Residential Retaining Wall Geotech

[Once20036]

I've been working as an engineer for the past 7 years, but mostly on large scale commercial & residential projects.
A friend has asked me to help design a retaining wall for his house.

The concrete design is easy, but normally on projects I have a full geotech report to give soil type, allowable bearing, active pressures, passive perssures, etc. Because this is such a small project, that isn't going to be an option here.

Are there standard/conservative design values to use in cases like this? Do you give a couple designs for the contractor to pick from once he determines if the wall is founded on clay, sand, silt, etc (this feels like it's asking for trouble)? Provide notes with the assumptions and request the contractor notify you if any of the assumptions are off?

Surely a geotech engineer isn't required on every project..

 

[SLTA]

The NC residential code lets me use 2000 psf without blinking. It also gives prescriptive requirements for walls like heights, rebar, footing, etc for different retained heights. Check your state's residential code for similar info. Definitely write notes with the assumptions and ask to be notified if you're off.

Or, call one of your geotech buddies, explain the situation, and ask for rough-but-conservative numbers to use.

Or, just totally overdo it.

 

[mogug]

IBC Table 1804.2 allows the following for clay (worst case):
Bearing = 1500 psf
Passive Pressure = 100 psf/ft
Sliding Resistance = 130 psf

For sand:
Bearing = 2000 psf
Passive = 150 psf/ft
Coefficient of Friction = 0.25

They don't mention active pressure but you can safely use 45 psf.

These are very conservative values.

 

[msquared48]

Make sure you spec a good granular backfill behind the wall with a perf pipe drain venting to the end of the wall to limit the pore pressure. Otherwise you could be kidding yourself, even at 45 psf.

I believe that 35 is the minimum, but 45 would be a good suggestion without knowing much about the soils.

As for the bearing pressure, use the 1500 psf value. If the wall is too high, the soil pressure will control and you may have to get a geotech or soils report from a neighboring project to justify raising the value. If you are only dealing with an 8 or 9 foot high wall though, this should not be a problem, assuming you are not on a property line, restricting the full of either the heel or toe.

Mike McCann
MMC Engineering

 

[FixedEarth]

The cost for a retaining wall soils report is much less than the additional expenses due to concrete, rebar, and excavation.

Let us say you over design the active pressure and under utilize the available passive resistance like IBC table suggest. Then how would you satisfy the following issues?

Should you use Type I, II or V cement?
Would you have a global slope stability issue?
Do you have unfavorable bedding planes?
How would you avoid cut/fill transition problems?
What is an appropriate surcharge earth pressure value?
Are the site soils suitable for backfill?

Just be aware of the geotechnical/geological risks.

http://www.FoundEng.com

 

[Once20036]

Thanks for all the pointers. I looked but didn't find the IBC table re: typical pressures.
Max wall height will be 10ft +/-, away from all property lines so there are no issues with having the toe that I want.
I`ll definitely be detailing the wall with granular backfill, weep holes, drain pipes, etc to eliminate/minimize water behind the wall.

FixedEarth: I tried to sell him on getting a geotechnical out to the site, but it doesn't look like it's going to happen. I`ll just need to do the best I can with what I've got...

 

[DST148]

Structural20036 - mogug has already mentioned in his post about IBC Table 1804.2 which gives allowable foundation and lateral bearing pressures.
For minimum design lateral loads for retaining walls you may refer to IBC 2003 Table 1610.1 which gives active and at-rest earth pressures for various soil classifications.
Since this a small house I would also suggest referring to local Residential Code.

 

[JennyNakamura]

Many cities and counties have their own typical or "standard" details and specifications for residential retaining walls within their jursidictions. You should look into that -- call the building department and/or do a Google search. I would not personally just blindly use a jurisdictional standard detail, but it is a good cross-reference to check against when your done with your own design based on your own assumptions.

 

[SLTA]

Structural20036: you're also looking in the wrong code. For residential, you need the IRC, no the IBC. You'll find a wealth of information in there.