Direct Shear 1

[JasonTaylor]

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?

 

[moe333]

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]

Thanks a lot. I'm just trying to wrap my head around the concept.

 

[bilal09]

also add..total stress=pore water pressure developed stresses + effective stresses ...

 

[fattdad]

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]

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]

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]

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.