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Undrained Shear Strength - Granular Soils

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nbr1

nbr1

Geotechnical
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Feb 29, 2008
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For the purposes of a slope stability analysis (static case only), would a free draining material (eg. clean gravel and/or sand)
have an undrained strength? I often see in reports that the undrained strength is equal to the drained strength.
For example, the following values may be reported:

Drained Phi = 30
Drained C = 0
Undrained Phi = 30
Drained C = 0

Would reporting a value of undrained shear strength be appropriate for this type of material, or should
the drained strength be reported only?
 
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Use drained. Freely draining ignores the fact that interstitial waters would persist.
 
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Any soil can have both drained and undrained strength. However, for clean gravel and sand, undrained strength is generally irrelevant to anything but a seismic problem.
 
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I understand which value would be used in the analysis. I'm just trying to figure out the proper protocol for reporting strength values.
So, if you reported an undrained strength the same as the drained strength.....would that technically be correct?

Or should you just not report an undrained strength at all?
 
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I think I would report the undrained strength as "n/a."
 
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I have always reported the undrained strength as equal to the drained strength for free draining materials.

Mike Lambert
 
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Other than seismic, why is anyone reporting undrained shear strengths of granualar soils (say, k<10-04 m/s)?
 
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Yeah - I guess - always get mixed up when dealing with decimals - I am looking at numbers bigger than 10-4 (or so . . .)
 
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