Seismic Lateral Earth Pressure for Retaining Wall Design

[ivanga7]

I am designing a retaining wall but I cannot seem to find a straight answer on how to calculate the Kh factor. I have read about it on different sources and they all seem to say something different. This is what I have seen:

[li]Kh = Sds/2.5[/li]
[li]Kh = 0.5*PGA[/li]
[li]Kh = PGA[/li]
[li]Kh usually ranges from 0.5 to 0.3[/li]

Any thoughts. The project is in California, so there is high seismic activity.

Thank you

 

[JoshPlumSE]

Yup, there doesn't seem to be a real consensus on this subject as far as I can tell. My thoughts on the subject:

1) My tendency is to lean towards the Sds/2.5.
2) Retaining walls that are designed well for regular loading seem to have performed pretty well in earthquakes in the past.... Despite the fact that very few of them were designed for seismic loading.

I'm glad the code is trying to define this concept better in recent cycles. Because there are projects where the retaining walls are pretty critical. That being said, for those projects (where liquifaction might be an issue or the entire site stability is based on a tall retaining wall) my tendency would be to let the geotech handle the wall.

For the more boring conventional retaining walls that I've designed, my tendency is to believe that seismic design of these walls isn't truly necessary.

 

[WARose]

If you have a geotechnical report/guy on the job......this is a question he should be able to answer. (Well, you would hope so anyway.)

 

[bridgebuster]

Google Mononobe Okabe plenty of design examples out there.

 

[JoshPlumSE]

Well, there are a couple of variations on Mononabe Okabe that can be made.

And, the fundamental question is what Kh to use as the basis for the M-O pressures? That's where the consensus seems to be lacking....

 

[WARose]

And, the fundamental question is what Kh to use as the basis for the M-O pressures? That's where the consensus seems to be lacking....

Chapter 8 of Braja Das's 'Principle of Soil Dynamics' is devoted to this subject. If you look at just about all the comparisons they cite (between experimental and theoretical values) they are all off by a pretty significant factor. Fortunately, they have comparisons between some of those values in charts.

Further complicating the matter is the fact the magnitude and distribution of the pressures on the wall depend on how the wall itself yields (i.e. rotation about the bottom, translation, or rotation about the top; Das addresses all 3 in the reference). You combine that with a large number of possible experimental input parameters (i.e. frequency input motion, etc) and you see why there isn't a consensus.

That's why I suggested a few posts back to rely (if possible) on a Geotech's opinion (like we do so many other times). You can always fall back on that (if something goes wrong)......and back check it yourself to see if it makes sense.

 

[KootK]

The city of Los Angeles has a policy statement on this: Link. In the absence of anything more official you could at least point to that. They take Kh = Sds/2.5.

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.