Fines content of structural fill
Fines content of structural fill
(OP)
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?





RE: Fines content of structural fill
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!
RE: Fines content of structural fill
RE: Fines content of structural fill
Engineers and their boilerplates. . .
f-d
¡papá gordo ain't no madre flaca!
RE: Fines content of structural fill
RE: Fines content of structural fill
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.
RE: Fines content of structural fill
RE: Fines content of structural fill
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!
RE: Fines content of structural fill
RE: Fines content of structural fill
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!
RE: Fines content of structural fill
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.