Sesmic slope stability

[Mccoy]

My current reading is Duncan & Wright's book on soil strength and slope stability (Wiley 2005).

They say, in chapter 10:

Reductions in strength of up to 20% caused by cyclic loading during an earthquake are probably offset by the effects of a higher loading rate during an earthquake compared to normal loading rates in static tests...

...Thus, there is some basis for not reducing the shear strength used in pseudostatic analysis, provided of course that the analysis is only used for cases where significant (more than 15 to 20%) strength losses are not anticipated

Bold is mine, to underline the nature of my question:

how to anticipate a significant loss of strength in dynamic conditions, without falling back to lab tests?

Quick clays? soft clays? Non-dense sand stringers below watertable? Else?

 

[moe333]

I would think quick clays and liquefiable soils as you mention. Also collapsible soils, and soft clays that are potentially liquefiable based on the recent procedures by various authors such as Bray and Boulanger. I think you would still need to do lab and insitu tests but not necessarily high end cyclic testing.

 

[Ron]

Mccoy...BigH is concerned about your well-being (as are the rest of us....but didn't know you were in the quake area)

You might check and answer his thread on the quake...

You've probably already checked, but H.B. Seed has quite a bit on this in ASCE and a couple of other journals.

 

[dgillette]

Don't have the book, but what this may refer to is work by HB Seed and, as I recall, IM Idriss in the late 1970s that showed significant permanent strains, though not complete failure, at cyclic shear stresses exceeding 80 percent or so of the peak strength, although there was not much change in the peak strength from the cycling (as measured in monotonic shear after cycling). What I do not recall is whether this applied to clays in general, compacted clay, normally consolidated clay, or what. The recommendation to reduce the strength like that is made in the documentation for the Makdisi and Seed method for estimating deformation of dams. (It's basicaly an improved Newmark chart solution.) That suggests that it applies to compacted clay fill, at least.

A few years ago, I had Duncan reviewing some work, and at the end, I realized I hadn't accounted for sensitivity in the calculations. I asked him about it, and he said that for that situation (clayey foundation that had to be NC under 20-90 m embankment, and clayey embankment that showed contractive behavior under high confining stress), he didn't think the sensitivity would be high, and it would be pretty much offset by the strain-rate effects. Unless a clay is highly sensitive, it requires pretty significant strain to get from peak strength to post-peak/softened strength to remolded strength. Triaxial tests normally don't produce that much strain.

Every time I turn around the seismic analysis gets more and more complicated. It makes my head spin.