Resistance Factors for Sheet Pile Design

[peelwvu]

Working on a fairly simple sheet pile design which is nice because it's my first one. It's for DOT and LRFD is required. I'm using Shoring Suite Software. I started out using Strength Case Load Factors for all of the loading. 1.5 for Earth Pressure. 0.75 for Passive Pressure, 1.75 for Live Load.

It's only and 8 ft. exposed height and water is assumed at the top of the wall as per the spec. I'm getting embedment lengths of 31 ft., which seems like a lot. I read in the manual that the program automatically adds 20% to the embedment length and that AASHTO requires 30%.

Should I be using service load factors, since a factor of safety is already being added by the program?

 

[r13]

I suspect you have specified inaccurate earth pressure coefficients. Suggest to read this paper/study. Link

 

[PEinc]

peelwvu, read AASHTO carefully and do not mix up discussions and requirements for ASD with those for LRFD. With LRFD, you already factored up the driving forces and factored down the passive resistance. Therefore, CivilTech's automatic 20% embedment increase should not be needed. For a cantilevered, non-gravity wall designed by ASD and unfactored service loads, the safety factor in provided by increasing the embedment depth. For LRFD the "safety factors" are already provided by the load factors and resistance factors.

http://www.PeirceEngineering.com

 

[peelwvu]

Thanks to both of you.

I've tried two ways. One using retaining wall strength loads for LRFD and not using the 20% increase in shoring suite. This seems to still give me a very conservative embedment.

I also tried ASD and adding 30% per AASHTO. This seems to be a lot more reasonable. A 23 ft. embedment for an 8 ft. wall as opposed to a 31 ft. embedment using LRFD Strength case.

 

[r13]

If you are correct on both cases, I think AASHTO LRFD has over played the load factors, as the difference is quite large.