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Effective Unit Weight of Soil in Lateral Load SoftwareHelpful Member!(5) 

k2p (Structural)
3 Mar 08 19:40
I normally see an effective unit weight of sandy soil below water table in range of 45 to 65 pcf for input of LPILE, GROUP, or COM624P. My understanding is that effective unit weight is basically a saturated unit weight minus water unit weight (i.e. submerged unit weight).

However, the geotechnical report of the current project I'm working on reports sand effective unit weights of 110 to 120 pcf, although these soil layers are all below water table. Is it possible that effective unit weight can be that high? I found that these values are not conservative to use.
Helpful Member!  fattdad (Geotechnical)
4 Mar 08 8:41
a bouyant unit weight of 110 to 120 pcf is WAY too high for soil.  This is more typical for a moist or saturated unit weight (i.e., where the water pressure is equal to or less than zero.

f-d

¡papá gordo ain’t no madre flaca!

Helpful Member!  hokie66 (Structural)
4 Mar 08 16:22
Your geotechnical report may be confusing "effective unit weight" with equivalent hydrostatic pressure.  But even so, the pressure would not normally be that high.  More like 80 to 100 pcf, dependent on whether active, passive, or at rest.
Helpful Member!  Mickney (Geotechnical)
4 Mar 08 17:03
If you have any doubt or questions about the geotechnical report, call the authors and ask.  In my experience, a lot of geotechnical reports are prepared from shells and include "standard" paragraphs that are not adjusted for a specific site.   

In my opinion, above the water table, a moist or total unit weight of 110 to 120 pcf for sands seems okay.  This also seems reasonable for a properly compacted sandy fill material.  Below the water table, I would subtract 62.4 pcf from the above values for an "effective" unit weight.

I hope this helps.



Helpful Member!  himat42 (Structural)
4 Mar 08 21:58
k2p
In piling for the horizontal loads,the horizontal sugrade reaction plays therole and for submerged (below water table)horizontal subgrade reaction is much lower than the dry sand subgrade reaction value.
Check Professsor Tarzeghi paper published in ICE Geotechnique and discussion by Professor Rowe.
HTS
Helpful Member!  jdonville (Geotechnical)
5 Mar 08 13:17
k2p,

I agree with Mickney - likely the geotech didn't change the effective unit weight or should have called it TOTAL unit weight. Contact the author for clarification.

Jeff
k2p (Structural)
6 Mar 08 17:29
Thanks so much to everyone. I really appreciate this.
Our final submittal was before my post, so we wouldn't have a chance to ask the question to the geotechnical engineer. However, we did use their values subtracted by water unit weight since they are more conservative.

Next time I'll make sure to ask them.

k2p

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