NCEES- Geotechnical depth, problem 514

[chess5329]

I hope somebody can provide a better detail on how they got to the solution.

I've checked my formulas and to get FS= CSR/CRR

Any help?

 

[wvgirl14]

This formula is in Goswami's All in one pg 211. "The cyclic stress ratio is defined as the ratio of the average cyclic shear stress produced in a soil (due to rapid loading) to the effective vertical stress. CSR = Tave/ov' Thus, if the cyclic stress ratio is specified, it can be used to estimate the average cyclic shear stress due to a particular earthquake event. This shear stress can then be used to calculate the factor of safety given by FS = Th,field/Tave where Th,field = cyclic peak shear stress required to cause initial liquefaction in the field." The F.S. formula F.S. = CSR/CRR is from Navfac Reference. I had to do a little digging on this one as well, because I like you went to the Navfac first. I hope this helps.

 

[terzaghi83]

OK, so I'm confused. In the NCEES sample exam problem listed, the solution calculates the FS as CSR* sigma prime sub v / cohesion of soil

Thsi same problem is in the NCEES 2007 sample exam and is solved the same way. The way you are suggesting, and teh way that is listed in the goswami all in one practice exam places the CSR * sigma v term in the denominator, not the numerator like NCEES.

Which is correct?

 

[civilized_naah]

OK, so I'm confused. In the NCEES sample exam problem listed, the solution calculates the FS as CSR* sigma prime sub v / cohesion of soil

Thsi same problem is in the NCEES 2007 sample exam and is solved the same way. The way you are suggesting, and teh way that is listed in the goswami all in one practice exam places the CSR * sigma v term in the denominator, not the numerator like NCEES.

Which is correct?

In the NCEES problem, the 'maximum allowable cyclic stress ratio' is given as 0.29. This makes it the CRR (cyclic resistance ratio - maximum allowable = resistance). That's why it goes into the numerator (ultimate) in the calculation of the FS. What's in the denominator is the actual shear stress induced by the earthquake, which corresponds to a CSR = 0.225, but it's given directly as 450 psf

 

[chess5329]

civilized, If you don't make this input, I think I wouldn't get........I think this makes sense!

Thanks

 

[terzaghi83]

Thank you very much!

 

[terzaghi83]

hmmm... just noticed that in the solution is states CSR= 0.29

Then puts the effective overburden * CSR into the numerator... This seismic stuff is driving me crazy.

So your saying that the solution is correct, but should label 0.29 as the CRR?