Taking Advantage of Concrete Shape

[DaveVikingPE]

Odd subject title...

I have a concrete retaining wall that's poured behind a sheet pile wall. The concrete will, naturally, take the shape of the sheet pile wall on one side.

To determine moment capacity, etc. of the wall, I am of the opinion that the conservative approach is to look at the wall in nice rectangular sections. However, I am being proded to consider the wall's geometry, that is, look at unit section that's not rectangular and consider the section modulus concerning bending.

Thoughts?

 

[dougantholz]

Dave,
This is one that I have run into time to time. Basically the owner is questioning your analysis, trying to take any conservativeness out of it they can.
To answer your query, let me first ask if the pile is on the tension face of the wall. I assume that it is, because the piles would be used as the temporary soil retainer. If that is the case, then it doesn't matter what the profile is, it doesn't affect the capacity of the wall. The only thing that would is the depth to reinforcement. The contractor can, provided depth to reinforcement rules are used, put the steel in the triangular area the piles give you - but that makes placing the dowels in the footing very hard. It is their option, but if I were building it, I would want it as easy to build as possible.
-Doug

 

[Ginger]

I feel that Dougantholz is correct up to a point.

In considering the concrete wall only, Dougantholz' point about the concrete in the return pans of the sheet piled wall not being of any structural value is only true if your tension face rebar is not placed into the pans.

If you place your bars into the 'in-pans' then you will increase the effective depth of your section in this area. One way to analyse this section is as a Tee beam with the pan acting as the downstand web and the narrower section of the wall at the 'out pans' forming the slab section of the beam. Doing this will increase the moment capacity of your wall section.

Beware though that doing this may lead you to design very thin sections of wall in front of the out-pans which may be susceptible to cracking due to differential movement along the wall face if you are not thorough with your analysis.

If the piles are to be left in, then they are doing most of the work as they will already have deflected under the active earth pressure behind the wall while you are constructing your concrete wall. Therefore, your new wall will only carry any additional wall loading which comes on after the concrete has been poured.