Direct shear -- Normal stress and residual 2

[phafan]

I am rarely in the lab, when I got the results, 2 questions I have:
1)I an comfortable with using the normal stress for 3 points at 700, 1400, 2800 psf (double the number), but I also see people use 1000, 2000, 3000 psf. I want to make sure both ways can get the same C & Phi, because I don't see anything wrong within them. AND why they use the different terms.

2)When run the test, undisturbed soil gains peak, then pull shear box back, run again, get the ultimate. HOW to get residual?

Thanks

 

[GeoPaveTraffic]

First, you really should review some geotechnical texts to get a better understanding of what you are seeing from the lab.

As for you quesitons:

1) Assuming the tests are being ran on different samples there is nothing special about doubling the normal load. The normal load should be determined on a case by case basis to cover the range of expected normal stresses for the specific project.

2) Getting good residual strengths takes a lot of time and paractice. The peak strength should be determined normally, then the sample should be sheared back and forth multiple times. Residual is reached when the shear stress is nearly idential on repeated shearings.

Mike Lambert

 

[phafan]

Thank you Mike

 

[dgillette]

I don't have the ASTM standard handy to check, but USACE EM 1110-2-1906 allows pre-cutting the shear plane with a fine wire (like the thinnest "string" on a guitar).