At-rest soil loads on a cantilever retaining wall?

[tngolfer]

I am designing an exterior wall of a water treatment plant. The plant floor will be higher than exterior grade (backfilled side is the interior). The liquid-supporting slab-on-grade floor slab will be placed roughly 18 ft. above exterior grade. The design will have to be a multi-step design:
1. Cantilever condition during construction and backfill
2. Restrained condition once the floor slab has set

The geotechnical report I'm reading recommends designing the cantilever wall for at-rest pressures. These are 60% higher than the active pressures I would usually use for a cantilever condition so my wall thickness, footing thickness, footing size, reinforcement, etc. are all much higher than expected. I want to get some other's input before contacting the geotechnical engineer. Do you agree with designing an 18 ft cantilever retaining wall for at-rest pressures? At that height I would expect the wall to tilt some creating an active condition.

 

[FixedEarth]

If you are retaining soil and you have no restraint at the the top of the wall, then use Active earth pressure. However, if you are retaining fluid, use k=1.0 and use the fluid's unit weight density. A sketch will help. A case can be made that the corners or L shaped intersection should be designed for "At Rest" earth pressure, since the corner is more rigid.

 

[hokie66]

It sounds like this wall will be cantilevered only during construction. You would use at rest pressure for the permanent condition. During the time while it is cantilevered, you don't want it to move, so you might want to consider building it as a braced wall during construction rather than cantilevered.

 

[JedClampett]

I'd brace the top of wall before backfilling and design for at rest pressures as a propped cantilever. Then pour your top slab, wait for it to reach strength, then remove your braces.
I'd just prefer to design it conservatively once for its permanent loads, rather than having it switch from a cantilever analysis to a propped cantilever analysis.

 

[JedClampett]

Once again, beat to the punch by hokie66.
But if he says it, you know its right.

 

[hokie66]

Thanks, Jed. But you said it better.

 

[spats]

Bracing the wall could be fairly expensive... big loads. Seems like you'd need a continuous waler at the top, tied to braces that are earth anchored. What's the extent of the wall? Does the savings on the wall and foundation justify the cost of the bracing?

 

[tngolfer]

The bay that is being retained (all 4 sides since the infill in on the interior side) is 37'x64'.

I'm leaning towards bracing the wall which will allow me to more efficiently place my steel in the wall for both the construction and final conditions which will be similar. It would be nice for the opposing walls to brace eachother but I think that would interfere with the backfille operation.

I also thought about not backfilling the volume and using a framed slab at the 18' elevation but the owner doesn't want a vacant space in his building. Maybe I'll run a cost comparison. If the framed slab option is cheaper it may sway him.

 

[burningDiesel]

Since it doesn't need to be filled with soil, and the owner doesn't want a vacant space, can't you fill it with something with a lesser lateral pressure, e.g. middle sized rocks, I'm guessing you can reduce your at rest pressures by factor 2. I have no idea what this would mean for your covering slab though.

 

[RFreund]

I'm thinking one design for cantilever with active pressure and one design for propped cantilever using at-rest pressures if some slight wall movement is acceptable during construction. Also for a short time frame the wall may not move much and it may not even see the full active pressure (not that this means you can design for less). I would think you would have a very large footing using at-rest pressure for a cantilever that tall. Also make sure you can support the top of the wall and truly have a propped - cantilever condition as it will be a rather large reaction. Or make it simple as others suggested an achieve the final condition and use one design.

EIT

http://www.HowToEngineer.com

 

[JedClampett]

There's companies that make styrofoam blocks that would pretty much remove your lateral soil pressures. Insulfoam

http://www.insulfoam.com/

gave a presentation to us a while back. Personally, I don't like anything that was invented in the last 100 years, but maybe you're more adventurous.