AASHTO Simplified Earth Pressure

[fromtherafters]

In working with Figure 3.11.5.6-6 in the AASHTO LRFD Bridge Design Manual, I am finding conflicting opinions on how to handle the increased embedment of 1.2D. I consulted a Geotechnical engineer who stated the 1.2D is performed to achieve a F.S.=1.5. I reviewed references stating the 1.2 factor is used to account for differences between the simplified method and the conventional method and is not a factor of safety. Those references would apply a factor of safety by reducing the passive pressure (as I would commonly do), find the point of moment equilibrium and finally increase the embedment by 20% when using the simplified method.

I would appreciate it if anyone could share their thoughts, experiences or perhaps references that reinforce either position. Thank you in advance.

 

[fromtherafters]

Any help is greatly appreciated.

 

[bridgebuster]

My understanding is "D" is minimum embedment for individual elements, such as soldier piles and "D_O" is for a continuous wall, such as sheet piling. The DOT that I primarily work with adds 30% to the minimum embedment.

They also require that K_P be reduced by 1.5 for permanent walls and 1.25 for temporary walls, which is an additional safety factor.

I hope I didn't muddy up the waters for you.

 

[fromtherafters]

bridgebuster - Thank you for the reply. I have had similar experiences and tend to agree with the method you outlined. I posted the question to see what others thought and if they had any additional information to support either case. I recently received comments indicating the additional embedment was to provide a factor of safety and reduction of the passive pressure was not required if an increase in embedment was implemented. Though I disagree with the comment, I would like a better understanding of the additional embedment outlined in AASHTO before committing to either approach. Thanks again for your input.

 

[bridgebuster]

I'm glad to have been of help. You can also contact AASHTO and pose the question to them.