Soils for Structural Fill

[ONENGINEER]

If a soil is classified as a poorly graded sand with gravel (< 3% fines and <29% gravel), will the soil be suitable for use as a structural fill.

How could an engineer assess the suitability of a soil for structural fill?

 

[oldestguy]

Let's suppose you were checking a building site with test borings and you found that soil at a relative density of dense. Would you build on it?

enuff said.

 

[ONENGINEER]

Oldestguy
Thank you for the answer. I would say that a granular dense material would be a good foundation.

But if the poorly graded sand with gravel (SPG) are imported materials, can they be compacted at site to a dense state without problems?

 

[ONENGINEER]

I have the answer and seek no more feedback on this thread. Thank you.

 

[Ron]

Yes and Yes.....feedback prohibition ignored.

Most of the soils I deal with are SP. When a little gravel is thrown in, it's gravy!

 

[oldestguy]

The question here comes down to the points of both shearing resistance and possible reduction in volume due resulting in settlement. Instead of soil, what would one do if the support were marbles? Sure shearing resistance may be less than site soils, but settlement, which usually controls, would be tolerable, even if in a loose condition. Just because the "compacted" density might be less than well graded soil, does not necessarily mean it is not capable of doing the job, where shearing is controlled, as with confinement.

I bring up a "test" that commonly is done on jobs, hand pushed rod penetration. If it hardly can be shoved in, it usually is good support. Try that in uniform sand and the rod will go in easily, but the stuff has a small range of density between loose and dense, thus a low potential for settlement, even if loose. Vibrate it and it is great stuff.

 

[ONENGINEER]

Thank you for the valuable comments.

Also from a fieldwork point of view, is SP with about <5% silt content would pose problems for compaction with conventional compactors. I understood some have compacted SP materials satisfactorily with a plus minus 3% from Op moisture.

Now that the thread receives further attention, I would be interested if others may have similar experience? And how about compacting SPG materials.

 

[Ron]

SP soils are not difficult to compact. With less than 5% fines, you have to look carefully at your Proctor curve...it might be awfully flat or even "double humped". As with any compaction process, the closer you are to optimum moisture, the easier the compaction will be.

Fine sands also tend to pump (build up excess pore pressure) if you are above optimum moisture. Be careful with that. Don't overkill on the compactor size either!

 

[BigH]

What I do - rather than worry about SP or GP or . . . at first glance is to look at the Ontario Ministry of Transportation gradations (these are what I was first exposed to). If the material meets a Granular A or B or even C, yes, it would be suitable for structural fill; with a caveat. For important structures or foundations such as a paper machine, for instance, I would use MTC Granular A - which is a road base material, usually crushed (can googl MTC Specification 10.10 - unless they changed the number). For lighter foundations - say a one story structure, Granular C - or even a material with as much as 20% fines would be fine - the more uniformly graded the better.

 

[oldestguy]

Big H is right about usage. I once was called to see why an under construction industrial building column footing was settling. The building did not have a roof yet. The area was being filed a few feet with sand (probably somewhat uniform, but I can't recall exactly). As large vibrating compactors were passing by the column compacting the new fill, I could see sand from under the footing actually flow out from under. Soil was somewhat saturated.

The fix was bring in a crane and lift the column up a little. Use a bulldozer and shove the footing off to the side. Excavate the uniform sand and backfill with a more well graded sand, compacted. Slide the footing back and re-set the column. Footing was about 7 x 7 ft. If you are careful, it can be done to as good as new.

Needless to say, to play it safe, lighter vibrating compactors on thin lifts were used thereafter.

 

[fattdad]

In the absence of minus 200 material, there's a chance of an irregular moisture-density relationship. At some point, if the moisture-density relations are irregular, the soil should be specified by relative density. You may recall that relative compaction ranges from about 80 to 105 percent, but relative density ranges from 0 to 100 percent. In the world of modified proctor, 100 percent relative compaction is about 100 percent relative density and 80 percent relative compaction is 0 percent relative density. It goes about 5 percent relative compaction is 25 percent relative density.

You may have to specify the placement using relative density criteria. That means you'd need to run the tests for minimuma nd maximum void ratio. There's an astm standard for these tests.

compaction will be via vibration and density testing can still be via nuke gauge, but I'd run a few sand cone confirmations.

it's been a while though. . .

I'm sure if placed at 75 to 100 percent relative density, you'd have a good structural fill.

f-d

¡papá gordo ain’t no madre flaca!

 

[cvg]

the biggest problem with relative density testing is that many of the labs dont have the equipment

 

[BigH]

I ran across a nice ppt on relative density but sadly forgot to document the URL. The attached graph is from the presentation - it was done on filter sands in the Fort Worth area of TX.

 

[ONENGINEER]

Checked and the materials do not fit easily into the Type A, B and C of Ministry of Transportation.
I wonder if there is any other way that one could predict if the SP materials with 29% gravel (minus 1.5"), 68% (mainly medium) sand and 3% fines would be suitable for structural fill.

 

[oldestguy]

1 engineer:

No one here is saying it is not suitable. The discussion is how to measure degree of compaction. In my footing case it was removed only because adjacent work was causing it to migrate when vibrated. If there was no nearby vibration,it would have stayed.

 

[cvg]

uniform sands and gravels have no cohesion, so they need to be well confined. once the confinement is removed, they might "run". they also may be relatively permeable. neither is a good characteristic of structural fills. so not all poorly graded granular materials are suitable for structural fill. you really need to evaluate the material to make sure.

 

[oldestguy]

cvg:

OK, if that is the case, I must have ok'd several hundred, maybe thousands of jobs with uniform sand as the structural fill DURING SOME 40 YEARS. Too late now I guess. Our main criteria to meet was no more than 5 % passing the No. 200 sieve for meeting frost requirements. GOSH For some reason no complaints or claims, Huh?

 

[cvg]

really depends on what kind of structure and groundwater conditions. I've seen failure of a structure caused by water flowing through a layer of uniform gravel and cobbles. will never try that again. Live and learn

 

[oldestguy]

Agree with your example. A basement dug for a clinic in dune country, south off Lake Michigan, where groundwater table was normally higher but down when built. Contractior-owner installed a perimeter underdrain syatem, backfilled with clear gravel. Water table came up to normal. Sump pump started to do its thing. After a year the basement floor was undermined so bad that it had to be replaced. Luckily the footings didn't settle. At the sump pump outlet there was a large delta. If this had been my job, the gravel would not have been allowed on the job and all backfill to drains would be graded filter sand. Needless to say, even this drastic action did not affect footings (to my knowledge).

 

[ONENGINEER]

cvg- Per your comment, 'sand and gravel ... may be relatively permeable. neither is a good characteristic of structural fills' Could you clarify why? I thought structural fill made of crushed aggregate could be one of the bests despite their high permeability - like those under railways.

and oldestguy, I understand the poorly graded sand with gravel would be a suitable fill but its sustainability on water issues and confinement issues would be problematic. Is that right?