Unit weights of urban fills

[NJGeotech1]

Hi all,
What would be the typical range of soil unit weights for urban fills, such as silty sands with a variety of concrete, brick fragments and other oversized particles; basically fragments of demolished structures. Fills like that are common in Philadelphia, and probably other large cities as well.
I don't think that blow counts obtained from this material are too reliable, as they may be inflated due to sampler trying to get past those oversized particles.
Thank you in advance for your help...

 

[TDAA]

Was there effort put into compacting the material? Are there voids? How much rubble?

If the material has had some effort put into it, I would try to figure what percentage the heavier material takes up, then use typical unit weights for each general material, times the percentage there, and summed up. This could get you close.

If you want an alternate, you could excavate some, put it in a truck and have it weighed, then get a volume of the hole – measure, or fill with known density material.

Or, perhaps someone has an approximation for you.

 

[GeoPaveTraffic]

What does it matter what the unit weight of the fill is?

Actual values could varry widely, I would say that values less than 100 pcf are rare, but possible; and that values over 135 pcf are rare, but possible.

 

[msucog]

Not to put words in your mouth, but based on the phrasing of your question, it sounds like you are trying to evaluate if this fill is well compacted enough to support something.
Old, undocumented fills could weigh just about anything--especially depending on which cubic foot you happen to sample. Whether you sample silty sands with brick pieces, 4' diameter boulders, bar joints, decayed trees, tires, etc., I hope that you have good luck, happened to sample the "worst" spots and don't plan to put anything structurally sensitive in nature on top of it.

 

[NJGeotech1]

I'm trying to calculate the effective stress in the soil layer beneath the fill. I used 120 pcf for my calculations which probably was a bit on higher side.

 

[msucog]

in that case, 120 pcf is reasonable for the unit weight in my opinion.

 

[fattdad]

How does the effective stress in the underlying soil layer affect the design? If this has to do with a settlement calculation, you can always run the numbers at 120 and also at 130, just to see how this affects the results.



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