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One-way slab acting as retaining wall

One-way slab acting as retaining wall

One-way slab acting as retaining wall

(OP)
Hello all,

I am assigned the task of designing a replacement retaining wall that is supported by a pile at each end (per project location, and client desire, we could not go about a simple retaining wall w/ footing). The retained soil height is 12'-0" and the span between piles is 11'-6". I am at a loss as to how this slab is to be designed (including the reinforcement), as there is an increasing load from soil pressure. Pile diameter was determined to be 30", and per existing soil conditions each pile is 18'-0" below ground for a total pile height of 30'-0", and the reinforcement cages have also been designed.

Would it be okay to turn the soil pressure into a point load acting 1/3 up from the base of the wall, or is this not correct in this case?

Thank you for your assistance.

RE: One-way slab acting as retaining wall

If the piles (are you sure they're piles and not cast in place piers?) have a lateral and moment capacity determined by the geotechnical report and there's developed bars sticking out to carry that lateral force and moment to the wall, you might be able to design this. It would be better to dowel a 30 inch wide grade beam into the piles and see if you can carry the moment and lateral force as bending and torsion in the grade beam. It still seems somewhat unstable to me.
I'm not sure what changes by converting the soil pressure to a point load, you're still going to have the same problems.

RE: One-way slab acting as retaining wall

You could vary the horizontal reinforcement linearly from top to bottom in accordance with the calculated active soil pressure. Theoretically, you could vary the thickness of the wall but the complication of forming it would probably make that idea uneconomical.

BA

RE: One-way slab acting as retaining wall

I'm not clear on the situation. Is this a building basement wall or a cantilevered exterior wall?

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

RE: One-way slab acting as retaining wall

Quote (markeng2692)

...okay to turn the soil pressure into a point load acting 1/3 up from the base of the wall...

No, at a minimum the lowest elevation of the wall needs to be designed for the highest pressure which is at a depth of 12'. The design needs to be conservative, soil pressure can vary from calculated values for several reasons. Also, perhaps a surcharge load will be applied at the surface just behind the wall - say a parked truck, construction equipment, etc.

There is a another issue. A one-way reinforced wall will only be loaded as predicted if the wall is 12' high. The wall should be more than 12' high to prevent soil from raveling under the wall (please see the attached sketch). With the wall embedded in soil there will be some cantilever support of soil behind the wall. Two-way reinforcing is required.

IMHO, the bottom line is don't skimp on wall design - there are too many variables.

www.SlideRuleEra.net idea
www.VacuumTubeEra.net r2d2

RE: One-way slab acting as retaining wall

If this is a cantilevered wall cantilevering from the piles, it's going to be all about the moments and shears generated locally, near the connection to the piles. The mid-field moments would be child's play compared to that. If this is the case, I'd highly recommend the distribution grade beam recommended by Jed.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

RE: One-way slab acting as retaining wall

I think SRE has correctly defined the OP's problem. A simple span between the vertical piles, which hopefully have been designed correctly.

RE: One-way slab acting as retaining wall

Phew, that's a relief.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

RE: One-way slab acting as retaining wall

If SlideRuleEra is correct, I misunderstood, although the piles still need to take the moment and lateral load.

RE: One-way slab acting as retaining wall

Thanks Hokie. Perhaps the OP can consider an updated version of a design my father came up with in the early 1960's. Instead of the considerable expense of constructing a 12'+ high, cast-in-place wall, use stacked, horizontal precast concrete members (they could resemble square concrete piling).



In the above diagram, the (red) piling resist soil pressure.
The (green) piling, spaced as needed between the red piling, keep the wall from settling when soils are poor.
The horizontal precast concrete members are stacked one-at-time, joint sealed - then backfilled, before placing the next horizontal member.

The photo below, which I took in the 1980's, is one of my father's walls. The horizontal members are salvaged 18" octagonal reinforced precast concrete bridge piling (from the 1930's). The vertical creosoted piling were very used, so have since been replaced more than once. This waterfront bulkhead, now 55 years old, is still in use.



www.SlideRuleEra.net idea
www.VacuumTubeEra.net r2d2

RE: One-way slab acting as retaining wall

Seriously! If it is a basement I see very difficult to cast that wall without removing the ground around both sides.
Do secant piling and than Shotcrete the wall to make your waterproof solutions from the inside.


Regards

MSc. Eng. Serguei Joa
Structural Engineer
Bouygues Batiment International, Cuba.

RE: One-way slab acting as retaining wall

(OP)
Thank you all for your input. I should've been clearer in explaining myself. It is an existing cantilevered retaining wall and is severely cracking in this section (as well as multiple smaller cracks along the length of the wall) and they are looking to repair it temporarily before the HOA approves costs and plans to redo the entire length of the wall(~60 feet long retaining heights that vary between 3' and 14'. This is for a single car detached garage (less of a garage and more of a shade structure placed in front of a retaining wall, above ground. I have attached a diagram of what the goal of the slab behind the wall should be.

However, currently trying to pressure my boss that providing temporary shoring and building a replacement cantilevered retaining wall may make more sense.

RE: One-way slab acting as retaining wall

Can the new wall lean against the existing wall such that the new wall could be considered laterally supported top and bottom? If so, then it should be a pretty straight forward design of a vertically spanning thing. If it's cantilevered, that takes me back to this:

Quote (KootK)

If this is a cantilevered wall cantilevering from the piles, it's going to be all about the moments and shears generated locally, near the connection to the piles. The mid-field moments would be child's play compared to that. If this is the case, I'd highly recommend the distribution grade beam recommended by Jed.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

RE: One-way slab acting as retaining wall

markeng2692 - Do you have current geotechnical soil data to use for calculating soil pressure?
From your description of wall damage, looks like the original soil assumptions severely underestimated soil pressure.

www.SlideRuleEra.net idea
www.VacuumTubeEra.net r2d2

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