Settlement of Clayey Structural Fill vs. Undisturbed Soil

[TAC]

I am doing a settlement analysis for a foundation being built on some old structural fill. The lower portion of the soil profile is glacial till silty clay that is probably A-6b soil. The original soils are over-conslidated and have an OCR of about 5 to 6. The upper portion of the soil profile is the same material obtained from a deep excavation elsewhere on the site. At the time of placement it was placed as structural fill and tested. I normally would not be concerned about the settlement of these materials for the loads we have, however, a portion of our building is on deep crushed stone structural fill and I need to calculate the settlement of the portion on the clay fill/clay side versus the crushed stone side. Does anyone have a feel for what the recompression ratio of the clay fill would be relative to the original clay soil? I would think it is somewhat higher, but I'm not sure how to quantify it.
Thanks!

 

[fattdad]

If there's no water table in the "silty clay" or crushed stone materials, I'd use elastic theory. Now, you just have to divine the soil modulus values. That said, even if the silty clay is 150 tsf and the stone is 300 tsf are you really going to have any real settlements.

On an unrelated note, I don't use the term silty clay, 'cause it's not defined by ASTM D2488 and very rare in D2487. It's widely used in our vernacular, but not very precise (to me at least). When I see it used by our consultants, I require supporting laboratory testing, as our consultants are contracted to use D2488 or D2487. Pet peeve rant now over. . .

f-d

ípapß gordo ainÆt no madre flaca!

 

[BigH]

Depends on the country . . . I admit that I am not a "fan" of the ASTM methods of classification . . . prefer the ones I grew up on (in Canada).

 

[aeoliantexan]

l not know how to determine that ratio without a testing program, but I would start by looking at the water content, density, and degree of saturation of the fill vs the native material. Then, depending on how I felt that day, assume a value like 1/2 or 1/3rd, do the calcs, and see how much differential settlement is predicted. Then decide whether to spend the time and money on a testing program.

An alternative might be in-situ testing such as cone penetrometer or dilatometer.

 

[BigH]

TAC - as a very rough estimate of Cc/1+eo - if you know the water contents you could check out Lambe and Whitman, Figure 22.1c (SI version, 1979) - you will find very little difference at low end water contents. I've normally taken Cr as about 1/10 of Cc.

 

[TAC]

Thanks everyone. The original clay soil and the structural fill clay soil have about the same moisture content and blow counts so it seems like I am safe or conservative assuming that the settlement for the material will be pretty uniform.