Swell Pressures on Retaining Wall 2

[VTenge]

Hey all - I have a project that has some deep basement walls retaining a maximum of 14'. Due to the poor soils on site, and the limits of the property lines, the geotech provided a diagram that shows the distribution of the horizontal earth pressures due to clay swelling (to be in addition to the backfill pressure). Does anyone know of a design software that can account for this? I've tried Risa Foundation, but it only evaluates braced walls at their top, and the basement walls extend past the floor diaphragm. There's probably a way of making a footing and wall separately, and piecing it all together, but I'd like to avoid the hassle if I can.

 

[r13]

I don't understand your problem, can you show a sketch?

 

[emmgjld]

Soil Expansive pressure against a wall gets quite messy. It is also very dependent on geology.
1) Is the expansive pressure to come from the backfill soil or from the 'retained' soil.
2) Is the 'retained soil' actual soil? very weathered rock? or slightly weathered rock? [all within 4 x the wall height]
3) What is the 'structural attitude'[strike/dip relative to the wall alignment] of the retained material within 4 x the wall height?
4) What kind of 'global' soil/rock water problems are possible?

 

[oldestguy]

I gave a star to the next above since he touched on a problem that is very difficult to deign for. It all depends on the moisture content of the expansive clay Some montmorilonites can expand 16 times the original volume.. If at all possible remove a large portion of that clay and use a non-expansive material such as sand or other non-expansive material. A simple standard retaining wall likely would have to be significantly reinforced to withstand these expansive pressures. Might even need tie backs anchored off to the area away from the wall. By the way, are there any other retaining walls in the area that you can observe for behavior as a general guide? What is the climate in your area and how do other walls behave? A big change from dry to wet seasonally can be a tough one to handle.

 

[VTenge]

These are good questions. I've attached the swell pressure diagram I was provided. I'm trying to use the 3' backfill zone currently (due to property line locations).

emmgjlf:
1) Swell pressures come from the existing clay soils behind the backfill.
2) Material directly behind the wall is a typical draining gravel with the geotextile fabric, french drain, etc.
3) I'm not sure I follow the question, but can't go back 4x the wall height (3' max backfill zone is what there's room for. I assume the soils beyond the property line are similar, but I don't really know. They didn't get any samples that far away.
4) Saturation shouldn't be an issue with the gravel backfill, and two drain systems (the geotech recommended at least 2, in case one is being worked on)

oldestguy: There aren't any walls around that I've seen to observe. This house is being dropped down to avoid issues with height limitations. Very high end construction, located in central Texas.

I'll post a loading diagram of what's going on here in one second.

 

[VTenge]

Here's the loading diagram. I know I don't show the surcharge lateral pressures, and it's not to scale, but I think it gets the point across. I used the 3' backfill zone swell pressure loading from the swell pressure diagram in the previous post.

 

[1503-44]

Thats a pretty hefty load. It makes for a very large bending moment. Why not just excavate and replace with sand. At least halfway down. It would eliminate a lot of that expansive pressure at the top of the wall. Expansive clays in TX are notorious for destroying foundations over much of the state.

“What I told you was true ... from a certain point of view.” - Obi-Wan Kenobi, "Return of the Jedi"

 

[oldestguy]

I'm just thinking a little. I wonder if some form of Styrofoam as backfill will be of any value. Perhaps some one will come on with experience on these things. With Styrofoam I can see how one season is ok, but repeated shrinking and swelling may not work so well in the long run.
Some kind of spring would do it maybe.

I've run unto a few cases where tie backs were installed without the OK of the adjoining property. Some were fought in court and others no problem. Maybe look into an easement to avoid this.

 

[1503-44]

There is some crushable stuff we use to allow buried pipe to expand a bit more freely in critical locations and when crossing fault lines. Its more water resistant than Styrofoam. Not sure if the cost or availability fits, but I'll try to find out more if you're interested.

I think sand would work pretty well though. And probably cheaper. Take a lot of load off that wall. That's what I'd do. Better to find a work around than risk such expansive clay. If its mot expanding into the foundation, its sticking to it and pulling it out, or breaking free and making gaps that fill in from the soil above, then pushing that much harder against the wall the next time it gets wet and wants to expand again. The constant and extreme variation of the TX dry and wet cycle is what does the damage. The PI of those clays is terrible.

“What I told you was true ... from a certain point of view.” - Obi-Wan Kenobi, "Return of the Jedi"

 

[1503-44]

RISA won't stop the expansion, if you stop lateral movement, it just expands upwards and messes up the pavements above.

“What I told you was true ... from a certain point of view.” - Obi-Wan Kenobi, "Return of the Jedi"

 

[VTenge]

I'm going with the foam route for sure. I still have the problem of finding a design program that can model this correctly.

 

[r13]

Use RISA, do a finite element analysis.