Fines content of structural fill 2

[lviescas]

I am an engineering geologist consulting as an interpreter between a civil engineer and an earthwork contractor. The engineer has specified that structural fill for roadways and under foundation footings cannot have more than 10% fines (passing the #200 sieve). The earthwork contractor would like to reuse site soils with a bit higher fines content (sound familiar?) Can someone explain where this value of 10% comes from? I understand the effects of moisture and frost on the fine component of soil, but the 10% seems to be written in stone. Why?

 

[fattdad]

I pretty much always strive to write an earthwork spec that'll provide for the best use of on-site soils during earthwork. I have no idea where the geotechnical engineer is on this project, but it's typically the onus of the geotechnical engineer to evaluate the site soils and determine those soils that are appropraite for earthwork during grading activities.

When trucking in structural fill, it's typically in the interest of the overall project to specify the best soil that's reasonably available so your trucking dollars are spent on the best quality materials.

Just some arbitrary 10 percent criterion makes no sense to me.

f-d

¡papá gordo ain’t no madre flaca!

 

[lviescas]

To answer fattdad: The civil engineer contracted a geotech to do a soils report. In that report the geotech recommended a structural fill gradation. It's the same exact gradation he uses on every report I have seen from him. Always the same 10% fines rule. Before I ask for a conference with the civil and the geotech, I wanted to do my homework. That means I ask you folks your professional opionions! You guys 'n gals never cease to amaze me.

 

[fattdad]

Please clarify whether this project requires off-site borrow or whether it's a cut-to-fill job. For the former, I'd be o.k. with the spec, for the latter (and knowing that <10 percent fines soil is not available) I'd be pi$$ed.

Engineers and their boilerplates. . .

f-d

¡papá gordo ain’t no madre flaca!

 

[lviescas]

to answer fattdad further: This is a cut-to-fill project. We want to use what we have on site and we are willing to process our excavated soil and rock to whatever gradation we need to meet the specification. Yes, it was engineering boilerplate, but I think I know the reason. I was told that much research has been done and geotechs are in agreement (mostly) that greater than 10-12% fines dramatically affects the engineering properties of soil (shear strength, permeability, plasticity). So the 10% isn't the shot-in-the-dark I assumed it was.

 

[jgailla]

You get what you pay for.
If you want real solutions, get your geotech on board.
If it's a simple site, your $2 grand is a small part of the equation.

 

[Ron]

The 10% is not arbitrary, but it is a "standard" recommendation, particularly in coastal plains soils. It is based on performance perspectives, particulary with ease of compaction. For the unified soil classification system, the "10%" level means little. As you get into AASHTO classifications, above 10% starts to get you out of the A-3 classification and transitions to an A-2-4 or similar.

 

[fattdad]

we don't worry about aashto classifications until we get within 3 ft of the subgrade. I'm referring to the Virginia Department of Transportation, where I work.

we and our design-builders use all sorts of fill that have greater than 10 percent fines throughout the coastal plain; also in the Shenandoah Valley where the residuum of limestone is typically CL/CH or ML/MH.

For tall embankments we require the slope stability be based on real subsurface and lab data, rather than chart takeoffs and the like. If the slope is "critical" we require a safety factor of 1.5. A critical slope is over 25 ft or a slope that supports a structure or is a real pain to fix.

Within a few feet of the subgrade our specs tighten up a bit (100 percent compaction, control on swell, design CBR/Mr values, etc.).

f-d

¡papá gordo ain’t no madre flaca!

 

[lviescas]

Fattdad-That was helpful point. So far, I have been recommending soemthing similar, and it is being met with approval. On this upcoming project however, very little of the re-used excavated soils will be for tall embankments or deep fills. Primarily we will be re-using excavated soils for structural fill beneath slabs-on-grade. Usually they are only about 2 feet deep.

 

[fattdad]

In my prior life, I did the geotechnical work for many industrial slabs on grade. To the best of my knowledge, they all performed just fine. Here was my basic approach:

Field exploration
Lab work (classification, percent sand, Proctors and CBRs).
Determine if there were any soils that just weren't any good for earthwork. If all that existed happened to be fat clay, then I had little choice but to expect cut to fill with fat clay. I'd certainly have natural moisture contents, classification, proctor and CBR data on that material though.
Correlate CBR to subgrade modulus. Use the ACI "bump" factor on subgrade modulus depending on the thickness of the subbase (we always recommended dense-graded aggregate for industrial floor slabs rather than open-graded aggregate).
Tell the owner if natural moisture content would be a problem during earthwork (i.e., too wet or too dry).
Anticipate soil modification (e.g., lime) if that would be a cost-effective way to address soil properties or natural moisture content issue).
Use settlement plates if the thickest fills were much more than 10 ft or so.
Be involved during the construction phase of the project.

I've also served as an expert witness on failed slabs. Usually, the earthwork testing was out of control (i.e., wrong proctor and inadequate compaction).

Good luck!

f-d

¡papá gordo ain’t no madre flaca!

 

[theCorkster]

The best part of less than 10 percent fines is that you'll not have to worry about soil expansion.

However, since water can move through this type of soil, I do not want to build a "bath tub" that will tend to fill will rainfall, runoff, landscape irrigation and seepage with time; this has led to problems with slabs and slab coverings.

Some areas of the north San Fransisco Bay area do not have such granular materials (unless quarry derived), and importing costs prohibit specifying such material unless there is strong justification. Therefore, use of the native materials is addressed in the design parameters and recommendations that are commensurate with the project requirements for site improvements and structural elements.