Bridge Abutment Pile assumptions - MSE wall 1

[bb_gb]

I am a new engineer and a question has been posed about bridge abutments using MSE walls that I am curious about. I have heard different design assumptions from other engineers for when the pile is considered restrained. For the length of pile driven through the existing strata, the pile is considered continuously braced. Some engineers do not consider this bracing through the retaining wall fill and some do.
Is there any guidance on backfill properties required to consider a pile continuously braced? This is particularly important when considering alternative backfill materials to ensure the pile is analyzed correctly.

Any suggestions would be appreciated,
Thanks.

 

[BridgeSmith]

I would say lacking research results to the contrary (which may exist; I haven't looked), bracing against buckling (for axial capacity) in the reinforced fill is questionable. Resistance to lateral loads, however, is very good.

 

[canwesteng]

I don't see how you can have one and not the other. Talk to your geotech - for example if they say neglect the top 3' or so for lateral resistance, neglect that for buckling restraint as well.

 

[bb_gb]

The geotech that we are working with on this project is considering the fill section to be braced, but now that we are looking at using a lightweight fill product, I don't feel as comfortable with that assumption. I reached out to another geotechnical engineer prior to writing this post and they said that they typically consider the fill section to be unbraced.
Until I can find evidence otherwise, I may have to conservatively assume it to be unbraced.

 

[BridgeSmith]

"I don't see how you can have one and not the other."

The eccentricity (movement) required for buckling is fairly small, perhaps too small for the resistance of backfill placed around the pile to be mobilized. As I said, I haven't looked at the research on this for steel piles. For lateral bending of the piles, the displacements are larger and the moment on the pile can be determined from a typical P-y curve analysis (L-Pile, All-Pile, etc.)

 

[Bobby46]

Check out FHWA Geotechnical Engineering Circular 12. They have a short section on pile buckling in soft soil.

 

[BridgeSmith]

I asked an engineer in our office what he did for the design on a similar project. We actually used a CMP "sleeve" backfilled with sand after driving to allow the contractor to build the wall first. He considered the piles fully braced.

Before you spend a substantial amount of time wrestling with this, is the reduction in the axial capacity substantial if you consider it unbraced through the MSE layer?

 

[pikku14]

Is your company/team designing the MSE wall or is it being done by a contractor? I'ts fairly common to isolate the piles within the MSE wall to avoid loading the MSE wall due to actions on the bridge. If you're not isolating your piles, then you'll need to ensure the MSE wall design has sufficient strength and liaise with the MSE designer to work out what the additional pressure on the wall will be.

Soil straps take some movement before the straps are fully engaged in any case so I'd be inclined to consider the piles as unbraced over the height for structural design regardless. If you're using bored piles or similar that shouldn't be an issue, but might be if you're using driven.