Cantilever Solider Beam in Drilled Shaft

[dcarr82775]

For a cantilevered steel soldier beam installed in a drilled shaft we have always used structural concrete (3ksi) to fill the shaft around the beam below the bottom of cut, and used lean mix to fill the shaft the rest of the way up.

We are looking at just using the flow fill over the entire height as it is still stiffer than the soil in this instance. I wanted to get a feel for what are other designers practices are with regards to this (concrete/flow fill/etc)?

 

[hokie66]

Stick with the structural concrete. This gives you the full section of the drilled shaft to resist overturning, while using the flow fill reduces this resistance. Same work involved, just a bit difference in cost of material...not worth it.

 

[PEinc]

Almost always use full height, lean mix, low strength concrete or flowable fill. The only time I use structural concrete is on permanent walls for highway projects (required by DOT) or when I am installing very stiff, conservatively designed, closely spaced, tiedback, soldier beams instead of underpinning on a small job where the owner or my client is willing to accept more risk.

http://www.PeirceEngineering.com

 

[born2drill]

I think the OP is talking about a soldier pile with the toe in soil, in which case it's a waste to use any structural concrete. Also, I'm unclear on why a pile with structural concrete in the toe would provide more overturning resistance than one with flowable fill.

Also, pouring structural concrete in the pile toe and then flowable fill above is definitely not the same amount of work as pouring flowable fill full height. It's a pain to build them that way, plus you're pretty much guaranteed to have a few piles where the structural concrete is poured too high, which makes lagging installation a nightmare. The price will reflect all of this.

 

[hokie66]

Are you saying the strength of a bored pile doesn't matter in resisting a lateral force?

 

[PEinc]

For a drilled-in, steel soldier beam, the strength of the cementitious fill usually doesn't matter very much. 100 psi or a little more is sufficient. The steel soldier beam resists the bending. The low strength cementitious fill is in lateral compression and provides bearing against the soil or rock around the embedded portion of the steel beam below subgrade.

http://www.PeirceEngineering.com

 

[dcarr82775]

Thanks for the comments.

My primary concern with the flow fill (or in this case a weak cement slurry) is that instead of utilizing a 30" width of shaft to push against the soil, I have something closer to the flange width of the beam itself. My thinking is that this will result in some movement of the beam withing the flowfill in addition to movement of the shaft in the soil resulting in greater overall movements of the soldier beam, but the bending in the beam will not change much.

 

[PEinc]

I almost always use a passive resistance width of 3 times the diameter of the concreted drill hole, unless specs say otherwise or the soil is soft. If the soil is soft, you probably should not be cantilevering soldier beams or you should use sheet piling.

http://www.PeirceEngineering.com

 

[msquared48]

If the pile is intended to resist lateral forces temporarily, as for temporary shoring, I see no problem with the lean mix taken to the bottom of the hole.

But it the shoring is intended to be permanent shoring, I would use the stronger concrete below the lowest excavation point.

How's that for the middele ground?

Mike McCann
MMC Engineering

 

[PEinc]

That's the usual compromise.

http://www.PeirceEngineering.com

 

[InDepth]

Just calc it out...for 1D for lean and 2-3D for structural. I think you'll find lower embedmnent and less tip deflection with structural concrete....but it just depends how you calc it. What's a penny for a bit more safety.