Retaining Wall Design? 1

[SteelPE]

Another simple one.

I have a question regarding retaining wall design. I have a proposed multistory structure that has one wall that is a retaining wall. Normally I would design the wall for two conditions, freestanding and propped (by the building). In this particular instance, I do not believe I will be able to develop enough resistance in the structure to resist the forces contributed by the wall (it will be cost prohibitive).

In thinking about this, if my wall is cast in place…. And then back filled prior to construction of the building (allowing for any movement to take place at the top of the wall) then the building is placed, will I end up with any loads from the wall being transferred into my structure (not counting surcharges)? To me it seems like the wall will be stressed and deflect. Then once the building is constructed on this deflected wall then there will be no additional load transfer from the wall into the building structure.

 

[a2mfk]

They may be right, unless the soils don't behave exactly as they are supposed to and still move slightly after the fact, and DO exert pressure. This is how I would have modeled it. I don't know if we can predict how soils interact with structures as well as we sometimes think we can...

So maybe in 5-10 years you will have a forensic job!

 

[SteelPE]

I'm all set with a forensic job. I can barely grasp what I am doing now.

Unfortunately I am still very involved even though we were not awarded the job. I was asked to help with some aspects of the job with my responsibilities growing day by day (don't know how my company came to that decision). This is how I know what is specifically happening with the design of the wall.

Ugh.......

 

[JAE]

There is always long term soil pressure.

It isn't a one time static thing.

BAretired is correct that it is really an indeterminate problem in that SOME of the active pressure will engage during construction (not the full design active pressure all at once).

After construction, due to thermal changes, moisture changes, near-sourced vibrations, etc. the soil will consoltidate over time and add to the pressure on the wall. And with the addition of the building frame, you then might be approaching a more "at-rest" condition than active.

This pressure will be shared by the wall stiffness and the building's frame stiffness in direct proportion to those stiffnesses.

The rate, and amount of the soil pressure is difficult to pin down - I'm not sure your average geotech could get a fully accurate number.

 

[SteelPE]

JAE/BA

So what you are saying is that even though you have the GC compact the soil to 95% due to environmental conditions, you may end up with more compaction or soil movement that gives you more pressure against your wall?

I find it hard to believe that a retaining wall is going to be more "stiff" than a steel brace frame or masonry shear walls. So I would be inclined to believe that you would end up with some soil loads induced into the LFRS of the building.

 

[a2mfk]

Maybe an afterthought at this point, but it was never mentioned- where is this project and what are the soils (most likely) at the site?

My entire career has been in Florida where we mostly have cohesionless sand, and you can't much dig a two foot hole without it collapsing continuously. I have always had my doubts about this idea of soil pushing against a wall, the wall moving away, and that is the end of it...

 

[hokie66]

JAE's point is well made. A compacted soil mass is not static. There will often be increased lateral pressure, and always there will be more settlement.

 

[SteelPE]

a2mfk

About a week ago I received the geo. report for the project. According to the report, they recommend an angle of internal friction of 32 degrees. From the test pits that they dug, you can see that the soil on site appears to be a gravel/sand or sand/trace of silt mixture. There are also some concerns with bedrock in the area but they do not believe that they will have bedrock problems within the building footprint.