Normal Loads

[DRQC]

I am starting a direct shear test for the first time. I have read several refereces but have no idea what normal loads to start with. I am dealing with an SC soil. Can anyone give me some idea what normal loads to start with and/or in what increments to increase them?

Thanks
DRQC

 

[GeoPaveTraffic]

Two things come into play when picking the normal load to run tests.

First if the sample is undistrubed, all selected normal loads should be at least equal to the normal pressure that the sample was under prior to collection.

Second, the normal loads should cover the range of normal loads of interest to the project.

 

[oldestguy]

For that material direct shear may not be suitable if the sample consolidates during the test. Tri-axial may be more suitable.

 

[fattdad]

all selected normal loads should be at least equal to the normal pressure that the sample was under prior to collection.

I respectivelly disagree. If you are considering strength of in-situ soil and the application is a cut slope, there may be a design field stress imposed on the soil element that is less than the current state of stress. You should consider both the existing stress and the proposed stress (whether greater or lesser) in defining the normal loading.

To BigH, while a TXC may also appropriate, just because there is some potential for consolidation, I'm not sure that would preclude the use of a DDS to evaluate the soil strength. Then again. . .

f-d

¡papá gordo ain’t no madre flaca!

 

[moe333]

In direct shear, the sample is consolidated to the specified normal load prior to shearing so that should not really be an issue. To me, the big issue is do you need a drained strength or undrained strength? Direct shear can really only give drained strengths, although there have been some methods using direct shear where undrained strengths could be approximated, but not reliably in my view. If you need undrained strenght, you would need to go to triax.

The other consideration is failure mode. Direct shear is basically a plane strain test where you are forcing the failure through a particular plane. Triax allows the shear to happen with more freedom, and you will typically obtain lower strengths compared to direct shear.