i suggest you hire a new testing firm. if it's the contractor doing the testing, i suggest you contact your insurance company to discuss your coverage. take a stiff probe rod over to the area and see how far you can get it in the ground. if it's compacted as you say, it shouldn't go more than a couple of inches at the very most in very sandy or very silty soil. bobcats do not compact fill...they move it around. i understand they are heavy and all that, but they aren't made to truely compact and shouldn't be relied upon for such. properly placed fill is 4-8 inches with compaction equipment (sheepsfoot roller). poorly compacted fill will eventually become saturated over time. if water happens to build up behind the walls, it's usually pretty grim. even saturated soils are often enough to cause problems. i don't know what kind of drilled piers you're using, but poorly compacted fill reduces the capacity of such foundations due to down drag.

it's not difficult to compact soil in 4-6" layers. i personally did it as a contractor and now see it NOT done everyday and go through the same speech. then, after the fill has to be removed and replaced, it becomes very apparent that it would've been easier done correctly the first time. even if it does work for 10 jobs, how much does the 11 job cost you if it doesn't work? besides, if it's well compacted, you shouldn't need drilled piers. i'm sure those things aren't cheap (even if it is "joe bob's foundation" doing the installation with a rigged up telephone pole auger.

 

[dirtsqueezer]

Well, a few things:

The settlement of your fill if only compacted with the tracks of a bobcat will be pretty noticable. I hope you're not installing any slabs on grade over that. But when I started the business I was told that fill compacted to 95% (modified) proctor would ensure less than 1" settlement per lift. This is probably very conservative, but I've used it as a pretty good rule of thumb for these seven years.

>100% compaction: I had the opportunity to test a site twice with about a year in between tests, as the property came under different ownership. First of all, just by consolidation (raiwater percolation, mostly), the compaction increased by 5%. Second, the compaction in the end, was actually higher than 100%. Also, modified proctors are higher than Standard proctors by about 3%. The impact of equipment in the field pretty much fairly matches that of the modified proctor hammer. The hammer for the Standard is much lighter. Thus it is possibly for the stronger field equipment to outperform the standard proctor hammer by, probably at least 3%. I have seen many standard tests go over 100%.

I've done a few different jobs with the 90% spec (as opposed to 95), mostly for paving, and haven't seen any real difference show up in the form of settlement (cracking). I have only been doing this, though, since 2000.

In sum, I'd say your biggest worry would be the SOGs on that poorly compacted subgrade. If it's just a crawl space, I'm sure the ants aren't going to mind, but loading that 4' of fill in any shape or form for structural purposed I think would be a bad idea.

 

[msquared48]

Dirtsqueezer:

"But when I started the business I was told that fill compacted to 95% (modified) proctor would ensure less than 1" settlement per lift. This is probably very conservative, but I've used it as a pretty good rule of thumb for these seven years."

How big were your lifts and what material - structural fill?

For deep fills - 10 feet or more - this seems like a lot of settlement to have to compensate for.


Mike McCann
McCann Engineering

 

[hokie66]

Not being one, I have always thought geotechnical engineering (we used to call it soils) is as much art as science. Apologies to Casagrande, Tschebotarioff, and other soils engineers. The relationship of settlement with time to degree of compaction is quite complex. Probably the best assumption you can make about the settlement is that the material will behave differently than assumed. Therefore, I think in a 10 to 15 ft deep, varying depth fill on a sloping site, you are using the right approach in taking the loads down through the fill on piers to the rock below.

Is this method the most economical? I would look at a suspended slab on columns rather than placing all that fill which serves no purpose in the finished structure. It is really just formwork.

 

[msucog]

i maintain (and have performed consols on sites to confirm--in my geology) that you will get little settlement of the fill itself if it is placed in 6" lifts and compacted to 95% standard Proctor compaction. i also ran numbers for 90% std and came up with minimal difference based on the consolidation test results. the problem is that once you dip below 90% std, the settlement may increase faster with small changes in compaction (so the difference between 95% & 90% would not be the same as 90% to 85%). if you see settlement after 10'+ of fill, it's likely consolidation of the underlying soils due to the area fill load. fill induced settlement numbers are big, ugly and happen within a couple of months after fill is placed (again, in my geology<---important factor since our residual soils are generally non-plastic silts). long term settlement is typically "minimal" for most geologies here (we have many bands of varying geologies in the piedmont) after the fill induced settlement occurs.
i seriously doubt you could ever see 1" settlement out of 1 lift (6"-8" thick) of soil compacted to 95% std or mod. maybe going from the loose state prior to compaction versus after compaction would give you 1 to several inches difference...but that's not settlement.
if the material is going in loose and thick, i would expect 10-15' of fill could settle 1-2+' easy over a couple of years (just in the fill--not including what happens below the fill) depending on the loading conditions. well compacted fill would settle very little (fill itself) and maybe there'd be an 1 to a couple of inches of fill induced settlement that would occur "instantaneously". you'd never see it unless you installed settlement pins prior to fill placement and monitoring them all the way up. most of it would be worked out as soon as you hit subgrade elevation.

i agree: geotechnical engineering is very much an art. i am constantly surprised as how ugly things really can get just because someone cut a few corners ("i been doin' this 25 years this way"<----typical mentality). sometimes, and quite often, you can do everything by the book and still have problems (but usually to a lesser degree than the job that cut corners or didn't follow the geotech's recommendations). so, i just always try to set myself up to be able to tell them "I TOLD YOU SO". people that cut corners will eventually get bit by it...especially these days when litigation is an everyday occurance. then, the profits from 20-30 jobs will go toward one lawsuit that probably wasn't even worth getting sucked in to. i'd rather take my chances on something that i can at least defend myself on. sometimes, people just have to go through getting sucked in to the terrible situation to learn better for the next time.

 

[shin25]

When a site needs fill, the fill material needs to be selected carefully. This becomes more important if the structure is sensitive to settlement.

As have been mentioned before, eventhough, the total amount of long term settlement depends on the initial compaction(in turn the void ratio), this also depends on the soil matrix. Under load, this soil matrix (soil structure) continuously remolds. This may very well be called as creep in the soil.

This rate depends on the type of soil used as fill. When selecting a soil type, you probably can use a good geotechnical book as a reference to guide you.

As I mentioned before, if at some elevation the site has the presence of water table, the seasonal variation of this water table also will contribute to significant soil settlement. For settlement sensitive structures,you have to consider this carefully as well.

Also, at a hill side, rain water runoff tends to transport soil from site. If not planned carefully, you may encounter (in the long run) some amount of excess settlemnt due to continuous loss of some material.

Hope this helps.

 

[fattdad]

Geotechnical engineering is a dicipline. Hiring field technicians that know how to properly take field density tests and relate them to the correct Proctor - that's an art!

As said before me: 95 percent relative compaction does not mean 5 percent remaining settlement.

You can certainly get more than 100 percent relative compaction. It's more likely when the field density test is related to the Standard Proctor moisture-density relations. Important to note: If the contractor is using thin lifts, multiple passes with the equipment and at the proper moisture content it's very possible to get greater than 100 percent compaction. Please note it's not likely at all to get greater than 100 percent compaction at moisture contents greater than the laboratory optimum moisture content. There's reasons for this but it may be hard to explain without a drawing.

Proper earthwork cannot be done using a Bobcat for compaction. If the test results indicate that there is proper compaction, either the wrong Proctor is being referenced or there is a problem with the field density test.

Good luck.

f-d

¡papá gordo ain’t no madre flaca!