Retaining wall design 1

[MacMerdin]

I'm designing a retaining wall. I'm going to assume that the soil behind the wall is compacted fill. Can I use a standard density or do I need to test the soil to be more accurate? Also, what angle of internal friction should I use for the backfill? Thanks.

 

[Gimbli]

First, that kind of recommendations should be included in the geotechnical report prepared for the design of the wall. And, the parameters to be used for your fill depend on what type of fill are you going to use. Be careful with retaining walls. Ask the geotech for recommendations, specially if wall is proposed on a slope, if there is new weight from new fill over soft compressive subsoil conditions, among many others.

PD You might overstress the wall compacting the backfill. Consult your geotech.

 

[MacMerdin]

Thanks Gimbli. What we are doing is taking an already built retaining wall (brick)and re-designing it with reinforced concrete with a brick veneer. Do I need to have a geotech investigation performed?

 

[Gimbli]

I do not understand. Are you going to demolish the existing wall to contruct a new one?

You should hire a geotech, if there is issues of concern like seismic considerations, if any, bearing capacity of soil. I dont know, i'm just thinking loud. I know to little about the project. Can you tell me something else; location, hieght of wall, geology, are you going to demolish and contruct new wall, are there going to be any additional loads other than the backfill, what is the project about?

 

[MacMerdin]

Yes, we are going to replace the existing wall with a reinforced concrete retaining wall with a brick veneer.

It's in Lancaster, Pennsylvania, so siesmic considerations shouldn't govern I think. Wall height is 12 feet at the apex. No additional loads. Thanks for your help.

 

[Gimbli]

You should also hire a structural engineer.

 

[DRC1]

For good granular material generally I use 120 pcf and phi=32 degrees. This assumes the material is sandy gravelly or processes agg. base. Be sure you have good drainage details for your backfill. For small walls use smaller plate compactors. Over compaction can cause high stress. design for Ko conditions.

 

[MacMerdin]

Thanks for the replies all. I'm going to use 120 pcf and 32. It's only an ornamental retaining wall at a university. Should I even spec compaction? Or even something smaller than 95% compaction?

Just curious: Why do you say to use Ko conditions? Isn't that for a restrained wall? I'm using a cantelever retaining wall.

Also, as far as the bearing pressure allowable, I have a table that states the worst case scenario as .5 TSF. But, it also states that the factor of safety is included. I need an allowable bearing pressure of 5000 PSF. Should I backfill with rock or something or should I try and extend the heel?

Thanks in advance.

 

[MacMerdin]

Sorry DRC1. You are actually correct in using Ko conditions. The wall is attached and restrained. Thanks again for the info.

 

[msucog]

i agree...120pcf...phi would be lower in my part of the world for "typical" site soils. we recommend using 28 since we're much more silty here in GA plus the geology is extremely variable. i would suggest putting minimum compaction of 95% in there unless geotech recommends something different. i personally am not a fan of plate compactors again, because we're much finer than straight sands...but small mechanical hand tamps work like a champ here. i have had projects were the contractor insists on running rather large compaction equipment next to the wall less than a week after it's cast, so i would have it somewhere in the contract documents so that if they fail the wall, they bought it. and drainage is an absolute must...i also agree that you should consult a geotech to look at the site conditions and assist with the design parameters. good luck.

 

[cvg]

note that a 12 foot high retaining wall is a large structure and will require a permit to construct. the permit will require an engineer to stamp and sign the plan and furnish calcs. This should really be done by someone with experience in large retaining wall design. Lacking that, you might start with a standard highway department retaining wall design which are generally very conservative designs. You could adapt that to your site and then apply the veneer. Follow the specifications required by the DOT.

 

[H57]

I was thinking the same thing. You say it is an ornamental wall, but if it is 12 feet high it is much more than just ornamental. You will develop large forces that must be considered or you may be taking your wall out to rebuild another.

 

[MacMerdin]

Thanks all for the replies. I guess saying it was ornamental was the wrong way of describing it. I'm getting help from my senior engineer here and we'll see what he says. Again, thanks all for the help.

 

[fattdad]

WARNING: There are just too many unknown variables for anybody on this forum to give you proper advice. You have no idea what the existing backfill is and you don't know what the wall/backfill is sitting on. Other than the knowledge that there is an existing wall we don't really know anything. Also, where's the water table?

Fortunatly, you have a senior engineer with a seal to oversee the design. That said, I'd strongly encourage you to take a few hand auger borings and collect samples of the wall backfill and the toe conditions before going much further. The best advice given so far is to hire a geotechnical consultant and base the design on Ko conditions.

f-d

¡papá gordo ain’t no madre flaca!

 

[MacMerdin]

"The best advice given so far is to hire a geotechnical consultant and base the design on Ko conditions."

I agree and we may just have to do this. That was my original question really. Do I need an investigation or are there standards that I could use. So, my answer is do the geotechnical study. Thanks again all.

 

[civilperson]

Reseach the original walls design. Probably was a geotech report done when originaly built and the recommendations still apply.

 

[UcfSE]

If you don't hire a geotechnical engineer and the wall fails, how can you justify assuming you didn't need one? That the same as saying you are knowlegdeable in geotechnical engineering, which doesn't sound true and may go against yours or your boss's license. How would you justify that if you get sued for the wall failure? There's more to it than just assuming some values. If that's all there was to it we wouldn't need geotechs.

 

[MacMerdin]

True. I just thought maybe I could assume worst case and get away with it. We have a meeting about what we are going to do with it today. Thanks again all for the replies.

 

[fattdad]

Worse-case scenario would be Ko=1 (i.e., phi=0) gammaB=65 and full water pressure. This is essentially a retaining wall supporting a heavy liquid (i.e., the soil mass has no shear strength). Now you'd have to worry about global failure - so you'd still need a geotechnical engineer (unless the wall is sitting on unfractured rock).

f-d

¡papá gordo ain’t no madre flaca!