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Home > Forums > Geotechnical Engineers > Activities > Soil testing engineering Forum

Direct ShearHelpful Member! 

thread261-323022
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JasonTaylor (Geotechnical)
31 May 12 10:42
Hello!

I've been reviewing the standard for the direct shear (D3080) to try and figure out what the purpose of submerging the sample in water is for. Is it to replace the excess pore pressure with water as the sample is consolidated?
Helpful Member!  moe333 (Geotechnical)
31 May 12 11:47
If the sample is not saturated, the measured strength would include suction forces due to partial saturation...it wouldn't be an effective stress shear strength. You also need to shear it slow enough that excess pore pressures don't develop....if they do, it wouldn't be an effective stress shear strength.
JasonTaylor (Geotechnical)
31 May 12 13:24
Thanks a lot. I'm just trying to wrap my head around the concept.
bilal09 (Structural)
16 Oct 12 14:43
also add..total stress=pore water pressure developed stresses + effective stresses ...
fattdad (Geotechnical)
17 Oct 12 13:24
While it may be good practice and required by the test, I feel it's unlikely to guarantee saturation. If you are running a clay sample in the DDS, some little bit of water head is not likely sufficient to obtain saturation. Sure, capillary forces may draw in some water, but to think this would reduce air voids to zero is optimistic. Consider just how much back-pressure is needed to obtain a saturation "B" value in the triaxial test.

f-d

¡papá gordo ain’t no madre flaca!

moe333 (Geotechnical)
17 Oct 12 17:42
Saturation can be enhanced (not guaranteed) by setting the lower half of the sample in water for a period of time and let the water wick up through the sample, then fully submerging.
JasonTaylor (Geotechnical)
18 Oct 12 8:26
Hmm, these are really good points! Here I thought this thread had been abandoned! :)

So we're just attempting saturation by submerging the sample in water, without actually being able to do it?
dgillette (Geotechnical)
19 Oct 12 9:38
Depends on material type - clean sand is very easy to saturate, with the time required increasing and the effectiveness decreasing as fines content increases. The materials where saturation matters the most are the most difficult ones to saturate, unfortunately.
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